Insulation Between And Below The Rafters – Design Guidance

Where condensation risk analysis indicates a risk of condensation a vapour control layer should be installed behind the finish. The two layers of insulation should be in contact to minimise air movement between them: timber battens fixed to the sides of the rafters may be used to restrain the insulation between the rafters.

The underlay should have a vapour resistance of less than 0.25MNs/g and may be draped over the rafters or the counterbattens. A rigid eaves carrier may be required to prevent ponding and avoid UV degradation. There is no need to ventilate beneath the underlay, but there must be sufficient air movement between the batten space and outside air to allow moisture to disperse: counterbattens (min. 38 x 50mm) may be used to form a deep air space above the underlay. Roof coverings such as tiles and slates will allow enough air flow through the laps, however, air spaces beneath tight coverings should be vented.

To avoid thermal bridging the roof insulation should meet the insulation in the walls: where a cavity wall is finished with a closer, additional insulation should be fitted from the top of the closer to the upper face of the rafters, where it will butt the Ballytherm insulation between the rafters (figure 30). At verges the wall insulation should extend to the top of the wall, the gap between the wall and the first rafter should be packed with insulation and the insulation under the rafter should be butted and sealed against the wall (figure 32).

At ridges, hips and valleys where roof planes intersect, the under-rafter insulation should be cut to form a continuous layer of insulation. Junctions may be sealed with expanding foam insulation.

a. additional insulation to avoid thermal bridging b. insulation between inner face of wall 23 and joist

Further Advice & Information

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Floor Insulation