Structural Elements
There is a growing trend to use ‘drop down’ loft traps instead of traditional ‘push up’ ones
always been the means of compliance in their respective Building Regulations. Approved Document Part C in England and Wales does not make any specific recommendations to control condensation in roofs but refers to BS5250 as a means of compliance with the Regulations. Building Regulations require roof space ventila-
tion of equivalent to a 10mm continuous gap at low level or 10,000mm2/m. The solution being opted for in many cases is the use of a breathable (vapour permeable) roofing membrane, with ridge, and perhaps eaves, ventilation. The recent cold winters have prompted the NHBC to update its Standards in line with BS5250, and specify 5,000mm2/m of high-level ventilation is included in cold roofs. The only situation where ventilation is not required is in warm roof construction, and then
only when using a vapour permeable (low resist- ance) underlay. Such constructions require a well- sealed ceiling and a separate vapour control layer with sealed laps. Well-sealed ceilings have been a consideration
since the introduction of BS5250: inevitably, a well-sealed ceiling minimises the risk of any moisture entering the roof space in the first place. The stricter airtightness levels make the creation of well-sealed ceilings even more important. Inclusion of an airtight membrane at ceiling
level is an effective solution, but so often, we find the loft trap compromises the ceiling airtightness. Figures from the Building Research Establish-
ment have shown that 50 per cent of vapour trans- fer from the warm home to the cold loft space occurs around the loft hatch cover, especially if the hatch is a conventional unsealed version.
The loft trap needs to incorporate integral
closed cell seals and insulation, to provide continu- ity of thermal insulation at ceiling level. They vir- tually eliminate heat loss by air movement around the trap door and water vapour migration into the loft space. And as insulation levels in the roof space
increase, there is a growing trend to use ‘drop down’ loft traps instead of traditional ‘push up’ ones. Under natural laws of physics, the weight of ‘hinge down’ loft traps can encourage the hatch to react to gravity and drop away from its frame, neg- atively impacting on the airtight seal. Thus care needs to be taken on product choice, as manufac- turers have become aware of this issue and are now engineering traps to overcome it. It is not a case of ‘build tight, ventilate right’, but ‘build tight and right’.
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