Roofing blanket thickness sets its R-value, and the two move together in fixed steps: 55 mm is R1.3, 75 mm is R1.8, 100 mm is R2.3 and 130 mm is R3.2. Which one you need comes from the Total R-value the roof has to reach, not from preference. The spacer depth then has to match the blanket, or the sheeting flattens it and you lose what you paid for.
In short
- Thickness and R-value are locked together. You cannot get a higher R-value in the same depth.
- The label R-value is the material. The code asks for the Total R-value of the whole roof.
- Coverage per roll drops as thickness rises, which changes the delivery as well as the price.
- A 100 mm blanket held by a 60 mm spacer is a 60 mm blanket.
Blanket is one of the few insulation products where the choice is genuinely simple, as long as you understand what the number on the label is and what it is not.
The range, thickness by thickness
These are the figures for our EcoRoof Roofing Blanket. Other brands use similar steps but not identical ones, so always work from the roll label of the product you are buying.
| Thickness | R-value | Roll | Coverage | Where it usually lands |
|---|---|---|---|---|
| 55 mm | R1.3 | 1200 mm x 15 m | 18 m² | Sheds, carports, patio roofs, noise control more than heat |
| 75 mm | R1.8 | 1200 mm x 15 m | 18 m² | Light commercial and warmer climate zones |
| 100 mm | R2.3 | 1200 mm x 10 m | 12 m² | The general purpose commercial choice |
| 130 mm | R3.2 | 1200 mm x 10 m | 12 m² | Cooler zones and higher Section J targets |
Notice the jump between 75 mm and 100 mm. The roll length drops from 15 m to 10 m, so coverage falls from 18 m² to 12 m² per roll. That is a third less area per roll for a 25 mm gain in thickness.
Material R-value against Total R-value
The R-value printed on the roll is the thermal resistance of that material on its own. It is not what the building has to achieve.
What the National Construction Code asks for is the Total R-value of the construction. That adds up every layer in the roof, the air spaces, the surface resistances on both faces, and then subtracts for the framing that bridges through the insulation. On a steel purlin roof that bridging deduction is real, which is why NCC 2022 Volume One Part J4 requires the Total R-value to be calculated in accordance with AS/NZS 4859.2.
So an R2.3 blanket does not produce a roof with a Total R-value of 2.3. It might land above or below that depending on what else is in the build up, whether there is a ceiling, and how much steel is passing through. Somebody on the project owns that calculation, usually the energy assessor on a house or the ESD consultant on a commercial job. Your job is to supply the thickness their calculation assumed.
Have a target Total R-value and no idea which blanket meets it? Send us the roof build up and the number the report asks for, and we will tell you what thickness that assumes.
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What actually decides the thickness
| Factor | Which way it pushes the thickness |
|---|---|
| Climate zone | Cooler zones need more. The requirement is set zone by zone, not nationally. |
| Ceiling or no ceiling | A warehouse with no ceiling lining has to do all the work at roof level, so it needs more. |
| Framing material | Steel purlins bridge heat past the insulation, so the calculation loses more than a timber roof does. |
| Roof colour | A darker roof absorbs more solar radiation, so the surface above the insulation runs hotter. |
| Acoustic requirement | Rain noise and reverberation control push thickness up independently of the thermal number. |
| Available depth | The spacer or batten depth on site caps what will actually fit at full thickness. |
That last row catches people out. If the spacers on the job are 85 mm, a 130 mm blanket will be crushed by 45 mm at every fixing line. Either the spacer depth goes up or the blanket thickness comes down. There is no third option that still performs.
Matching the spacer to the blanket
On a commercial roof the blanket sits over safety mesh, and the sheeting is fixed above it. Without something holding the sheet clear, the screws pull the roof sheet down onto the blanket and squash it.
The Roof Razor Insulation Spacer exists for exactly this. It lets the blanket recover to full thickness between the safety mesh and the metal deck, cuts through the insulation to bolt straight to the purlin so it will not work loose over time, and is wide enough to walk on during install. It comes in depths to suit the common blanket thicknesses, and its 1200 mm length matches typical blanket width so the installer keeps a line of sight to the purlin.
