Mould and mildew
- A problem in Danish buildings?
In recent years, there has been an increased focus on mould in the home. Despite several medical, biological and construction studies, the effects of mould on the indoor climate are still unclear. There is a lack of clear threshold values for when mould is unacceptable in the home. This ignores people who are actually hypersensitive to mould. It requires in-depth specialist medical knowledge to assess these conditions and as long as there is no consensus within the medical expertise, it is extremely difficult for "the rest of us" to assess the health aspects. There is also disagreement among advisors about how the building should be examined and which analysis methods are best to use when mould is present. It is understandable that consumers find it difficult to relate to fact or fiction.
This article describes the issues surrounding mould from a biological and building technology perspective. Please also refer to the latest BYG-ERFA Foundation experience sheet on mould.
In the following is a review of:
What is mould?
Moulds originate in nature and are found everywhere. Just like plants and animals, moulds (along with other types of fungi) have their own biological group. Moulds are primitive fungi that are unable to form fruiting bodies.
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| Mould on contact prints | Mould originates from nature |
Under favourable conditions, the mould spores will germinate and grow as hyphae (cell threads), forming a mycelium (tissue of the fungus' cell threads) that can typically be seen as black, whitish, brownish or blue-green colonies on building materials, foodstuffs etc. In the mycelium, millions of new spores are formed and spread by the wind. During the winter months, when nature is dormant, the concentration of mould spores in outdoor air is very low. During spring, nature awakens from hibernation and mould spores can again be detected in outdoor air. In late summer, the highest concentrations of mould spores are seen, with many thousands of spores/m3 of outdoor air.
Why does mould occur in buildings? - Growth conditions
The mould spores are brought into the home with the outside air and if the right conditions are present in the home, the spores will germinate and mould growth has started. A prerequisite for mould growth is the presence of moisture. Moisture can occur in many ways. Typically, dampness is caused by poor living habits, condensation due to thermal bridges, leaking water installations, leaky building envelope, flooding and fire-fighting water. Finally, combinations can of course occur. When the relative humidity in the home exceeds 75-85% for a prolonged period of time, mould growth is certain to occur. Temperature is also important. Most moulds grow best at 15-28 degrees C. Some types can grow at low temperatures (5-10 degrees C) while others can grow at high temperatures (50 degrees C).
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| Mould growth on floor joists After water damage | Black mould on roof window |
Where does mould occur in the building?
Mould growth can occur anywhere in a building if the right conditions are present. Building materials consisting of organic materials such as plasterboard, wallpaper, wood, veneer etc. are particularly vulnerable, but mould can also thrive on wet concrete and brickwork. Mildew, which is caused by black-pigmented mould, is typically seen at lower temperatures on e.g. thermal bridges, window slits, cold water pipes, etc. Inadequate cleaning in the home can also provide a breeding ground for mould growth. House dust consists of organic materials that mould can feed on.
Is mould harmful to indoor air quality?
Mould consists of and contains components that under certain conditions can be a nuisance in the indoor environment. In addition, actively growing mould can release volatile organic compounds (VOCs) into indoor air and release fungal toxins (mycotoxins) into the material they grow on. Some mould species can release substances that give off a characteristic pungent and unpleasant mouldy odour that is familiar from damp summerhouses, basements, clothes left in damp conditions, etc.
If mould is present in the home for a longer period of time, it can cause indoor air quality issues such as mucosal irritations in the eyes, nose and throat. Symptoms such as tiredness, headaches and difficulty concentrating can also be attributed to mould growth in the home in some cases. However, these symptoms can also be caused by other conditions such as asthma, psychosocial conditions (poor working conditions, bullying, conflicts, etc.)
It is unlikely that either mycotoxins or VOCs will be released in such high concentrations in a home that they could cause indoor climate problems. However, it is known that mould spores and fragments (particles) from mould can cause discomfort for people who are hypersensitive (allergic) to mould. Mould spores and hyphae contain both foreign proteins and carbohydrates (glucans) that can be allergenic to hypersensitive people. It will therefore be difficult or impossible for these people to stay in homes with mould. If you want to know more about these conditions, you can download the Danish Health Authority's leaflet on mould.
How should you proceed?
We should not accept mould growth in the home beyond what is normal. If a tenant suspects mould growth, landlords should always take the problem seriously. Start by investigating if there is a problem that requires action. Often it will be a minor problem that can be solved by simple cleaning. In other cases, it may be appropriate to seek advice from one of the consultancies that have experience in inspecting a building for mould.
Think of your home as a place with both indoor and outdoor zones. Mould in an outdoor zone such as an attic or crawl space an attic or crawl space is less significant than if mould is found in an indoor zone such as the living area of the home, where you spend many hours during the 24 hours of the day.
