3.1. Introduction

Buildings provide shelter from climate for their occupants. Local resources, culture, climate and building traditions have significant effects on building design and construction. Since buildings have long lifetimes and use a significant portion of national assets, the construction industry is regulated by international or national codes and guidelines. Building codes are usually intended to ensure the quality of buildings: their safety and health properties and also the sustainable use of natural resources, such as energy, to reduce the environmental impact of the built environment. With increasing scientific understanding of the problems represented by moisture and dampness and recognition of the widespread nature of these problems, all countries must pay proper attention to how the quality of construction affects these problems. As construction practices vary among countries, it is difficult to create guidelines that are applicable in all regions. Nevertheless, many problems have the same origin and their solutions are thus similar in principle. The purpose of this chapter is to characterize the sources of and solutions for moisture and dampness problems in buildings and discuss the role of ventilation in controlling moisture and providing a healthy indoor environment.

Ventilation is intended to remove or dilute pollutants and to control the thermal environment and humidity in buildings. It must be sufficient either to remove pollutants and humidity generated indoors or to dilute their concentrations to acceptable levels for the health and comfort of the occupants and must be sufficient to maintain the building’s integrity. A number of reviews (Seppänen, Fisk, Mendell, 1999; Wargocki et al., 2002, Sundell, Levin, 2007) have shown an association between ventilation and health, although the precise exposure–response relationship differs by study. As exact values for ventilation cannot be identified and limit values have not been set for all pollutants, it is seldom possible to determine the necessary ventilation rates and the associated risks on the basis of pollutant concentrations. Selection of ventilation rates is based on epidemiological research, laboratory and field experiments, odour perception, irritation, occupant preferences, productivity, and experience.

Ventilation can be provided by various natural and mechanical methods. These usually improve health but may also have adverse effects (Seppänen, Fisk, 2002; Wargocki et al., 2002) if not properly designed, installed, maintained and operated (Mendell, Smith 1990; Seppänen, 2004; Mendell et al., 2007), as ventilation can then allow the entry of harmful substances that degrade the indoor environment. Ventilation also affects air and moisture flow through the building envelope and may therefore lead to moisture problems that degrade the structure. Ventilation changes pressure differences across a building and may cause or prevent the infiltration of pollutants from building structures or adjacent spaces. While ventilation is used to control humidity, under certain circumstances it can result in very high or very low humidity. Humidity is an important parameter when considering limiting the use of outdoor air ventilation.

In non-residential buildings and in hot climates, ventilation is often integrated with air-conditioning, which complicates the operation of these systems. The addition of humidifiers to ventilation systems or as stand-alone units can introduce excess humidity, chemicals (used to treat the water in humidification systems) or microorganisms that grow on components in humid locations, such as drip pans in air-conditioning units or humidifiers.

In some studies, the prevalence of symptoms of the sick-building syndrome has been associated with the characteristics of the heating, ventilation and air-conditioning system. On average, the prevalence of such symptoms was higher in air-conditioned than in naturally ventilated buildings, independent of humidification (Mendell, Smith, 1990; Seppänen, Fisk, 2002). The evidence suggests that better hygiene, commissioning, operation and maintenance of air-handling systems is particularly important in reducing the negative effects of heating, ventilation and air-conditioning systems (Mendell, Smith, 1990; Sieber et al., 1996; Seppänen, Fisk, Mendell, 1999; Mendell et al., 2003, 2006, 2008). Thus, moisture and microbial contamination – not only in the building structure or surfaces, but also in heating, ventilation and air-conditioning systems – has adverse health effects.

Exposure to pollutants in indoor air generated by indoor activities and emitted from indoor materials or ventilation systems can have a variety of effects with a broad range of severity, from the perception of unwanted odours to cancer (e.g. from radon). The effects can be acute or longer-term. The positive and negative effects of ventilation and ventilation systems include the following (European Collaborative Action on Urban Air, Indoor Environment and Human Health, 2003; Seppänen, Fisk, 2004).

• Ventilation dilutes the concentrations of (or disperses) airborne viruses or bacteria that can cause infectious diseases. Thus, higher ventilation rates reduce the prevalence of airborne infectious diseases (Fisk at al., 2002; Li Y et al., 2007).
• Some microorganisms can grow in cooling-coils and drip pans, as well as air humidifiers and cooling towers, thus causing respiratory diseases or symptoms, such as legionnaires disease and humidifier fever (Flannigan, Morey, 1996). Further evidence was provided by a blinded intervention study that showed significant reductions in respiratory and other symptoms during periods of ultraviolet irradiation of coils and pans in office buildings (Menzies et al., 2003).
• Ventilation rates below 10 l/s per person are associated with significantly higher prevalences of one or more health outcomes or with worse perceived air quality in office environments (Seppänen, Fisk, Mendell, 1999).
• Ventilation rates greater than 10 l/s per person, up to approximately 20–25 l/s per person, are associated with a significant decrease in the prevalence of symptoms of sick-building syndrome or with improved perceived air quality in office environments (Seppänen, Fisk, Mendell, 1999; Sundell, Levin, 2007).
• Improved ventilation can improve task performance and productivity in the office environment (Seppänen, Fisk, Lei, 2006).
• Ventilation rates below half an air change per hour are a health risk in Nordic residential buildings (Wargocki et al., 2002; Sundell, Levin, 2007).
• Ventilation rates up to 9 l/s per pupil in schools improve performance in school tasks (Wargocki, Wyon, 2006a,b).
• In comparison with natural ventilation, air-conditioning (with or without humidification) is often associated with a statistically significant increase in the prevalence of one or more sick-building syndrome symptoms in office buildings (Seppänen, Fisk, 2002).
• Indoor humidity is influenced by ventilation rates. Ventilation usually reduces indoor moisture levels. Very high indoor humidity is associated with increased growth of microorganisms such as mould and bacteria (Institute of Medicine, 2004).
• Relative humidity greater than approximately 50% increases indoor dust mite levels. Low ventilation rates may thus increase the prevalence or intensity of allergic and other symptoms (Flannigan, Morey, 1996).
• Pathogenic microorganisms can be transported by ventilation systems, and some evidence indicates that inadequate ventilation rates in general hospital rooms influence exposure to airborne infectious agents, such as tubercle bacilli (Menzies et al., 2000). Insufficient information is available, however, to specify minimum ventilation requirements for hospitals, schools and offices in relation to the spread of airborne infectious diseases (Li Y et al., 2007).
• Increased risks of lung cancer, heart attack, heart disease and stroke have been linked to exposure to environmental tobacco smoke and to radon decay products. Ventilation rates usually reduce the indoor concentrations of these agents.

