Rigid Insulation Board
Rigid insulation boards (PIR or phenolic foam boards) are a common way to insulate the sloping ceiling of a converted loft. They are fitted between and often below the rafters, with plasterboard fixed over them. They insulate well for their thickness, and the Energy Saving Trust notes that insulating at rafter level costs much more than standard loft insulation and is a job for a professional.
What to Know About Rigid Insulation Board
- Good thermal performance for its thickness
- Gives a flat surface for plasterboard
- Widely used for the sloping ceilings of converted lofts
- Needs careful fitting and ventilation to avoid gaps and condensation
- The thickness needed depends on the product and your rafter depth
Price Guide
From about £35/m² (materials only)
Typical UK price guide, last reviewed September 2026. A guide, not a quote.
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Frequently Asked Questions
What thickness of rigid board do I need?
It depends on the product, the depth of your rafters and the U-value the roof needs to reach under Part L of the Building Regulations. Your designer, the installer or your building control body can calculate the build-up for your roof. Remember that thicker insulation at the rafters can reduce headroom.
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