Roof Insulation
Roof and ceiling insulation are essential components of a building’s thermal envelope, helping to reduce heat loss and gain, improve energy efficiency, and provide a comfortable indoor environment.
Including roof and ceiling insulation can save up to 45% on heating and cooling costs and along with walls are the critical areas to focus on when trying to improve the insulative capacity of your home.
Pitched roof with flat ceiling
Warmer climates
In warmer climates with high humidity for some to all of the year, including reflective insulation under the roof to support the reflection of radiant heat is important. We are trying to keep that heat from the sun out of our building by reflecting it away. If the home is to be airconditioned often and the internal temperature is to remain cool, reflective foils can also be used on the outside or warm side of bulk insulation.
Cooler climates
Reflective insulation can be used on the inside of bulk insulation to keep warm air inside the home. This of course will be preferred in cooler climates. To prevent the development of mould, an appropriate moisture barrier should be allowed to prevent condensation from settling.
Check out this Bunnings video showcasing how bulk insulation is installed in a flat ceiling pitched roof
Raked ceiling
A raked ceiling has no accessible ceiling space between the ceiling and the roof. As there is less space, insulative material is needed to be of higher R-values to allow for sufficient insulation.
The manufacturer of the insulation should specify installation clearances, however as a guide for raked ceilings 190mm minimum clearance is needed for R3.0 bulk batts, while 60mm is the minimum for R3.0 PIR foam boards.
Appropriate insulation materials for raked ceilings include:
- Bulk insulation: polyester or fibreglass batts
- Rigid foam boards: PIR or XPS boards
- Composite insulation: foil-faced polystyrene boards. A minimum of 20mm reflective air space is needed for the foil face.
Insulation is often installed between rafters to make efficient use of the space between ceiling and roof, however, for a home with exposed rafters on the ceiling, more clearance is required for the insulation.
Flat roof
If the pitch of the roof is less than 5 degrees, then the risk of condensation not draining efficiently increases. In this case condensation must be prevented from forming in the first place.
If the roof is unventilated a trained consultant must check if the roofing complies with the NCC by completing a hygrothermal analysis.
Structural insulated panels (SIPs)
Structural insulated panels (SIPs) have a metal structural skin on both faces and a dense closed-cell foam core of PIR, PUR or XPS foam. No space is needed for air around the insulation and so the condensation risk is mitigated. An appropriate R-value depends on the site and climate zone and influences the risk of condensation.
Composite roof built up from conventional materials
A roof structure with roof battens and rafters separating the steel roofing from the ceiling will require bulk insulation to be installed between these battens and rafters. Again, reducing any air gaps to none will mitigate the risk of condensation and hence mould. Often the insulation is compressed slightly to ensure there are no air gaps, however slightly reducing the effective R-value. This compressing along with climate zones will affect the choice of R-value.
Flat membrane roof (on lightweight structure vs on suspended concrete slab)
For a lightweight structure, a membrane is attached to a substrate (structural ply or cement sheets) made of either heat-welded or bonded poly sheets or a liquid membrane applied onto the substrate. Similarly to the composite roof built from conventional materials, bulk insulation will be tightly fitted in between the rafters to ensure no air gaps are present. Some compression may occur also reducing the R-value. Some designs may need thermal breaks to prevent thermal bridging.
For a suspended concrete slab, the membrane layer adheres to a dense rigid foam board insulation layer which is also attached to the concrete slab. The R-value must be appropriate to ensure the slab temperature does not fall below the dew point, which could lead to condensation on the ceiling.