How do you test for mould?
A mould inspection in a building should always be destructive. This is the only way to adequately uncover hidden mould growth. Particularly in damp buildings, care must be taken in hidden structural elements where moisture has difficulty escaping and therefore provides a breeding ground for mould growth. After water damage, extra care should be taken to check cavities in partitions, floors, etc. If these areas are left to dry out, mould growth will often occur after just 1-2 weeks.
There are several methods of analysis for the detection of mould. None of the methods can be directly compared. It is therefore very important that the person performing the analyses knows both the strengths and weaknesses of the chosen method. The following are brief examples of methods for detecting mould in the home. There are significant price differences between the analysis methods mentioned.
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| Mould on exterior walls Thermal bridge | ProClean test |
Contact print - is a small plastic dish with a culture medium (usually tomato juice gel, called V8) that can be imprinted on a surface, transferring mould growth, spores etc. to the culture medium and then brought back to the lab for cultivation for approximately 7 days. The resulting mould colonies can then be counted and named
The following criteria are used to describe the extent of mould growth:
- Poor growth < 10 colonies
- 10 < moderate growth < 50
- Massive growth > 50 colonies
Air analyses - are analyses collected directly from the air using special collection equipment (slit samplers) or on agar plates with culture medium. The air analyses can give the investigator a picture of the extent to which indoor air is contaminated with mould and if so, whether it originates from the home or has been introduced from outdoor air (nature).
Material samples including tape prints - is an analysis method where you take e.g. wallpaper samples or tape prints from the surface. wallpaper samples or tape prints are taken from the surface and then under a microscope you can see what and how much mould growth is present on the examined surface.
The following methods are all so-called rapid methods, performed by swabbing the surface with a cotton swab and subsequent chemical analysis:
Mycometer - Analyses on an enzyme that is characteristic of mould. Operates with 3 categories, A: 25 and 450.
ATP - analyses for the chemical adenosine triphosphate, which is found in all living cells. ATP numbers below 100-200 (not yet finalised) will usually be acceptable.
ProClean - is a protein test that requires no instrument, but gives a clear violet colour reaction if proteins are present.
Remediation after mould infestation
It is important that the cause of mould growth is found and removed. If necessary, a proper drying process should be initiated. It is often wise to hire a professional drying company that has both the experience and the right equipment for the job. When starting a mould renovation, it is important to set aside the necessary time to both plan the process and inform everyone involved. Then the renovation can be carried out in peace and quiet, with due consideration for quality control and safety in the execution.
Choice of method
Before starting a mould remediation project, a decision must be made whether the mould-infested materials should be discarded or whether they can be cleaned and reused. This will usually be based on an economic assessment.
As a general rule, mould growth must be removed, not just killed or dried out. The dead mould spores and particles from the mould will contain foreign proteins and carbohydrates that will be allergenic to sensitive individuals.
Remediation methods
When repairing mould infestations, both mechanical and chemical methods can be used and are often performed in combination. The method chosen will always depend on the specific damage, building conditions, building materials, etc. Mechanical methods include techniques such as grinding, planing, milling, steam cleaning (dry steam), high-pressure cleaning with hot water, ice removal, etc. For larger jobs, it is advisable to test the chosen method on a small area to avoid costly re-treatments.
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| Ice cleaning of roof rafters | ProtoxBiox Treatment of |
Disinfection neutralises and removes mould growth. For disinfection, use the following ProtoxHysan and/or ProtoxBiox. Neither of the 2 mentioned agents leave chemical residues that can subsequently have any negative impact on the indoor climate.
In some cases, it may be necessary to prevent mould growth after disinfection to avoid a resurgence of mould growth. This will typically be on a thermal bridge, damp foundations, wood veneer underlay, etc. In these cases, the product is used Protox mould containing fungicide. A degassing study carried out by Eurofins Miljø A/S shows that no degassing can be detected from the product.
How to use ProtoxHysan, ProtoxBiox and Protox mould?
Please refer to the technical data sheet and product descriptions for the individual products:
- ProtoxHysan – Product description, application and indoor climate report.
- ProtoxBiox – Product description, application and BST report.
- Protox mould – Product description, application and indoor climate report.
How to prevent mould?
- Check the house/building for penetrating moisture and make sure to repair all moisture damage - check gutters, drains, wells, windows, doors, roofing, guttering, etc.
- Do not dry clothes in your home - even in the bathroom, hang damp towels outside on the clothesline after bathing.
- Use the cooker hood when cooking, check that the extractor is working properly.
- Ventilate frequently but briefly to avoid cooling of surfaces.
- Turn up the heat.
- If there is dew on the double glazing, it is a sign of excessive humidity in the indoor air. Get a hygrometer and check that the humidity never exceeds 60% relative humidity.