The characteristics of ventilation include the rate, the type of system, the contaminants in indoor air and the physical characteristics of the indoor environment. These characteristics affect human responses individually and collectively. Also, ventilation can affect other aspects of the indoor environment that in turn affect health, including thermal conditions, indoor humidity, pressure differences across the building envelope, draught and noise.

The relationships between absolute humidity (moisture content of the air, measured in grams of water per kilogram of dry air), temperature and relative humidity are shown in Figure 2, where the relative humidity of air is presented as curves, temperature is on the vertical axis and the absolute humidity on the horizontal axis. The following are examples of how relative humidity is affected by temperature, i.e. heating.

Figure 2

Psychometric chart of relations between air temperature, absolute humidity (water in grams per kilogram of dry air) and relative humidity in the air.

• When outdoor air at a temperature of −8 °C and 100% relative humidity, which is usually very high in winter (point A in Figure 2), is brought to an indoor temperature of 20 °C, its relative humidity decreases to 15%. Thus, cold outdoor air used in ventilation is effective in carrying moisture from indoors.
• When air at a temperature of 15 °C and an absolute humidity of 5.5 g of water in 1 kg of dry air (corresponding to a relative humidity of 50%) is heated to a temperature of 18 °C, its relative humidity decreases to 40%. Thus, heating can help to prevent high relative humidity.
• When air at a temperature of 20 °C and a relative humidity of 58% (point B in Figure 2) is cooled to 15 °C, its relative humidity increases to 75%; when the temperature is decreased to 11 °C, the relative humidity reaches 100%. Water condenses on surfaces at such temperatures. Thus, when the temperature in houses decreases locally (e.g. at window panes, unheated sections of the house or poorly insulated walls), the relative humidity rises and accelerates microbial growth.

3.2. Sources of moisture

Moisture has become a major cause of building damage: it has been estimated (Bomberg, Brown, 1993; Ronald, 1994) that 75–80% of all the problems with building envelopes are caused to a certain extent by moisture. Haverinen (2002) conducted cross-sectional analyses in the Finnish housing stock and found that 38% of detached houses and 25% of apartments had notable or significant moisture problems. A study of 420 buildings in Sweden (Wessén, Honkanen, Mälarstig, 2002) showed that moisture problems, resulting in vivid microbial growth with microbial or chemical emissions from building materials and microbial metabolites, were present in 65% of the buildings. The presence of house dampness or mould (i.e. damp spots, visible mould or mildew, water damage and flooding) was reported by 38% of participants in a Canadian study (Dales, Burnett, Zwanenburg, 1991). These estimates show that moisture problems are a serious issue. They also have a strong economic effect, as repair of extensive problems is expensive. Pirinen et al. (2005) estimated that the cost of repairing microbiological damage that resulted in adverse health effects in Finland was €10 000–40 000 per case.

Phenomena related to water intrusion, dampness and excess moisture are not only harmful to the health of a building’s occupants, but they also seriously affect the condition of the building structure, which may diminish the indoor air quality of the building.

In addition to the risks of wooden structures rotting and microbial growth, building materials may also be degraded by chemical processes induced by moisture. For example, installation of hard surface flooring or even textile carpets over concrete that is not sufficiently cured and therefore still has excess moisture can result in chemical reactions that produce alcohols and other volatile organic compounds. Flooring is often installed before the concrete is sufficiently dry, because of pressure to complete a construction quickly.

In a study of moisture problems in 291 Finnish houses, Pirinen (2006) found that about two thirds of moisture problems could be found by non-destructive methods, such as careful visual inspection. The remaining third were hidden in the structures and required comprehensive investigations with destructive methods, such as drilling holes, opening structures or taking material samples. Rainwater or groundwater or plumbing leaks and dampness caused by capillary suction were the commonest causes of the problems; however, a detailed technical examination showed a wide variety of 82 mechanisms.

Examples of the commonest moisture-related problems in buildings are:

• rainwater or groundwater leaking into the enclosure (roof, walls, windows or foundation), often resulting in mould growth, peeling paint, wood decay or corrosion;
• plumbing leaks and spills, perhaps resulting from improper design, installation, operation or maintenance (e.g. failure to inspect and repair plumbing leaks);
• water wicking (capillary suction) through porous building materials (such as concrete or wood) from a moisture source (such as rainwater or plumbing water) to a material that does not tolerate wetting;
• rainwater, condensation or plumbing water running along the top or bottom of a material (bridging), for example, along the top for some distance or clinging to the bottom of a truss, rafter, I-beam, floor joist or suspended ceiling track before falling or being absorbed by a porous material;
• infiltration of warm, moist outside air through cracks and holes in the enclosure during warm, humid weather – which can cause condensation on materials that are cooler, because they are part of the air-conditioning system – have been cooled by air-conditioned indoor air or are in cool basements or crawl spaces;
• exfiltration of warm, moist indoor air through cracks and holes in the enclosure during cold weather, which can cause condensation in wall and ceiling cavities and attic spaces;
• intentional or accidental vapour barriers in the wrong place, which can lead to condensation in the building enclosure;
• unvented or poorly vented sources, such as swimming pools, spas, aquariums, dishwashers, combustion devices, kitchens and baths, from which water may condense in the building enclosure or, if indoor humidity levels are high enough, on materials in the space itself (e.g. ceilings, walls, furniture, cold-water pipes or air-conditioning air supply diffusers);
• insufficient dehumidification by heating, ventilating and air-conditioning systems, which may result in levels of interior humidity that are high enough to cause mould to grow on furniture, walls, ceilings or air-conditioning supply diffusers;
• poor condensate drainage due to heating, ventilation and air-conditioning system deficiencies; condensation from cooling coils may overflow drain pans or leak from condensate drain lines; and
• enclosure of wet materials in building assemblies during construction by materials that are prone to moisture problems and grow mould, delaminate or do not cure properly.

