A roofing blanket is a roll of glasswool insulation bonded to a reflective foil facing, laid under metal roof sheeting. The glasswool handles heat and sound. The foil reflects radiant heat and blocks water vapour from reaching the cold underside of the sheets. It is sold by thickness, and thickness sets the R-value.
In short
- Two materials doing three jobs: thermal, acoustic and vapour control.
- Thickness sets the R-value, and coverage per roll drops as thickness goes up.
- It only performs at full thickness. Compression is the enemy.
- The facing must be impermeable, and it faces down into the building in most of Australia.
Roofing blanket goes by a few names on site. Building blanket, roof blanket, blanket and foil. They all describe the same thing: bulk insulation with a facing, supplied in rolls wide enough to run across purlins or battens in one pass.
What it is made of
The bulk layer is normally glasswool, spun from molten glass into a mat of fine fibres. The glass itself is not a good insulator. What does the work is the air trapped between the fibres, held still so it cannot carry heat by convection. That is the whole principle, and it explains almost every rule about installing it: squash the mat and you remove the air, and the R-value goes with it.
The facing is a foil laminate bonded to one side. On the EcoRoof Roofing Blanket that facing is a light duty Ametalin foil. The facing does three jobs at once. It reflects radiant heat, it gives the blanket a finished surface where the underside is exposed in a shed or warehouse, and it stops water vapour from inside the building reaching the steel above.
That last point is the one that gets skipped. The Australian Steel Institute is specific: to stop moisture reaching the underside of the roof sheet and the blanket getting wet, the foil must be impermeable to internal water vapour. A blanket without a working vapour barrier is a sponge sitting under a cold steel sheet.
Facings come in different duties. A light duty facing is fine where the underside will be lined or is out of reach. A heavy duty facing is used where the blanket is exposed in a warehouse and will take knocks, or where the installer needs a tougher surface to work over. Some facings are also rated for bushfire prone construction. Check what the specification asks for before you compare prices, because two rolls of the same thickness can be very different products.
What it actually does
Thermal
A bare metal roof gains heat fast in summer and loses it fast at night. Blanket between the sheet and the building slows both directions. The Australian Steel Institute also notes it moderates the extremes of roof space temperature, which improves the efficiency of ducted air conditioning running through that space. In a warehouse with no ceiling, the blanket is the entire thermal envelope of the roof.
Acoustic
Rain on an uninsulated metal roof is loud. Blanket absorbs some of that impact noise. It also solves a quieter problem: as sheets heat and expand they drag across the battens, build up friction, then release with a pop. BlueScope notes that a membrane or blanket between the battens and the sheet reduces that friction and smooths the expansion and contraction cycle.
The Australian Steel Institute adds a condition worth knowing. Installing the blanket above the battens, hard against the underside of the roofing, is not just about keeping it dry. It is also a requirement for the acoustic benefit to work, because it stops air moving over the top of the blanket.
Condensation control
Blanket keeps the vapour barrier warmer than the roof sheet. BlueScope puts it this way: adding a blanket layer between the reflective membrane and the roof sheeting insulates the membrane from the cold sheet and further reduces the risk of condensation. Warmer surface, less condensation. The full mechanism is in condensation in metal roofs.
Thickness, R-value and roll size
Blanket is specified by thickness in millimetres, and each thickness has a rated R-value. Coverage per roll drops as thickness goes up, because a thicker blanket fills the roll faster.
| Thickness | R-value | Roll | Coverage | Rolls for 500 m² plus 10% |
|---|---|---|---|---|
| 55 mm | R1.3 | 1200 mm x 15 m | 18 m² | 31 |
| 75 mm | R1.8 | 1200 mm x 15 m | 18 m² | 31 |
| 100 mm | R2.3 | 1200 mm x 10 m | 12 m² | 46 |
| 130 mm | R3.2 | 1200 mm x 10 m | 12 m² | 46 |
Those figures are for EcoRoof. Other brands differ, so always work from the roll label rather than a rule of thumb. Note what the last column does to a delivery: the same roof needs half as many pallets again once you step up past 75 mm.
The R-value printed on the pack is the material R-value. The number a building has to satisfy is the Total R-value of the whole roof construction, which adds up every layer including air spaces and then subtracts for thermal bridging through the framing. A blanket labelled R2.3 does not produce a roof with a Total R-value of 2.3.
Where it goes in the roof
Blanket is laid over the structure and under the sheeting, facing down towards the building. On a house with timber battens, the Australian Steel Institute states it should sit above the battens against the underside of the roofing.
On a commercial roof the blanket goes over safety mesh that has already been fixed across the purlins. The mesh protects the crew and carries the blanket. To stop the sheets crushing it, an insulation spacer such as the Roof Razor Insulation Spacer holds the blanket at full recovery between the mesh and the metal deck. Spacers come in depths to match the blanket, commonly 60 mm, 85 mm and 115 mm. Match the depth to the blanket, or you have paid for thickness you are not getting.
