Blanket, sarking and batts are not three ways of doing the same job. Blanket is bulk insulation with a foil facing that sits under the roof sheeting. Sarking is a thin membrane with no bulk layer, draped over the battens for radiant heat and moisture control. Batts are bulk insulation cut to fit between framing, normally above the ceiling. Most roofs use two of the three.
In short
- Different positions in the same roof, not competing products.
- Sarking adds very little R-value on its own. Its job is radiant heat, water and vapour.
- Blanket is the only one of the three that goes under the sheeting and adds real R-value.
- A warehouse with no ceiling has to do the work at roof level. A house usually does it at ceiling level.
The confusion is understandable. All three are sold as roof insulation, all three come in rolls or packs, and a supplier will quote you on whichever one you name. The difference is where each one sits and what physics it is using.
The three side by side
| Roofing blanket | Sarking or membrane | Ceiling batts | |
|---|---|---|---|
| What it is | Glasswool with a foil facing bonded on | Thin laminate, foil or vapour permeable | Glasswool cut to framing widths |
| Where it sits | Under the roof sheeting, over battens or mesh | Draped over battens, under the sheets | Between ceiling joists, above the lining |
| Main job | Bulk R-value, plus acoustic and vapour | Radiant heat, water and vapour control | Bulk R-value for the living space |
| Typical R-value | R1.3 to R3.2 | Very low on its own | R3.5 to R6.0 |
| Cuts rain noise | Yes, noticeably | A little, mostly by stopping sheet friction | No, it is below the roof cavity |
| Installed by | The roofing crew, as the sheets go on | The roofing crew | Insulation installer, after lock up |
Blanket
Blanket is the only one of the three that puts real bulk insulation between the roof sheet and the building. That matters most where there is no ceiling to insulate: a warehouse, a factory, a shed, a carport. In those buildings the roof plane is the entire thermal envelope.
It also does three jobs in one pass. The glasswool handles heat and absorbs sound. The foil facing reflects radiant heat and blocks water vapour from reaching the cold underside of the steel. The Australian Steel Institute is specific that the foil has to be impermeable so internal vapour cannot reach the roof sheet.
Position matters. The Australian Steel Institute states the blanket should be installed above the battens, against the underside of the roofing. That stops air moving over the top of the blanket, keeps it dry, and is a requirement for the acoustic benefit to work. Full detail is in what is a roofing blanket.
Sarking and membranes
Sarking is a thin sheet. There is no bulk layer, so it adds very little R-value by itself. What it does is control radiant heat and manage water and vapour, and those are not small jobs.
For it to work at all it needs an air gap. BlueScope recommends draping a reflective membrane between the battens to leave at least 20 mm of air space above and below, choosing a membrane with a vapour barrier classification to AS/NZS 4200.1:2017 that will not shrink under a dark roof and is reflective on both sides. A foil pressed flat against steel reflects almost nothing.
There are two families and they are not interchangeable:
- Impermeable foil. Blocks vapour. This is what sits under a metal roof to keep internal moisture away from the sheet, and it is what a blanket's bonded facing is doing.
- Vapour permeable membrane. Lets vapour pass while stopping liquid water. NCC 2022 Housing Provisions clause 10.8.1 requires a minimum vapour permeance for membranes in external walls, at least Class 3 in climate zones 4 and 5 and Class 4 in zones 6, 7 and 8.
VapourTech RWC is an example of the second family: a Class 4 vapour permeable weather barrier for roof sarking and wall wrap, rated for use in climate zones 2 to 8 and for all bushfire attack levels. Putting a permeable membrane where the design called for an impermeable one, or the reverse, is one of the more expensive substitution mistakes.
Ceiling batts
Batts are bulk glasswool cut to friction fit between framing. Above a flat ceiling they are the main thermal layer in most Australian houses, and they reach much higher R-values than a blanket because there is depth available: R3.5 through to R6.0 in our ceiling range.
