When it comes to mould infestation, a distinction should be made be-tween the following:
a) constructional influencing factors such as insufficient or inadequate thermal insulation, thermal bridges, poor moisture buffering of materials, damaging water-permeable areas in the building envelope, other leaks, moisture in a new building and rising moisture due to insufficient damp course or soil moisture sealing;
b) usage-related influencing factors such as insufficient or improper heating and ventilation and
c) other influencing factors such as water ingress through accidents or flooding.
Moisture damage is often caused by an unfavourable combination of dif-ferent influencing factors.
A basic prerequisite for understanding the mechanisms taking place is the knowledge of the relationship between the surface temperature and the surface moisture in relation to indoor air climatic conditions (see In-fobox 4). The phase diagram of air explains the processes occurring on a cool wall in more detail.
In order to assess the causes of moisture damage and mould infestation, it is very important to establish the room climate situation through the professional determination of surface temperature and surface moisture (see Infobox 5).
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Causes of mould infestation in buildings
INFOBOX 4
Air phase diagram
The figure shows the relative indoor air humidity (in %) as a function of the room air temperature (x-axis) and the water vapour content of the air (y-axis). Graph according to Willis Haviland Carrier.
For example, indoor air at 22 °C and a water content of 10 g/m³ has a relative humidity of 50 % (point A). If the surface temperature of the inner wall is also 22 °C, its air humidity will also meas-ure 50 %. During winter, however, the low outside air temperatures mean that the inner surface tem-perature of the outer walls are lower (assuming a surface temperature of 14.5 °C by way of example), whereas the indoor air temperature is kept con-stant at 22 °C through indoor heating.
In the proximity of the wall surface, the absolute water content of indoor air remains the same as in the middle of the room (in this example 10 g/m³).
However, indoor air cools down when approaching the wall. This means that the air condition chang-es when approaching the wall, as shown in the figure, parallel to the abscissa up to point B. Thus a higher relative humidity of 80 % prevails in the proximity of the wall which favours mould growth.
A further cooling of the inner wall surface under these conditions would mean further cooling of the air thus reaching the dew point (at about 11 °C, point C). When the temperature drops below 11 °C, the state of the air follows the saturation line (up to point D). The result is water vapour condensa-tion on the cool surface.
INFOBOX 5
Determining the indoor climate situation
The surface temperature and moisture are decisive in assessing the possibility of mould infestation.
The SURFACE TEMPERATURE of the walls should be measured at various locations and potentially at different times. A single measurement only indicates potential thermal bridges under certain conditions (significant inside/outside temperature difference). Long-term measurements provide more meaningful results. These should be car-ried out by professionals who have the necessary equipment and experience for the evaluation.
Additional information can be obtained from knowledge of the structure of the building (ther-mohygrometric calculations).
The SURFACE MOISTURE on the inside of external walls due to hygrothermal effects is generally not determined by humidity measurements but calcu-lated from the indoor air humidity and the meas-ured room air and surface temperatures.
The room occupant can easily check the temper-ature as well as the relative humidity in the room and in critical areas such as corners and in the immediate vicinity of the outer walls. A simple electro-thermo-hygrometer that is cheaply avail-able in DIY warehouses are perfectly adequate to check whether the room temperature is sufficiently high and has been sufficiently ventilated. How-ever, their measurements only provide a rough estimate.
The WTA leaflet ‘Measurement of water content or moisture of mineral building materials’ contains information on the measurement of the equilib-rium moisture content of materials (WTA leaflet 4-11, 2016).
3.1.1 Inadequate thermal insulation
Inadequate thermal insulation means that the inside of external walls cools down at low outdoor temperatures and increased surface moisture forms due to condensation of indoor air humidity.
The appearance of mould growth on the inside of exterior walls and ceil-ings depends on their surface temperature and moisture. These in turn are influenced by the heat transfer coefficients (U value) of the exterior wall and the heat transfer resistance (Rsi value) on the inside of the exter-nal wall (see also Section 3.1.3) as well as the room air temperature and humidity.
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Causes of mould infestation in buildings
Under steady-state conditions – in practice only approximately achieva-ble – the surface temperature can be calculated as follows:
Q
si= Q
i– U R
si(Q
i– Q
e)
Qsi [°C] Interior surface temperature Qi [°C] Room air temperature Qe [°C] Outdoor air temperature U [W/(m² K)]* Heat transfer coefficient Rsi [(m2 K)/W]* Interior heat transfer resistance
*W = Watt, K = Kelvin
The U value characterises the insulation level of a building component of the outer envelope, e.g. the external wall. A high U value means high heat transfer, i. e. poor insulation.
