by Denise Henkenhaf, eZED Ltd | 10 June 2014
Fungus needs only moisture and food to flourish
Mould is a form of fungi and only a fraction of the approximately 150,000 fungi specicies identified to date. Commonly known as multicellular fungi, it can be found on decaying food, in food processing as “natural” flavours, cheese maturing agents, medical substances (Penicillin) and unfortunately in most New Zealand houses.
Mould as a fungi has a simple reproduction strategy. It requires moisture and food. The food can be readily found in the air-borne particles, dust and building materials. Growth-supressing agents can be applied to surfaces or impregnated into a material, however these offer only partial protection, if the component is under constant high humidity mould will still grow on the surface. Structural decay will be delayed as the mould will not use the material as food until the chemicals dissipate. The surface of structural steel is no exception, mould will happily grow on the surface even though the steel is not a food source.
Thus mould only requires moisture to thrive in our building environment as their food from air borne particles is so abundant. Once the relative humidity is above 80%, spores will start to germinate into hyphae. This is generally temperature independent, different types of spores will germinate at different temperatures, some even close to freezing point. Once these spores grow into myscels (many hyphae) they will release mycotoxines into the air. These mycotoxines have been shown to be damaging to our health. Respiratory diseases are only one example of mould related health issues.
Mould-build up is caused by high humidity levels
The recent update in Clause E3 of the NZBC stipulates that a temperature difference of 5 to 7ºC (internal to external) is sufficient in combination with ventilation to avoid humidity/condensation. A factually correct statement, however, this is not really practically achieved if the indoor temperatures are to be in the healthy range of 15 to 20ºC. Even in the Auckland region, outdoor temperatures can drop to freezing point and it is unlikely that indoor temperatures would be a low as at 7ºC and be a comfortable living environment. On the contrary, we aim to keep our indoor spaces warm by (attempting to) heat it to 20ºC. Hence the temperature difference between indoors and outdoors on a cold frosty morning is more likely to be 10 to 20ºC and condensation commonly occurs. Fresh air ventilation is also reduced in winter as occupants try to keep out the drafts.
We add additional moisture to our indoor air by cooking, showering and breathing. Adding reasonable fresh air ventilation will help to manage these moisture loads, however we will likely still maintain an average relative humidity around 50% (Auckland ambient is approximately 70%, Lower South Island it can be 30%). The dew point for 20ºC at 50% RH is 9.26ºC, materials within the building construction will likely have temperature profiles below this figure during winter months. In some cases these low temperatures occur on internal surfaces such as window frames, hence we see condensation beading and mould growing here if left unchecked. Most of the time we don’t immediately see the problem as it is hidden inside our wall or roof construction and is not noticed until the mould growth is well advanced.
A Problem by Design
There are a number of design factors that contribute to having low internal surface temperatures. Essentially the only means of defense is to keep the internal temperature up and avoid having low surface temperatures that allows the localised humidity to be raised above 80%RH. If adopting minimum code R-Values from clause H1 of the NZBC, the heat losses are often still too high to be able to afford heating effectively and avoid low surface temperatures.
Geometrical and structural thermal bridges can be assessed with isothermal analysis software to ensure internal surface temperatures are kept well above the critical dew point temperatures to avoid surface condensation and mould.
Simply adding insulation without taking other measures can exacerbate the problem as it will move from being a interior surface problem to a hidden problem which is essentially far worse as it will go on unnoticed. Warm indoor air can infiltrate the building fabric as energy moves outward, this causes condensate to form within the construction once the air has cooled down to the dew point temperature. If this happens over a period of a few days you can be certain that spores will have germinated. And where air gets in, it will get out again – enabling the spores and toxins to enter our indoor environment. An airtightness layer to the interior face of the insulation will prevent this from happening and should be considered a must for all buildings if all mould risk is to be avoided. Obviously an adequate level of insulation is required to work with the airtightness layer otherwise a cold surface temperature can still occur and condensation will still result.
A more detailed study of mould risk can be determined through hygrothermal analysis using software such as WUFI®. In some circumstances the temperature gradient and humidity gradient might still cause critical levels of relative humidity, despite using an airtightness layer. Having a mixture of impermeable surfaces and diffusion open materials can create a complex scenario that needs to be studied for year round climate effects. Seek our expert advice to avoid having a problem by design and ensure that condensation and mould risk is kept in check.
These images from WUFIBio shows how moisture builds up in a typical well insulated skillion roof construction for a cold lower South Island location, with and without an airtightness layer. If a vapour check such as INTELLO® is introduced the problem can be completely mitigated as can be seen on the Mould Growth graph on the right image.
Copyright eZED Limited 2014