The technical causes of failure to control water damage, dampness or moisture are often closely connected to the climate. The prevailing temperature, humidity, rain and wind conditions regulate much of the principles and practices of construction, such as the foundation, insulation, structure of the building envelope and ventilation system. Indoor humidity is also physically connected to the outdoor climatic conditions. Therefore, the problems of building moisture and dampness, microbial contamination, repair and control vary with the climatic zone. Nevertheless, regardless of the climate, the prevention and control of moisture problems and their subsequent effects should be addressed in the early phases of building construction and in sustained maintenance of a building. Methods for controlling moisture are discussed in section 3.9.

3.3. Mould and mites as indicators of building performance

Measures can be taken to prevent mould, even though all the mechanisms by which microbial growth affects health are yet unknown. As described in section 2.2, the primary factor in the control of mould growth is moisture, especially relative humidity indoors and in structures. (Note that relative humidity above the substrate is the same as water activity, which is often used to describe mould growth conditions.)

The presence of mould and mites in a building is associated with the relative humidity of indoor air. As noted in section 2.2, mites require a relative humidity in excess of 45–50%. Therefore, to prevent the multiplication of dust mites in houses, the relative humidity during the heating season should be below this value.

Indoor relative humidity can be readily controlled by ventilation or air conditioning (see sections 3.4 and 3.8), by keeping it below the limit values for mould growth or mites. Even under such conditions, however, relative humidity can be higher on colder internal surfaces. This should be avoided by relevant hygro thermal design, which includes heat and moisture transfer analyses, application of the necessary thermal insulation and avoidance of thermal bridges. Relative humidity is much more difficult to control in structures than in indoor air as it is often dominated by outer climatic factors. Close consideration must be given to the hygrothermal performance of building assemblies, including mould growth, for the prevention of mould and moisture problems. Limit values for relative humidity on surfaces and structures and the mould growth models described in this chapter should be applied in the design of new buildings, in repairs and in field investigations of existing buildings, following the principles of hygro thermal design.

Even if the lowest relative humidity for germination of some species of fungi is 62–65%, experiments on common building and finishing materials indicate that susceptible surfaces can be kept free of fungal growth if the relative humidity is maintained below 75–80%. Mould fungi do not grow below a relative humidity of 80% (Adan, 1994) or below 75% within a temperature range of 5–40 °C (Viitanen, Ritschkoff, 1991).

After an extensive analysis of published data on laboratory experiments, Rowan et al. (1999) recommended that the relative humidity be maintained below 75% to limit fungal growth in buildings. Johansson et al. (2005), after a review of the literature, described the material-specific critical moisture conditions for microbiological growth: the critical relative humidity (maximum long-term relative humidity allowed for non-growth) was 75–90% for clean materials and 75–80% for contaminated or soiled materials (Table 2).

Table 2

Critical relative humidity for various groups of materials.

In reality, relative humidity and temperature on surfaces and in building structures change all the time, and mould formation is a time-dependent process that can be described by relative humidity, temperature and building material. Obviously, the relative humidity of the outdoor climate does not permit the relative humidity in building structures to be kept below 75–80%. Therefore, a single limit value is inadequate for the hygrothermal design of building structures. In order to describe mould growth formation, dynamic models are needed that can take into account fluctuations in relative humidity and the time required for mould growth on a given material. To assess dynamic mould formation, dynamic data are required on the relative humidity and temperature in the structure or material studied, which can be either measured or calculated with simulation tools.

Viitanen et al. (2000) provided a differential equation (Eq. 1) for varying temperature and humidity conditions of pine and spruce. The equation (and the model based on it) predicts mould growth from an index, M, that varies between 0 and 6. A value of 0 indicates no mould growth; 1 indicates that some growth can be detected under a microscope, 3 that some mould growth can be detected visually, and 6 that very heavy, dense mould growth covers nearly 100% of the surface. The model takes into account temperature, surface relative humidity, the type of wood and the quality of the wood surface; it can also account for delay in mould growth due to temporarily low relative humidity. Equation 1 is as follows:

where M is the mould growth index, t is time calculated in days, T is temperature (0.1–40 °C), RH is relative humidity (%), W is the wood species (pine = 0, spruce = 1) and SQ is a factor that describes the quality of the wood surface (a resawn surface after drying = 0, a kiln-dried surface = 1). The coefficients k₁ and k₂ define the growth rate under favourable conditions and the upper limit of the mould growth index. This model is useful for analysing wooden building structures; however, it can be used only if the value of the mould growth index to be used as a design criterion has already been decided. Arguably, a mould growth index of 1 (some growth can be detected under a microscope) would be the relevant choice in most cases.

Another widely used mould growth model, which is applicable to many materials, is the isopleth model and its extension, the transient biohygrothermal model (Krus, Sedlbauer, 2007). Isopleths show the mould growth rate and the lowest boundary lines of possible fungus activity (lowest isopleth for mould). Isopleths are given for three categories of substrates (Figure 3), which differ in the availability of nutrients for fungi. The common building materials belong to categories 1 and 2. If we consider only the fungi that are discussed in the literature, because of their possible health effects (e.g. A. fumigatus, A. flavus and S. chartarum), the lowest isopleth for mould is slightly higher than that given in Figure 3 (Krus, Sedlbauer, 2002).

Figure 3

Isopleth systems for three categories of substrate to determine the influence of the substrate on the formation of mould. Source: Adapted from Sedlbauer (2001). Note. days, germination time in days; mm/day, germination rate; LIM, lowest isopleth for mould (more...)

