The attic roof of course has three roof voids, those at either side at low level and the triangular roof void above the attic room. Airflow must again be introduced at low level, i.e. at the eaves, and allowed to flow unobstructed from the lower roof void up between the rafters to the upper roof void, where exhausting from the ridge is particularly important because of the usually steep pitch nature of the attic roof.
The area of rafter which forms the sloping ceiling of the room must be carefully considered. When attic trussed rafters are used then this rafter may well be 200 mm deep which, when 100 mm of insulation is used between the rafters, should allow adequate ventilation space above the insulation from one roof void to the next. However, unless the designer can be absolutely certain that the insulation will not slide down the sloping ceiling, some controls may need to be introduced to ensure that adequate ventilation space of 50 mm between each rafter is maintained.
On traditionally constructed attic roofs where the timbers may be smaller, or in the type of construction illustrated in Figs 6.19 and 7.15, the timber used for the rafters will be relatively small. To ensure that adequate insulation thickness and air space are maintained, it may be necessary to specify a minimum rafter depth of at least 175 mm, thus giving room for 100 mm of insulation, an insulation controller and a 50 mm air space. In the next section, this subject of insulation control is again highlighted in the discussion on sheet material used for stability bracing in attic roof construction.
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