Mould Resistance In Construction Materials

The aim of mould resistant products is to lessen the long-term hazards that builders and architects have to deal with, but it’s key to understand how firms can demonstrate that their produce is in fact completely resistant to mould. Here’s the down and dirty on it…

It might come as some surprise to certain people but the world we live in is overrun with mould. Some authorities estimate the number of types of mould, or fungi, at approximately 300,000 or possibly more. The majority of these fungi have useful functions such as breaking complex substances down into compounds of simpler structure. One example of this is yeast, a single-celled fungus which makes the manufacture of bread, beer, and wine possible. Even most multi-celled (“hyphae”) moulds help our planet by decomposing organic matter so that it can be recycled and reused by other organisms on Earth.

Outdoor Mould Prevails

The built-up environment, without a doubt, has its fair share of mould. However, recent research has discovered that the majority of moulds found indoors are in fact outdoor species. In their “Diversity and Distribution of Fungi on Residential Surfaces” study, Rachel I. Adams and researchers sampled fungi from 3 types of surfaces likely to support growth and therefore possible contributors of fungi to indoor air: drains in kitchens and bathrooms, sills under condensation-prone windows and skin of human inhabitants. Their samples were taken at a university residential complex with no mould issues.

Taken together, the results from this research suggest that, in healthy structures with no reported mould problems, indoor surfaces are not a major source of indoor fungi, but in fact the reverse: surfaces seem to be mostly depositional environments which collect many of the same fungi common to both outdoor and indoor air.

So, most indoor mould isn’t actively growing since it’s outdoor mould that has a preference for outdoor conditions in order to multiply and propagate. According to the Centers for Disease Control (CDC), construction areas are among the top eight environments with high mould populations. Other places with typically high mould counts include antique shops, greenhouses, saunas, farms, mills, flower shops and summer homes.

The Value Of Sampling For The Presence Of Mould Is Questionable

In most cases, even when mould exists in buildings, it is not growing to the point where it can be seen with the naked eye. In addition, the growth of mould is significantly affected by humidity and, as so many of our buildings these days are air conditioned or use dehumidifiers, the opportunity for mould growth is reduced.

The CDC does not recommend sampling for mould since allergies are the afflictions generally associated with mould and people’s susceptibility can vary significantly as the amount or variety of mould, sampling and culturing are unreliable factors in determining one’s health risk. If a person is susceptible to mould and mould is either seen or smelled, there is a possible risk to health; so, regardless of the kind of mould that is present, it should be removed.


Mould In Petri Dishes


One variety of mould in particular, Stachybotrys Chartarum, may cause respiratory health issues, particularly in small children, and is one of the test cultures required by the Environmental Protection Agency’s (EPA) “Test and Quality Assurance Plan” for testing building materials for mould resistance.

Businesses who sell building products are now, more than ever, having their products certified as mould resistant based upon various tests that they carry out. These usually involve the introduction of mould cultures to the products, under carefully controlled conditions where moisture levels are above 85%, and subsequently either looking for visible evidence of mould or measuring the amount of mould growing through the use of microbiological plating methods. Different tests are used for different types of materials. Plasterboard is typically tested using one of two methods.

Testing Methods

One test, ASTM D3273, measures the resistance of plasterboard to mould over a four week period, with visual effects being recorded on a weekly basis. In an environmental chamber, samples of the product to be tested are suspended over soil that has been inoculated with several different types of fungi. After 4 weeks, the samples are inspected under magnification to verify whether what was observed over the four week period was in fact mould.

This method of testing aims to simulate real world conditions as it is based upon indirect inoculation. However, according to a document released by the United States Gypsum company, there are 6 limitations to this approach. These include:

  • Results do not accurately represent field conditions and highly variable field conditions.
  • Tests only for 3 types of fungi.
  • The method was originally aimed at testing for mould resistance of paints and other similar coatings.
  • Difficult to reproduce the same results from one laboratory to the next.
Another type of test is ASTM D6329, where each sample is gathered into its own petri dish while the level of humidity is controlled to accommodate the types of mould that were introduced into the sample. With this method, the rating aims to produce more robust results by not having to rely on visual measurements, but rather measuring the degree of fungal growth through the use of plating techniques and therefore reducing the risk of cross-contamination and increasing accuracy.

