Habitable roof insulation

Why is it important?

Room in roof is becoming much more common as buildings are built on smaller plots. It is important to be aware of potential moisture problems, and to ensure the roof insulation forms a complete wrap by joining with the wall insulation.

How is it installed?

Rafters should be filled with 175mm Knauf Rafter Roll insulation and then underdrawn with 40/12.5mm thermal laminate sheets. By installing the thermal laminate sheets it prohibits the cold bridging that occurs if plasterboard is fixed directly to the timber rafters.

When using attic trusses to form the roof, the bottom chord of the trusses act as the floor joists. Therefore they need to comply with Part E of the Building Regulations and should be filled with 100mm Earthwool Acoustic Roll to reduce the transfer of sound between floors.

Breathable roofing membrane should be used over the rafters with a minimum of 25mm gap between the membrane and the insulation allowing the void above the insulation to breathe and thereby reducing the risk of condensation forming on the underside of the membrane. The roofing membrane and insulation should never be in contact.

Hints & Tips

  • When using attic roof trusses, it is important at the design stage to position windows/roof lights in a way that allows for the simplest truss design.

  • Ensure that where the roof insulation and floor insulation meet at the eaves, the insulation is continued through to meet the cavity insulation, reducing thermal bridging at this junction and forming a complete house wrap.

  • Ensure insulation between rafters is fitted snuggly, as too loose or too compact a fitting can affect thermal performance.

  • Ensure thermal laminate boards are fitted with all joints tightly butted together as any gaps will affect thermal performance.

  • Consider using wood fibre insulation such as STEICO flex between rafters instead of mineral wool due to its benefits regarding summer cooling and ability to help regulate air humidity.