How to turn a lovley old house with solid walls into a warm efficient home




My home was built around 1804. The walls are solid 9" brick. As discussed earlier, my extension was designed to envelope the rear of the house making the old rear wall an internal wall, eliminating the need to insulate it.

Obviously I have no cavity within the wall that I could fill so I had two options - external wall insulation or internal wall insulation. I ruled out external wall insulation immediately. The lovely old brickwork of my house is a key feature and I had no intention of covering this up.

When external insulation is fitted the old wall is within the envelope of the house, it is kept dry and also acts as a thermal mass. Alas things are the other way around if one installs internal insulation. Effectively, the old wall is now outside of the warm house envelope and there is a very real concern that interstitial condensation could occur. This is where the warm air from the house condenses onto the wall behind the insulation because that is the first cold surface the warm air comes into contact with. Some people fit a vapour barrier behind the insulation board to prevent vapour getting to the cold wall surface and condensing. However, this is only of use if the vapour barrier is 100% unbroken with no areas of air leakage. Floor to wall joints, ceiling to wall joints, joints between boards, risk of puncture for services etc makes the idea of a perfect vapour barrier retrofitted to an old house very unlikely in my opinion. Condensation and dampness behind insulation boards will lead to wet walls and mould growth, which though not apparent for some time is a problem in the making. I have seen such damp and mould behind cladded walls in old houses and it's not nice. My preference, if it were possible, was to take the opposite route; accept that a continuous, unbroken vapour barrier is not feasible, but be confident that any dampness is managed. People often talk of lime being 'breathable' - moisture can pass through the material, but such claims are never quantified - how breathable? It all sounds a bit 'airy fairy'. I was looking for something more quantifiable and measurable.

wall condensation, interstitial, condensation, wufi, breathable, vapour permeableThe Glaser test is a system of measuring and predicting moisture and condensation within building materials. I am not an expert on this, I just knew of its existence. When researching, this lead me to something called WUFI - as a layman I will describe this as an improved and more detailed Glaser Test. WUFI is modelling software developed by The Fraunhoffer Institute of Building Physics in Germany. I had a quick look at their free demo software and was very impressed. The software takes very specific weather details for a given location, takes into account building exposure and orientation, models the exact properties of different building materials within the wall and then predicts if over a period of many years there is likely to be a build up of moisture at any given point within the structure. This was exactly the kind of scientific detail I was looking for rather than some salesman promising a vague notion of 'breathability'. I was in no position to buy or use this WUFI software, however.

I was keen to find a very thin insulation product to minimise loss of space in my rooms. A little good fortune came my way as I researched further in search of such a highly insulating yet thin product. Aerogel is a product I had heard of years ago. It's thermal insulation properties are second to none - check out some youtube clips, it's amazing stuff - I had never seen this stuff as anything 'usable', however, they were just small, brittle chunks. Fortunately though I now found a couple of companies beginning to use it within usable insulation materials.

Aerogel is manufactured in the USA and there was a fairly new product called Spaceloft that was a fibre matt impregnated with aerogel. I could only find a couple of companies doing anything with it, but I chased them up and was eventually put in touch with a guy that worked for Aspen Aerogels, the company that made the insulation. He became interested in my project and as he was booked on to a course at the Fraunhoffer Institute to learn about WUFI software he chose to use my home to model.

We listed out every material within the proposed wall structure. I planned to use the spaceloft aerogel insulation, a magnesium silicate board to be plastered over with fibrelime and painted with keim paints (more on each of these in my article covering the fitting of the insulation). All of these products very deliberately have a vapour open quality meaning that they allow vapour to pass through quite naturally - 'breathable', if you will, but rather the term breathable being used in a vague sense these products can all be backed up with exact vapour permeability figures, allowing them to be modelled accurately by the WUFI software. The results of the WUFI test on my building showed how by using fully vapour open products rather than trying to create a barrier to vapour, the natural movement of vapour is stable and not building up over a period of years. This gave me the confidence that this was the right route to go.

A following article about the fitting of the wall insulation discusses the products used in more detail. The point of this article was to raise the awareness of interstitial condensation as a potential problem as more and more people are looking to improve insulation of walls where there is no cavity. Any WUFI test is specific to a building but if you would be interested in seeing more details of my WUFI report then feel free to contact me.

aspen aerogel, wufi, condensation, wall, breathable,

© Christopher Thompson