The ever increasing levels of thermal insulation required by thermal regulations can result in a higher risk of condensation forming on those elements not enclosed by this insulation, e.g. roofing underlays; cold roof decks and external cladding or tiling.
In flat roof construction thermal insulation can be placed above and below the structural roof deck or waterproofing layer. When placed below the structural deck; ventilation is required to the underside of the deck. This ‘cold deck’ construction is considered problematic in the humid, temperate climate of the UK where the effective ventilation of restricted roof voids may be impractical, particularly in sheltered, built-up areas. It is universally recommended that cold deck flat roof construction should be avoided. Where the insulation is placed above the structural deck (warm deck), and either above or below the waterproofing; there is no need for additional ventilation.
In pitched roof construction thermal insulation can be placed horizontally at ceiling level (cold roof); inclined along the line of rafters (warm roof) or positioned within the roof structure to form an insulated void (room-in-roof) which can result in warm and cold roof areas. Cold roof voids are typically required to be ventilated; however, where vapour permeable underlays are specified, this requirement may be relaxed. The enclosed void of a warm pitched roof does not require to be ventilated.
With the introduction of vapour permeable (plastics) underlays the need to create ventilated voids above inclined insulation and the requirement to ventilate cold roof lofts may be reduced. Rather than actively ventilating to the outside, entrapped water vapour diffuses through the vapour permeable underlay; negating the need for forming cavities or voids below the underlay. However, with some, air-tight and fully interlocking roof tiling systems there may be a need to provide a ventilated void directly below the slating or tiling system to facilitate this diffusion through the roof construction. Always check with manufacturer’s literature and any third party accreditation.
In all roof constructions the risk of condensation can be considerably reduced by specifying an effective vapour control layer (VCL) at ceiling level. This VCL should be lapped and/or sealed at joins; penetrations and openings. Access hatches and recessed light fittings are particularly problematic requiring perimeter sealing and, in the case of light fittings, impermeable enclosures to reduce the risk of vapour transfer into the colder roof construction.
Related reading on theNBS.com
Ventilation of the loft space
Vapour permeable underlays for slate and tile roofing
Specifying Reinforced Bitumen Membranes
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