Q: I am considering having cavity wall insulation but have read that it can cause damp and condensation. Would be interested in your views.
A: Cavity wall insulation is a great idea if you have a clean cavity. It is true that filling the cavity with insulation can cause damp and this is usually due to lumps of mortar (known, endearingly, in the trade as snots) lying on the cavity tie.
As the wall was built it is possible that mortar fell in the cavity and came to rest on the ties.The process is that rainwater hits the external skin and penetrates to the snot sitting on the tie. That is not a problem in an open cavity as the ventilation will dry any moisture penetration before it reaches the inner skin.
Fill the cavity with non-breathing insulation and that will stop any ventilation. The rainwater will still hit the external skin and penetrate to the snot but now cannot be evaporated away and continues to penetrate to the inner skin and emerges as a damp spot.
Ideally you will check that your cavity is clean before filling it with insulation. Reputable insulation companies will do this for you as a matter of routine and give you an honest answer. Less reputable companies may say they have done it and give a more or less honest answer. Distinguishing between the two is the usual process of asking for and taking up references – from customers that had their cavities filled at least 12 months previously.
Alternatively, get someone like Dyno Rod to check the cavity for yourself (other drain cleaning companies are available). It is a quick, simple process of drilling a few 15mm holes in the wall, sticking in a little camera and seeing what the cavity is like. Companies like Dyno Rod use cameras for checking drains all the time and are set up to do this for you.
As to condensation, this occurs when warm air meets a cold surface – glass of chilled Chablis in a warm pub – the condensation appears on the outside of the glass. It is far more likely to occur with uninsulated walls as they are the cold surface.
Insulating the cavity allows the inner skin of the wall to become warm, essentially pushing the dew-point back into the wall where there is then a possibility of condensation within the brickwork – known as interstitial condensation. If this is going to occur it is generally on the inside surface of the external skin of brickwork.
The effect of interstitial condensation will vary with the insulation. If polyurethane foam is used it becomes bonded to the brickwork and prevents any condensate running down the wall, holding it in the brickwork where frost action could cause damage. Blown fibres tend to overcome the problem by allowing any condensate to trickle down the wall to the damp-proof course and away out of the wall. As a consequence the majority of companies offering cavity insulation use blown fibre rather than polyurethane. Again, a reputable company will sort this out and it is generally not a problem.
