Why do we use clay plaster in our internal wall insulation systems? – Can I Just Ask? | Ep.27

Clay plaster is the preferred substrate for BTE’s internal wall insulation systems, and the reasons stack up across performance, embodied carbon, and practicality. The fundamental goal when fixing insulation boards to an existing wall is achieving full, continuous contact with no voids — and the preferred method is to apply plaster to the wall first, allow it to dry fully, and then dry-fix the boards onto that flat surface. The alternative — using a notched trowel to adhesively fix boards directly — is widely used but has an inherent problem: the furrows left by the notched trowel don’t always close up fully under the board, meaning the very method intended to eliminate gaps can systematically create them.

The choice of clay rather than lime plaster is primarily an embodied carbon decision. Retrofitting a building to reduce its energy use and carbon emissions while stripping back walls and relaying them in high-embodied-energy lime plaster is a contradictory position — the carbon cost of the lime is unnecessary when clay performs at least as well and often better. Clay requires minimal processing, has a very low embodied carbon footprint, and is slightly more effective at moisture buffering than lime — which matters in an internal wall insulation context where managing the movement of moisture through the system is a key design consideration.

The third advantage is reversibility. In historic buildings particularly, the ability to remove an insulation system without causing damage to the underlying fabric can be a significant factor — and clay plaster, being water-soluble, can simply be washed off if the boards are ever taken down. What’s left are a handful of fixing holes, nothing more. Taken together — low embodied carbon, good moisture buffering, full-contact bonding when applied correctly, and easy reversibility — clay plaster is a straightforward choice for a system that’s trying to do things properly from the outset.