Two widths cover most construction. 430 mm batts suit framing at 450 mm centres, 580 mm suit 600 mm centres. Get that wrong and you leave a gap on every run, which the code does not allow: clause 13.2.2(1) requires insulation to abut or overlap adjoining insulation and form a continuous barrier.
Batts do nothing for rain noise, because they sit below the roof cavity rather than under the sheet. If noise is the complaint, the fix is at roof level.
Which combination suits which building
| Building | Usual combination | Why |
|---|---|---|
| House with a flat ceiling | Sarking under the sheets, batts above the ceiling | Bulk R-value sits at ceiling level where there is depth for it |
| Warehouse or factory, no ceiling | Blanket under the sheeting, over safety mesh | The roof plane is the only place to put insulation |
| Shed or workshop | Blanket, or sarking if the budget is tight | Blanket also takes the edge off rain noise and radiant heat |
| Raked or cathedral ceiling | Blanket or board under the sheeting | There is no roof cavity to fill with batts |
| Reroof on an existing house | Sarking or blanket while the sheets are off, batts from inside later | Under sheet layers can only be added when the roof is open |
Where people get it wrong
- Expecting sarking to replace bulk insulation. It cannot. It has no bulk layer. Where a design calls for an R-value at roof level, sarking alone will not deliver it.
- Skipping the air gap. A reflective surface with nothing in front of it is a shiny sheet. Draping between battens is what makes it work.
- Batts instead of blanket in a shed. There is no ceiling to lay them on, and batts wedged against the underside of a metal roof get wet and sag.
- Mixing vapour classes. Impermeable and permeable membranes solve different problems. Check the specification before substituting.
- Insulation into the gutter. Whichever layer it is, keep it back from the drip edge. Wet insulation held against steel drives corrosion from the underside, which usually is not detected until the sheet perforates.
Not sure which layers your job needs? Send us the roof type, whether there is a ceiling, and the R-value the report asks for. We stock all three families and ship Australia-wide.
Browse roofing blankets, building membranes and ceiling insulation, or get in touch.
Frequently asked questions
What is the difference between sarking and roofing blanket?
Sarking is a thin membrane with no bulk insulation layer, so it controls radiant heat and moisture but adds very little R-value. A roofing blanket is bulk glasswool with that foil bonded to it, so it does both.
Do I need sarking if I have a roofing blanket?
Usually not under a metal roof, because the blanket's bonded foil facing is already doing the membrane's job. Where the design calls for a separate membrane as well, follow the specification.
Can I use batts instead of blanket under a metal roof?
No. Batts are made to sit between framing, not to be squeezed under roof sheeting. Under a metal roof they compress, they can get wet, and they will not stay put.
Does sarking add R-value?
Very little on its own. Its contribution comes from the reflective surface facing an air space, which is why it has to be draped with a gap rather than pressed flat.
Which is better for rain noise, blanket or batts?
Blanket. It sits directly under the sheeting where the noise is generated, and it also removes the friction between sheet and batten that causes the popping noise as a roof heats and cools.
Can I have both blanket and batts?
Yes, and on a house with a flat ceiling that is common: something under the sheets and batts above the ceiling. They are counted in different parts of the calculation.
What is a vapour permeable membrane?
A membrane that lets water vapour pass through while stopping liquid water. NCC 2022 requires a minimum vapour permeance for membranes in external walls in climate zones 4 to 8, at least Class 3 in zones 4 and 5 and Class 4 in zones 6, 7 and 8.
Can sarking be added to an existing roof?
Only by lifting the sheets, because it belongs between the sheeting and the structure. On an existing roof the practical retrofit is at ceiling level.
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
- NCC 2022 Housing Provisions, Part 10.8 Condensation management and Part 13.2 Building fabric
- AS/NZS 4200.1:2017, Pliable building membranes and underlays
- Product data from the manufacturers of the products listed above
General information for trade and owner builders. Which layers your building needs comes from the specification or energy report for that project.