By use of the germination time and growth rate, the isopleth model allows an assessment of mould growth similar to that allowed by the model of Viitanen et al. (2000). Application of the isopleth model indicates the degree of germination (between 0 and 1) and growth in millimetres per day when the degree is one. The biohygrothermal model (Sedlbauer, 2001) allows an even more accurate assessment of mould growth, as it describes moisture transfer at the spore level, thus taking into account interim drying of fungal spores during transient microclimatic boundary conditions. Application of this model also gives mould growth in millimetres. The disadvantages of the isopleth models are that they can be used only for assessing cumulative mould growth and do not allow assessment of mould growth decrement (as in the model of Viitanen et al.) when conditions became dry.

Dynamic mould growth models are very useful in assessing the hygro thermal performance of building assemblies. Predicted mould growth also correlates reasonably well with measured concentrations of spores (Pasanen et al., 2001), but such correlations are limited and can be application-specific. Mould growth models can support field investigations when the spore concentrations of material samples are measured. The availability of predicted mould growth makes it possible to conclude whether the measured spore concentration is typical of the structure or material being studied or if it indicates some failure in moisture performance, such as accidental leaks.

The main challenge of field investigations is to decide which contaminated materials should be removed and which can be left in building assemblies with a reasonably low risk of indoor climate problems. Spore concentrations in material samples can be measured easily, and mould growth can be predicted, but little is known about how indoor concentrations can be predicted from these results, even when epidemiological indoor spore concentration guidelines are available. For example, a limit value of 500 CFU/m³ for airborne fungal spores in indoor air in urban areas in winter is used in Finland (Ministry of Social Affairs and Health, 2003). This value was derived by comparing buildings with and without moisture problems; concentrations below 500 CFU/m³ were found to be typical in Finnish buildings without moisture problems in winter. This value would be a straight-forward guideline for design if a method of calculation existed. However only limited information is available on fungal spore transport – for example, penetration factors must be determined by laboratory measurements (Airaksinen et al., 2004a). Thus, in order to realize the possibilities of mould growth models fully, relevant methods for calculating fungal spore levels and indoor air target values should be available.

3.4. Ventilation performance

Ventilation (outdoor airflow into a building) must be adequate to remove and dilute pollutants and humidity generated indoors, although the first alternative for improving indoor air quality should be control of pollutant sources. Ventilation should be energy efficient and arranged so that it does not degrade indoor air quality or climate and does not cause any harm to the occupants or to the building. Ventilation rates should be based on pollution loads, moisture generation and use of the building. To the extent possible, outdoor pollutants should be removed from the air before the air is brought inside the building.

The concentration of indoor air pollutants or moisture can be used to calculate the ventilation rate needed for dilution of the pollutants or moisture to an acceptable steady-state level. Removal of humidity generated indoors can be calculated for the steady state from Equation 2. The equation assumes that the concentrations at different locations indoors are equal, that the indoor concentration equals the exhaust air concentration and that the moisture contents of the building and the interior materials are in equilibrium. In the following equation:

q_vSUP is the volume flow rate of supply air (in m³/s), G is the indoor moisture generation in the room (in g/s), υ_IDA is the moisture per volume of indoor air in the room (in g/m³) and υ_SUP is the moisture per volume of supply (outdoor) air (in g/m³).

Ventilation removes humidity at a rate of q_vSUP × (υ_IDA−υ_SUP). More humidity is obviously removed (ventilation is more effective) at low outdoor air humidity. As cold air contains less absolute humidity than warm air, humidity removal is most effective at low outdoor temperatures. In hot climates, the outdoor humidity may be higher than the indoor humidity. In such cases, ventilation brings in humidity, and humidity is usually removed by air-conditioning (condensation on a cooling coil).

While high indoor relative humidity is problematic in mild and hot climates, very low relative humidity may be a problem in a cold climate. In a cold climate, low outdoor humidity during winter combined with overheating may decrease the indoor relative humidity to levels that provoke skin symptoms and nasal dryness and congestion (Reinikainen, Jaakkola, 2003). Kalamees (2006) measured an average relative humidity during winter of 26% in 94 Finnish houses and 32% in 27 Estonian houses. Humidified indoor air imposes a serious vapour pressure load on the building envelope; in hot climates, a similar effect is caused by dehumidification.

All relevant pollutants should be checked to determine which are the most critical. As a rule, source control is preferable to ventilation. Equation 2 above is valid for a steady-state situation (default situation); it also assumes that all pollution generated in a room is carried out with the airflow: no other pollutant sinks are assumed to be present. When the emission period of moisture is short, the stationary equilibrium concentration may not be achieved, or the airflow can be reduced for a given maximum concentration.

Kalamees (2006) measured average indoor moisture generation rates of 5.9 kg/day per house and 1.9 kg/day per person in 101 Finnish houses. Indoor humidity was calculated with Equation 2 with reasonable accuracy from data on ventilation and standard moisture generation from people, pets, house plants, cooking, showering, saunas and drying clothing.

Usually, moisture generation indoors is not continuous or in a steady state but is intermittent. To take this into account, a dynamic calculation with relevant indoor humidity generation profiles and material data (including moisture buffering effects) is needed. Many simulation tools are available for such calculations. The capacity of building and interior materials to absorb and release moisture has a significant effect on indoor humidity fluctuations and may have consequences for moisture damage and dampness. Moisture buffering effects are especially strong at low ventilation rates (Kurnitski et al., 2007). The recent general trend has been towards buildings with significantly lower moisture capacity, and this, together with generally reduced ventilation rates, may affect the prevalence of dampness-related problems.

3.5. Ventilation systems

Ventilation systems can be classified as natural, mechanical or a mixture of the two.

3.5.1. Natural ventilation

Natural ventilation usually uses no fans to move the outdoor air into and out of a building. Thus, the ventilation rates in natural ventilation depend on the size and distribution of the openings in the building envelope and on the magnitude of the driving forces (pressure differences), which are the stack effect and wind pressure. The stack effect depends on the height of the building (stack) and the temperature difference between indoor and outdoor air. Wind pressure depends on wind speed and direction. As the forces for air movement depend on the weather conditions, the openings in the building envelope must be controlled according to the weather as well as to pollutant and moisture generation. If the indoor environment is conditioned, the energy used to heat, cool, humidify or dehumidify the ventilation air can be significant. Therefore, ventilation rates that are too high should be avoided. High ventilation rates are not a problem in climates where indoor environments are not conditioned.

