A metal roof in Australia is insulated as a system, not as a single product. Working from the structure up, the usual order is safety mesh over the purlins, an insulation blanket laid over the mesh, spacers or battens to hold the blanket at full thickness, a reflective facing to control vapour, then the roof sheets.
In short
- The blanket goes above the battens or purlins, hard against the underside of the sheeting.
- Nothing performs if it is compressed. Spacers exist for this reason alone.
- Metal sheeting fixed straight to metal purlins needs a thermal break of at least R0.2 under NCC 2022.
- Insulation that reaches into the gutter will rot the roof from the inside out.
- Roof insulation goes in before the sheets. There is no second chance without a reroof.
Get the order wrong and the roof still looks finished. It just performs badly. The blanket sits crushed to half its thickness, the foil faces the wrong way, or the insulation hangs into the gutter and wicks water back under the sheets. None of that is visible from the ground, and none of it is cheap to fix later.
This guide sets out what each layer does, where it sits, and the rules that actually apply in Australia.
The layers, in order
| Layer | What it does | Where it sits |
|---|---|---|
| Safety mesh | Stops a worker falling through the roof opening during sheeting and later maintenance. Also carries the insulation. | Fixed over the purlins, before anything else goes on. |
| Insulation blanket | Thermal and acoustic performance. Slows heat moving in and out, and cuts rain noise. | Rolled out over the mesh, foil facing down towards the building. |
| Spacers or battens | Hold the blanket at its full thickness so it delivers the R-value you paid for. | Between the blanket and the underside of the sheeting. |
| Reflective facing or sarking | Vapour control and radiant heat control. Keeps internal moisture away from the cold steel. | Bonded to the blanket, or draped separately over battens. |
| Thermal break | Breaks the steel to steel path where sheeting meets a metal frame. | Between the sheeting and the purlin, batten or rafter. |
| Roof sheeting | Weather. Everything above is what stops the roof behaving like a frying pan and a fridge on the same day. | Screwed through to the purlins or battens. |
Why the order matters
Each layer only works in one position. Bulk insulation slows conducted heat, so it belongs between the hot surface and the cool one. A reflective facing only works next to an air space, so it belongs where there is a gap to face into. A vapour barrier only works if it sits on the warm, humid side of the assembly, which in most of Australia means below the insulation, facing the building.
Move any of them and they stop doing their job. A foil pressed flat against steel reflects almost nothing. A blanket laid under the purlins instead of over them leaves the whole steel frame bridging straight to the outside. A vapour barrier installed above the insulation traps moisture inside the insulation instead of keeping it out.
The Australian Steel Institute is direct about the blanket position: it should be installed above the battens, against the underside of the metal roofing. Laid that way it stops air moving over the top of the blanket, which keeps the blanket dry and is also what makes the acoustic benefit work.
Commercial and residential build up differently
On a commercial or industrial roof with steel purlins, safety mesh comes first. It is a fall control for the crew laying sheets and it doubles as the tray that holds the blanket up. The blanket is rolled across the mesh and the sheets go over the top. Without something holding the blanket clear of the sheet, the screws and the sheet pans squash it flat at every fixing line.
That is what an insulation spacer is for. The Roof Razor Insulation Spacer is an Australian made metal bracket that lets the blanket recover to full thickness between the safety mesh and the metal deck. It cuts through the insulation to bolt straight to the purlin, so it will not work loose from vibration and movement over time. It is wide and stable enough to walk on and can be trafficked the same as a purlin, it comes pre-loaded with screws, and its 1200 mm length matches typical building blanket width so the installer has a direct line of sight to the purlin. It suits roofs up to and including 30 degrees pitch, with options for cyclonic regions.
On a house with timber battens the sequence is simpler. Battens go on, blanket or sarking goes over the battens, sheets go on top. There is usually no safety mesh on a domestic pitched roof, and the bulk of the thermal work is done later by batts above the ceiling. The layer under the sheets is doing radiant heat, condensation control and noise.
