Floor Insulation Types
Insulation requirements for flooring vary depending on the type of flooring. As you can imagine, if your floor is raised, your house has one more surface exposed to the outside temperature. Sometimes floor systems require insulation also.
Suspended floors
According to the NCC, raised flooring exposed to the outside of the house requires a specific R-value depending on the climate zone.
For timber floors and suspended slabs, insulate the underside of the flooring exposed to air. Edges are advised to be insulated also especially for suspended slabs.
Whenever foil-faced boards are used to insulate the floor the risk for condensation must be considered. Any subfloor installed must also consider appropriate ventilation as specified in the NCC. In addition, floor insulation can be in the form of appropriate underlay and carpet or lay insulation boards under floor finishes.
In climate zone 1 as experienced in the top end of Australia including Darwin, Broome and North Queensland, insulate with cyclone-rated products and foil facing outwards on the building envelope. This will keep that warm air out of the house.
Timber floors
The R-value for timber is approximately 0.25, so most timber floors require additional insulation. Bulk insulation can be installed and tied down with nylon cord or wire, although if you have a likelihood of pests you might have to reconsider. Alternatively, a thin fibre cement sheet or any impervious sheet such as XPS or PIR, can be installed below the joists.
An expanding spray foam such as PUR can provide good R-values while adhering well to most overhead surfaces without additional fixes. If the above surface is smooth, such as a concrete slab once formwork has been removed, may need a primer to allow the foam to adhere, or some additional fixing to allow it to cling. You can bring this up with your installer to find what suits your situation best.
If you have a building in a hot climate that you know will not be air-conditioned, perforated foil or concertina-type batts attached to the side of the joists with non-conductive staples is appropriate. This keeps hot air out while reducing the risk of condensation.
Suspended concrete slabs
Typically suspended concrete floor slabs are approximately R0.30 in thermal performance, though the floor’s thermal resistance depends on the climate. The most accurate R-value can come from thermal modelling.
Suspended concrete slabs can be insulated by adding rigid foam boards with or without a foil face. Foil-face boards will require special fixings while care must be taken to reduce condensation risk. This is something the manufacturer’s technical information and installation guide should indicate.
Slab-on-ground
In climate zone 8 or when in-slab heating or cooling is installed, the NCC requires the vertical edges of the slab-on-ground must be insulated. In other climate zones edge insulation is still advisable though not required in the NCC.
Nearly 80% of heat loss occurs at the edges of slab-on-ground flooring. Often slab edge insulation is sufficient alone. The edge insulation will be installed before the slab is poured, but not under concrete edge footing beams. A vapour barrier membrane will also be included, and the placement of such can be advised through the manufacturer’s directions or the installer’s advice.
Where groundwater is present, the underside of a slab will also need to be insulated. This insulation must have high compressive strength and be moisture resistant, or it will lose its function as an insulative material and can potentially lead to structural failure. In most climate zones around Australia and in all states except Tasmania, termite protection is very important for slab-on-ground flooring.