When is an Insulation Manufacturer not and Insulation Manufacturer headlines the October issue of Insulate Magazine. Possibly the best front cover for an Insulation publication EVER.
www.insulatenetwork.com - Hygrothermal performance of building components and building elements - may appear to show no risk. However, the fact that BS 5250 does not support the construction type means that we believe insulation manufacturers should label any condensation risk analysis for a hybrid flat roof as ‘not recommended’. Consider as well another reason to doubt the appropriateness of this analysis of a hybrid flat roof solution. Section 4.2 of BS 5250 states: ‘BS EN ISO 13788 considers only the risks arising from the diffusion of water vapour through the building fabric; it does not take account of the much greater risk of condensation occurring as a result of air leakage, which transports water vapour through gaps, joints and cracks in the building fabric.’ By its very nature, a hybrid roof that features insulation fitted between timber joists introduces the potential for air gaps. Section A.3 is clear that, ‘the rate at which moisture is transported by air movement, where it occurs, is much greater than that of transportation by diffusion.’ A condensation risk analysis cannot adequately allow for the standard of workmanship, and a high level of workmanship in itself would not mitigate potential issues. If anybody is to make a judgement on the levels of workmanship, and whether it makes the construction method any more acceptable for a particular project, it should be the designer, building inspector or end user. The insulation manufacturer can advise on the interpretation of a condensation risk analysis for a hybrid roof, but first and foremost should promote the fundamental principles of good roof design outlined in BS 5250. the correct VCL position, some might ask if a hybrid roof is acceptable if the VCL is moved to ceiling level behind the internal plasterboard finish. While technically that might satisfy the basic requirements, we would urge practical thinking: are light fittings or other services going to be installed in the ceiling? Can the continuity of the VCL – vital to its performance – be guaranteed? It only takes the building’s next owner to want to change something and that VCL could be compromised as part of any works to the ceiling. Will anybody make sure it is restored to its original condition? If a VCL cannot be relied upon to mitigate the risk of condensation then the roof design cannot be considered appropriate, and there are precious few alternative solutions. Despite the best efforts of many to promote the advantages of service voids – airspaces between ceiling and VCL, where services can be installed and the ceiling altered without risking the integrity of the VCL – few seem willing to employ them. There are several reasons why: extra time and materials to construct, restrictions on headroom etc; all of which serve to highlight why it is best to keep things simple. One of the positive aspects of a warm roof is that the VCL can be installed with the confidence that it will perform for as long as the roof performs – hopefully for the lifetime of the building. That’s why we referred to it as a good example of fabric first construction, and it’s why we believe it should always be preferred over a hybrid alternative. Considering one of those principles is to ensure 28 www.insulatenetwork.com
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