Read the pack, then divide
Insulation batts are sold by the pack, and every pack states the area its batts cover. That number is the one that matters, because it changes with the R-value, the batt width and the brand: a pack of R2.5 wall batts might cover 11 square metres while a pack of R6.0 ceiling batts covers 5. The calculation is simply the area you need to fill divided by the pack coverage, rounded up.
packs = (area × (1 − framing) × (1 + waste)) ÷ pack coverage, rounded up
The framing deduction matters. Batts go between the studs and joists, not over them, and the timber takes up about a tenth of the wall. Ordering on the gross area gives you a spare pack or two on every job, which is fine if you like spare packs.
Which batts
Width is set by the framing. Studs, joists and ceiling battens at 450 mm centres take a 430 mm batt; framing at 600 mm takes a 580 mm batt. The batt is slightly wider than the gap so it holds itself in place by friction. Thickness is set by the R-value and by the depth of the framing: a 90 mm stud wall will take an R2.5 batt, and a thicker R2.7 or R3.0 batt may need a 90 mm high-density product or a deeper wall. Ceilings are not constrained the same way, which is why ceiling batts are much thicker.
| Location | Typical R-value, Victoria | Typical pack coverage |
|---|---|---|
| Ceiling, new build to NCC 2022 | R5.0 to R6.0 | 5 to 8 m² |
| Ceiling, top-up of an old house | R4.0 or more | 7 to 10 m² |
| External walls, 90 mm studs | R2.5 to R2.7 | 9 to 12 m² |
| Internal walls, acoustic | R2.0 to R2.5 acoustic | 9 to 12 m² |
| Under a timber floor | R2.0 to R2.5 | 8 to 11 m² |
R-values are typical outcomes, not rules. A new house has an energy report that sets them, and the building surveyor checks them at frame stage.
Installing
Batts should fill the space without gaps and without compression, with the edges touching the framing. In ceilings they go between the joists or battens, and keep clear of downlights unless the fittings are rated for contact, and clear of the eaves so the roof space can breathe. In walls they go in after the wiring and plumbing and before the plasterboard, with services cut around rather than squashed behind. Wear long sleeves, gloves, glasses and a mask: modern glasswool is much kinder than it used to be, but it is still itchy.
Worked example
A single-storey house with a 150 m² ceiling and 45 m of external wall at 2.4 m, with 20 m² of windows and doors, framed at 450 mm centres. Ceiling: 150 × 0.9 × 1.05 = 141.8 m² of batts, in R5.0 packs covering 8 m², so 18 packs. Walls: 108 − 20 = 88 m², × 0.9 × 1.05 = 83.2 m², in R2.5 packs covering 10 m², so 9 packs. Buy 430 mm batts throughout.
Common questions
How many packs of insulation do I need?
Net area after framing, plus 5 percent, divided by the coverage printed on the pack.
What width batts?
430 mm for 450 mm framing, 580 mm for 600 mm framing.
What R-value in Victoria?
New builds typically R5.0 to R6.0 ceilings and R2.5 to R2.7 walls under NCC 2022. Check the energy report.
Do you deduct framing?
Yes, about 10 percent. Batts fill the gaps, not the studs.
Quantities only. R-values and installation details come from the energy report, the NCC and the manufacturer's guide.