Climate adaptation of pitched wooden roofs

In this thesis, wooden roofs are defined as roof structures where the load-bearing structure consists of wood. Such structures are normally built with a ventilated air cavity between the underlayer roof and the roofing. The main purpose of this air cavity is to transport excessive moisture away from the roof structure as well as to transport heat and avoid snow melt when there is snow on the roof. Hence, the design of the air cavity is crucial in relation to pitched wooden roofs being adapted to the local climate.\nWooden roof structures might have a favourable carbon footprint compared to other building materials. Hence, increased focus on the carbon footprint of building materials and components makes wooden roofs more relevant, including for large buildings. Changes in climate can provide more intense precipitation in the form of torrential rain in parts of the country. Climate-adapted solutions must both protect against the ingress of water and ensure rapid drying-out of moisture in the structure.\nThe SINTEF Building Design Guides (Byggforskserien) serves as a collection of standard building designs that fulfil the Norwegian building regulations. The collection declares a maximum roof length from eaves to ridge of a pitched, ventilated wooden roof of 15 m. The minimum roof pitch is set to 10-15°. The guidelines are based on long-term experience in the Norwegian climate. Roofs with larger spans and lower angles must be planned in detail for every building project, which is not very efficient.\nThe objective of this thesis is to increase the knowledge about moisture safety and air flow through the air cavity of pitched wooden roofs. Experimental research, field measurements and numerical simulations have been used to assess and characterise the driving forces and resistances for roof ventilation.\nAnalysis of the SINTEF Building Defects Archive shows that moisture from precipitation and indoor air leakages is the dominating source of building defects for pitched wooden roofs. This is critical when we bear in mind the anticipated climate changes with wetter and warmer climate in Nordic countries. An airtight vapour retarder and use of balanced ventilating is proposed as an effective means to increase moisture safety. These means are also important in the climate-adapted roofs of tomorrow.\nAir movement inside the thermal insulation layer is found to significantly increase the thermal transmittance of roof and wall structures. The effect may also redistribute moisture inside the insulated layer causing increased moisture in the colder parts of the structure with an increased risk of condensation and moisture damage. Dividing the insulation layer using a vapour-open convection barrier is therefore suggested to increase the moisture safety for roof structures with more than 200 mm of insulation.\nIncreased interest in renewable energy production increases the relevance of using facades and especially roofs for energy production. The possibility to combine PV (Photo Voltaic) systems as roofing and to harvest solar energy, hereafter called BIPV (Building Integrated Photo Voltaic), is therefore relevant. Such systems can lead to building- physical challenges. In particular, hazards due to downfall of snow and ice during the winter and ventilating requirements during warm sunny periods to ensure low temperatures and thereby higher efficiency are challenges that need to be dealt with.\nThe air change rate of the air gap between the roofing and the underlayer roof is given by the driving forces and resistances. The driving forces are given by wind and thermal buoyancy. The resistances are given by the air passing inlet and outlet and the different obstacles inside the air cavity. This thesis and the included papers include studies of both the driving forces of wind and the resistances inside the air cavity. The height of the counter and tile battens as well as the edge design of the tile battens is found to influence the air change rate of a specific air cavity. The driving forces of wind are studied by analysing previous data from field measurements. The wind pressure at the facade was affected by the wind approach angle. A value for the average wind pressure difference coefficient given eaves-to-eaves ventilated air cavities is proposed as Δcp =0.7.\nThe field investigation of the ventilated wooden roof at the ZEB Test Cell Laboratory shows a strong correlation between the wind speed and the air speed inside the air cavity. In addition, long periods of lower temperatures compared to the ambient temperature onthe lower facing of the roofing material have been found. There is a risk of increased moisture content during these periods. Use of dynamic valves which open the ventilating systems during periods with dry-out conditions and close during periods with moistening conditions is a possible solution to the problem.\nThis thesis also looks at the possibility to construct long, climate adapted roofs. The study included winter conditions with snow on the roof. Roof insulation thickness and the thermal transmittance affect the snow melt potential of the roof structure to a large extent. A review of the ventilating guidelines of different cold climate countries in Europe as well as Canada and USA (Washington) reveals similar guidelines regarding air cavity design compared to the Norwegian guidelines.\nExperimental data established in the thesis is used in a stationary model to calculate the snow melt potential of pitched wooden roofs in order to develop a basis for roof ventilating guidelines adapted for future wooden roofs. Given a 30 m long roof and insulation thickness of 350 mm an air cavity height of approximately 160 mm was proposed for roofs with combined underlayer roofing and wind barrier in order to avoid snow melt problems.\nThe work shows that pitched wooden roofs adapted to the Nordic climate of tomorrow need:\n1) Increased climate adaptation and moisture safety by improved air cavity design.\n2) Convection barrier when insulation thickness exceeds 200 mm.\n3) More knowledge and relevant documentation if BIPV roofing is used.

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