Felt lap vent
A felt lap vent is normally a moulded plastic product which is designed to allow air to pass through the lap between two layers of the felt which lies on top of the rafters, beneath the battons which hold the tiles or slates. It is a retrofit solution for houses which have cold roofs and suffer from moisture build up in the attic or loft space from moist warm air rising from the living spaces below. These houses are normally older constructions where roof felt has been used which is not breathable rather than newer construction that may use a membrane that prevents water ingress but lest moist air out ie breathabe mebranes.
In a cold roof scenario, the insulation and air tightness barrier is normally in line with the ceiling of the upper most rooms (rather than the pitch of the roof). Keeping the uninsulated roof space ventilated can be important to prevent moisture build up, mould and potential damage, particularly where non breathable felt or bitumen products are used. Increased moisture from warm air and activity in living spaces below can build up in the cold roof space above, condense on surfaces and with out good air flow can cause structural damage over longer periods.
Felt laps are one way of introducing air flow into a cold roofs to remove moisture build up, other options are tile, slate, soffit, fascia and ridge vents. In some cases where levels of insulation are increased above the ceiling level this can restrict air flow that traditionally would have come through the sofits or barge board ventilators. Felt laps can be installed higher up the roof pitch, where ever layers of felt over lap, thus not being restricted by the increased levels of insulation and without the need to remove or replace roof surfaces such as slate or tiles.
For further information on the ventilation of cold roofs guidance should be sought in Aproved Document C "Site preparation and resistance to contaminants and moisture" section C2.
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