In cold and hot climates, the configuration and size of openings are a significant aspect of ventilation design. As the weather fluctuates constantly, the challenge of designing natural ventilation is to harness forces determining air movement, so that the flow into a space is maintained at the desired rate.

Often the term natural ventilation is applied to buildings that are ventilated haphazardly by relying on the leakage (cracks or porosity of structures) of the building combined with window openings, which can result in poor comfort and uncontrolled heat loss. Natural ventilation needs careful design and construction in order to supply adequate ventilation rates, although the occupant will usually have to adjust the ventilation openings when necessary. Inadequate ventilation rates can lead to serious dampness and health problems and moisture damage. Like any other technical system, natural ventilation has advantages and disadvantages. Its advantages include:

• its suitability for many types of buildings in mild or moderate climates;
• the associated open window environment, which is popular, especially in pleasant locations and mild climates;
• high airflow rates for cooling and purging if there are enough openings;
• short periods of discomfort during periods of warm weather;
• no need to provide space for a ventilation plant;
• minimum maintenance;
• usually, a lower cost of installation and operation than that of mechanical ventilation; and
• the absence of fan or system noise.

The disadvantages of natural ventilation include its:

• lack of suitability for severe climates, where the ingress of very cold air causes discomfort, condensation and high energy loss;
• inadequate control over the ventilation rate, which can lead to poor indoor air quality and excessive heat loss;
• varying airflow rates and patterns of airflow;
• impracticality of fresh air delivery and air distribution in large, deep, multiroom buildings;
• inadequacy in cases of high heat gains;
• unsuitability for noisy and polluted locations;
• security risk if ventilation depends mainly on openable windows;
• impracticality of heat recovery from exhaust air;
• obligation to adjust openings as necessary;
• inability to filter or clean incoming air;
• requirement for large-diameter ducts and restrictions on routing air in ducted systems; and
• increased occurrence of dampness and fungal growth in humid climates.

Various techniques or combinations of techniques are used to provide natural ventilation, including single-sided ventilation, cross-flow ventilation, wind towers, stack ventilation and atrium ventilation. They are described briefly in Box 2. In practice, controlled natural ventilation is best suited for buildings located in mild-to-moderate climates away from inner city locations. The buildings should be designed from the beginning for natural ventilation. This type of ventilation cannot be used in deep, multi-room buildings if it is constrained by climate and outside noise and pollution. The building types for which natural ventilation is suitable include dwellings (individual and apartments), small-to-medium-sized offices, schools, small-to-medium retail premises, recreational buildings, ware-houses and industrial premises.

BOX 2

Types of natural ventilation. Single-sided ventilation (see diagram A) is the process by which air enters a building on the same side of a space as it leaves. Single-sided ventilation through a small opening is driven by random turbulent fluctuations (more...)

With natural ventilation, the required ventilation rate cannot be maintained in all weather conditions. Also, the building envelope cannot be made airtight, as is common practice in buildings with mechanical ventilation – to reduce energy losses due to infiltration and exfiltration. Figure 4 illustrates some of the problems related to natural ventilation systems that rely on temperature differences as a driving force. As the figure shows, under some weather conditions, pressure differences will reverse the airflow (blue arrows), and the exhaust air stacks, which may be contaminated, become supply routes and spread the pollutant into living rooms.

Figure 4

Under some weather conditions, the flow in the stack may be reversed (blue arrows) in natural ventilation systems that rely on temperature differences as a driving force.

3.5.2. Mechanical ventilation

Mechanical ventilation is based on the requirement that the ventilation rate is maintained in all weather conditions without involving the occupants of the building. When ventilation is provided by a mechanical supply and exhaust system, the building envelope can be made airtight, and energy losses due to infiltration and exfiltration can therefore be reduced. A tight building envelope also improves sound insulation and reduces the transfer of external noise into the building. The energy efficiency of ventilation can be further improved through heat recovery from exhaust air, demand-controlled ventilation depending on occupancy, moisture or air quality factors. Air supplied for ventilation can be cleaned of outdoor air pollutants. Also, heating and cooling can easily be combined with mechanical ventilation systems. Moreover, mechanical ventilation systems may also control pressure differences over the building envelope and prevent moisture damage in building structures. It can be used in any type of building and allows freedom in architectural design, which may be the major reason for its rapid acceptance in modern building practice.

3.5.2.1. Mechanical exhaust ventilation

In mechanical exhaust ventilation systems, air is exhausted from rooms with greater pollutant generation and lower air quality. Air infiltration through the building envelope brings outdoor air for ventilation into the building. In apartment buildings, exhaust from the different floors can be connected to the same duct (Figure 5) if the pressure drop in the exhaust grille is high enough to prevent airflow from floor to floor. A central fan serves all the apartments. Room airflow can be controlled by adjustable grilles, according to humidity or the concentration of carbon dioxide or other pollutants, or by occupancy sensors.

Figure 5

Mechanical exhaust ventilation system serving one or several apartments.

The advantages of mechanical exhaust ventilation are a constant ventilation rate and the small negative pressure in the building, which prevents moisture migration into external walls and prevents condensation and (consequently) mould growth. It has several disadvantages, however. Air infiltrates through the building envelope, which creates draughts in winter in cold climates. Also, it is difficult to recover heat from exhaust air, and recovered heat cannot be used to heat ventilation air, although it can be used to preheat domestic hot water with a heat pump. As the exhaust usually comes from kitchens and bathrooms, the ventilation supply airflow is not evenly distributed in the bedrooms and living rooms. Furthermore, the distribution of outdoor air for ventilation depends on leakage in the building envelope.

3.5.2.2. Mechanical supply and exhaust ventilation

In mechanical supply and exhaust systems, the air is supplied via ducts and fans to bedrooms and living rooms in residential buildings and typically exhausted from kitchens, bathrooms and bedrooms. In other types of buildings, it is supplied to all occupied spaces. Exhaust air may flow through a heat exchanger before it is discharged outdoors. In the heat exchanger, a major part of the heat is recovered and used to heat the outdoor air for ventilation. This is usually the most economical use of recovered heat, as the need for heating is met by the available heat. Another use of recovered heat is for domestic hot water.

