So obsessed are we with ensuring our homes are properly aired even the building regulations devote pages to how this can be achieved in new houses and extensions to older homes. Attics must be properly ventilated, kitchens, bathrooms and utility rooms must all have extractor fans, and even the need for double glazing unravels a little when you have to provide trickle vents in the window frames. So in these post Kyoto, environmentally friendly times we’ve reached the bizarre situation that requires new homes and extensions to be well insulated and efficiently heated, and then filled with holes so that all that valuable energy can be ejected into the atmosphere to protect us from a few damp patches. In modern parlance I think this is called a lack of joined-up thinking.
As every lover of period homes knows, it’s important that the damp that inevitably accumulates in houses is dealt with and traditionally this was achieved by natural ventilation, a wonderfully euphemistic term for draughts. I had my first practical demonstration of this way back in the 1970s when my father very carefully improved the insulation in our Jacobean home to make the most of the newly installed central heating. Secondary double glazing, draught-proofing strips and underlay below the carpets to stop draughts rising through the floorboards were all dutifully installed and we snuggled up toasty warm come October ready to brave the winter ahead.
The problem came when we tried to light the first fire of winter. The sitting room fire simply wouldn’t draw, instead billowing smoke back into the house for several minutes before going out altogether. The simple fact was there was no air coming into the house so the fire couldn’t burn and pressure from the cold column of air in the chimney pots simply drove smoke back into the house; with hindsight we were lucky not to suffer carbon monoxide poisoning. The problem was eventually solved by banging a small section of cast iron rainwater downpipe through the hearth into the cellar, and hey presto the fire drew more magnificently than ever, just leaving us to dig out the toasting fork and crumpets.
This illustrates perfectly the problem is not just damp, but the way we expect to use our houses today. Sleeping in your overcoat and bobble hat is no longer considered acceptable, and as fossil fuel runs out and green house gasses increase it’s only right insulate our homes as well as we can. The problem is the average family of four produces a bucketful of water every 48 hours just through showering, washing and cooking and that water has to go somewhere; without adequate ventilation it condenses on solid walls, magnifies any damp problems and accumulates on roof timbers with potentially dire consequences. But I’m not convinced that simply ejecting all this damp air from the house is right, because with the damp goes our expensive warm air, almost literally throwing the baby out with the bath water. So what are the alternatives?
The obvious solution is not to create as much damp in the first place. The RICS does give advice (some available via its website, rics.org) some of which would be embarrassingly obvious if it wasn’t so readily ignored. Trying to keep pots and pans covered when you’re cooking sounds trite but it actually makes good sense; less energy is used for cooking and less water vapour is driven off into the house. Trying not to run showers for any longer than you have to is also common sense, especially if we continue to experience water shortages. The basic message is a little common sense goes a long way.
The thing we haven’t adopted in this country is heat exchangers, which are already mandatory in some northern European countries. These are really glorified extractor fans which remove the heat from the air as it passes out of the building, then used to help run the central heating system. In fact heat exchanges are little more than dehumidifiers, which are something I think the owner of every period house ought to own. Dehumidifiers can be bought from DIY or electrical stores for less than the price of installing an extractor fan. No, they’re not particularly attractive but you can probably find a place in a hallway, ideally as near to a kitchen and bathroom as possible. Yes they’re noisy, but you don’t have to run them 24/7, simply flick them on as you go to work. And I promise you’ll be astonished how much water they’ve collected by the time you get home.
Actually you’ve already got what amounts to dehumidifiers all over your house, but most people call them windows. Dehumidifiers work simply by blowing air across a chiller unit for a few minutes, then turning the chiller unit off so that the water that has condensed can run off into a collection bucket. During cold, winter nights or even cold, windy days single glazed windows in particular become very cold and any damp in the house condenses onto them before running to the edges to leave that lovely black mould. I’d guess that in a typical family house wiping down the windows with a squeegee after a frosty night probably collects about a litre of water, or if the weather’s kind enough you can just leave the windows ajar until the condensation has dried off.
A side effect of keeping the air in your house dry is that it will actually feel warmer. A lot of things make human beings feel the cold, and temperature is only one of them. As damp levels rise in a house you’ll feel colder, partly a result of the damp condensing and evaporating from your skin, which also makes you notice draughts more.
So what if you follow all this advice and you still have a problem? The first thing to check is that the problem is actually background damp appearing as condensation. The easiest way to be sure of this is the black mould I mentioned earlier. This can only grow with clean water, so water pushing through a building from outside, whether penetrating or as rising damp won’t support it, because the water will have inevitably collected salts and other contaminants as it passes through the fabric of the building. Any number of things could make part of a building cold enough to cause condensation; windows I’ve already mentioned, and solid walls, particularly 9″ brickwork, are prone to problems. Concrete lintels above doors and windows are other problem areas, and “dirty” cavities in brick cavity walls (where mortar has been allowed to bridge the gap in the wall cavity) show up as distinct blotchy patches across outside walls. If the problem has suddenly appeared, as usual when in diagnosing faults ask yourself what changed. Have you just added double glazing, or a shower? Or are the children now having a shower each rather than sharing the bath? In my experience the problems are usually that simple.
The type of damp I’m talking about in this article is to do with how much water there is in the air, measured as relative humidity. As well as a damp meter for measuring the moisture content of walls and timbers most serious surveyors also have a meter for measuring relative humidity, expressed as a percentage and adjusted to make the readings directly comparable regardless of temperature. A relative humidity level in a building of less than 50% is terrific, but rare. Anything over 75% will almost certainly be setting up condensation and creating breeding grounds for rot and wood-boring insect attack. The Building Research Establishment reckon that you’re pretty safe from wood-boring insect attack with relative humidity levels in a house maintained below 75%, so this is where the humidistat is set on modern extractor fans, which turn on when relative humidity rises above this level and off when it falls below. In fact you can adjust this on most modern extractor fans and dehumidifiers, and I would suggest 65%-70% as being about right. And don’t worry about the old wives’ tale about houses getting too dry, since the sort of dehumidifier you or I are likely to buy just won’t be powerful enough.
And although everyone bangs on about building ventilation I think the time has come to accept that if we want healthy, warm buildings the only way those conflicting desires can be resolved is to put less damp into the building in the first place or at the very least remove it more efficiently than through simple ventilation.
