Combating Condensation
The solution to condensation issues remains simple, says Tim Pullen, ventilate your home effectively and remove cold spots — but applying these rules can be tricky
TSimply Sustainable
Homes.
he most often used illustration of condensation is a pint of cold beer in a warm pub. Condensation will appear on the outside of the glass almost immediately thanks to the temperature difference between the beer and the pub — as well as the relatively high level of humidity in the room. What goes on in a house is largely similar.
There are any number of people and companies that will give you ‘ top 10 tips’ for dealing with condensation and mould growth. But really these tips all amount to two things: increase the heat in the house and increase the ventilation, often supplemented with a dehumidifier.
Warm air can carry more moisture than cold air and increased ventilation will get that warm, moist air out of the house. But that tends to be expensive as we are then heating the house with the direct purpose of throwing the heat out of the window. So, are there more efficient, more effective ways of dealing with it?
But first, a note on dehumidifiers. They draw air across a cold surface (as an air conditioning unit does), which causes moisture to condense out of the air, with the drier air then returned to the room. The water that is condensed out is caught in a con- tainer, but in some of the cheaper dehumidifiers that container is not fully sealed. The water in the container can then be warmed back to room temperature, at which time it evaporates back into the house, making the whole process pointless. Ideally the condensate is ducted directly outside, or at least caught in a properly sealed container.
Minimise Moisture Content
Obviously drying the laundry on a clothes horse in front of a gas fire in a house that already has a damp problem would be a bad idea. The reality is that just living in a house increases the moisture content and there is not a whole lot we can do about that. The problem is that we use a lot more water than was once the case, and a much higher proportion of that is hot water. We are putting more moisture into the air than older houses were designed to deal with. We have installed new double-glazed windows, carefully sealed-off the chimneys and air bricks to avoid draughts and given the moisture nowhere to go.
The reality is that we are not going to live a Victorian lifestyle, taking a bath every Christmas, and we need modern levels of energy efficiency, so we have to deal with the problem in different ways.
Mould Growth
Mould growth is the physical evidence of condensation. In some cases, often older houses, it is the only evidence as old timber and old plaster (especially lime) will absorb the moisture and hold it until conditions allow it to be evaporated away. Consequently, mould will often occur in areas where there is poor air circulation — top corners of rooms, behind wardrobes, sofas and the like. The fact that
“The importance of insulation and eliminating cold walls to reduce condensation cannot be over-emphasised”
condensation is not occurring in other parts of the room implies that better air circulation would be helpful and that spots where the mould is occurring are cold spots.
But it does not necessarily confirm that condensation is the cause. The possibility of external water penetration needs to be eliminated first. Cracked or leaking rainwater goods (sometimes evidenced by plants growing in the walls), poor pointing around brickwork and poor seals around windows can all allow water penetration. It may not be enough to cause a visible damp patch but may be enough to cause a cold spot, encouraging condensation and mould growth.
Introducing Insulation
Condensation occurs at the confluence of warm, moist air and a cold surface. Eliminate the cold surface and the problem goes away. Mould Growth Control Ltd sells a product called Sempatap Thermal, which is applied to the inside of a house’s external walls, effectively creating warmer walls and reducing the prevalence of mould.
The use of thin internal wall insulation products as pure insulators will not usually deliver results. But typically they are not advertised as that. What they really do is provide a warmer internal surface to the wall, eliminating any cold spots which can be enough to prevent condensation forming.
Cavity insulation will also work but requires a bit more care. In my own case, a couple of years after the cavity walls were filled we had mould growing above the window in the bathroom. The heating was good and the ventilation great, so this was a classic case of cold-spot mould growth. It turned out that the lintel over the window prevented the injected cavity wall insulation reaching that area. Fixing it was a simple matter of injecting a tiny bit more insulation.
The idea of putting more heat in the house, so that it can carry more moisture, is sound, but if we then leave cold walls we could be making the problem worse. The importance of insulation and eliminating cold walls to reduce condensation cannot be over-emphasised.
Improving Ventilation
Improving ventilation can be as easy as opening the window in the bathroom when you take a shower. But that may be less acceptable if you want to have a hot, relaxing bath with patchouli oil, candles and rose petals.
The ventilation requirements of the Building Regulations and the advent of whole house mechanical ventilation have pretty much eliminated condensation as a problem in modern houses. In rare cases, it does still happen but these tend to be poorly built houses that contravene the Regulations. What this tells us is that the combination of insulation and ventilation works. The problem is the balance, especially in older homes.
Unusually, perhaps, the Building Regulations have it about right. The combination of trickle vents in windows, extract fans in wet rooms and cooker hoods that vent to outside will be enough, if they are done right.
Whole house mechanical ventilation will work better, as it runs 24/7 and is controllable, but it can be tricky to conceal the ducting in older homes and it can be expensive. (Although a solution such as positive input ventilation (PIV), that works by gently supplying fresh, filtered air into a property from a unit installed in the loft area and a distribution diffuser mounted in the ceiling, is a good retrofit solution.) If that is not an option then getting vents in the right places and extract fans to run long enough are key factors.
For example, putting trickle vents in windows on one side of the room is less effective than vents on two sides, as that allows cross-ventilation. While extract fans need to run-on for five or 10 minutes after you have left the bathroom to get rid of all the excess moisture.
Conclusion
The comforting thing, perhaps, is that condensation and mould growth are not a new problem — even in older houses. Many people have been down that road and many companies have solutions to offer, but ultimately it will always come to ventilation and eliminating cold surfaces.
Designing an effective ventilation system can be a complex business. It does not appear to be easy to work out how much ventilation is needed. But, according to a Which? survey, 64% of people successfully dealt with a condensation problem themselves, without expert help. The reason being that we have been there before and trickle vents and extract fans are all standard. Tell a company such as Vent Axia how big the bathroom is and they will tell you which extract fan to install and how long it needs to run to be effective.
We already live in relatively warm houses and we don’t really want them any warmer, so adding more heat can only be of limited benefit. But conversely, warming the walls up with insulation can make a significant difference. We all know that cold beer in a warm pub will produce condensation. Ask yourself why a glass of red wine does not. The answer is that the wine, the glass and the pub are all about the same temperature. That is what we need to achieve in the house. H
“According to Which? 64% of people successfully dealt with a condensation problem themselves”