Thermal insulation must not be confused with heat storage. The high-er heat storage capacity of heavy building mathigh-erials used for walls (solid walls) has a higher chance of compensating for temperature fluctuation than lightweight building structures and thus also ensures better buff-ering of the room air temperature (see Section 3.1.7). In addition, solid structures can improve summer heat protection (avoiding excessive room heating). In the case of mould prevention, however, the thermal insula-tion of the outer envelope as well as sufficient ventilainsula-tion and heating are the decisive factors, not the heat storage.
3.1.2 Thermal bridges
Thermal bridges are localised areas in the enclosing surfaces (walls, ceilings, floors) of a building which enable an increased heat loss to out-door or unheated spaces. They lead to a reduction of the interior surface temperature of building components. Thermal bridges may be due to the spatial (geometrical) conditions (e.g. corners, see Figure 11 right and Fig-ure 12) or the use of building materials with very different thermal con-ductivity (e.g. supporting pillars in a wall, wooden beams in a converted attic, etc., see Figure 11 left). The consequences of thermal bridges dur-ing the cooler seasons cause – in addition to heat energy losses – a lower interior surface temperature of the affected building components, an in-creased surface moisture and thus an inin-creased risk of condensation and mould infestation along the wall.
3.1.3 Increased resistance to heat transfer
The free flow of air (convection) is obstructed in corners within a build-ing generatbuild-ing an increased resistance to heat transfer. Warm indoor air does not reach the corners in a room sufficiently well. In addition to the thermal bridge effect, this leads to an additional reduction in the surface temperature, especially in corners of external walls and thus to an in-crease in surface moisture within the wall corner. For this reason, mould growth is fairly often observed in the corners of external walls.
Furniture, curtains and the like do not represent much resistance to in-door air humidity, thus it progresses to the walls behind furniture. At the same time, however, the heat in the room – due to reduced convection and radiation heat transfer – is very inefficient at getting behind the fur-niture and curtains. Furfur-niture and curtains thus cause an increased re-sistance to heat transfer. The combination of these two effects further in-creases the relative humidity on the wall behind the furniture (see Figure 12). This can lead to mould growth.
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Causes of mould infestation in buildings
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Furniture, curtains and other furnishings should always be ar-ranged or installed a few centimetres away from the wall if the ex-ternal walls are insufficiently well insulated so that the warm in-door air can flow unobstructed behind these furnishings and thus heat up the cold wall. The air flow also removes moisture from the wall surface. It is also helpful to put furniture on feet so that an improved ventilation is maintained. In planning home furnishings more attention should be paid to the fact that built-in wardrobes (or kitchen cupboards) are not installed directly on poorly insulated external walls without a sufficient air gap to the room. The arrange-ment of furniture on external walls is usually unproblematic in well-insulated, low-energy and passive houses of modern design with sufficient ventilation.Additional resistance to heat transfer is problematic especially in poorly insulated old buildings (see Figure 12, left). While only 70 % surface moisture occurs at the surface of an unfurnished external wall with a surface temperature of 15 °C, the reduction in temperature of 11 °C behind a cupboard causes 89 % of surface moisture on the out-side wall and condensation water at the external corner at 6 °C (see Figure 12, left).
Furniture on the external wall/corner is not critical in well-insulated buildings (see Figure 12, right), unless there is an additional increase in indoor humidity caused, for example, by insufficient ventilation (60 % relative humidity in the example of Figure 12). At 20 °C room air temper-ature and 50 % relative indoor humidity, a surface humidity of only 73 % occurs even in the external corner behind the cupboard. However, an in-creased indoor humidity of 60 % already leads to a surface moisture of 88 %, which allows mould growth. Figure 12 shows the theoretically cal-culated values under steady-state conditions. In practice, these specifica-tions do not always apply precisely.
3.1.4 Inadequate or improper heating
Heating causes an increase in room air temperature and thus reduces the relative water content of the air at the same absolute water content. In ad-dition, heating the room also increases the surface temperature of the in-ternal wall surface. Both effects contribute to avoiding mould growth.
If individual rooms such as bedrooms, guest rooms or storage rooms are heated just a little or not at all, the risk of mould growth increases in re-verse. Reduced room air temperature not only ensures an increased rela-tive room humidity, but also lower surface temperatures (see Figure 13). In bedrooms, additional moisture is released through breathing and sweating
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Causes of mould infestation in buildings
(transpiration). This increases humidity and the possibility of water va-pour condensation on cool walls. Bedrooms especially are often ventilated insufficiently or incorrectly (for proper heating and ventilation, see expla-nations and information boxes 9 and 10 in Chapter 4).