| Step | Commercial, steel purlins | Residential, timber battens |
|---|---|---|
| 1 | Safety mesh fixed across the purlins | Battens fixed to rafters |
| 2 | Blanket rolled over the mesh, facing down | Blanket or sarking laid over the battens |
| 3 | Spacers fixed through to the purlins | Sheets fixed through the battens |
| 4 | Sheeting fixed to the spacers or purlins | Ceiling batts installed later, from inside |
| Who installs it | The roofing crew, as the sheets go on | Roofer for the sarking, insulation installer for the batts |
Choosing the blanket thickness
Roofing blanket is sold by thickness, and thickness sets the R-value. Our EcoRoof Roofing Blanket is a useful reference because the whole range sits on one product:
| Thickness | R-value | Roll size | Coverage per roll | Typical use |
|---|---|---|---|---|
| 55 mm | R1.3 | 1200 mm x 15 m | 18 m² | Sheds, carports, noise control |
| 75 mm | R1.8 | 1200 mm x 15 m | 18 m² | Light commercial, warm climates |
| 100 mm | R2.3 | 1200 mm x 10 m | 12 m² | General commercial roofing |
| 130 mm | R3.2 | 1200 mm x 10 m | 12 m² | Cooler zones, higher Section J targets |
Two things follow from that table. Thicker blanket means fewer square metres per roll, so a jump from R1.8 to R3.2 is not just a price change, it is a delivery and handling change as well. And the R-value on the label is the material value. What the building has to meet is a Total R-value for the whole roof construction, which is a different number that adds up every layer and every air space and then subtracts for the framing that bridges them.
Match the spacer depth to the blanket. A 100 mm blanket held by a 60 mm spacer is a 60 mm blanket with extra cost.
The thermal break rule people miss
If metal roof sheeting is fixed straight onto metal purlins, metal rafters or metal battens, the steel becomes a bridge that carries heat past the insulation. NCC 2022 deals with this in the Housing Provisions at clause 13.2.3(7). A roof with metal sheeting fixed directly to those metal members, and with either no ceiling lining or a lining fixed directly to the same members, must have a thermal break with an R-value of not less than 0.2 installed between the sheeting and its supports.
The same idea applies to walls at clause 13.2.5(5), where a metal framed wall in the building envelope needs an R0.2 thermal break at every point of contact between the cladding and the frame. Clause 13.2.3(3) also sets out ways to deal with thermal bridging in metal framed roofs more generally, including raising the R-value between the frames or running a continuous insulation layer.
Products are made specifically for this. Trade Select ThermalBreak 7 from Ametalin provides an R0.2 thermal break in situ, carries a Group 1 fire performance classification, and complies with AS 3959-2018 for construction in bushfire prone areas, which makes it usable across all BALs.
It is a cheap item at build time and an expensive one to add later, because adding it means lifting sheets.
Air gaps, foil and vapour
A metal roof radiates heat to the night sky and gets colder than the air under it. Warm moist air from inside the building drifts up into the roof space, hits that cold steel, and condenses. BlueScope's technical bulletin on roof condensation describes exactly this cycle, and it is the reason the facing on a roofing blanket matters as much as the glasswool.
The foil facing has to be impermeable so internal water vapour never reaches the underside of the sheet. Where a separate reflective membrane is used instead of a faced blanket, BlueScope recommends draping it between battens to leave at least 20 mm of air space above and below, and choosing a membrane that has a vapour barrier classification to AS/NZS 4200.1:2017, that will not degrade or shrink at the temperatures under a dark roof, and that is reflective on both sides.
The air gap is not a detail to skip. A reflective surface with no gap in front of it is just a shiny sheet. Draping the membrane between battens is what creates that gap.
One detail causes more damage than any other. Do not let the blanket overhang the last batten into the gutter. Wet insulation acts as a wick and draws water through a large area, holding it against the underside of the sheet. Corrosion that starts on the underside is usually invisible until the sheet perforates. Trim the insulation back well clear of the drip edge.
Roof colour and solar absorptance
Insulation is not the only lever on a metal roof. How much solar radiation the roof surface absorbs in the first place changes how hard the insulation has to work, and the code now puts a limit on it. NCC 2022 Housing Provisions clause 13.2.3(10) states that in climate zones 1 to 5, the solar absorptance of the upper surface of a roof must not be more than 0.64.
In practice that rules out the darkest roof colours in the warmer half of the country. It is worth raising early with the client, because roof colour is usually chosen for looks and then discovered to be a compliance item.
Ventilation is part of the system
Sealing a roof space tight and insulating it heavily traps the moisture the building generates. The Australian Steel Institute puts it plainly: practices that restrict roof space ventilation can lead to trapped internal water vapour and moisture problems in metal roofs. Ventilation at the eave or fascia with matching ventilation at or near the ridge is the standard arrangement, so air can enter low and leave high.
