Question 1

What is interstitial condensation?

Accepted Answer

Interstitial condensation is condensation that forms within the body of a building element (wall, roof, or floor construction) rather than on its internal or external surface. It occurs because the indoor air contains water vapour at higher pressure than the outdoor air, causing vapour to migrate through the construction from inside to outside (driven by the vapour pressure difference). As the vapour moves through the construction, it encounters progressively lower temperatures. If the temperature at any plane within the construction falls to the dew point of the vapour at that location, the vapour condenses into liquid water at that internal plane. The result is moisture accumulation within the insulation layer, between the insulation and the structural deck, or at other interfaces - invisible from the surface until damage becomes significant.

Question 2

What damage does interstitial condensation cause?

Accepted Answer

Interstitial condensation can cause significant structural and fabric damage: it wets insulation, reducing its thermal performance (wet mineral wool, for example, has a fraction of its dry thermal conductivity); it promotes timber rot and fungal decay in timber rafters, joists, and studwork within insulated roof and wall constructions; it corrodes steel fixings, wall ties, and structural connections embedded in the wall; and it can cause internal finishes to delaminate, plaster to blow, and tiles to detach as moisture accumulates behind them. In flat roof constructions, interstitial condensation within the roof build-up is a leading cause of premature failure. The damage accumulates slowly over years and is not visible until major remediation is needed.

Question 3

How is interstitial condensation risk assessed?

Accepted Answer

The traditional assessment method is the Glaser method (BS EN ISO 13788), a steady-state calculation that plots the temperature and vapour pressure through the construction at each layer, identifies the plane where vapour pressure equals saturation pressure (the dew point plane), and calculates the mass of condensate deposited. If condensate can dry out fully in summer, the risk is manageable. If the winter condensation rate exceeds the summer drying rate, the construction has a condensation risk that must be redesigned. The Glaser method is a useful screening tool but oversimplifies real conditions (it uses single average temperatures rather than dynamic climate data). More accurate assessment uses hygrothermal dynamic modelling software (WUFI or similar) which simulates actual moisture and heat flows through the construction over time using real climate data. For complex or high-risk constructions (warm flat roofs, highly insulated timber frames), dynamic modelling is advisable.

Question 4

How is interstitial condensation prevented?

Accepted Answer

Interstitial condensation is prevented by controlling the movement of water vapour through the construction: a vapour control layer (VCL) on the warm (inside) face of the insulation restricts vapour from entering the cold part of the construction where it would condense. On the cold (outside) face of the insulation, a breathable membrane (vapour-permeable underlay) allows any vapour that does penetrate to escape to the outside rather than accumulating. This combination - vapour control layer on the warm side, breathable layer on the cold side - is the standard modern approach for insulated timber-frame walls and pitched roof constructions. The vapour resistance of the inner layer should be at least five times greater than the vapour resistance of the outer layer (the '5:1 rule' as a simple guide). Getting the order of layers wrong (putting an impermeable membrane on the outside of insulation, trapping vapour) is a common installation error that can cause serious condensation problems.

Interstitial Condensation

Just so you know...