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Cumberworth Retrofit: Internal Wall Insulation (IWI)

Why internal wall insulation (IWI) matters

To radically address carbon emissions in the UK we are going to have to undertake an extensive retrofit of existing buildings.   External wall insulation is not a viable option for many buildings in the UK and so we will need to get to grips with how we use internal wall insulation (IWI) effectively and safely. There is currently much ongoing research and debate about the dangers of interstitial condensation when using IWI, with concerns about moisture and mould building up behind and within the installed insulation. The forthcoming AECB CarbonLite Retrofit programme is seeking to address these issues among other and share best practice from projects around the UK.

In many ways, as unsexy and unglamorous as it may be, thinking about the best ways to install internal wall insulation is at the current cutting edge of sustainable building in the UK. It is in this context that we are attempting to approach the IWI requirements at our Cumberworth radical retrofit project.

As we said in the last blog, the risk factor of interstitial condensation is different for different construction types. For this reason, our insulation strategies at the Cumberworth project are tailored to the different construction types.

Barn (solid stone, rubble filled)

For the barn internal walls, we are trialling for the first time in the UK, a new innovative insulation material called TecTem from Knauf AquaPanel in Germany. It is designed for internal insulation applications, is vapour open and deals robustly with the common moisture issues of internal insulation systems. The product is not, at present, available on the UK market.

A positive aspect of TecTem is that you can, in theory, apply up to 200mm thickness to the internal face of walls because it ‘breathes’ in the way that the original construction did. Its thermal conductivity is 0.045 W/mK,.  We do not know yet the final costings, but it does go a long way to reducing risk factors of interstitial condensation.  It is particularly useful in situations where masonry walls are most at risk of moisture build up, so with the help of AECB Carbonlite we have identified these areas.  We have parged with a weak sand and cement coat the internal face of the external wall to help minimise wind driven moisture and to give a flat surface for adhering the insulation. We have used water inhibitor in the parging for the bottom 900mm to inhibit rising damp. The East wall is particularly damp for the bottom 300mm and so we have decided to use 600mm high Foamglas slabs as an impervious tanking and insulating solution.

We are very excited to be trialling a product that appears to offer a significant advance for this application, with the only main disadvantages potentially being cost and difficulty of applying it.  It is a very delicate material, soft and quite crumbly.  The product is already attracting a lot of interest from the AECB CarbonLite researchers, who have installed moisture monitors throughout the project, to monitoring moisture levels in the Cumberworth retrofit walls.

1990s extension (cavity wall construction)

The risks of moisture build up and interstitial condensation are not as great in the 1990s extension and so we have developed a different IWI  strategy  for that area. There had been significant water ingress during the re-roofing operations resulting in high moisture content in the plaster and masonry. Normally it would be expected that the moisture in the wall could dry easily through the block to the cavity, however the inner leaf was constructed with a foam insulated block. This type of block which was popular in the 1990s in Yorkshire was supplied with either expanded polystyrene or polyurethane bonded to one face which acted as cavity insulation. The presence of the plastic foam insulation will have affected the drying potential so modelling was undertaken using WUFI to check that the wet plaster and masonry could dry in an acceptable period of time. This moisture levels in this construction are also being monitored to compare to the prediction in the modelling.

We are leaving the existing gypsum plaster in place and parging (applying a weak sand and cement mixture) in areas where there are gaps in the plaster (eg between intermediate floors) and then using proprietary insulated studs  mechanically fixed, with low thermal conductivity mineral wool insulation between, followed by an Intello Plus vapour control barrier, service void and plasterboard.

On site

Reducing thermal bridging in structural steel

Our structural engineer Stuart McCormick from SGM has been helping us rationalise the space in the top floor which was really badly laid out.  Most of the first month on site has been taken up with altering the roof trusses to accept new doorways through.  We’ve had to put big steels at the intermediate floor level to support the truss allowing us to chop them out so we could put doorways in.

Unfortunately however those steels create bad thermal bridges to the external wall. To reduce this we’ve used Foamglas surrounding the ends of the steel.  This  has a high compressive strength  and is consequently often used in foundations to minimise thermal bridging.

Bill Butcher, Director, Green Building Store

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