NCC 2022 made part of this mandatory. Housing Provisions clause 10.8.3 sets roof space ventilation requirements in climate zones 6, 7 and 8, with opening areas that change with roof pitch. Range hoods and bathroom fans must discharge to outdoor air rather than into the roof cavity under clause 10.8.2.
The full mechanism, the warning signs and the fixes are covered in condensation in metal roofs.
Six mistakes we see on site
- Blanket compressed at the purlins. No spacer, no full R-value. The certificate says R2.3, the roof performs closer to nothing at every fixing line.
- Insulation into the gutter. Covered above. It is the one that eats roof sheets.
- Safety mesh skipped, then retrofitted. Mesh has to go on before the sheets. Once the roof is closed, the cost of adding fall protection multiplies.
- Foil facing the wrong way. In most Australian conditions the facing goes down, towards the building. Tropical detailing differs, and BlueScope notes that in places like Darwin and Cairns a membrane may be needed both above and beneath the blanket. Follow the manufacturer's instructions for the climate.
- No thermal break on a steel framed roof. Clause 13.2.3(7) is not optional.
- Gaps at the edges. Clause 13.2.2(1) requires insulation to abut or overlap adjoining insulation and form a continuous barrier. A 20 mm gap running the length of a roof is a hole in the envelope.
Ordering a roof package
Six numbers get a roof package priced properly:
- Roof area measured on the plane, not the footprint, plus the pitch
- Purlin or batten spacing
- Target R-value or the Total R-value from the energy report
- Climate zone and the site postcode
- Whether safety mesh is in scope
- Access for delivery, and whether there is a forklift on site
What to send and what to expect is set out in ordering insulation online.
We stock the whole build up, not one part of it. Roofing blankets, safety mesh, spacers, sarking and thermal break products from Fletcher Insulation, Kingspan, Ametalin, Pyrotek and Proctor Group. We supply trade and owner builders, and the whole build up can be ordered online for delivery across Australia.
Browse roofing blankets, roofing and safety mesh and building membranes, or send us the roof plan and we will price the package.
Frequently asked questions
What insulation goes under a metal roof in Australia?
A glasswool insulation blanket with a foil facing is the standard answer, held at full thickness by spacers or battens, over safety mesh on commercial roofs. Reflective sarking may be used on its own where the design only calls for radiant heat and vapour control.
Does insulation go above or below the battens?
Above. The Australian Steel Institute states the blanket should sit above the battens against the underside of the roofing, which stops air moving over the blanket and is also required for the acoustic benefit.
Do I need a thermal break under a metal roof?
You do if the metal sheeting is fixed directly to metal purlins, rafters or battens and there is either no ceiling lining or a lining fixed to those same members. NCC 2022 Housing Provisions clause 13.2.3(7) calls for a thermal break of at least R0.2 in that case.
Can I install roofing blanket on an existing roof?
Only by lifting the sheets, because the blanket belongs between the sheeting and the structure. On an existing roof the practical retrofit is usually ceiling level insulation, or a sheet replacement that includes the blanket at the same time.
How thick should a roofing blanket be?
Thickness follows the Total R-value the building has to meet, which depends on climate zone, roof type and what else is in the construction. Common commercial choices run from 55 mm at R1.3 up to 130 mm at R3.2.
Does a roofing blanket stop rain noise?
It reduces it. Blanket under the sheeting absorbs sound and also removes the friction between sheet and batten that causes the popping noise as a metal roof heats and cools.
What size insulation spacer do I need?
Match the spacer depth to the blanket thickness so the blanket recovers fully. Spacers are commonly available in 60 mm, 85 mm and 115 mm depths.
Does roof colour affect insulation requirements?
It affects compliance. In climate zones 1 to 5, NCC 2022 Housing Provisions clause 13.2.3(10) limits the solar absorptance of the upper surface of a roof to no more than 0.64, which restricts the darkest colours.
Sources
- Australian Steel Institute, Building with Steel Factsheet #3, Blanket and Foil insulation in Metal Roofing
- Australian Steel Institute, Building with Steel Factsheet #2, Vapour Barriers in Metal Roofing
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- NCC 2022 Housing Provisions, Part 13.2 Building fabric and Part 10.8 Condensation management
- AS/NZS 4200.1:2017 and AS 4200.2:2017, Pliable building membranes and underlays
- Product data from the manufacturers of the products listed above
This article is general information for trade and owner builders. It is not a substitute for a specification prepared for your building. Check the current NCC edition adopted in your state and the manufacturer's installation instructions for the product you are using.