Colorbond steel is a coated steel product, not an insulation system. On its own it does nothing for heat, condensation or noise. What does that work is the layer underneath: a roofing blanket or a reflective membrane over the battens, a thermal break where the sheeting meets a steel frame, and a spacer to hold the blanket at full thickness.
In short
- The steel is the skin. The insulation is a separate purchase and a separate line on the quote.
- Blanket goes under the sheeting. It has to go on before the roof is closed.
- Metal sheeting on a metal frame usually needs an R0.2 thermal break under NCC 2022.
- Colour changes how much heat the roof takes on, but it is not insulation.
- Condensation is a roof problem, not a plumbing problem. Plan for it up front.
People search for Colorbond roof insulation expecting to find a product called that. There is not one. Colorbond is the roof. The insulation is whatever you put under it, and that decision is usually made in the same week as the roof quote, often without much thought, and it is very hard to change afterwards.
This guide covers what belongs under steel sheeting, what the code requires, and what the colour does and does not do. For the full system across commercial and residential, see our hub guide on metal roof insulation in Australia.
What Colorbond steel actually is
Colorbond steel is BlueScope's prepainted steel, on the market since the 1960s. The base steel carries a corrosion resistant metallic coating, then a pre-treatment, a primer and a baked-on topcoat. It is a durable, well engineered skin for a building.
What it is not is a thermal barrier. Steel conducts heat readily. A bare steel roof gets hot in the sun and cold at night, and it does that whichever colour you choose. Everything you want from a comfortable roof, less heat coming in, less heat escaping, no drips, less noise, comes from what sits beneath the sheets.
The one distinction that matters
- The roof keeps water out and lasts thirty years.
- The insulation controls heat, condensation and noise.
- They are quoted separately, installed at different moments, and one does not substitute for the other.
What goes under the sheeting
Working from the sheeting down, a typical steel roof carries some or all of these.
| Layer | What it does | When you need it |
|---|---|---|
| Thermal break | Interrupts the metal to metal path where sheeting meets a steel purlin or batten | Where NCC 2022 clause 13.2.3(7) applies, see below |
| Roofing blanket | The bulk insulation layer, laid over the purlins before the sheets go on | Nearly always on a metal roof |
| Reflective membrane or sarking | Radiant heat and a drainage plane for condensation | Where a separate membrane is specified instead of a faced blanket |
| Spacer or batten system | Holds the blanket at full recovered thickness instead of letting it squash | Whenever the blanket would otherwise be pinched at the purlin |
| Safety mesh | Fall protection during and after installation | Commercial work, and residential where the risk assessment calls for it |
| Ceiling batts | The second thermal layer, at ceiling level rather than roof level | Where there is a ceiling and a roof space between |
The order is not negotiable and neither is the timing. Blanket, membrane, spacers and mesh all sit under the sheeting, so they go on while the roof is open. Once the sheets are screwed down, adding any of them means lifting sheets.
The thermal break rule
This is the requirement most often missed on a steel roof, and it is the easiest for a certifier to spot.
NCC 2022 Housing Provisions clause 13.2.3(7) requires that 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 is installed between the sheeting and its supports.
| Your roof | Thermal break under 13.2.3(7)? |
|---|---|
| Steel sheeting on steel purlins, no ceiling lining | Yes |
| Steel sheeting on steel purlins, lining fixed to those same members | Yes |
| Steel sheeting on timber battens | The clause is written for metal supporting members |
| Steel sheeting on steel, ceiling lining on separate members with a roof space between | Read the clause against your build up, the conditions matter |
The same idea shows up in walls at clause 13.2.5(5), which calls for an R0.2 thermal break at all points of contact between lightweight cladding and a metal frame in the building envelope. Clause 13.2.3(3) sets out broader ways to handle thermal bridging in metal framed roofs.
R0.2 sounds trivially small next to a blanket at R1.8. It is meant to be. The thermal break is not insulating the roof, it is breaking one conductive path. The blanket between the purlins and the thermal break at the purlin are two different items and one does not replace the other. Our separate guide covers the R0.2 requirement in plain English.
Does the colour do anything?
Yes, but less than people hope, and in a specific way.
Colour changes solar absorptance, which is how much of the sun's energy the surface takes on rather than reflects. A light roof runs cooler than a dark one in the same sun. BlueScope builds solar reflectance technology into the range: 21 of the core Classic Colorbond colours carry Thermatech technology, and the Coolmax product is positioned as the highest solar reflectance prepainted steel in the range.
The code puts a number 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. That rules out the darkest colours in the hotter zones, on the buildings the code covers.
What colour does not do is stop conducted heat, stop condensation forming on the underside, or quieten the rain. A pale roof with no insulation is still an uninsulated roof. Treat colour as a useful adjustment on top of the insulation, not a substitute for it.
Condensation under a steel roof
A steel roof radiates heat to a clear night sky and can fall below the temperature of the air beneath it. Moist air touching that cold underside gives up its water. That is the drip that shows up in winter with no rain in sight, and it is the reason wet insulation and rusting screws turn up on roofs that never leaked.
