What Sort of Moisture Analysis Should I Have? – Can I Just Ask? | Ep.19

There are two main methods of moisture analysis used in UK building projects, and choosing the wrong one for the application can lead to seriously misleading results. The simpler and more widely used method is the Glaser analysis, governed by BS EN 13788 — a dew point calculation that looks at vapour pressure on either side of a wall and identifies at what point within the build-up the temperature drops to dew point. It has legitimate uses, particularly for lightweight structures like pitched roofs and timber frame walls where rain ingress isn’t a significant factor, but it was never designed for solid wall insulation and falls well short when applied to it.

The more rigorous approach is a hygrothermal analysis in accordance with BS EN 15026, typically carried out using software such as WUFI or Delphin. Where Glaser only models vapour diffusion in one direction, WUFI incorporates a far broader range of variables — wind-driven rain, air leakage from the interior, orientation of each elevation, external weather files specific to the building’s location, and the moisture buffering capacity of the materials themselves. This matters enormously for internal wall insulation, where Glaser will almost always flag a condensation risk and suggest a vapour barrier as the fix. In practice, a vapour barrier on the inner face of a solid wall severely restricts inward drying, and WUFI analysis consistently shows that this leads to year-on-year moisture accumulation over five to ten years — with knock-on consequences for any embedded timber and, in cold climates, freeze-thaw damage at the external face.

The practical takeaway is straightforward: Glaser is adequate for simple lightweight structures, but for internal wall insulation or flat roofs — ventilated or otherwise — a full WUFI-type analysis is the only way to understand what’s really happening within the build-up. Relying on Glaser in these contexts doesn’t just give an incomplete picture; it can actively point designers toward solutions that make long-term moisture problems worse.