Insulating skillion or flat roofs

Insulating a home with no ceiling space is best done when re-roofing or renovating. A lot of heat is lost through an uninsulated roof, so even if you have a skillion roof or cathedral ceiling, it's worth considering your options. Before starting any work, we recommend you consult a qualified builder or roofer.

Ceilings with exposed rafters

If your skillion roof has exposed rafters, you can simply fit insulation between the rafters and line with plasterboard (or similar) on battens. Some rigid insulation even comes with plasterboard already attached that can be glued to your original ceiling. Try to use bulk insulation of the same thickness as the depth of the cavity, ideally with an R-value of R3.6 or higher. If the rafter depth is less than 190mm, consider making more room for insulation by fixing additional spacer battens to the underside of the rafters. For sloping ceilings, be aware that you’ll lose some ceiling height at its lowest point.

Ceilings with hidden rafters

If your skillion roof has hidden rafters and you’re planning on re-roofing or renovating, this is the time to fit insulation.

  • When re-roofing - fit insulation from above during the re-roofing work. This is best done by the roofer. Choose a roofer who is experienced and comfortable with installing insulation well.
  • When renovating - remove the ceiling lining to fit in insulation from underneath. If the rafter depth is less than 190mm, and you want to fit insulation with a recommended R-value of R3.6 or higher, you may need to fix additional spacer battens. Afterwards, line with plasterboard (or similar) on the battens.

Whichever way you do it, leave a 25mm gap between the insulation and the flexible roof underlay to avoid moisture problems.

Loose-fill insulation in skillion roofs

While it may seem like a good idea to pump loose-fill insulation into skillion roofs, it can be difficult to assess whether the existing roof and underlay are in adequate condition. It may also be difficult for you to maintain the gap required between the insulation and the roof underlay. Ask a qualified builder or roofer for their advice on this or possible alternatives before you start.