Thermal Insulatior

Loadbearing layer

Vapour Vapour

Fig. 14: Schematic drawing of building performance parameters

Protective layer/ waterproofing layer Ventilation cavity

Thermal insulatior

Loadbearing layer

Protective layer/ waterproofing layer Ventilation cavity

Thermal insulatior

Loadbearing layer

Fig. 15: Section through cold roof

Fig. 16: Section through cold roof

Cold deck

The cold deck is a double-skin roof construction consisting of a lower, enclosing and thermally insulating skin with a separate airtight membrane, and an upper, weatherproof skin designed to carry wind, snow and imposed loads. Between these two there is a ventilation cavity - the size of which is determined by building performance parameters - with appropriate inlets and outlets. The cross-sectional area of this ventilation cavity must be min. 1/150 of the roof area, the minimum depth must be 100 mm. The total area of inlets/outlets must be at least half the size of the minimum cross-sectional area of the ventilation cavity itself. This ventilation arrangement ensures a balance in the vapour pressure between interior and exterior climates, especially in winter, and that in summer the temperature rise caused by solar radiation ("stuffy" climate) is dissipated by convection. One specific example of a ventilated roof is the Davos-style roof; the ventilation cavity in this roof is designed as a crawl space which enables the waterproofing to be inspected from inside.

The layer of i nsulation is placed over the l oadbearing layer and must consist of a vapour-permeable material (mineral or glass wool). Incorporating the ventilation above the thermal insulation obviates the need for a vapour barrier on the inside of the insulation. However, such a vapour barrier is included with a l oadbearing layer that is very open to diffusion (timber or steel) and this acts as a diffu-sion-retardant airtight membrane. The layer of i nsulation need not be vapour-permeable because it is positioned above the ventilation cavity. However, it requires its own loadbearing layer (double-skin construction). Gravel or sheet metal are suitable materials for the protective layer above the i nsulation; the minimum roof pitch for a sheet metal roof covering with double welt joints must be 3%. The fall in the cold deck is usually achieved within the ventilation cavity (i oadbearing layer or waterproofing layer). Such an inclined boundary surface promotes the thermal currents in the ventilation cavity.

Fig. 16: Section through cold roof

Heat

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Project Management Made Easy

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