Havuvanerirakenteiden kosteusteknisen toiminnan perusteet

In Finland plywood is seldom used as an outer sheathing of the wall insulation, whereas in North America OSB (oriented strand board) is a typical outer sheathing board. Both plywood and OSB are wood and glue composite materials. Before applying structures used in other climates to our environmental conditions, their moisture performance should be closely studied. The objective of this research was to study the fundamentals of the moisture performance of structures having outer sheathing made of spruce plywood. Experiments were done to determine the vapour permeability of (spruce) plywood as a function of moisture content and the drying rate of plywood outer sheathing when exposed to different outside temperatures. In numerical simulations, mould growth was studied, for the first time, together with transient heat and moisture transfer models. Vapour permeability of wet plywood can be 15 - 20 times higher than that of the dry material. When the moisture content increases, also the drying effect of the plywood sheathing increases highly. 9 mm thick plywood made of spruce meets the Swedish and Norwegian requirements set for the vapour permeance of the outer sheathing layer. In the conditions that the outer sheathing layer is typically exposed to, plywood has significantly better vapour permeability than OSB. In Finnish climate, it is recommended to use a vapour retarder in the structures having plywood outer sheathing, even though the diffusive moisture loads from the indoor air could be kept low enough without one. The vapour retarder layer ensures the long-term vapour and air tightness of the structures. Continuous air exfiltration (indoor air flow out through the structure) causes a moisture risk to all structures in cold climates. It is recommended to maintain indoor underpressure to avoid convection caused high local moisture loads in the structures. The drying rate of a wall structure can be significantly increased by using exterior insulation made of vapour permeable porous material that is open to ventilation cavity. 30mm thick mineral wool layer on the outer side of a 9 mm plywood sheathing improves the drying capacity of the wall to correspond to that of a wall with highly vapour permeable porous wood fibre board layer as outer sheathing. The moisture performance of a residential house wall structure in cold climates is safe when the wall has a vapour retarder, and the outer sheathing is made of 9 mm spruce plywood adjacent to a ventilated air space behind the siding. A vapour permeable exterior insulation open to the air cavity improves the drying capacity and moisture performance of the wall during accidental moisture loads.