Loft Conversions Insulation – Design Guidance

Where the existing construction includes an underlay with a high vapour resistance, such as a 1F bituminous felt, there is a risk of condensation forming on the underside of the underlay and damaging the roof. To minimise that risk there must be 50mm min. air space between the insulation and the underlay, vented to remove moist air to atmosphere and prevent a harmful build up.

To assist with setting the insulation at the correct depth and prevent it blocking the air spaces, timber battens may be fixed to the sides of the rafters as stops: with a draped underlay, the air space should be 75mm deep when measured at rafters to give the 50mm minimum depth at the centre of the air space. Each air space should be vented with a vent opening equivalent to a 25mm continuous opening at the base and 5mm at the top: that requirement applies to all air spaces, including those formed by roof windows and other penetrations.

If the rafters are not sufficiently deep for the required insulation and the cavity, their depth may be increased by fixing battens to the undersides of the rafters. Alternatively, a second layer of insulation may be fitted across the underside of the rafters.

In loft conversions it is often difficult to extend the insulation between the rafters as far as the wall head, also the practicalities of creating usable space results in the construction of stud walls between the joists and rafters.
Those walls should be insulated with Ballytherm boards cut to fit between the studs (figure 34). To prevent thermal bridging, the insulation between the rafters should extend as far as the rear face of the stud wall. Where a horizontal ceiling is formed beneath the apex of the roof, the ceiling insulation must be fitted tightly between the rafters.

Gable walls should be insulated with Ballytherm Insulation board fitted behind the plasterboard lining. It is good practice to improve the thermal performance of the ceiling beyond the stud walls by laying insulation across the ceiling joists, taking care not to block any eaves vents.

To minimise heat loss through service penetrations the plasterboard lining may be set on battens to form a cavity for running services. At roof windows the insulation should be butted tightly to the back of the frames. Gaps at penetrations should be sealed with expanding foam.

A. timber battens used as insulation stops B. vertical insulation extended up wall face C. insulation between inner face of wall
and joist

Further Advice & Information

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Roof Insulation