Breathable vs. high performance insulation for solid walls

Tuesday night was 'Thermal Comfort in Older Houses', a Transition Cambridge Energy Group event, organised by myself and others. The first speaker was Andrew Mitchell from Natural Building Technologies and he discussed at some length why insulation can sometimes cause damp in solid wall houses, at least if it isn't a breathable material like the NBT products. So I have learned a little more to add to what I wrote before on 'Why insulation causes damp'.

This was not the only subject of the evening however: my post on the Transition Cambridge Media Blog is more comprehensive.


To recap very briefly what I said about solid wall insulation before:

• Don't use internal insulation in areas prone to wind driven rain because the walls will be colder and take longer to dry
• Beware cold surfaces where condensation can occur, especially if the insulation has sealed draughts so the air in your home is more humid than before. Cold surfaces can occur in gaps in the insulation.

So far so good but there are other potential issues of concern too. Moisture moves through walls in complicated ways. It is possible for water vapour to be travelling in one direction due to difference in vapour pressure while liquid water is travelling in another due to capillary action or water absorbent materials. Most solid wall insulation systems rely on keeping the insulation dry. They may not rot when wet but they won't insulate very well either and dampness leads to mould and serious health issues. So to keep the insulation dry there is always a vapour barrier between it and the outside weather or inside air.

The proponents of breathable insulation products argue that no matter how well the junctions are designed and installed, there is always scope for water to get into the wrong place and once that happens any vapour barrier works to stop the water getting out again. As Andrew said, the key thing is to make sure that moisture escapes faster than it gets in. The trouble is there are a lot of ways that water can get in.

1) If you don't have a damp proof course, damp comes up from the ground. Of course most houses do have a damp proof course, and if yours does not then you should be very careful.

2) Moist air can seep in over time, through small gaps or tears in the vapour barrier. When our (celotex) insulation was installed, the specification included that the blocks of insulation, which have a vapour barrier on one side, are fitted close together and the joints taped up with special tape. Even so, I can imagine that there are some places where the tape does not hold or there are small gaps or it may be that a few years in the future someone bangs a few nails in the wrong place. Also, perhaps we will need a new pipe or cable to come through the wall and so someone will make a hole and perhaps not seal it properly. I still don't see how this can make much difference, however, under circumstances when the humidity is well managed.

3) Liquid water can get in catastrophically through spills or bad plumbing. I should imagine the worst possible would be a minor plumbing leak that seeped water slowly over a long period without being noticed. This sort of wet can make timbers rot quickly and most houses with solid walls also have timber floor joists. Ultimately your floor could collapse. On the other hand, this sort of problem is serious even without a vapour barrier. It is similar to the situation we found when we pulled up our old floor boards. The joists in some areas were rotten because the damp course had been breached by builder's rubble thrown into the void under the floor. Insulation was nothing to do with it.

So why don't we just all use breathable insulation all the time, to be on the safe side? There are two main reasons. The first is that foam insulation is cheaper, at least at the moment. The second is that it is more effective so you don't need so much thickness. For example Kingspan insulated plasterboard has thermal conductivity around 0.021 W/m.K while NBT Pavadry is about 0.043 - so the same thickness insulation will lose twice as much heat in the NBT case.

However, Andrew has an argument about that too. Firstly, solid walls are usually better insulators than is assumed in the standard models so you don't need so much extra as is normally calcualated. This is true, though walls vary greatly. The EST has found that solid walls are on average 30% better than is normally assumed. Secondly, when you take the inevitable thermal bridging into account - those places where the heat can take a short cut because of junctions or holes in the wall for pipes and so on - the high performing foam insulators don't work out much better than the others. The chart below shows how the presence of the bridging limits the overall performance of the wall.

U-values for a wall with different thicknesses of insulation with and without thermal bridging. Assumes the wall alone is U-value 1.4 and the bridged wall is U-value 2.0. The conductivity of pavadry is 0.042 and Kingspan is 0.023.

One more point about the breathable insulation choice is that you have to be careful what you put on the top - if it is covered with standard plaster and gloss paint it won't work. So you need to use lime plaster and vapour permeable paint.

I am still sitting on the fence on this. I can see lots of cases where naturally breathable materials would be a wise choice but I can't quite justify using them all the time, with the increased cost and thickness that entails and the pain of having to use lime plaster and special paints.