mouldy wall

What causes Mould growth behind plasterboard?

Picture of Bryan Hindle CSRT CSSW GradIOSH

Bryan Hindle CSRT CSSW GradIOSH

Bryan is Technical Director of Brick-Tie Limited

What causes mould growth here?

The video above shows the practical consequences of a failure to control humidity in a kitchen below ground. This isn’t due to groundwater penetration in this case. High humidity causes mould growth Just as does penetrating rainwater or ground water. we were there as part of a standard damp and timber survey for a homebuyer.

This is a back-to back house in Leeds, West Yorkshire. Only parts of the front wall have any walls with high ground against them; the three neighboring houses (left, right and rear), have cellars too, which are on the same level.

Obtaining dry walls for decoration is a cosmetic approach. This is what has been done here, probably because the homeowner didn’t think the cellar was very damp – many are dry so this is common. It risks ground water ingress. That is fine as long as the client understands the possible damage ground water leaks can cause. The video is not about the pros and cons of basement waterproofing; more on what causes mould growth.

The role of Humidity and Temperature

The kitchen is rather cold and lacks effective mechanical extraction. This inevitably leads to excess humidity. The contractor who installed the dry linings has included some PVC soffit vents in the face of the plaster board. Why? He isn’t around to talk to, but the assumption is that he wanted to allow the cavity behind the plaster board to ‘breath’ and stay dry.

The vents actually worked against this premise for the following reasons (these are rules too):

1.    Walls don’t ‘breath’. This is a misleading term, related to permeability and evaporation.
2.    Adding water vapour to air will increase the vapour pressure of the air.
3.    Water vapour production in an occupied house means that the vapour pressure in a house is almost always higher than it will be outside
4.    Vapour will always travel down it’s pressure gradient from high to low
5.    In the absence of water vapour being added or subtracted from a body of air, lowering its temperature, will always increase the Relative Humidity of that air.
6.    High Relative Humidity causes mould growth

Following the simple rules above, we can see that the vapour pressure in a kitchen, where there is a kettle, washing machine, sink full of pots and a hob, will be high. Vapour pressure is just one way of stating the amount of water vapour in air – expressed as pressure.

The kitchen is reasonably warm (when the house is occupied), so the relative humidity is not too high in the kitchen itself – there is little visible condensation and no visible mould.

The amount of water vapour being produced by the walls, which in any case are covered by a plastic sheet behind the plasterboard, will be low, because they are cold and it is heat which drives evaporation (along with air-movement and the ‘dryness’ or otherwise of the air-flow).

Vapour pressure seeks balance, so the vapour pressure in the void behind the plasterboards will ebb and fall with that of the kitchen (see 4 above).

Let’s look at temperature. This is because the kitchen, like most homes, is heated by convection. The warm radiator transfers heat to the air touching it (very little heat is actually ‘radiated’ and air just lets that radiation pass straight through it anyway). This air will expand and lose mass as the oxygen, water and nitrogen molecules and elements get all excited and start to dance. This drives convection. The air circulates, losing energy to all it touches; you, me, the ceilings (mostly) and walls get warmer.

In the light if the above the cavity behind the plasterboard (maybe not that bit right behind and above the radiator) will be colder than the kitchen itself.

Thus the relative humidity will be higher behind the insulating plasterboard (rule 5). The insulation is provided by more than the U value of the plasterboard. It is also a feature of the reduced transmission of heat via convection. The plasterboard is a wind-break to the warm draft from the radiators.

A kitchen is quite a dynamic environment, from a vapour pressure point of view. Kettles boil and Vp rises quickly. Then it may fall. If condensation has happened on cool surfaces evaporation should be swift. However, condensation in an enclosed draft free space, where it is cold will be slow. The chances are that in a busy household the evaporation may not be complete by the time the kettle is on again. Perhaps this time the pasta is boiling in its salty broth. More condensation on a still damp service will act to keep the plasterboard cold (the slight heat deposited by the condensate will be lost during the endothermic evaporation phase).

Cool dark areas with high relative humidity – this causes mould growth.

What could have been done?

You cannot keep peaks of vapour pressure from happening in a kitchen. This is especially true of a back to back, with only one window for ventilation. Better purge ventilation, via throwing open a door or window is fine in summer, but in winter this is not a comfortable option. A really good extract system would be good. That way the generally high vapour pressure and the peaks, would be reduced. In this case the provision of a good vapour control layer and the omission of the surface vents would have been a massive improvement.

We hope you are better informed about the invisible dynamics of the internal environment. How these can sometimes conspire to ruin, well intentioned plans, is worth knowing.

Did the mould matter?

Yes, because the smell was bad and it just made the cellar seem a damp and unpleasant place to live in. Our MD is not a ‘toxic mould’ scaremonger and is usually quite relaxed about hidden mould. However, the surface area in this case was very high (many square meters). The provision of the PVC vents in effect made the living space readily accessible to the mould spores and volatile organic compounds emitted by it. This caused the smell and could be unhealthy too, if persons of reduced immunity or allergic sensitivity to mould.

Hygroscopic buffering

The above case study also sheds some light on current discussions about the use of hygroscopic buffering materials in old houses. There are those who would sanction filling voids like these, with natural fibres to readily absorb water vapour. The expectation being that they then release it over time. The purpose is to lower overall humidity levels, as an alternative or adjunctive to mechanical ventilation.

Careful design will be crucial if this is done. A slight increase above expected water vapour production, combined with a lower than expected air and material temperature, could lead to hidden mould problems.

Colder voids will be prone to condensation and of course high humidity causes mould growth – take note!

The repair was a complete strip-out and the opportunity was taken to waterproof the basement against ground water too…. But that’s for another video.

For the best advice from real damp and timber experts call Leeds 265 2752 York 566577 or Sheffield 224 5121 or email us  via our contact page

We are not doctors and try to avoid commenting on the health implications of mould. However, for an occupational hygienists point of view, read the review below – it is fascinating.

If you have mould in your house (other than on cheese or bread), there can a number of reasons – read on for a further guide:

Why is my house mouldy?


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