Loft Conversions Insulation – Design Guidance
If the construction includes a 1F bituminous felt or other underlay with a high vapour resistance, then condensation becomes a risk. To minimise condensation on the underlay, there must be a minimum 50mm cavity between the underlay and insulation, vented to prevent buildup of moist air.
Timber battens may be fixed to the rafter sides as stops so insulation is set at the correct depth without blocking air spaces. Draped underlays require a depth of 75mm measured from the rafters to ensure the desired 50mm minimum space between the cavity and loft insulation. All air spaces, even spaces created by roof windows and other penetrations, must be vented with the equivalent of a 25mm continuous opening at the base and 5mm at the top, even spaces created by roof windows and other penetrations.
Sometimes the rafters are too shallow to accommodate both the cavity and loft insulation. Battens or a second insulation layer may be fitted across the underside of the rafters to compensate. With a loft conversion, it is often challenging to extend the insulation to the wall head while maximizing usable space. This can lead to the use of stud walls between the joists and rafters.
Cut insulation to fit between the stud walls (Figure 34). Ensure that the insulation between rafters extends to the near face of stud walls to prevent thermal bridging.
Where a horizontal ceiling is formed beneath the apex of the roof, tight fitting of the product is essential. Gable walls must be insulated with Ballytherm boards installed behind the plasterboard lining. Improve the thermal performance of the ceiling beyond the stud walls by laying insulation across joists, being careful not to obstruct eaves vents.
Service penetrations introduce chances for heat loss , but can be minimised by installing the plasterboard lining on battens, and creating a cavity for running services. For roof windows, the insulation should butt tightly to the back of the frame, while gaps at penetrations should be sealed with expanding foam. More general advice on ordering, storing and handling Ballytherm insulation products in available here.
A. timber battens used as insulation stops B. vertical insulation extended up wall face C. insulation between inner face of wall