Vapour barrier

In conventional roof structures a vapour barrier is needed to prevent the transmission of moisture from the warm side into the structure in cold conditions.

The partial pressure difference of water vapour causes moisture to move towards the colder environment and in winter conditions vapour first condenses in the structure in the form of water, which then possibly freezes. Condensed and frozen water melts at latest in spring causing leaks. A sufficiently impervious vapour barrier prevents the movement of moisture from the interior into the structures. It is highly important to seal the vapour barrier tightly at points where there are penetrating structures and components. Thermal insulation of all ventilation ducts in the ventilated roof space also helps to eliminate potential condensation problems.

Roof vapour barrier

Steep roofs usually have a structure (trussed structures) that does not provide an even base for the vapour barrier. The use of bitumen membrane as a vapour barrier is often impossible. In these cases sufficiently resilient airtight membranes (e.g. aluminium/polythene laminates, with reinforcement netting, not just plastic films) should be used as a vapour barrier. Special attention must be given to the proper sealing of penetrations, and ready-made lead-through sleeves should be used in these structures. As vapour barrier structures are rather fragile anyway and they are easily pierced, ventilation should function well at all times.

In low pitch roofs the significance of the vapour barrier is particularly great, as ventilation gaps are generally rather small. In timber roofs the height of the ventilated space should be at least 200 mm. In mineral wool insulated roofs the slabs have grooves for ventilation, and in roofs insulated with light aggregate, air flowing through the air gaps between the granules ventilates the structure. The vapour barrier should always be installed on an even, firm base, e.g. a concrete vault or concrete elements. If the supporting structure is profile sheetmetal, sufficiently firm material (hard mineral wool or building board) should be used as underlay for the vapour barrier.

PRODUCT FOR LOW PITCH ROOFS

Elastomer bitumen membranes