There is an increased risk of interstitial condensation occurring in certain types of construction, including:

• “Cold roof” construction, that is, flat or pitched roofs constructed with insulation placed in the voids between timber joists or rafters, above a plasterboard ceiling and beneath a plywood roof deck or sarking felt and external finish. Warm moist air that penetrates between the insulation and the joints/rafters will meet the cold underside of the roof deck/sarking. This causes condensation and could lead to the rotting of the deck and the tops of the joints/rafters.

• Timber or steel framed walls with insulation between the framing members, and sandwiched between plasterboard internal linings and external sheathing board. Warm moist air that penetrates between the framing members and the insulation will meet the cold inner surface of the sheathing. This again causes condensation and can lead to rotting or corrosion of the framing.

• Insulated internal ‘dry lining’ of solid walls. This consists of insulation between battens fixed to the walls and lined internally with plasterboard, or of thermal board fixed directly to the walls on battens or plaster ‘dabs’. Warm moist air that penetrates behind the lining board will meet the cold inner surface of the external wall, resulting in condensation and potentially rotting or corroding the battens.

In all of these cases, the insulation could become saturated and therefore ineffective.

There are several ways of avoiding interstitial condensation:

• “Warm roof” construction, that is, placing the insulation on top of the roof deck but beneath the waterproof finish. This ensures that the underside of the roof deck is around the same temperature as the heated space underneath.

• Incorporate polythene vapour barriers or damp proof membranes into framed wall and roof constructions, on the inside of the insulating layer and framing, before the wall or ceiling linings are fixed. Alternatively, proprietary lining board (plasterboard backed with aluminium foil or impermeable paper) may be used.

• Use a vapour-based “breathing” construction that is airtight but vapour permeable. In this type of construction, the vapour resistance of the inner layer must be at least five times the vapour resistance of the outer layer, so that any water vapour that condenses within the construction will find its way to the outside. Vapour balanced construction is becoming more common, especially for insulated roofs and framed walls.

The risk of interstitial condensation should be checked when new build or refurbishment projects are specified.

If you require further information, or wish to arrange for an Interstitial Condensation Analysis to be carried out, please contact us.