Insulation Of Joisttype Roofceilings

When the ceiling finish is applied directly to its bottom surface, a roof framing member is called a joist rather than a rafter. This type of construction is found in flat roofs and some "cathe dral" or sloping ceilings. When such roofs are insulated between the ceiling and the sheathing, condensation problems can occur because the space between the insulation and the sheathing is divided into small compartments which are difficult to ventilate. Thus, any moisture which

Insulating joist-type roof-ceiling between the ceiling and sheathing.

roof sheathing

3 '/2" (89 mm) recommended ventilation space cross members soffit vents insulation batts vapour barrier gypsum board roof joists

-57

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Alternative method of insulating joist-type roof-ceiling between the ceiling and sheathing. This method can be used where the slope is at least 1:6, the joists run in the same direction as the slope, and the ventilation space is continuous from eave to ridge and is vented in both directions.

holes in web to cross ventilate betwen joists

2 V2" (63 mm) minimum ventilation space holes in web to cross ventilate betwen joists

2 V2" (63 mm) minimum ventilation space

leaks through imperfections in the air/vapour barrier is not dissipated but accumulates and condenses. Measures to prevent this are shown in Figures 126 and 127.

Another approach to avoid condensation problems in joist-type roof-ceilings is to place the insulation above the sheathing, as is commonly done with flat roofs (Fig. 128).

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