Retrofit Roulette: Why We Keep Getting Moisture Wrong

The UK is in a race to Net Zero, but are we sprinting towards a damp and mould crisis? Dr. Sarah Price, Technical Author of PAS 2035 and Research Fellow at Leeds Sustainability Institute, delivered a stark warning to the HBN community: "It isn't all about just sticking insulation in and hoping for the best."

In a seminar that bridged complex building physics with brutal industry realities, Sarah unpacked why - despite standards like PAS 2035 - we are still seeing "retrofit roulette" in British housing.

The Physics of Failure

Sarah brought the group back to basics with a concept that often gets ignored in the rush to insulate: Holding Capacity.

Air at 20°C with 52% humidity is comfortable. But take that same air and put it against a cold, uninsulated surface at 10°C? You hit 100% relative humidity. Saturation. Condensation. Mould.

This is why thermal bridging (leaving gaps in insulation around windows, reveals, or stairs) is so dangerous. It creates cold spots that act as magnets for moisture.

The "Joist End" Problem

One of the most specific and terrifying risks in retrofit is rotting the ends of timber joists where they sit in solid masonry walls. When you add Internal Wall Insulation (IWI), the wall gets colder, the joist end gets colder, and moisture accumulates.

Sarah shared recent WUFI modelling research comparing insulation types in this high-risk zone. The winner? Wood Fibre.

"We looked at mineral wool, PIR, and wood fibre. Wood fibre came out top. Why? It's hygroscopic and capillary active. It can store moisture, spread it out, and draw it back to the surface to dry."

The "Vapour Open" Myth

A critical point raised during the Q&A was the industry's misunderstanding of "vapour open" materials. Just because mineral wool is vapour open (breathable) doesn't mean it's safe. It is hydrophobic - it hates water. It can't manage moisture; it just lets it pass through (or sit there).

To protect traditional buildings, we need materials that are capillary active - materials that actively manage water transport, like lime, hemp, and wood fibre.