Most homes built before around 1919 in South Wales are solid-wall — stone or brick, without a cavity — plastered internally in lime, not gypsum. These buildings were never designed to be waterproof. They were designed to let moisture in and back out again, evaporating through breathable materials before it could cause harm.
What "breathable" actually means
Lime render and lime plaster are vapour-permeable — moisture passes through them and evaporates from the surface. Cement render and modern gypsum plaster are far less permeable. When a solid wall is sealed with cement or gypsum, moisture that would once have evaporated gets trapped inside the wall instead, often reappearing as damp somewhere else, or causing decay in timber in contact with the wall.
Why this causes so much misdiagnosis
A generic damp survey trained on modern cavity-wall construction can misread a breathable wall's normal moisture behaviour as a fault, or recommend a fix — like a chemical damp-proof course or cement render — that actively makes a solid wall's moisture problem worse.
What this means for your home
- If your home is pre-1919 or solid-wall, any diagnosis needs to account for how the wall is built, not just what a moisture meter reads.
- Cement render, gypsum plaster, and cavity-wall insulation retrofitted into a solid wall are common causes of "new" damp in old buildings.
- The right fix usually means restoring breathability — lime-based materials — not adding more barriers.