3.1.5 Increased indoor moisture production
High moisture production through cooking, washing, etc. leads to higher absolute indoor humidity and thus to higher surface moisture.
Table 6 gives an overview of the quantities of moisture produced by var-ious indoor activities and furnishings. These are empirical values and moisture levels can vary significantly upwards and downwards in indi-vidual cases.
The amount of moisture produced by room users can be as high as 6 to 12 litres a day for an average 3-person household (see Figure 14).
Table 6
Moisture production by activities of room users or by furnishings in rooms at a room air temperature of 20 °C Humidity source Moisture production per hour or day
or per m² per hour
Humans, light activity 30–40 g/h
Drying laundry (4.5 kg drum)
Spin-dry: 50–200 g/h Dripping wet: 100–500 g/h Cooking/showers per person 270 g/d each
Indoor plants 1–5 g/h*
Water surface Open aquarium: approx. 40 g/m2/h **
Covered aquarium: approx. 2 g/m2/h
* Can also be significantly higher according to the number and type of houseplants
** Grams per square metre per hour, depending on environmental conditions.
Source: Fraunhofer Institute for Building Physics, Holzkirchen, amended
Additional sources of moisture such as drying laundry, many in-
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door plants or a indoor water fountain should be avoided especial-ly with a high relative humidity indoors (see Figure 15) in the cold season. If there is an increased indoor humidity level, relative hu-midity must be reduced by increased ventilation and, if necessary, heating (see notes in Chapter 4).
Figure 15 shows indoor air humidity values common in buildings over the year. Data were collected for normally used, non-mould infested rooms. It shows a typical trend over the year with indoor air humidity lower in winter and higher in summer. In order to avoid mould, indoor air humidity should not exceed the stipulated common values perma-nently, especially in poorly insulated buildings in winter. In our latitudes however, with the exception of alpine regions, winter situations where the indoor air is very dry (less than 20 %) over a longer period of time is only the exception. If a humidifier is used in such cases, the relative hu-midity should be checked with a hygrometer so that it does not reach ele-vated values (see Figure 15) otherwise the risk of mould growth
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Causes of mould infestation in buildings
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3.1.6 Inadequate or improper ventilation
Chapter 4 gives a detailed description of ventilation.
Ventilation is the most effective way to remove moisture from the home.
The efficiency of ventilation has long been expressed by the air ex-change rate. It indicates the volume of air exex-changed and replaced by outdoor air per hour in relation to the volume of a room. Today, user- and area-specific outdoor air volume rates are used instead of the air exchange rate to calculate the air exchange for ventilation systems in particular.
Especially at low temperatures in winter, the outdoor air has a low ab-solute humidity even at a high relative humidity (e.g. rainfall) (see Table 7). If ventilation takes place at –10 °C outdoor temperature and the cold outdoor air is heated up to 20 °C indoors, the relative humidity of the out-door air is reduced by the heating originally from 80 % to only 9 % (see Table 7). This provides a large capacity for absorbing the moisture stored in indoor materials which is released to the now dry indoor air and trans-ported by ventilation back outside. In practice, however, indoor air hu-midity levels below 20 % relative huhu-midity are only rarely achieved over a longer period of time since moisture-buffering materials release mois-ture into the indoor air during the ventilation process (moismois-ture buffering by desorption).
Table 7
Theoretical relative indoor air humidity at different outdoor air temperatures by heating outdoor air of 80 % humidity to 20 °C indoor temperature at a constant absolute humidity (ignoring moisture buffering)
Outdoor air temperature [°C]
Relative outdoor humidity [%]
Absolute humidity [g/m³]
Theoretical relative indoor air humidity at 20 C [%]
–10
80
1.7 9
0 3.9 21
10 7.5 42
20 13.5 80
Time and again, one hears the assertion that walls “breathe” and
i
an air exchange takes place through them. But this is physically not possible, unless the walls have leaks and cracks.
There is no air exchange from indoors to outdoors through structur-ally intact walls. Also, the amount of moisture transported through the walls by vapour diffusion is negligible compared to the amount removed by ventilation. Vapour-tightness of a wall structure there-fore has only minimum influence on indoor air humidity and quali-ty. The term “breathing” walls, which is often used in this context, can only be seen in connection with moisture buffering (see Chapter 3.1.7), but not supporting air exchange in the building.
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Causes of mould infestation in buildings
3.1.7 Moisture buffering of building materials
Different amounts of humidity are released through the use of indoor spaces throughout the day. Part of this humidity is absorbed, stored and released by the building materials in the room. This is called mois-ture buffering or humidity regulation. When the relative humidity is in-creased, the material adsorbs ambient humidity (adsorption) and trans-ports a part by diffusion into deeper, drier layers of the component.
