Interstitial Condensation Calculator — Dew Point Analysis

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.

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.

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).

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.

It can. A full-fill cavity with no warm-side vapour control layer will often FAIL the steady-state Glaser test (BS EN ISO 13788) — this calculator's default cavity-brick build-up flags a risk at the cold face of the insulation under standard 21°C/60% internal, 0°C/90% external winter conditions. In practice serious problems are less common than Glaser predicts, because the method is conservative: it ignores the drying effect of cavity ventilation, the moisture buffering of the masonry, and seasonal re-evaporation in summer. The real-world risk is reduced by a continuous warm-side VCL, a ventilated/drained cavity, vapour-open outer leaf, and good airtightness. Add a warm-side VCL in this tool and the same build-up clears. For a definitive answer on a marginal wall, use transient (BS EN 15026 / WUFI) analysis with the manufacturer's declared vapour-resistance figures.