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Flat roofs in cold climates - climatic limits for building flat roofs with a permeable vapor retarder

Presentation held at Cold climate HVAC 2009, Sisimiut Greenland, 16-19 March 2009
: Bludau, C.; Künzel, H.M.

2009, 8 pp.
International Conference on Cold Climate, Heating, Ventilating and Air-Conditioning <6, 2009, Sisimiut>
Fraunhofer IBP ()

Building in cold climate zones requires a high insulation thickness and a good protection against interstitional condensation. Applying vapour tight membranes on both sides of a roof construction offers no drying potential in case of leakages or initial moisture. Therefore the use of slightly permeable membranes at the interior side is often preferred because they allow condensed or existent moisture to dry out. However, drying to the interior space will only happen if the solar heat gains of the roof are high enough to inverse the vapour flow for a sufficient time period. This paper looks to the climate limits of building flat roofs with moderately permeable vapour retarders. The balance of interstitial condensation and subsequent drying is investigated using an adapted hygrothermal simulation tool for heat and moisture transport in building components. For the simulations a black surfaced flat roof construction with mineral fibre insulation and moderate vapour retarder is considered. Special interest is set on the drying potential in
relation to the building's location in Northern climate zones. The results can be shown by two limit lines around the North Pole, one for vapour retarders with a constant permeability and the other for those with humidity controlled permeability. These lines are not parallel to the Arctic Circle. They reflect the regional climate conditions which are influenced by the Gulf Stream and other meteorological phenomena.