How do we prevent interstitial condensation with internal wall insulation? – Can I Just Ask? | Ep.35

Interstitial condensation is one of the most frequently raised concerns around internal wall insulation, and understanding what it actually is — and how it behaves differently depending on the materials used — is fundamental to designing a system that works safely over the long term. In any wall there is a point where the temperature drops low enough that moisture in the air condenses: the dew point zone. In an uninsulated solid wall this sits within the masonry itself, and because old solid walls are relatively vapour-open, moisture can cycle in and out without causing significant harm. Add insulation to the inside and the picture changes: the masonry behind the insulation gets colder, and the dew point zone shifts inward to sit at the interface between the insulation and the wall. That’s interstitial condensation — and it’s a real consideration that needs to be designed out, not ignored.

The approach taken with vapour-permeable, hygroscopic materials like wood fibre fundamentally changes how that interface behaves. A standard steady-state Glaser calculation will flag significant condensation risk at this interface and suggest the system shouldn’t be used — but what it cannot model is the difference between liquid water formation and adsorbed moisture. With wood fibre, individual water molecules bond to the enormous internal surface area of the material rather than accumulating as liquid. The surface area available within a wood fibre board is vast enough to hold substantial quantities of moisture in this adsorbed state without forming liquid water, without significantly changing the thermal conductivity of the material, and without creating the conditions that cause damage to the surrounding fabric. This is why hygrothermal analysis using dynamic software like WUFI is essential for these specifications — it captures the actual moisture behaviour of the materials rather than defaulting to a worst-case static model.

The more nuanced point is that actual liquid condensation at the interface is only part of the picture. Moisture risks exist below the condensation threshold too: mould growth doesn’t require liquid water on a surface — elevated relative humidity in that zone, sustained over time, is sufficient. A well-designed system using capillary-active materials smooths the moisture peaks at the critical interface, allows drying to occur in both directions where possible, and keeps conditions below the thresholds for both liquid condensation and mould growth. Internal wall insulation always increases the stress on the wall fabric to some degree — the goal is to understand and manage that stress, not to pretend it doesn’t exist.