R-values, in plain English
An R-value measures how well a product resists heat flow, and higher is better. What really counts is the Total R-value: every layer in the roof and ceiling added together, air gaps included. Where foil is involved you'll see an 'up' figure for winter and a 'down' figure for summer, and both matter.
Two traps. American R-values look much bigger because they use different units, so don't compare them with Australian ones. And the building code's tables don't simply add foil's R-value to the batts; they use the labelled R-value of the bulk insulation. Insulation sold here must meet AS/NZS 4859, and reflective insulation AS 4200.1.
Ceiling batts or a roof blanket?
| Ceiling batts | Blanket under the roofing | |
|---|---|---|
| Where it goes | On the ceiling, between the joists | Directly under the roof sheets, mostly on metal roofs |
| When it goes in | Any time there's access to the roof space | When the roofing goes on or is replaced |
| What it helps with | Heat in summer and winter | Heat, plus rain noise when held hard against the sheets (Lysaght) |
| Also called | Bulk insulation | A roof blanket, or anticon blanket |
Do you need both?
On a metal roof, often yes. BlueScope says the highest thermal performance comes from a blanket and foil under the roofing combined with ceiling insulation. One doesn't replace the other.
How much insulation do you need?
For new building work, the 2022 code sets minimum ceiling R-values by climate zone. For a standard pitched roof with a flat ceiling and nothing under the roofing, it's R3.5 in climate zone 4, R3.0 in zone 5 and R4.0 in zone 6.
In warmer zones the roof colour changes the answer. In climate zone 2, the minimum climbs from R3.0 for a roof with solar absorptance up to 0.32 to R4.0 for one up to 0.64, because darker roofs soak up more of the sun's heat.
Those rows are from the 2022 edition, and the 2025 code began rolling out state by state from 1 May 2026, so new work in your state may follow different figures. They're written for new work, but they make a useful yardstick for an existing home.
Before anyone goes into the roof space
Your Home says AS 3999 requires an electrical safety check by a licensed electrician before insulation goes into an existing home. Insulation has to keep clear of flues and older halogen downlights, though most LED downlights sold now are IC rated and can be covered. Foil conducts electricity, so it must not touch cables. And some homes built before 1990 used loose-fill asbestos as ceiling insulation, so if you're not sure what's up there, have a licensed asbestos assessor look before anyone disturbs it.
What to ask an installer
- What's the labelled R-value, and does the product meet AS/NZS 4859?
- Who does the electrical safety check, and is it in the price?
- What will you do around downlights and flues?
- How will you avoid gaps and squashed batts?
- Is old insulation coming out, and could any of it be asbestos?
What's realistic for you
You can
- Look through the manhole with a torch, without climbing in, to see what's there now
- Count your downlights and check whether they're halogen or LED
- Note rooms that run hot or cold, to show the installer
Leave to a roofer
- The electrical check, by a licensed electrician
- Laying batts without gaps or squashing
- Fitting a blanket under the roof sheets
- Anything that might involve asbestos
Where we'd spend first
Before buying anything, we'd find out what's already up there and get the electrical check done, because those two things shape the whole job. On a metal roof that's being replaced, we wouldn't skip the blanket: it's the one chance to fit it without lifting the sheets again, and it's what keeps the rain noise down. And we'd compare quotes on the labelled R-value, not on thickness.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.