Moisture Safety in Wood Frame Buildings - Blind evaluation of the hygrothermal calculation tool WUFI using field measurements and determination of factors affecting the moisture safety

Sammanfattning: Due to increased awareness of climate change and higher energy costs, well-insulated buildings have become more common. Furthermore, interest in the use of wood in building to produce more carbon dioxide-efficient buildings has increased. However, thicker thermal insulation in building envelopes increases the risk of high relative humidity levels and the risk of mold-related damage in wood frame buildings. In order to predict the risk of moisture damage it is important to have a properly verified, user-friendly and reliable calculation tool that can be used in the design phase. The first part of the thesis presents a blind validation method that can be used in order to verify heat and moisture calculation tools in a reliable manner. General results and findings from blind validations using a one-dimensional transient heat and moisture calculation tool are summarized and presented. The comparisons include measurements and calculations of temperature and relative humidity in wood frame walls and ventilated roofs carried out in Northern European climates. The thesis shows and discusses examples of how the validated tool can be applied as a tool in the moisture safety design process in practice. Furthermore, a parametric study is presented in which moisture-critical positions in traditional Swedish wood frame designs in Northern European climates are investigated by using hygrothermal modeling. Traditional Swedish designs are compared to more energy-efficient designs with thicker thermal insulation, and alternative designs and important factors affecting the risk of mold growth in well-insulated wood frame constructions are presented. In general, the comparisons of measured and blindly calculated values show a good correlation. The results indicate that the validated tool can be used during the moisture design process in a reliable manner. However, factors such as the influence of impaired temperature readings on relative humidity have to be taken into account. There is also a need for developing outdoor climate boundary conditions that include critical periods and variations between different years. Unexpected human behavior, poor workmanship and poor design may have major influences on the hygrothermal conditions in the wood frame construction. Several unexpected leakages caused by driving rain penetrating deep into different wooden frame walls, on the inside of the air gap, were noticed. It has to be discussed and further investigated what appropriate safety margins should be used in future hygrothermal calculations. It has been found that there is a higher risk of moisture-related damage in thicker insulated walls and roofs. However, this risk can be reduced by choosing more suitable designs in which well-ventilated air gaps behind the cladding and exterior vapor-permeable moisture-proof thermal insulation boards are of great importance in walls. Wooden roof constructions were found to be very sensitive to all kind of leakages, both from exterior precipitation penetrating the roofing felt and interior humid air penetrating cracks or poorly executed joints in the interior vapor membrane. The ventilation rate in a cold roof air gap or cold attic must, primarily, be sufficiently high to remove all moisture. However, high ventilation rates decrease the temperature which, in turn, increases the relative humidity and this may cause damage. An unnecessarily high ventilation rate in the cold attic or air gap in roofs should therefore be avoided. This is possible to achieve if the exterior and interior membranes are water- and vapor-tight. The results also show that exterior insulation, on the outside of tongued and grooved wooden roof boarding limits the risk of damage.