Hygrothermal Performance of Windthight roof constructions

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 measurements show further a clear relation between rising wind speed, wind direction and the increasing heat flux as well as the decreasing inside surface temperatures. Aims of the project - Relevance (results and end products) 1. Design of durable, economically and environmentally optimized, energy efficient light weight roof and wall constructions 2. Development of new air tightness standards (wind and air) and new building regulations 3. Determination of an additional value for the calculation of the heating load against the basic value of the wind speed v10 according to ÖN B 4013. 4. Determination of the fault tolerance of standard light weight construction systems bei air leakages and ventilation in reference to the increased heat losses and moisture insertion caused by the ventilation through the thermal insulation. 5. Determination of the necessary wind tightness of light weight constructions subject to tolerable heat losses caused by ventilation through layers of thermal insulation with a low flow resistance (e.g. mineral wools), as a base for a standardised value. 6. Determination of the necessary flow resistance of a thermal insulation layer by non ventilated sealed roofs subject to not avoidable leakages (ageing process, workmanship mistakes) and allowed heat losses. Prospect 1. cooperation with producers of thermal insulation to develop convenient thermal insulating materials 2. publication of manufactures instructions for airtight layers of light weight constructions

During the last decades the insulation level of constructions has increased from around 5 cm up to 40 -50 cm. This development focused on the reduction of energy demand for heating and cooling. In practice severe moisture problems on the interior surfaces and inside the constructions showed up and an intensive research on the impact of air flow on the performance started. This research leads to performance criteria for the internal air barrier. Since 2007 in Austria all building codes demand an overall air tightness of the constructions which are the boundaries for heated or cooled spaces. This overall air tightness limits the impact of air flow on the energy demand. At the same time the local air tightness is handled by state of the art ways to produce an continues air barrier the assure that warm, moist air cannot enter the construction and condenses on the cold outside in a way rotting or other building failures can occur. A still unsolved problem is the impact of the wind tightness on the performance of the construction. For the heat loss and the moisture performance a perfect air barrier to the inside is always a positive measure. To enhance the wind tightness on the outside would decrease unnecessary heat losses but on the other hand also might increase the risk for condensation because of reducing the cross ventilation of a roof. This research project focused on the necessary wind tightness of the constructions and the possible performance criteria for wind tight fault tolerant roof constructions. Based on the experiences with a case study on 50 single family and 2 multi family houses with air and wind barrier problems a test house had been build to test different roof constructions under real wind loads. Based on the experimental results from the field test a first validation of a simulation tool has been achieved. To validate the software for different flow paths inside the cavity of a construction a laboratory test setup has been installed to measure the heat flow through a roof segment with different cross ventilation possibilities. The simulation tool HAM3D-VIE was able to reproduce the measured heat flows for all investigated flow paths. Based on the validated model the impact of different wind tightness levels could be investigated regarding the overall heat loss and the amount of condensate inside the construction. From the results the necessary level of wind tightness can be formulated. The wind tightness coefficient of the construction has to be less than 1.10-6 m 3 /m2 Pa0,5 . Looking at the results of this investigation it was clearly shown that wind tight constructions put a strong demand on the fault tolerance of the roof construction. A fault tolerant constructions tolerates small leakages in the air barrier. Such a construction is able to dry to the outside or by the help of solar radiation to the inside. Therefore the project ended with the development of an enhanced Glaser method to give practitioners a simple design tool for risk assessment of light weight roof constructions. The method has been published 2010 on the IAQVEC conference. During the project three different solutions for wind tight light weight roof constructions have been developed together with companies. We thank the companies ELK, Bramac, Bauder, Austrotherm, Saint Gobain - Isover and Fontana Beteiligungs AG for their support with building materials, setup of the test house and interest on the research results.