However, no mould testing method is perfect – in this case, according to ASTM, the static chambers limit the amount of material that can be tested, therefore producing results that may not always be truly indicative of what would happen in the field.

The Move Towards a Standard

The EPA has carried out a project with the goal of making the testing protocols for plasterboard and other building materials more standardised and more accurate. In its December 2008 brief, the EPA wrote the following:

Removal of growth substrates from building materials, or the incorporation of antimicrobial agents in the manufacturing of building products may prevent mould growth and the spread of biological contaminants. There are several building products readily available that can reduce mould growth in the indoor environment.


However, there is no nationally accepted testing and verification program to guide consumers and building professionals on how to select or specify the best building products for their needs. To help fill this need, EPA started an ESTE project to verify the performance of microbial resistant building products. EPA has contracted with RTI International to develop the protocol and perform the testing needed to verify these products.


Testing for mould

When the EPA alludes to this testing approach in its Test and Quality Assurance Plan it refers to it as ASTM D6329-98(2008). In this plan, the test is designed to run for twelve weeks, i.e. 4 times longer than ASTM D3273. In this case, the number of colony-forming units (CFUs) taken from samples collected during weeks 1, 6 and 12 is counted and compared to the baseline level of CFUs present at the beginning of the test.

Considering the strengths and weaknesses of each test method, it would seem that only products that get through either of the tests with absolutely no fungal growth whatsoever can truly be described as mould resistant.

The Use of Multiple Standards

According to Kendall Clark, R & D Program Manager for Georgia-Pacific Gypsum, the company had products that, using the ASTM D3273 test protocol, met the “no visible growth” criterion. The company then put the products through the ASTM D6329 test for twelve weeks, resulting in “no growth” there also.

Clark states:


“Mould can grow on regular plasterboard within 2 weeks. Our DensArmor Plus and DensShield products are manufactured to show no mould growth in the ASTM D3273 test (28 days). In an effort to assess the robustness of our product, we ran the 12 week test and showed no growth for that period. This type of performance makes our products extremely well suited for applications such as installation of the wall panels before the building is fully enclosed or “dried in”. This sequencing allows the building project to proceed on an accelerated pace.”


As for the reasons why the company decided to proceed with the ASTM D6329 test, here’s what Clark had to say:


“The test was conducted to demonstrate the durability and robustness that is designed into our products that we do not feel is fully captured and reflected by the industry standard ASTM D3273 test. The product selected for the test was standard production product. No reformulation was involved in the test product.


The decision to conduct the testing was based on our belief that while the current industry standard test (ASTM D3273) is a reasonable measure of mould resistance for a relatively short period of time (28 days); it does not provide a good reflection of long-term mould resistance and durability that would provide a higher level of confidence relative to the product’s ability to withstand real world conditions seen on construction sites.”


The impact that all of this has on architects, building owners and contractors is influenced by numerous factors that are unique to each one, however Clark suggested that there is a common benefit:


“The extended testing period is meant to demonstrate the durability and robustness of the product design. Each of the groups… will likely experience an unexpected moisture event during the product’s life that will be atypical with respect to magnitude and/or duration.


The extended test indicates that our product can provide resistance to mould growth for a period that is three times longer than the industry standard test; thereby providing greater confidence for the architect, builder or owner that the product will not need to be replaced due to mould growth.”


When evaluating products, therefore, it appears that the tests used by manufacturers to support their claims of mould resistance are paramount in assessing their products’ expected performance. When all factors, including the products’ pricing, are taken into consideration, the use of “somewhat” or “marginally” resistant products will add only marginal value to buildings.