Study of ventilated low-slope and large span wooden element roofs in the current and future climate

Abstract

Finland's building regulations and guidelines rely on ventilation to ensure the hygrothermal performance of structures. For wooden roofs, height of ventilation cavity and area of ventilation openings have been determined depending on roof slope and roof area in guideline. This causes difficulties in practical implementation in low-slope roofs with large span, because the guideline leads to large ventilation openings that can be challenging to be implemented. In practice, roof element suppliers have produced roofs with slightly smaller height of ventilation cavities and areas of ventilation openings. This study examined the hygrothermal behavior of the ventilated wooden roof, where the role of airtightness of vapor barrier and ventilation rate were investigated. The ventilation rates of the simulation model were set based on the results of longterm continuous measurement. First, to ensure the applicability of used model, the mold index calculated from measurements was compared to simulated with the design weather data. Next, hygrothermal behavior was evaluated based on mold index using the design climate data (current and future) for airtight and 'loose' structure with various ventilation rates. Results shows that focusing on airtightness is important. However, the larger ventilation rate has an unfavorable effect on mold index which is emphasized in future climate. Thus, revision of the design guidelines is proposed to restrict the ventilation openings and unnecessarily effective ventilation. In addition, air tightness guidelines should be set more precisely from the perspective of moisture safety.