Coaction trainer Eric Fewster shares his personal experience on the challenges of considering moisture risk when retrofitting old buildings…

Internal wall insulation (IWI) is often required in retrofit, but it’s not just a case of considering u-values and heat loss!

With old buildings, we also need to consider the moisture risk, as when you add more insulation on the inside, the original wall gets colder and the wall's moisture content tends to increase – adding more insulation, or insulation of the wrong type, means at some point there will be too much moisture and the wall can’t dry out as fast as it gets wet, giving rise to the risk of degrading the existing structure.

An example of this comes from a two-storey solid-walled sandstone house which had many different wall types, most of which were some form of solid sandstone . Some of the existing walls already had 50mm of either XPS or EPS insulation internally, sometimes together with an impermeable membrane – coring out some of the existing XPS insulation from a west-facing wall revealed humid conditions behind (Photo 2).

While the project team were only considering the use of vapour-open materials in order to avoid or exacerbate the moisture problem, it was not clear how much insulation could be applied, and what the airtightness strategy should be. As the house was in a fairly exposed location, it was felt that some WUFI hygrothermal modelling was needed. This involved getting stone samples cored out (Photo 1) from different walls and checking their absorptivity before running the analysis.

The WUFI model (Photo 3) helped to firm up the solution which was kept the same for all of the solid walls - a layer of lime plaster (to dub out the uneven walls), then 15mm of Diathonite, followed by 80mm wood fibre, with 10mm of lime plaster as the airtightness layer. This gave a U-value range of 0.31 – 0.34 W/m2K for most walls.

You can read more about this phased retrofit project HERE.

Coaction’s RETROFIT DELIVERY course covers the reality of tackling the practical challenges thrown up by retrofit projects, approaches to minimise risk and reduce unintended consequences.