Rafter level roof insulation: between and under phenolic boards

Description

Rigid phenolic insulation installed in between rafters leaving a 50 mm airspace between top of insulation and underside of roof covering and rigid phenolic insulation bonded to plasterboard installed on the underside of the rafters. A thickness of 100 mm between rafters and 42.5 mm of insulated plasterboard under the rafters would be needed to achieve a U-value of 0.20 W/m2.K.

Unit cost of installation (3 bed semi)

The basic cost of the phenolic foam would be ~ £1800 +VAT (5%). Fitting costs are extra and would depend on the complexity of the roof.

Bulk cost of installation

Data not available

Indicative cost and carbon saving

Maintenance

None.

Lifetime

Lifetime of roof.

Technical suitability

Rigid phenolic insulation typically has BBA certification and the 50 mm air space can be reduced if a breathable membrane has been used in the roof, as long as there is a nominal gap between the insulation and the breathable membrane to allow the membrane to breath. The use of a breathable membrane that is laid taught over the rafters will require additional counter battening and hence will raise the roof above that of adjacent roof slopes. Dependent on the designed insulation value of the construction and the available rafter depth and headroom, different approaches can be taken including insulation over the rafters, which will also require counter battening - however this is likely to require very large fascia boards which may not be acceptable visually, and raising the ridge of the roof will require Planning Permission. Additionally, pattern staining in the position of rafters can be caused if rafters are left as uninterrupted cold bridges. For this reason, solutions relying solely on insulation between rafters should be avoided.

Non-technical constraints

None.

Potential concerns with fuel poor

No specific issues.

Ancillary benefits

Trigger points for work

Loft conversions, for which Building Regulations Approval will be required.

Technical Guidance Available

National Insulation Association Ltd (NIA) 01428 654011 http://www.nationalinsulationassociation.org.uk/