The full build up, layer by layer, is in our guide to metal roof insulation in Australia.
Blanket, sarking or batts
| Product | What it is | Typical use | Where it sits |
|---|---|---|---|
| Roofing blanket | Bulk glasswool with a foil facing | Thermal, acoustic and vapour control in one product | Under metal roof sheeting |
| Sarking or reflective membrane | Foil laminate, no bulk layer | Radiant heat and vapour control where the bulk insulation sits elsewhere | Draped over battens, under the sheets |
| Ceiling batts | Bulk glasswool cut to fit between joists | The main thermal layer in most houses | Above the ceiling lining |
They are not alternatives so much as different positions in the same roof. A house can run sarking under the sheets and batts above the ceiling. A warehouse with no ceiling at all has to do the work at roof level, which is where blanket earns its place. The full comparison is in blanket, sarking or batts.
Installation points that matter
- Do not compress it. An R2.3 blanket squashed to 40 mm is not an R2.3 blanket. Spacers or battens hold the thickness.
- Keep it out of the gutter. BlueScope warns that blanket protruding into a gutter acts as a wick and can draw water through a large area of insulation. Wet insulation against steel causes corrosion from the underside, which is usually not detected until the sheet perforates. Trim back well clear of the drip edge.
- Lap and seal the facing. Sealing overlaps with the correct tape improves the vapour control performance of the system, and matters most where the difference in temperature and humidity across the roof is large.
- Install to AS 4200.2. The Australian Steel Institute references installation in accordance with AS 4200.2:2017 for pliable building membranes and underlays.
- Do not seal off the roof space. Ventilation still has to work. Restricting it traps internal moisture in the roof.
- Do not leave it out overnight uncovered. A roll that gets rained on before it goes up is a problem you take into the building with you.
Handling and storage
Rolls arrive compressed and expand once the wrapping is cut, so plan where they go before you open them. Keep them dry, off the ground and out of direct weather. Glasswool handling calls for the usual precautions: long sleeves, gloves, eye protection and a dust mask, and wash with cool water afterwards rather than hot, which opens the pores.
On a commercial site the rolls come by pallet and need a forklift or a crane. Nobody carries a 90 m² pallet up a ladder, and getting the delivery access wrong is one of the more common ways a roof day gets lost.
Buying it
Order by area plus a waste allowance for laps and trimming, then check the roll width suits your purlin or batten run. Our roofing blankets range covers residential and commercial, and we also carry the mesh and spacers that go with it. Working out quantities is covered in how much insulation do I need, and what to send us is in ordering insulation online.
Not sure which thickness the job calls for? Send us the roof plan and the target R-value and we will price the package and get it on a truck to you.
Get in touch or browse the roofing blankets range.
Frequently asked questions
What is a roofing blanket?
A roll of glasswool insulation with a reflective foil facing bonded to one side, installed under metal roof sheeting to provide thermal insulation, sound absorption and a vapour barrier.
Which way does the foil face?
In most Australian conditions the foil faces down, into the building. Tropical detailing can differ, so follow the manufacturer's instructions for your climate.
What R-value roofing blanket do I need?
It depends on the Total R-value your roof has to achieve, which comes from the climate zone and the building class. Common thicknesses run from 55 mm at R1.3 to 130 mm at R3.2.
Can roofing blanket be used on walls?
Yes. Products like EcoRoof are rated for metal roof and wall applications, installed behind a lining, and some are also suited to under slab concrete soffits.
How much does a roll cover?
Coverage depends on thickness. A 1200 mm x 15 m roll covers 18 m², while a thicker 1200 mm x 10 m roll covers 12 m². Check the label before ordering.
Does roofing blanket stop condensation?
It reduces the risk by keeping the vapour barrier warmer than the roof sheet and by blocking internal moisture from reaching the steel. It works alongside ventilation, not instead of it.
What is the difference between roof blanket and sarking?
Sarking is a foil membrane with no bulk insulation layer, so it controls radiant heat and vapour but adds very little R-value. A blanket is bulk glasswool with that foil bonded to it, so it does both.
Can roofing blanket get wet?
It should not. Wet glasswool loses performance and, held against steel, can drive corrosion from the underside of the sheet. Store it dry and trim it clear of gutters.
Sources
- Australian Steel Institute, Building with Steel Factsheet #3, Blanket and Foil insulation in Metal Roofing
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- AS 4200.2:2017, Pliable building membranes and underlays, Installation requirements
- Product data from the manufacturers of the products listed above
General information for trade and owner builders. Confirm the specification for your building and follow the manufacturer's installation instructions.