In an apartment building, a mechanical supply and exhaust system can be centralized (Figure 6) or decentralized For heat recovery to be beneficial, the heat recovered must exceed the additional energy expended by the fans. Where carbon is the basis for comparison and where the unit-embodied carbon is higher for electricity than for the primary heating fuel source (e.g. natural gas), its use in a moderate climate might be difficult to justify without a carefully designed ductwork system, an efficient heat exchanger and energy-efficient fans and motors. In decentralized systems, more components require maintenance and are more widely spread out, but ventilation is easier to control by demand.

Figure 6

Centralized mechanical supply and exhaust system with heat recovery in an apartment building.

3.6. Outdoor and other sources of pollution related to ventilation

Outdoor air used for ventilation may also be a source of pollution, containing particulate matter, particulates of biological origin (e.g. microorganisms and pollen) and various gases (e.g. nitric oxides and ozone). In naturally ventilated buildings, all pollutants in outdoor air enter the building. In mechanically ventilated buildings, it is possible to reduce the concentrations of pollutants in outdoor air before ventilation air enters. Nevertheless, the ventilation system may also be a source of indoor air contaminants.

3.6.2. Pollutants in air-handling equipment and systems

Several studies have shown that the prevalence of symptoms of sick-building syndrome is often higher in air-conditioned buildings than in buildings with natural ventilation (Mendell, Smith, 1990; Seppänen, Fisk, 2002). One explanation for the association between sick-building syndrome and mechanical heating, ventilating and air-conditioning systems is microbial and chemical pollutants, which are emitted by heating, ventilating and air-conditioning components and ductworks. Microbial growth on wet surfaces in air-handling systems can be a major source of pollution in old buildings. Mendell et al. (2007) corroborated their earlier findings (Mendell et al., 2006) that moisture-related heating, ventilation and air-conditioning components such as cooling coils and humidification systems, when poorly maintained, can be sources of microbiological contaminants that cause adverse health effects in occupants, even if the causal exposures cannot be identified or measured. Poor condition and maintenance of such systems in general has been identified as a risk factor (Mendell et al., 2003, 2006).

It has been also shown that sensory observations of chemical emissions from ventilation systems and heating and air-conditioning components are significant and play a major role in the perceived indoor air quality of a space (Fanger, 1988). The measured emission rates of volatile organic compounds from materials vary considerably (Morrison, Hodgson, 1996; Morrison et al., 1998). Materials with large emissions of volatile organic compounds are used in duct liners, neoprene gaskets, duct connectors and duct sealants, whereas high-surface-area materials, such as sheet metal, have lower emission rates. The emission of volatile organic compounds may increase when components and surfaces become dirty due to poor maintenance. This hypothesis is supported by the results of several field studies, which showed an association between indoor air problems and the dirtiness of heating, ventilation and air-conditioning systems (Sieber et al., 1996; Mendell et al., 2003). Siber et al. (1996) reported that an increased risk of multiple respiratory symptoms was related significantly to poor cleanliness of such systems (with a risk ratio (RR) of 1.8, to dirty filters (RR = 1.9), to debris in the air intake (RR = 3.1) and to dirty ductwork (RR = 2.1). All are indicators of sources of chemical pollutants in heating, ventilation and air-conditioning systems. Another major source of pollution is used air filters loaded with dust, which can react with other chemicals in the outdoor air and generate new chemicals that pass into the ventilation air through the filter (Wargocki, Wyon, Fanger, 2004).

The importance of clean air-handling systems has been recognized in national guidelines and standards in many countries (Verein Deutscher Ingenieure, 1997; Finnish Society of Indoor Air Quality and Climate, 2001; REHVA, 2007a,b; Pasanen, 2007).

Specific criteria for ventilation systems with regard to hygiene and other important factors are given in Box 3. More information is given in European Standard EN 13779 (CEN, 2007a) and ASHRAE standards 62.1 and 62.2 (ASHRAE, 2007a,b).

BOX 3

Specific criteria for ventilation systems. Ventilation air should be distributed to the rooms of the building according to their design and use. In practice, therefore, the system should be designed and constructed so that airflow can be measured and (more...)

3.7. Ventilation and spread of contaminants

3.7.1. Ventilation and pressure differences in buildings

Ventilation, the stack effect and wind affect pressure differences over building structures. Pressure difference is a driving force for the airflows that transport water vapour and gaseous or particulate contaminants. One of the most important issues in respect of healthy buildings is keeping structures dry and preventing condensation of water in and on them. Convection of air and moisture through the building envelope can impose severe moisture loads on structures. In cold climates, the water content of the air is usually higher indoors than out. If the pressure is higher indoors, air with a high moisture content will flow into the cold structure, and water vapour may condense. The opposite is true in hot climates. Thus, exfiltration in cold climates and infiltration in hot climates can cause moisture accumulation or condensation, leading to microbial growth on materials, reduction of thermal insulation properties, changes in other material properties and even structural deterioration. The relative humidity at the junction of floors and external walls in multi-storey platform timber-frame houses can be high, resulting in a risk of mould growth and rot fungi when there is positive air pressure inside the building (Kilpeläinen et al., 2000). The simulation results of Janssens and Hens (2003) showed that, even when a roof design complies with condensation control standards, a lightweight system remains sensitive to condensation because of air leakage through the discontinuities, joints and perforations that are common to most construction methods.

The risk of condensation in buildings in cold climates can be reduced by higher exhaust flow rates than supply flow rates. In severe climates, especially, a pressure slightly lower indoors than outdoors (by not more than 20 Pa) can help avoid damage to structures caused by moisture. In hot, humid climates, the problem is reversed, and the supply airflow should be greater than the exhaust airflow. In a cold climate, buoyancy will cause a significant stack effect in winter. If the temperature difference is 40 °C, the pressure gradient in the room is about 2 Pa/m. In a two-storey house, this will lead to a positive pressure of 5 Pa on the first floor at ceiling height and −5 Pa at ground floor level when the air leakage flow paths are equally distributed. Figure 7 shows the measured and calculated results for a typical Finnish house with an average building leakage rate of four air changes per hour at a 50 Pa pressure difference; the air tightness of the building envelope is typically expressed by the airflow rate through the building envelope when the pressure difference between the outside and the inside is 50 Pa.

