Property: Doctor on the house - Don't tell Einstein, but it's heat that gets you warm

If you think all you need is good insulation then you'll feel cold and you'll get mould, writes Jeff Howell

NOW HERE'S a thing - insulation doesn't make your home warmer.

No, only heating can make your home warmer; insulation is there to stop the heat escaping. Insulation can equally be used to keep things cool, as with insulated plastic picnic containers and those fantastic Australian polystyrene gadgets for putting cans of beer in - they call them beer coolers but they don't really do any cooling as such; they just slow down the rate of warming.

But there is a common misconception that if you put a bit of insulation in your home then you don't have to worry about heating. This can cause all sorts of problems, such as nasty black mould growing on your bathroom ceiling and nasty green mould growing on your leather jacket in the wardrobe. These unwelcome life forms are a symptom of condensation and - forget whatever else you've been told about it - condensation is God's way of telling you you're not heating your home enough.

Now, there are two interesting advertising campaigns going on at the moment: one is a government-funded effort, featuring an Einstein lookalike, which tells owners of cavity-walled homes that it would be good if they had their cavities filled with insulating foam. The other is from the Science Museum in London, which is a quiz featuring a snowman, some children and a tweed jacket.

In the first the Einstein figure is, for reasons which escape me, being squashed up in the wall cavity. The message seems to be that if you have foam squirted into your cavities then you are in some way being very scientific.

As with most adverts that use the image of Einstein, this is complete nonsense: building science, inexact though it may be, developed the cavity wall as a means of keeping out wind-driven rain, and if you fill the cavity with foam then you no longer have a cavity. It may be a neat place to put some insulation, but it is definitely not the right place.

If you fill the cavity then you do not just risk rain penetrating across it, but you cut off the heat leaking through to the outside skin of brickwork, making it more prone to condensation and hence to the rusting of the galvanised steel wall ties. One of the earlier cavity insulation foams, urea formaldehyde, has even been suspected of corroding wall ties in its own right, and is the subject of a Building Research Establishment inquiry.

I am much more impressed with the Science Museum advert. What happens, the children ask, if you put the jacket on the snowman? Will it get warmer and melt? Or will the jacket keep the snowman cool? The answer is not given on the poster; you're supposed to take your kids to the Science Museum to find out. But the idea is brilliant: to get people to think about concepts of heating and cooling and insulation, and so help them make their own rational decisions throughout their lives.

In a temperate climate such as ours, buildings need heat because the indoor air is always moister than the outside air, and there will always be a condensation risk unless you keep objects and surfaces above dew point temperature. If people were educated to understand this then the country would save millions of pounds, and we wouldn't have to take Einstein's name in vain to achieve it.

q You can contact Jeff Howell at the Independent on Sunday or by e-mail: Jeff@doctoronthehouse.demon.co.uk.