Tips From Our Experts

Gretchen asks: I’m looking to add under floor insulation for a warmer home. How big of an impact will this have, what product is best and what other issues should I be aware of?

Floor Insulation

Gretchen asks:

I live in the western suburbs of Brisbane. It’s hot in summer and can get very cold overnight in winter (not infrequently close to 0°C, sometimes below), despite being pleasant during the day. My home is nearly 3m off the ground and I have a timber floor and 2.8m ceilings. The house gets very cold in winter – e.g. down to 12°C overnight which then takes significant time to warm up during the day even when I open the house up. I have good insulation in the walls and ceilings as well as curtains on most windows and a reflective coating on others (to reflect heat back into the house). I’m now looking at under floor insulation. Before going to this expense I’m interested to find out:

  • how big an impact this is likely to have
  • the best products to use – e.g. styrofoam board, batts etc. and
  • other issues to be alert to either in simply achieving a warmer house generally, or with respect to underfloor insulation specifically.

Many thanks.

Our expert’s answer

Thanks for your question. Being nearly 3m off the ground can make it difficult to keep the heat in an uninsulated floor. Any breath of wind will see you losing heat fast. Check out the Your Home website which shows 10-20% heat loss through an uninsulated floor. From that same document they say:


Floors do not always require insulation.

Raised timber floors should have subfloor access, with soil clearance of around 400mm below the lowest timbers. This provides sufficient access to install insulation. Foil or bulk insulation works well, but in either case care must be taken to ensure it is well supported and will not sag or fall down in time. Access by vermin also needs to be considered. Insulation board can be laid beneath floor finishes if there is no subfloor access.
Exposed subfloor (pole home) (left) and enclosed or ventilated subfloor (brick, brick veneer, timber frame) (right).

As you can see, there are a number of methods you can use to insulate timber floors. I understand that under timber floor insulation needs to allow the timber to ‘breathe’ or you will run into problems with moisture buckling the floor boards. Small gaps in the insulation can allow this or moisture permeable insulation can be used. I believe one of these products would be AIR-CELL Permifloor® 500 and this can be used to retrofit old floors as it fits neatly between the 450mm joists.

A Google search should locate where you can buy this kind of thing. You might also like to look into draughtproofing. This can be a very cost-effective way to reduce energy ‘leakage’. Use a 200mm length of cassette tape (you’re not really going to listen to it again, are you!), or an incense stick, on a windy day to help locate areas to seal. I hope this helps.

For more information about insulation, see the 17 Things™.

John Knox

John Knox

The technical advice provided by the Alternative Technology Association as part of the ‘Ask an Expert’ forum/website, in relation to home and building sustainability improvements, is general in nature only. The impacts of specific energy or water saving or sustainability products will depend on how and where the are used and located, and particularly for larger investments, we would always suggest users seek a specific assessment from an appropriately qualified building designer or service / product supplier.

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