Increased Thermal Losses caused by Ventilation through Compact Pitched Roof Constructions – In Situ Measurements

In the case of non ventilated compact roofs the wind tightness of the construction is usually warranted by a windproof underlay membrane and the flow resistance of the thermal insulation, as well as sealed eaves and ridge details. Because of the current construction practice of wind tight layers in Austria there are numerous small leakages in the eaves, the ridge and the underlay area. Because of this and the low density and length of the thermal insulation in common Austrian constructions the wind induced pressure differences between the eaves and loft area cause an air flow which cancels the thermal insulating effect of the rock wool partially or completely. During periods of high wind speed this leads however to uncomfortable low operative temperatures caused by an increased heat loss and lower surface temperatures. Different in-situ measurements (thermal performance and air propagation) of compact pitched roof constructions of single occupancy houses and apartment buildings in Austria were made. Validation results are presented as a comparison between measurements without wind and a dynamic simulation based on the measured in- and outdoor climate data. It was clearly shown by the measurements, that there is an air flow through the construction of the pitched roof area in such a way that the effect of the thermal insulation is lost. Especially the wind induced ventilation due to leaky eave or ridge details and unsealed underlay caused a tenfold increase of the heat flux nearby the eaves. The in-situ measurement