DPM placement and vapour control layer requirements

Where the DPM is on the warm side of the insulation it will also act as a vapour control layer (VCL) and prevent interstitial condensation forming on any of the layers on the cold side of the insulation. Where the DPM is placed at a lower level than the insulation and no VCL is provided there is a risk of interstitial condensation which may accumulate year-on-year. However, with insulation of high vapour resistance, a vapour control layer may not be necessary but designers should use condensation software to determine if there is a risk of a year-on-year build up of interstitial condensation (see paragraph 8.2).