Insulation is one of the cheapest upgrades for energy savings, and one of the easiest to under-order. The quantity depends on two things: the area to cover, and the depth or performance you're aiming for. Get the target right first, then the material count is simple.
R-values in Australia
Insulation is sold by its R-value: the higher, the better it resists heat flow. The NCC sets minimum total R-values for roofs and ceilings, walls and floors by climate zone, and new homes also need an energy rating, 7 stars under NCC 2022 in most states. In practice, ceilings commonly take R3.5 to R6.0 batts, 90 mm stud walls R2.0 to R2.7, and suspended timber floors R1.5 to R2.5.
Batts, blankets and loose-fill
Glasswool and polyester batts are the usual choice: easy to fit between ceiling joists and studs, and simple to count by the pack. Roof blankets go under metal roof sheets and add insulation, condensation control and less rain noise. Loose-fill (blown) insulation suits ceilings with poor access but needs a specialist installer.
Ceiling batts
Ceiling batts sit between the ceiling joists or the bottom chords of the trusses. A higher R-value means a thicker batt: R4.0 glasswool is around 175–200 mm thick, depending on the brand. If you're topping up an old, thin layer, lay the new batts across the joists.
Counting packs
Packs = area × (1 + waste) ÷ coverage per pack. Coverage varies with the R-value, the batt width (made to suit 450 or 600 mm framing) and the brand, so read the pack. Allow about 5% for cutting around joists, downlights and hatches.
Things that go wrong
- Blocking eaves ventilation. Fit eaves baffles first, so air can flow from the eaves.
- Squashing insulation under boards. Compressed insulation loses much of its value. Raise storage boards on legs.
- Covering recessed lights that aren't rated for contact with insulation.
- Forgetting the ceiling manhole: insulate and weatherstrip it too.
- Covering transformers, exhaust fans and flues. Keep the clearances the product and AS/NZS 3000 require.
- Not sealing air leaks first. Gaps around pipes, cables and light fittings let warm, moist air into the cold space.
Walls and floors
Wall batts go into the stud frame before the plasterboard, with wall wrap on the outside of the frame. Existing brick veneer and cavity brick walls are harder to insulate and may suit a pumped-in product from a specialist. Suspended timber floors can be insulated from below with underfloor batts or boards held in with straps. Each of these has its own detailing, so treat them as separate items on the quote.
Safety and access
Wear gloves, long sleeves, eye protection and a dust mask. Work from boards across the joists, never on the ceiling between them, and keep good lighting. Check for old wiring before covering it, and keep insulation clear of chimneys and flues.
A worked example
A 48 m² ceiling in R4.0 batts at about 6.5 m² per pack: 48 × 1.05 ÷ 6.5 = 7.8, so 8 packs. Check the coverage on the pack you're buying.
The insulation calculator works out packs of batts for the R-value you pick.
Common questions
Can I put too much insulation in a ceiling?
Not in terms of heat loss, but you must keep the ventilation paths at the eaves open, and leave access to junction boxes, water tanks and hatches.
Does insulation need a vapour barrier?
The bigger concern in most of Australia is condensation under metal roofs and inside walls in cooler climates. NCC 2022 added condensation rules for wall wraps and roof space ventilation, especially in climate zones 6, 7 and 8. Follow the product maker's details.
Pricing the job
Materials are only part of the price. Clearing stored items out of the roof space, fitting eaves baffles, moving or protecting cables, insulating pipes and tanks, boarding for storage, and disposing of old insulation all take time. List them separately, so the client can see what they're paying for and decide on the optional items.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.