What is interstitial condensation?

Interstitial condensation occurs within the fabric of a building (inside walls, roofs, or floors) when warm, moist air from inside the building passes through the construction and reaches a point where the temperature drops below the dew point. Moisture condenses at this interface, potentially causing rot in timber, corrosion of metals, and degradation of insulation performance.

How does a vapour control layer (VCL) prevent condensation?

A VCL (vapour barrier) is a membrane installed on the warm side of the insulation that restricts the passage of water vapour into the cold part of the construction. By reducing the amount of moisture reaching the cold surfaces, it prevents the vapour pressure from reaching saturation point. The VCL must be continuous with all joints sealed — even small gaps allow significant moisture transfer.

Where should the VCL be positioned?

The VCL should always be on the warm side of the insulation — typically between the plasterboard and insulation in walls, or below the insulation in roofs. If placed on the cold side, it traps moisture within the insulation. The rule is: the vapour resistance should increase from outside to inside (i.e., the inner layers should be more vapour-resistant than the outer layers).

What is the Glaser method?

The Glaser method (BS EN ISO 13788) is the standard calculation method for interstitial condensation risk. It plots temperature and dew point temperature through each layer of the construction. Where the dew point line crosses the temperature line, condensation will occur. The method uses monthly average temperatures and humidity to assess whether condensation accumulates over a year or dries out in summer.

Can condensation occur in insulated cavity walls?

Condensation in insulated cavity walls is rare because the insulation is in the middle of the cavity with brick on both sides. The main risk is in timber frame walls, internally insulated solid walls, and flat roofs where the temperature drops sharply across the insulation layer. Cavity wall construction naturally manages moisture well — one reason it's been the dominant UK construction method for over 100 years.

TradeCalculator

Interstitial Condensation Calculator — Dew Point Analysis

Analyse condensation risk through wall and roof layers. Plots temperature gradient against dew point at each interface to identify risk locations.

Wall/Roof Type

Determines layer build-up

Internal Temperature

Typically 21°C

External Temperature

Winter design: 0°C typical UK

Internal RH

60% typical occupied house

External RH

90% typical UK winter

How We Calculate This

This calculator performs a simplified Glaser-type analysis by plotting the temperature gradient and dew point through each layer of the construction. Where the dew point exceeds the actual temperature, condensation will occur.

The formula

Dew point = (237.7 × α) ÷ (17.27 - α)

where α = (17.27 × T) ÷ (237.7 + T) + ln(RH/100)

Temperature at interface = T_internal - (R_fraction × ΔT)

Vapour pressure at interface = VP_internal - (VP_fraction × ΔVP)

If the vapour pressure at any interface exceeds the saturation vapour pressure at that temperature, condensation occurs. A VCL on the warm side shifts the vapour pressure drop to occur before the temperature drops, preventing condensation.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.