Figure 7

Pressure differences across a building envelope at 40 °C (red) and 20°C (blue) temperature difference between indoor and outdoor air. Source: Kurnitski (2006). In: Rakentajain kalentari, Rakennustieto Oy, 2006. Note. The left-hand side (more...)

Field measurements and computer simulations of air pressure conditions in typical residences in a cold climate show that there is almost always a positive and a negative air pressure difference across the building envelope of detached houses and apartment buildings (Kalamees et al., 2007). In winter, continuous positive pressure is found at ceiling level on the top floor and negative pressure at floor level on the bottom floor. Wind influences air pressure peak values but not the average values. For detached houses, Kalamees et al. (2007) recommended a design value of ± 10 Pa air pressure difference across the building envelope. They concluded that it is difficult to control air pressure differences in normal and leaky houses (air leakage rate, ≥ 4 air changes per hour at 50 Pa) with ventilation, as a ± 15% difference in airflow had only a minor influence on the air pressure difference. Pressure can be effectively controlled by ventilation only in extremely airtight houses (building leakage rate < 0.5 air changes per hour at 50 Pa).

Pressure differences are used in certain spaces (including buildings for human occupancy) designed for overpressure in relation to outdoor or adjacent spaces. Clean rooms, rooms for sensitive electronic data-processing equipment and operating theatres in hospitals are examples of such spaces. In buildings with no special requirements or emissions, ventilation systems are designed for neutral pressure conditions. Pressure conditions should be monitored continuously in spaces where there are heavy emissions of impurities. Also, the air pressures in stairways, corridors and other passages should be designed so that they do not cause airflow from one room or apartment to another.

3.8. Moisture control in buildings

Moisture control in buildings includes measures for choosing building materials and measures for controlling indoor humidity by ventilation (see section 3.4). In most climates (e.g. Europe), adequate ventilation, heating with adequate moisture control and thermal insulation of building structures will be enough to keep relative humidity on and in building structures within acceptable limits. Thus, microbial growth and dampness are usually indicators of construction faults, moisture damage, or malfunction of ventilation or heating systems.

Developments in building assemblies and materials and the requirements of good indoor climate and energy performance have resulted in wall, attic and floor assemblies that, once wet, take longer to dry and are more likely to allow condensation. Many contemporary assemblies contain materials that are more likely to support mould growth than the simple, often massive assemblies used in eras when the energy performance of a building was considered less important. When materials, methods and equipment change each year, it is no longer practical to wait for rules of thumb to emerge from generations of successful and failed buildings. Assemblies must be based on a better understanding of their thermal, moisture and airflow properties and those of their materials.

To control moisture, to ensure long building life and good indoor air quality, three goals must be set and met:

• control liquid water;
• manage indoor humidity levels and condensation; and
• select materials and hygrothermal assembly designs that minimize mould growth (see performance indicators in section 3.3) and other moisture problems.

For effective control of liquid water intrusion, measures must be applied during the design and construction phase, as well during operation and maintenance. Barriers to water entry should be established and maintained with capillary breaks in the building enclosure. Moisture migration by capillary action can be broken by an air space or water-impermeable material, such as drainage planes behind the cladding of wall assemblies. Precipitation shed from a building should be deflected by continuous, effective site drainage and a storm-water runoff system. Plumbing leaks can be prevented by locating plumbing lines and components where they are easy to inspect and repair, are unlikely to freeze and, if they leak, will not soak porous insulating materials. Building materials that are sensitive to moisture should be kept dry during transport and on-site storage; if they accidentally get wet, they should be dried before being enclosed into building assemblies.

Effective control of condensation requires careful design of building assemblies and heating, ventilation and air-conditioning systems. Continuous thermal barriers should be designed, installed and maintained to ensure that interior surfaces (including inside building assemblies) remain warm or cold, as designed. Building air pressure relationships should be controlled between indoor spaces and outdoors and between spaces to minimize the flow of warm, humid air towards cold surfaces. This affects and is affected by the design of both the enclosure and the heating, ventilation and air-conditioning system. Continuous air barriers should be designed and maintained to minimize infiltration of cold or hot, humid outside air into building assemblies or interior spaces. Also, heating, ventilation and air-conditioning systems should be designed, installed and maintained to manage indoor relative humidity and to effectively exhaust known sources of moist air.

Building assembly design should include selection of materials that minimize mould and other moisture problems – that is, building materials, equipment and design assemblies that can withstand repeated wetting in areas that are expected to get wet, such as spaces where water is used (kitchens and bathrooms) and below-grade wall and floor insulation and finishes.

Moisture control does not mean the elimination of water. Many materials can safely get wet, as long as they dry quickly enough. Others are so easily damaged by water that they must never get wet, like paper-faced gypsum board. Concrete, while porous and clean, is resistant to mould growth, contains no nutrients for mould or decay organisms and is stable when wet. It contributes to moisture problems only because it can wick liquid water to more vulnerable materials, and it takes a long time to dry.

Hygrothermal assembly design is an essential element for preventing moisture damage and guaranteeing longer service life for buildings. Several gaps in everyday design practice can affect potential moisture problems. Hygrothermal design consists of selecting and dimensioning structures, making detailed drawings and selecting specifications for critical joints and other documents and rules for operation and maintenance (Lehtinen, 2000). In many cases, hygrothermal design is conducted by engineers (architectural, structural and heating, ventilation and air-conditioning design) and has not been established as a separate area. To assess and predict the long-term hygrothermal performance of a building envelope, calculations or experimental investigations are needed. Because laboratory and field experiments are expensive and time-consuming, calculations and simulations are increasingly used to assess the hygrothermal behaviour of building components. During the past few decades, computer programs for simulation have been developed, and advanced commercially available simulation tools for personal computers have replaced laboratory tools; they are easy to use and include databases of building materials and climatic data (Kalamees, 2006).

