A thermal break is a thin layer of insulation placed where metal touches metal, so heat cannot run straight through the frame and past everything else. NCC 2022 requires one with an R-value of not less than 0.2 where metal roof sheeting is fixed directly to metal purlins, rafters or battens, and at every point of contact between external cladding and a metal wall frame.
In short
- It is about thermal bridging, not about adding R-value to the whole roof.
- R0.2 is the minimum the code names, and it is a small number for a reason.
- It applies to metal on metal. A timber frame is not the same problem.
- Cheap at build time, expensive later, because adding it means lifting sheets.
Of all the insulation requirements in the code, this is the one most often missed on site, because the product is thin, cheap and easy to leave in the ute.
What thermal bridging actually is
Insulation slows heat down. Steel does the opposite. Put a steel purlin through the middle of an insulated roof and you have built a shortcut: heat travels along the steel, past the insulation, and out.
The insulation between the purlins still works. The problem is that the calculation for the whole roof has to account for the steel running through it, and that deduction is significant. It is why NCC 2022 Volume One Part J4 requires the Total R-value, including the allowance for thermal bridging, to be calculated in accordance with AS/NZS 4859.2, rather than just adding up the layers.
A thermal break interrupts that shortcut. It does not need to be thick, because it only has to break the metal to metal contact. That is why the number in the code is R0.2 and not R2.
The roof rule, clause 13.2.3(7)
NCC 2022 Housing Provisions clause 13.2.3(7) sets it out. Where metal sheet roofing is fixed directly to metal purlins, metal rafters or metal battens, and there is either no ceiling lining or a ceiling lining fixed directly to those same members, a thermal break with an R-value of not less than 0.2 must be installed between the sheeting and its supporting members.
Read the conditions carefully, because they define when it bites:
| Situation | Thermal break under 13.2.3(7)? |
|---|---|
| Metal sheeting on metal purlins, no ceiling lining | Yes |
| Metal sheeting on metal purlins, lining fixed to the same members | Yes |
| Metal sheeting on timber battens | The clause is written for metal supporting members |
| Metal sheeting on metal battens with a separate ceiling below on its own framing | Check the clause against the actual build up, the conditions matter |
Clause 13.2.3(3) also deals with thermal bridging in metal framed roofs more broadly, including approaches such as raising the R-value of the insulation between the frames or running a continuous insulation layer.
The wall rule, clause 13.2.5(5)
The same idea appears in walls. Clause 13.2.5(5) requires a thermal break with an R-value of not less than 0.2 at all points of contact between the external cladding and the metal frame, where a wall forms part of the building envelope and has lightweight cladding fixed to a metal frame.
Note the phrase "all points of contact". It is not one strip somewhere convenient. It runs the length of every stud that the cladding is fixed to.
How it works on commercial jobs
On Class 2 to 9 buildings the compliance route runs through NCC Volume One Section J rather than the Housing Provisions, and the thermal bridging allowance is handled inside the Total R-value calculation. In practice the same product does the same job at the same junction, but the number it has to satisfy comes from the ESD or building services consultant rather than an energy assessor.
Two things follow for anyone supplying or installing:
- The consultant's calculation assumed a particular thermal break. Substituting it changes their number, not just the price.
- On commercial work the product usually has to carry a fire classification as well. A thermal break that meets R0.2 but has no Group number can be rejected.
More on how the two pathways differ is in commercial vs residential insulation specification.
What a thermal break product looks like
It is a thin, tough strip or roll designed to sit between the cladding and the frame and survive being screwed through. Trade Select ThermalBreak 7 is the one we stock for this: an extra heavy duty sarking that provides an R0.2 thermal break in situ, carries a Group 1 fire performance classification, complies with AS 3959-2018 for construction in bushfire prone areas so it is usable across all BALs, and also acts as an acoustic dampener. It suits timber frame and commercial steel frame construction.
Because it is a sarking as well as a thermal break, on many jobs it is doing two things at once, which is part of why it is a cheap way to satisfy the clause.
Getting it right on site
- It goes between the sheeting and the support, not somewhere near it. The point is to break metal to metal contact at the fixing line.
- Run it the full length. A break that stops short leaves a bridge at the ends.
- Do not confuse it with the bulk layer. The blanket between the purlins and the thermal break at the purlin are two different items, and one does not replace the other.
- Check the R-value on the product data, not the thickness. Thin products can be rated R0.2 in situ; thickness alone tells you nothing.
- Keep the paperwork. The certifier compares the built work against the specification, and a thermal break is easy to check and easy to prove missing.
Steel framed roof or wall on your job? Ask us for the thermal break at the same time as the blanket. It is a small line item that stops a compliance problem at inspection.
Browse building membranes and commercial insulation, or send us the detail.
Frequently asked questions
What is a thermal break in construction?
A layer of insulating material placed where two conductive elements meet, so heat cannot pass straight through the junction. In a metal roof it sits between the sheeting and the purlin, rafter or batten.
When is a thermal break required under the NCC?
NCC 2022 Housing Provisions clause 13.2.3(7) requires one where metal sheet roofing is fixed directly to metal purlins, rafters or battens and there is either no ceiling lining or a lining fixed to those same members. Clause 13.2.5(5) applies the same requirement to metal framed walls with lightweight cladding.
What R-value does a thermal break need?
Not less than R0.2 for both the roof and wall clauses above.
Why is R0.2 so low compared with a blanket?
Because it is not there to insulate the roof. It is there to interrupt the metal to metal contact at the fixing line. The bulk R-value comes from the blanket or batts.
Do timber framed roofs need a thermal break?
The clause is written for metal sheeting fixed to metal supporting members. Timber conducts heat far less readily than steel, which is why the requirement targets metal framing.
Can sarking be used as a thermal break?
Only where the product is rated to provide R0.2 in situ and the specification allows it. Some heavy duty sarkings are rated for exactly this, so one product covers both roles. A standard foil is not automatically a thermal break.
Can a thermal break be added after the roof is on?
Not without lifting the sheets, because it sits between the sheeting and the frame. That is why it belongs on the first order rather than the variation.
Does the thermal break need a fire rating?
On commercial work the specification often names one, such as a Group number or suitability for a bushfire attack level. Meeting R0.2 is not enough on its own if the project asks for a fire classification as well.
Sources
- NCC 2022 Housing Provisions, clauses 13.2.3(3), 13.2.3(7) and 13.2.5(5)
- NCC 2022 Volume One, Part J4 Building fabric
- AS/NZS 4859.2, Thermal insulation materials for buildings, calculation of Total R-value
- AS 3959-2018, Construction of buildings in bushfire-prone areas
- Product data from the manufacturers of the products listed above
General information for trade. Clause numbers refer to NCC 2022. Check the edition adopted in your state and confirm the requirement for your build up with the certifier or consultant on the project.