When ambient humidity is reduced, moisture from the interior of the building material is emitted to the surrounding air (desorption). Since in-door conditions constantly change, so do humidity regulation and tem-perature of the material. How quickly a material can adsorb or release moisture depends on the material properties (e.g. sorption capacity and diffusion resistance).
The buffering effect of materials is usually limited to a depth of a few mil-limetres of the component, only those material parts close to the indoor space contribute significantly to buffering. Furniture (uncoated wood furniture, upholstery) also has an influence on indoor air humidity.
Humidity buffering of building materials reduces daily humidity varia-tions, depending on the buffering capacity of the material (see also Info-box 6).
The humidity-buffering effect of all these materials together leads to a reduction of humidity fluctuations from which the indoor climate and comfort can benefit. It cannot be concluded directly from this as to what extent this also has an influence on mould growth. If damp peaks are buffered by the sorptive effect of the walls, which would have led to condensation or increased surface moisture without buffering, mould growth can be prevented.
Sorption properties of the wall material primarily compensate humidity peaks. The mean humidity content of the air remains largely unchanged and can only be reduced by active ventilation (see Chapter 4).
INFOBOX 6
Example of moisture buffering
The effect of moisture buffering can be measured by introducing a typical daily humidity load into a defined room with different wall coatings (diagram left) and determining the relative indoor humidity (diagram right).
The case study simulated the humidity production of a family of two adults and two children in a 65 m² apartment. It observed a particularly high humidity production between 6 and 8 am due to cooking and showering and once again in the evening between 6 and 10 pm. This high humidity load increased even further due to washing, cooking and the production of humidity by the occupants. Observations further
concluded that the timber lining in the chosen example reduced the relative indoor humidity from approx. 60 % to approx. 40 % due to the buffering of the timber lining. Painted plaster would have reduced the relative humidity even further, albeit with a higher fluctuation range.
These are just examples. In individual cases, the buffer effect depends largely on the type and struc-ture of the wall surface materials and on the type of paint (breathable, sealing) etc.
Daily trend of humidity production (left) and the resulting changes in indoor air humidity (right) of a room with a paint coated wall, typical interior plaster (green line) and a room with timber lining (tongue and groove, T + G formwork, yellow line);
( Image source: Fraunhofer Institute for Building Physics, Holzkirchen)
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Causes of mould infestation in buildings
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3.1.8 Moisture in the building structure due to leaks and rising damp
Moisture finds its way into the building structure in various ways.
Moisture can penetrate the building from the outside. Driving rain can force moisture into the structure via outer walls that are insuffi-ciently protected against rainwater ingress. Typical cases of damage to buildings that allow water ingress are leaky connecting joints – in window reveals and frames etc. – or leaks in the roof area. Rising and laterally penetrating moisture can arise through masonry, founda-tions and basement walls that have been insufficiently sealed from the ground.
Large amounts of water from damaged household water pipes, leak-ing heatleak-ing pipes, burst hose connections or poorly sealed fittleak-ings in showers or bathtubs can be released and enter the building structure.
Increasingly recurring floods in individual regions repeatedly lead to a significant moisture accumulation in buildings.
All moisture damage to the building structure requires immediate action to eliminate the causes of the moisture input and to dry the affected building areas in order to avoid the occurrence of visible and hidden mould infestation (see Chapter 6).
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3.1.9 Trapped moisture
The content of humidity in building materials during constructing pro-cess (trapped moisture) poses a problem if occupants move into new-ly built homes or existing buildings after large-scale refurbishment too soon, especially, if they have not been adequately ventilated after com-pletion. Components (predominantly concrete and screeds, but also plas-tered walls and ceilings) often contain large amounts of water immedi-ately after construction.
Trapped moisture has a negative effect on thermal insulation properties and thus on energy consumption. However, its influence on the indoor air humidity (which is often significantly increased by trapped moisture over a longer period) is even more important.
New buildings or buildings that underwent large-scale refur-bishment require intensive ventilation because of the increased trapped moisture, but also because of the chemical substances potentially emitted by the building materials. An indoor space with trapped moisture should definitely be subject to a long inten-sive ventilation before occupants move in. Depending on the con-struction and structure, the drying phase can take up to several years. Intensive heating can be a supporting factor.
3.1.10 Flood damage
Buildings situated near the water are repeatedly exposed to flood dam-age. The frequency and extent have been increasing for years due to cli-mate change and missing flood plain areas in nature. Water ingress often leads not only to mould growth, but also to microbial and chemical con-tamination by correspondingly polluted water.
For further details see Chapter 6.
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Causes of mould infestation in buildings