Mould

Assessement, causes and remedial actions

Assessment of existing mould damage by visual inspection

The original external wall in historic buildings often consists of inorganic building material and may therefore be considered as robust from a mould perspective. However, there are also adjoining frames, beams, windows, doors, added insulation and surfaces containing organic compounds, that needs to be inspected.

Extended mould growth on building materials may be visible to the naked eye. However, often growth cannot be seen, cf. Figure 4–11(A). Growth not visible to the naked eye and growth inside building structures may be as problematic as visible growth, as there is a risk of adverse effects on the indoor environment, which can pose health risks to those in the building. Mould can theoretically grow anywhere in a building, provided the conditions for growth are suitable. Some types of structures are more favourable than others for mould growth, and some materials are more susceptible to mould growth than others. In houses in Northern Europe, mould typically grows inside sealed building structures, e.g as shown in Figure 4–11(B). Extensive mould growth may be present in buildings even though there are no clear visual signs. Typical indications of moisture related problems can be damp surfaces, dried out water stains and rusty nails in the construction.

Figure 4–11:  (A) Illustration of how the visible impression of mould growth varies. All samples shown have the same extent of mould growth, although the growth is not visbile to the naked eye in all cases. (B) Example on how mould growth may be hidden within the building structure. In this example, interior surface materials and insulation was removed before mould growth was detected.

Table 4‑3: What to look for, and where with regard to mould growth

For mould to grow on a material, nutrients in the form of simple carbohydrates must be present in the material. All materials with organic compounds are therefore at risk for mould growth. However, the susceptibility for mould growth varies, some materials can withstand higher moisture loads than other. This can be described as the critical moisture value, RHcrit, further discussed in (RIBuild Deliverable D2.2, 2019). Also, materials may be contaminated by organic substances during production and construction, for example by dust, soil, surface treatments etc. and the susceptibility may then be changed.

Some fungi produce pigments in their hyphae and spores that can cause discolouration of surfaces on which they grow, while others lack this pigment and therefore can’t be seen by the naked eye. The production of pigments by fungi is a species-specific trait, but can also be dependent on the nutrients available, or the growth phase of the fungus (Gadd G. M., 1980),  (Eagen, Brisson, & Breuil, 1997), (Fleet, Breuil, & Uzunovic, 2001).  

Causes of mould growth

Adding internal insulation to the building façade will make the original wall structure colder, reducing the drying potential. This in turn may increase relative humidity (RH) locally and thereby the risk for mould growth and condensation. An increased RH level increases the risk for mould growth on existing, historic materials and on new materials. The risk of high moisture levels may increase further if the internal insulation admits air leakage of humid indoor air into the wall construction, especially if the ventilation is unbalanced (internal overpressure). In time, problems with rot may also develop if access to moisture increases (see ‘wood rot’).

When mould growth has been detected (e.g. if users of the building become ill), the cause of the mould growth should be found and remedied, and damaged material should be replaced or mechanically treated, e.g. by grinding, planing or blasting. Note that residues from the process must be collected and removed to minimize the risk of future mould growth.

Mould in buildings may have negative effect on the perceived indoor environment, for example, by the production of odorous substances. Also, human health may be adversely affected due to the spread of particles, toxins and volatile organic compounds from the mould fungi to the indoor air. The costs associated with this growth, i.e. due to renovation, are substantial. Therefore, both economy and health can be used as arguments for reducing the risk of mould growth in buildings.

Mould fungi are widely spread across different environments on the Earth and there is no natural place where air and materials are free from spores. When favourable conditions are present, spores (also called conidia) will germinate and a small germ tube will develop; if the favourable conditions prevail, a hypha will be formed. A hypha is a tubular cell structure which extends at the tip. By continuously branching during growth, the hyphae form a mycelium. Eventually, specialized structures (conidiophores) develop from the hyphae and from them the spores are produced and dispersed.

The main environmental factors affecting mould growth in building structures are humidity and temperature; moisture being the crucial factor. Suitable conditions for the growth and reproduction of different mould fungi vary. Some thrive at relatively low relative humidity (RH = 75%), while most fungi require higher values of RH (90-95 %) for optimal growth in room temperature. Different building materials vary in their susceptibility to mould growth; some can withstand high moisture content better than others.

Mould growth is the result of a complex interaction between all these factors; environmental factors and duration, material properties and the characteristics of mould fungi present (Blackburn, 2000). To prevent mould growth in buildings, these interactions should be considered during the design, construction and maintenance of a building.

Remedial actions if mould growth is identified

In general, specialists are needed to execute remedial actions if mould growth is identified, as it is difficult to ensure that the mould is complete removed.

