ProofTherm is Proofshield’s ultra-lightweight insulating base coat, applied to the internal face of external walls to create a thermal barrier that reduces heat loss, raises surface temperatures, and makes rooms noticeably warmer — without any external works or scaffolding.
In properties with solid wall construction — typically pre-1920s brick, stone or block with no cavity — heat moves freely from the warm interior to the cold exterior through the wall fabric. Standard internal plasters do not interrupt this heat flow. Rooms on the cold side of the house remain difficult to heat, expensive to keep warm, and uncomfortable in cold weather.
External solutions — such as external wall insulation board systems — are effective but expensive, disruptive, and require planning consideration in many cases. ProofTherm internal plastering addresses the same problem from the inside, at a fraction of the disruption and cost.
ProofTherm replaces the sand in a conventional plaster base coat with expanded perlite — a volcanic mineral with a honeycomb microstructure of micro-air bubbles. Perlite has a thermal conductivity of 0.058 W/mK, making it significantly more thermally resistant than sand and cement.
Applied at 20–25mm to the internal face of an external wall, ProofTherm creates an uninterrupted insulating layer that reduces the rate at which heat passes through the wall. The perlite base coat is vapour permeable — it allows moisture to move through it — and contains polypropylene fibres for added flexural strength.
ProofTherm is applied as a base coat at up to 20mm depth. Once cured, it is finished with a breathable skim coat of decorative plaster in the same way as any plastered wall. The finished surface is indistinguishable from a standard plastered wall — smooth, paintable, and ready for decoration. The only difference, in fact, is that the wall is now well-insulated and highly breathable.
Because ProofTherm is applied at a greater depth than a standard plaster base coat, it does result in a small reduction in room dimensions — typically 20mm per treated wall. We will confirm the practical implications of this at the free survey.
No. ProofTherm is a specialist insulating base coat, not a conventional backing plaster. It is applied at 20mm — thicker than a standard plaster coat — and uses expanded perlite as its primary aggregate rather than sand. It requires a trained installer and is finished with a lime-based, breathable skim plaster.
Applied at 20mm per wall, the reduction in room dimension is small but real. On a treated wall, you will lose approximately 20mm. On a typical room treating two external walls, this equates to 40mm in total across the width or length of the room. We discuss this clearly at the survey stage.
The product cost is higher than sand and cement. However, we price ProofTherm installations at the same overall level as a conventional replaster job — we absorb the material cost difference. There is no premium to the customer.
Yes. ProofTherm is finished with a skim coat or decorative plaster that requires painting. We will advise on drying times before decoration at handover.
ProofTherm is most effective on external-facing walls where heat loss through the wall structure is the issue. We will confirm which walls would benefit most at the survey.
Book a free, no-obligation survey. We will visit your property, assess your walls, and tell you exactly which Proofshield system is right for you — and what it will cost. No pressure, no hard sell.