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In a cold roof construction, where permitted by building regulations, insulation is placed between the rafters and below the deck. This is to ensure the deck remains the same temperature as outside of the building.
In a cold roof construction, the Sika-Trocal membrane is installed directly to the structural decking (usually timber). Then there must be ventilation between the deck and the insulation (traditionally a generic mineral wool) to prevent moisture vapour being transmitted from the building outwards where it could condensate in the roof construction (interstitial condensation).
Cold roofs are generally considered as ineffective for energy use reduction and building performance and providing for the ventilation is problematic. Sika-Trocal membranes can be used in cold roofs but our advice will be to avoid designing them in the first place.
With a cold deck timber roof there is always the risk of condensation occurring within or above the insulation, with consequent damage to the timber construction.
Because of this risk, current building regulations require a minimum air gap of 50mm between the top of the insulation and the underside of the deck. The air gap must then be ventilated by the use of airways created in the vertical faces at opposite ends of the roof. Ventilation on the top of the roof surface tends to encourage moisture entry, failing to keep the roof space clear of condensation.
This is not a recommended option.
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