When building materials have been affected by mould, various remedies or methods are used to get rid of the mould and sometimes the material is only dried without any further action. There are studies concluding that treatment with chemical agents do not stop or eliminate mould growth, and that the release of particles and toxins from mould-damaged materials even increase by drying.

In a study conducted in Sweden 2010 a number of remediation methods for removing mould were tested (Bloom, Must, Åmand, Peitzsch, & Larsson, 2010). It concluded that none of the remediation methods could eliminate viable mould growth on the tested building materials. No decontaminant eliminated the toxins completely from the damaged building material.

Primarily, the study emphasizes the importance of working preventively with moisture safety throughout the construction process and administration to prevent mould damage. If damages are found, the preferred method of remedy should be, as stated before, replacement of damaged materials or mechanical treatment of surfaces, e.g. by grinding, planning or blasting. Note that residues from the process must be collected and removed to avoid mould residues from remaining.

To treat surfaces and mix materials with mould inhibitors such as e.g. asphalt or fungicides is not recommended as these products can fortify the development of an odour and/or health hazard to the indoor air. The precautionary principle is recommended as an approach.

Mould prediction models

VTT model

This model was first presented in a version aiming to predict mould growth on wood (Hukka & Viitanen, 1999). It was later modified to handle also other materials. The latter version is often referred to as “the new VTT model” (Ojanen, et al., 2010). The model is available as a postprocessor in the hygrothermal simulation programs DELPHIN and WUFI. In this study, the partners used either DELPHIN or excel files, programmed after published version.

The material susceptibility parameters to be chosen in the model are shown in Table 4‑4. The description differ slightly between the published description (Ojanen, Peuhkuri, & Viitanen, 2011) and DELPHIN. Two additional parameters linked to material properties can be chosen in the model; Wood species (W=0 for pine or W=1 for spruce) and surface quality (SQ=0 for sawn surfaces, SQ=1 for kiln dried surfaces). In addition, a parameter (Cmat) is used in the model in relation to the effect of the duration of unfavourable conditions (Table 4‑5).

Table 4‑4: Sensitive classes in the VTT model. The descriptions vary some between the model as it is described in the published model (Ojanen, Peuhkuri, & Viitanen, 2011)  and in DELPHIN (Bauklimat-Dresden, 2019).

Table 4‑5: Description of Cmat

The outcome of the model is a dimensionless value, denominated Index. It describes the mould growth intensities, with values between 0 (no growth) and 6 (heavy and tight growth, coverage about 100%). The partners in this study used the index 1 as limit values, according to (Ojanen, et al., 2010). However, in other publications other limit values can be used.

The PJ-model

The PJ-model is a static Isopleth model. The model has not previously been described as a model. It is a part of the application of a standard test method for assessing the critical moisture level for mould growth on building materials in (SIS-TS 41, 2014). In this study, version 1.0 is used. However, the PJ-model is under development and later version may be published.

The material parameter input to the model is RHcrit, which is the tested critical moisture level for a material. A specific product can belong to one of five material classes, from which growth limit curves are constructed.

RHcrit according to the method/model has been published for some materials (Johansson, Ekstrand-Tobin, Svensson, & Bok, 2012). However, as different products have their specific RHcrit, according to the model, material data needs to be provided for each product through testing.

If the RHcrit is not known, Class A is recommended to be used. This is also in line with recommendations from the Swedish National Board of Housing (BBR 2014:3, 2014). It is recommended that the material producers provide and communicate data of RHcrit. Some producers in Sweden have performed tests at RISE. As this is commercial information, it is not referred to in this study. We have no knowledge about whether testing have been performed at other laboratories for other materials.

CML-method

The test was performed according to ‘Laboratory method for assessment of the lowest hygrothermal conditions required for mould growth’ (SIS-TS 41, 2014), also named the CML-method (Critical Moisture Level-method) (Johansson, Ekstrand-Tobin, & Bok, 2014). It is based on different standardised methods for determining the mould resistance of building materials and on laboratory testing (Johansson, Ekstrand-Tobin, Svensson, & Bok, 2012) and is validated in field studies (Johansson, Svensson, & Ekstrand-Tobin, 2013). The method is also described and discussed in (Johansson, Ekstrand-Tobin, & Bok, An innovative test method for evaluating the critical moisture level for mould growth on building materials., 2014) and is published in (Johansson, 2014).

If RH values are below the lower growth curve, no mould growth is expected. If it exceeds the upper limit growth curve, mould growth is predicted. Values in the zone between the upper and lower curve represent a “yellow-light case”. However, the model also predict mould in these cases to be on the safe side (Johansson, Svensson, & Ekstrand-Tobin, 2013).

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