The layer under the sheeting is what manages it. BlueScope's guidance in Corrosion Technical Bulletin CTB-11 is that where a separate reflective membrane is used, it should be draped between the battens to leave at least 20 mm of air space above and below it, and it should be a membrane with a vapour barrier classification to AS/NZS 4200.1:2017. Installation is covered by AS 4200.2:2017.
Two things follow. The membrane needs to be the right classification, not just any foil. And it has to be installed with the drape, because a membrane pulled tight against the underside of the sheeting cannot drain and cannot ventilate. Our full treatment is in condensation in metal roofs.
Rain noise
Steel roofs are loud in heavy rain, and this is one of the most common reasons homeowners ring after the fact. A bulk layer directly under the sheeting is what changes it, because mass and fibre absorb the drumming that a hard reflective surface alone will not. This is another argument for the blanket rather than a thin foil on its own, and another reason the decision has to be made before the sheets go down.
Choosing the thickness
Two questions settle it.
What is the target? On a house or a commercial building the R-value comes from the energy report or the specification, not from preference. On a shed or a workshop with no formal target, you are buying comfort, and the choice is a trade off against cost and the depth available.
What will actually fit at full thickness? This is where roofs go wrong. A blanket only delivers its rated R-value at its recovered thickness. Squash it at the purlin and you have paid for insulation you are not getting. That is what the spacer is for. Our guide to roofing blanket R-values works through the usual thicknesses.
What goes wrong
- Assuming the roofer's quote includes it. Sometimes it does, often it covers a thin sarking only. Read the line items and ask what R-value is being supplied.
- No thermal break on a steel framed roof. Clause 13.2.3(7) is not optional where it applies, and it cannot be fixed later without lifting sheets.
- Blanket crushed at the purlin. Rated R-value assumes full recovered thickness. Use a spacer.
- A membrane pulled tight to the sheeting. No drape means no drainage and no ventilation.
- Choosing a foil when the problem is noise. A thin reflective layer will not quieten a steel roof. Bulk will.
- Relying on colour. A pale roof helps with solar gain and does nothing else.
We supply the package, your roofer lays the blanket
AWBM supplies roofing blanket, thermal break, reflective membrane, spacers and safety mesh across Australia. We do not lay blanket, it goes down as the sheets go on and that is your roofer's work. What we install is batts, wall wrap and sarking, so the ceiling and the walls of the same house we can take on from start to finish.
Or browse roofing blankets, building membrane and accessories.
Frequently asked questions
What insulation goes under a Colorbond roof?
A roofing blanket laid over the purlins before the sheets go on, held at full thickness by a spacer or batten system. Add a thermal break where the sheeting fixes directly to a metal frame, and a reflective membrane where the specification calls for one instead of a faced blanket.
Does Colorbond steel come with insulation?
No. Colorbond is the prepainted steel sheeting. Insulation is a separate product, quoted and installed separately, and it goes on before the roof is closed.
Can I add insulation under an existing Colorbond roof?
Not under the sheeting without lifting sheets, because that layer is fixed under the roof. Where there is a roof space and a ceiling, you can still insulate at ceiling level, which is far less disruptive.
Do I need a thermal break under a Colorbond roof?
You do where 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.
Does a light Colorbond colour keep the house cooler?
It reduces how much solar heat the roof takes on, which helps. In climate zones 1 to 5, NCC 2022 clause 13.2.3(10) limits roof solar absorptance to no more than 0.64, which rules out the darkest colours. Colour is an adjustment, not a replacement for insulation.
Why does my Colorbond roof drip when it has not rained?
That is condensation. The steel radiates heat to the night sky, drops below the temperature of the air underneath, and moisture in that air condenses on the cold underside. It is managed with the right membrane, installed with a drape and an air gap, not with more sealant.
Will insulation make a steel roof quieter in rain?
A bulk layer under the sheeting will make a noticeable difference. A thin reflective foil on its own will not, because the noise is absorbed by mass and fibre rather than reflected.
What R-value should I use under a steel roof?
Take it from the energy report or the specification if the job has one. Where there is no formal target, choose against the depth available and check the blanket can sit at full recovered thickness, because a crushed blanket does not deliver its rated R-value.
What does AWBM supply and install for a steel roof?
We supply the whole roof build up Australia wide: blanket, thermal break, membrane, spacers and safety mesh. Blanket is supply only, because it goes on with the sheeting and the roofing crew lays it. Batts, wall wrap and sarking we supply and install. Send the roof area and the purlin centres and you will get a price.
Sources
- NCC 2022 Housing Provisions, clauses 13.2.3(3), 13.2.3(7), 13.2.3(10) and 13.2.5(5)
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Australian Steel Institute, Building with Steel Factsheet #3, Blanket and Foil insulation in Metal Roofing
- AS/NZS 4200.1:2017 and AS 4200.2:2017, Pliable building membranes and underlays
- COLORBOND steel, sustainability and thermal efficiency, BlueScope, for the coating system and Thermatech solar reflectance
- Product data from the manufacturers of the products listed above
General information. COLORBOND, Thermatech, Coolmax and Activate are registered trade marks of BlueScope Steel Limited. AWBM is not affiliated with BlueScope and does not supply Colorbond steel. We supply the insulation that goes under it. Always check the current NCC provisions and your project specification, since requirements vary by building class, climate zone and jurisdiction.