Can I use my parge coat as an airtightness layer? – CIJA? | Ep.37

In this episode of Can I Just Ask…, we tackle a question we hear almost daily from people planning internal wall insulation: can I use my parge coat as my airtightness layer? A parge coat or levelling plaster can be a great way to improve airtightness on masonry — but only if it’s the finished, exposed layer. Add insulation over the top and that parge coat now sits on the cold side of the insulation, which largely defeats the purpose. Airtightness works by stopping warm, moisture-laden air moving through to the cold side of the insulation, so on an internal wall build-up it needs to sit on the warm side. We talk through the methods that achieve this: surface plaster as the airtightness layer on direct-plastered systems (and how to seal properly around back boxes and conduit), airtight membranes clamped behind battens, and taped wood fibre board joints. The golden rule throughout: keep your airtightness layer on the warm side of the insulation. Watch the full explainer below.