Trade & construction Australia
Insulation Calculator Australia
Insulation is bought by the pack, and each pack states how many square metres it covers at its R-value. This calculator turns the area to insulate into the number of packs, after allowing for the timber framing that the batts do not cover.
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Measure the ceiling, wall or floor you are insulating.
Joists or studs the batts do not cover. 5% to 15% is typical; check your framing.
Printed on the pack. It depends on the R-value and batt size.
Packs to buy
—Area to cover
—
Area per pack
—
Packs before wastage
—
Estimated cost
—
The working
How many insulation batts do I need?
Divide the area to be insulated by what one pack covers, after taking off the part of the area taken up by framing, and add a little for waste. A 150 m² ceiling with 7.8 m² per pack needs about 20 packs, with a 5% allowance and 5% of the area taken by joists.
What R-value do I need?
The National Construction Code sets minimum R-values for ceilings, walls and floors by climate zone, and the requirement for a new build differs from the advice for retrofitting. The Your Home guide explains the options by climate zone.
Ceilings are the best place to start for an existing house, because most of the heat is lost through the roof.
Safety
Insulation near downlights, flues and electrical wiring has clearance rules, and loose-fill and some foil products conduct electricity. Do not install in a roof space unless you know where the wiring is, and consider an electrician or a qualified installer.
To estimate lining, use the plasterboard calculator.
How this is calculated
The rule, step by step, so you can check any figure by hand.
area to cover = area × (1 − framing share)packs = area to cover ÷ coverage of one pack × (1 + wastage), rounded up
Insulation is specified by its R-value, which is its resistance to heat flow, and the National Construction Code sets minimum totals for roofs, walls and floors by climate zone. A higher R-value is a thicker batt, which covers less area per pack, so the same room needs more packs.
The framing share is the part many people miss. Joists and studs occupy part of the area, and the batts fit between them, so subtracting the framing before dividing by the pack coverage avoids over-ordering. Anything missed is an air gap that reduces performance, so cut batts to fit snugly.
A worked example
A 150 m² ceiling with 5% framing, packs covering 7.8 m², 5% wastage.
- Area to cover, 150 × 0.95142.5 m²
- Packs before wastage18.3
- With 5% wastage20
About 20 packs.
Use the coverage printed on your pack, not the figure in this example.
Batts, blown and foil
Glass wool and polyester batts fit between framing, blown-in cellulose or wool suits irregular spaces, and reflective foil handles radiant heat in sunny roofs. Each has a different rating and installation method, so check the manufacturer's data sheet.
What it leaves out
Ventilation, vapour barriers, sealing of gaps and the compression of batts reduce actual performance. Moisture control needs a design in wet climates.
What this calculator does not do
- Quantity only, not a thermal design
- Coverage depends on the product
- Safety clearances apply near wiring and downlights
- Required R-values depend on climate zone
Common questions
How many insulation batts do I need for 100 m²?
About 13 to 14 packs at 7.8 m² each after allowing for framing and wastage.
What R-value do I need?
It depends on your climate zone and where it is going. The National Construction Code and the Your Home guide give the figures.
Should I subtract the joists?
Yes. Batts fit between framing, so only the gaps between joists need insulation.
Is it safe to insulate myself?
It can be, but wiring and downlights need clearances. If in doubt, use a qualified installer.
Where should I insulate first?
The ceiling usually gives the biggest improvement in an existing house.
Where these numbers come from
- Your Home — Australian Government guide to sustainable homes Independent guidance on insulation, materials and design
- National Construction Code (ABCB) The code that sets minimum building requirements in Australia
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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