Wall Coatings

Textured wall coatings are supposed to keep water out of your home. In fact, they can keep it in

Q. I was given a demonstration by a company's sales manager of the way in which their coating keeps out moisture but still permits the wall to breathe. The demo consisted of a transparent cylindrical container. Halfway down was a segment of porous building block that had been covered by the wall-coating material. I was asked to fill the top of the container with water. There was no evidence of water passing through the block - it remained dry. Then air was pumped into the bottom of the cylinder. Bubbles of air were seen to be emerging from the block of building material and coating. Pump air - bubbles. Cease pumping, no bubbles - no water passed from top to bottom. The work is also independently insured.

A. This is a demonstration that air under pressure passes through the coating, not that moisture vapour not under pressure would do so. So it does not mimic real conditions, and sounds like a clever sales trick. The problem with these coatings is that if you get condensation forming within the depth of the wall - i.e. the wall becomes damp from within, with liquid moisture - then it will be unable to escape. As the demonstration shows, liquid moisture will be trapped within the wall.

Q. We have a 17th-century farm house which the previous owners had spray coated with one of the "textured wall coatings" that you warn against. I do believe it is creating a damp problem as it is cracking and water is now seeping into the structure. And, as you have also warned, we found that the "guarantee" was worthless as the company was no more! Anyway, what hope for the house? Can this stuff be removed, and at what sort of cost?

A. These textured coatings sound like a good idea, as they offer the promise of long-term waterproofing of the walls. What the salesmen do not point out, however, is that many dampness problems are caused by water vapour produced inside the house, and that the coating will have the effect of trapping this moisture in the walls. Also, as you have found, if the coating cracks, then rainwater will be able to get in, and will then be trapped behind it.

The good news is that the coatings can be removed using chemical methods. There are two different types of coating, that respond to different strippers. Both are painted on with a brush, and washed off with water. Strippers Paint Removers (01787 371 524) will provide you with a trial pack of both types for £12. When you have found which one works best, it will cost around £2 per square meter to treat the house.

Many more readers' questions like these are answered in my book, "The Sunday Telegraph Guide to Looking After Your Property" - click on books towards the bottom of the menu for more information...

The best way to keep your home dry is to make sure your gutters, rainwater down pipes and drains are clear and in good condition, and to pay attention to internal heating and ventilation. Probably the worst way is to apply paint or waterproofer to the outside of your walls, as these can stop the building from "breathing", which, in turn, can lead to a build-up of moisture within the fabric of the wall itself.

Modern external textured wall coatings are an extreme and usually irreversible version of the above, and seem to be especially troublesome - readers have reported that the coatings have cracked and allowed water to penetrate from the outside; the moisture is then unable to escape by evaporation, and shows up as damp patches on the inside. And a recurrent theme of these complaints is that when attempts are made to get the contractor back to fix the problem, under the terms of their "guarantees", they have - surprise, surprise - galloped off into the sunset, leaving a "So long, suckers" note pinned to the office door.

Textured coatings are marketed using the following logic: Damp comes from outside, so wrapping the house in a waterproof coating must keep it dry. Sounds reasonable enough. Except that the basic premise is wrong - most damp does not come from the outside; it comes from within, produced by the everyday human activities of cooking, bathing, sweating and breathing. So enclosing your home in an impermeable skin can produce the same effect as leaving your used swimming kit in a plastic bag under the bed - when you discover it several weeks later, it is still damp, and has become a breeding-ground for assorted moulds and bacteria, exuding a characteristic odour which some people refer to as "damp". ("Dampness" itself, being just another term for water, actually has no smell - it is only the organisms that thrive on damp materials that produce a tell-tale pong.)

Another analogy for the problems caused by external waterproof coatings is that of going for a walk wearing a rainproof kagoul or anorak. All experienced hikers know that these garments are less than ideal, because, even on a dry day, the moisture produced from the body's exertions is trapped inside, and your clothes can end up wringing wet.

The other problem with a waterproof raincoat is that when it does rain, the water all runs down and soaks your trousers and shoes. Something similar happens with waterproofed buildings. The water which would have been absorbed by a whole wall now flows down and is concentrated at ground level, overloading the drains and drenching the footings.

So, in a temperate climate such as Britain's, it is better to have a building exterior that behaves like an overcoat rather than a raincoat. Any incident moisture is absorbed over a large area during wet conditions, and then dries out later when the sun comes out. To ensure that old brick buildings continue to perform like this, it is also preferable that any repointing is done with mortar made from lime rather than cement; strong cement mortars are themselves impermeable to moisture, as well as being too hard to accommodate the slight movements that all buildings are subject to.

And if you really must paint the outside of your walls, then the only totally breathable coating is limewash, in its natural virgin white, or coloured with earth pigments.

These materials have been used for thousands of years, and are very cheap, so nobody can flog them to you for a huge profit. And they certainly don't come with a paper guarantee.