6 years on from my ‘exemplar’ woodfibre external wall insulation (EWI) project and hairline cracks have appeared at the bottom of the reveals, just above the sills, mainly to the more exposed windows. The cracks run along the edge of the window reveal bead and the render has been weathered away where it is thinly applied over the plastic. The solution would appear to be a more robust reveal bead which allows more movement and also a thicker render coat at this critical point. The render bead was specified by NBT back in 2014 but there are better solutions available such as APU.
The installer is no longer trading and there seems no facility for redress through the (2014) Green Deal Home Improvement Fund who contributed £6,000. The insulation system was Diffutherm by Pavatex with Baumit Nanopor render all provided by NBT who are now owned by Soprema and who no longer support historical NBT projects, so there’s nobody left to offer support – a situation the industry can’t repeat too often if it wants to avoid poor public perception.
The repair was actually quick and easy, simply removing any lose render with a screw driver and brush and then applying Sikaflex EBT (kindly suggested by Chris at Back to Earth). I guess the system will need keeping an eye one and likely to need minor maintenance every so often which is the case with a traditional rendered wall, but it does highlight the increased maintenance required. An EWI system needs to last decades and ideally 60 years or more (with periodic ‘recoating’) to be truly sustainable and it would be interesting to see whole life costs (both monetarily and in terms of carbon).
Another issue has been mould growth during the colder months (which clears up in the summer) but does suggest that the silicate render specified by NBT (due to its high breathability) may not have been the optimum solution and a silicone product may be more suitable. EWI limits heat escaping and so the render is colder than for a traditional sand cement render applied directly to the masonry which may result in more mould growth and the granulated surface to the EWI may also attract more dirt – there are certainly many examples of tired looking EWI systems.
That said, the EWI works fantastically well in keeping the building warm and comfortable and I wouldn’t be without it.