According to Burke and Yverås (2004), the use of simulation tools in hygrothermal design by consulting companies is limited because they are too expensive, too difficult to learn and too time-consuming to run. This is perhaps the reason why, as a rule, hygrothermal dimensioning in consulting companies is limited to calculation of U-values and moisture accumulation in structures due to water vapour diffusion on the basis of the Glaser method (Lehtinen, 2000). Nevertheless, consulting companies have had to use hygrothermal simulation programs in complicated renovation cases that require sophisticated analyses. In the design of new houses, the solutions already developed, tested and standardized are easier to use, accounting for the limited use of simulation tools.

In research, simulation tools are used to solve hygrothermal problems of building assemblies. There are good examples for crawl spaces (Kurnitski, 2000a; Airaksinen, 2003), attics and roofs (Salonvaara, Nieminen, 2002; Kalagasidis, Mattsson, 2005) and churches (Häupl, Fechnerm, 2003; Schellen et al., 2004). Such research leads to solutions for typical building envelopes in certain climatic areas (Burch, Saunders, 1995; Karagiozis, 2002; Mukhopadhyaya et al., 2003) and the hygrothermal performance of renovated building envelopes, such as internal thermal insulation (Cerny, Madera, Grunewald, 2001; Häupl, Jurk, Petzold, 2003). It is important that such results be taken into account in revision of building codes and guidelines.

3.9. Measures to protect against damage due to moisture

Moisture control is the main method for controlling mould and mites. The reasons for mould and mites are the same all over Europe, but the methods of control may differ. The effectiveness of a measure to control health determinants indoors can vary by climate, existing building construction, and heating, ventilation and air-conditioning systems.

Wet surfaces in air-conditioning and ventilation systems are always at risk of microbial contamination and pollution. Air-conditioning systems should be designed and operated so that microbial growth is avoided. Guidelines are available for hygienic design and operation of air-conditioning and ventilation systems (REHVA, 2007a,b), but the maintenance of air-handling systems is often neglected. It is important to keep all surfaces of an air-handling system clean.

The effectiveness of actions to control moisture also depends on whether they are for new or existing buildings. A wider variety of control methods can be applied in new constructions (Table 3) than in existing buildings (Table 4). Measures to control moisture may increase or decrease the construction or operation costs of a building, as indicated in the tables. In new constructions, most of the proposed measures reduce the operating costs by lowering energy consumption but slightly increase construction costs. It is particularly important to have an adequate ventilation rate in buildings when the thermal performance and airtightness are improved to meet energy performance criteria.

Table 3

Methods for controlling moisture in new buildings with better building codes.

Table 4

Methods for controlling moisture in existing buildings.

The measures against moisture damage described in Table 3 are grouped into three categories. The first group covers building construction. Structures should be designed, constructed and maintained so as to withstand the indoor humidity without harmful condensation of water vapour that migrates to the structure.

The second group covers ventilation. Its effect is twofold: it can remove indoor generated moisture directly and reduce the level of moisture. In some climatic conditions (summer in some coastal areas), the outdoor moisture content may be high, and ventilation is not effective. The third group of measures covers heating. Effective central heating, preferably by cogenerated district heating, decreases indoor relative humidity and decreases the generation of moisture and pollutants from combustion.

In existing buildings (Table 4), the primary means for controlling moisture is changing consumer behaviour and improving ventilation. The measures for dealing with the building envelope are expensive and not as cost effective in the short term as those for ventilation. In the long term, however, ventilation measures may be more costly. Additional measures to reduce moisture can be taken by the occupants. Indoor relative humidity can depend on the air circulation in a room. Soft furniture, bookshelves close to exterior walls and carpets can increase the relative humidity indoors locally and cause mould growth. The relative humidity in carpets may be 10% higher than in room air; textile surfaces and carpets are good reservoirs for microbiological contamination and are very difficult to clean when contaminated. Heavy wall-to-wall carpeting is a risk factor in a humid climate.

Indoor air dehumidification can be used, in which dehumidified air is circulated through a cooling coil, where water is condensed and drained. After cooling and dehumidification, the air can be heated with the same device back to room temperature. These devices are particularly useful for temporary applications. From the standpoint of energy use, it is important that air be dehumidified to a relative humidity not much lower than the limit value for the growth of mites (45%).

The main methods for preventing moisture in buildings are summarized in Figure 8.

Figure 8

Controlling moisture through building design and construction. (1) Drain rainwater and surface water from near the building. (2) Drain the foundations with an underground pipe. (3) Install a tilted (rather than a flat) roof in wet climates. (4) Install (more...)

3.10. Conclusions and recommendations

While microbial growth and health outcomes are consequences, their common denominator is undesired moisture behaviour (i.e. excess moisture in building assemblies or on surfaces). The reasons for the presence of mould and mites are the same all over the world, but the methods of control may differ. Water intrusion, dampness and moisture-related phenomena are not only harmful for occupants’ health but are also a serious risk to building structures. In addition to risks of rotting wooden structures and microbial growth, building materials can also be degraded by chemical processes induced by moisture.

Moisture control, including ventilation, is the main method for containing mould and mites. The problems of building moisture and dampness, microbial contamination, repair and control practices vary by climate zone. Regardless of the climate, however, the prevention and control of moisture and the subsequent effects should be addressed in the early phases of building construction and by sustained maintenance.

Effective moisture control includes control of liquid water, control of indoor humidity levels and condensation and selection of materials and hygrothermal assembly design that minimize mould growth and other moisture problems. Dynamic simulations of heat and moisture and mould growth models are useful for assessing the hygrothermal performance of building assemblies. Methods should be further developed to allow calculation of fungal spore transport through building assemblies.

It is evident that a sufficient flow of outdoor air for ventilation is necessary: to remove indoor-generated pollutants and moisture from indoor air or to dilute their concentrations to acceptable levels for occupants’ health and comfort; and to maintain building integrity. Ventilation can be provided by either natural or mechanical means, but both need careful implementation to avoid malfunctioning. Failures in ventilation may lead to serious health problems and damage to building construction.