What compression costs you
Bulk insulation works because of the still air trapped between the fibres. Compress the mat and you remove that air. The material has not changed, but the performance has.
The loss is worst exactly where you least want it, at every purlin and every fixing line, and it runs the full length of the roof. From the ground the roof looks identical. In the calculation, and in the building, it is not.
There is a second cost. NCC 2022 Housing Provisions clause 13.2.2(1) requires insulation to abut or overlap adjoining insulation and form a continuous barrier. A roof that is at full thickness in the bays and flattened over every purlin is not the construction the calculation assumed.
Thickness changes the delivery
Work an example on a 900 m² warehouse roof, allowing 10 per cent for laps and trimming, so 990 m² to cover.
| Blanket | Coverage per roll | Rolls needed |
|---|---|---|
| 75 mm, R1.8 | 18 m² | 55 |
| 100 mm, R2.3 | 12 m² | 83 |
| 130 mm, R3.2 | 12 m² | 83 |
Stepping from R1.8 to R2.3 adds 28 rolls to the same roof. That is more pallets, more space on site, more lifts and more handling time. Worth knowing before the crane is booked. The full method is in how much insulation do I need.
Specifying it on a drawing
A blanket specification that will not come back with questions names five things:
- Thickness in millimetres and the material R-value
- Facing type and duty, because a light duty facing and a heavy duty facing are different products at the same thickness
- Whether the underside will be exposed or lined
- Spacer or batten depth, matched to the blanket
- Any fire requirement, such as a Group number or a bushfire attack level
Leave the facing out and you will get quoted on the cheapest one. If the blanket is exposed in a warehouse, that is the wrong product.
Frequently asked questions
What R-value is 100 mm roofing blanket?
On our EcoRoof range, 100 mm is R2.3. Other brands are similar but not identical, so check the roll label of the product you are buying.
What is the highest R-value roofing blanket?
In our range the thickest is 130 mm at R3.2. Higher Total R-values are usually reached by combining the blanket with other layers rather than by going thicker still.
Can I use two layers of blanket to get a higher R-value?
Only if the design allows for the depth and the spacers hold both layers at full thickness. Two compressed layers can perform worse than one uncompressed layer.
Does the blanket R-value equal the roof R-value?
No. The label figure is the material R-value. The code asks for the Total R-value of the whole construction, calculated in accordance with AS/NZS 4859.2, which includes air spaces, surface resistances and a deduction for thermal bridging.
What thickness blanket goes under a shed roof?
Sheds are commonly done in 55 mm or 75 mm, where the aim is taking the edge off the heat and the rain noise rather than meeting a habitable building target. If the shed is being lined and used as a workshop or studio, go thicker.
Do I need a thicker blanket if the roof is dark?
A darker roof absorbs more solar radiation, so the surface above the insulation runs hotter. In climate zones 1 to 5, NCC 2022 Housing Provisions clause 13.2.3(10) also limits the solar absorptance of the upper surface of a roof to no more than 0.64.
What spacer depth do I need for 100 mm blanket?
Match the spacer to the blanket so it recovers fully. Spacers are commonly available in 60 mm, 85 mm and 115 mm, so a 100 mm blanket is normally paired with the 115 mm spacer.
How many square metres does a roll cover?
18 m² for the 1200 mm x 15 m rolls in 55 mm and 75 mm, and 12 m² for the 1200 mm x 10 m rolls in 100 mm and 130 mm.
Sources
- Product data from the manufacturers of the products listed above
- NCC 2022 Volume One, Part J4 Building fabric
- NCC 2022 Housing Provisions, Part 13.2 Building fabric
- AS/NZS 4859.2, Thermal insulation materials for buildings, calculation of Total R-value
General information for trade and owner builders. The R-value your building has to meet comes from the specification or energy report for that project. Confirm product figures against the roll label.