A metal shed gets hot because there is nothing between the steel and the air inside it. The sheet absorbs solar radiation, heats up, and radiates that heat straight down into the space. Insulation under the sheeting fixes it, and the same layer cuts the rain noise and stops the underside of the roof sweating on cold mornings. The catch is that it has to go in while the sheets are off.
In short
- A bare metal roof is a radiator. There is no thermal mass and no cavity to slow it down.
- Blanket under the sheeting is the standard fix, 55 mm or 75 mm for most sheds.
- The same layer handles heat, rain noise and condensation.
- Retrofitting means lifting sheets, so do it during a build or a reroof.
Anyone who has opened a shed door at three in the afternoon in February knows the problem. What is less obvious is that the same building is uncomfortable in winter for a related reason, and that both come down to one missing layer.
Why sheds get so hot
Three things stack up.
The roof absorbs solar radiation. Steel heats fast, and a dark roof absorbs more than a light one. In climate zones 1 to 5, NCC 2022 Housing Provisions clause 13.2.3(10) limits the solar absorptance of the upper surface of a roof to no more than 0.64 on buildings the code covers, which tells you how much the colour matters.
The hot sheet radiates downward. Once the steel is hot it re-radiates into the space below. You feel that as heat coming off the ceiling even when the air is moving.
There is nothing to slow it down. A house has a roof cavity, a ceiling and insulation. A shed usually has a sheet and then air. No thermal mass, no cavity, no delay.
Insulation under the sheeting attacks all three at once. Bulk insulation slows the conducted heat, and the foil facing reflects radiant heat back rather than letting it into the room.
The other problem, condensation
The same shed that cooks in summer often drips in winter. The mechanism is straightforward: the roof radiates heat to the night sky and ends up colder than the air under it, and any moisture in that air condenses on the cold steel.
In a shed the moisture comes from the ground slab, from wet gear, from a vehicle brought in after rain, and from anything with a wet process. BlueScope's technical bulletin on roof condensation describes the cycle, and it is the reason a shed can drip on a clear morning with no rain for days.
The blanket helps here too, because the foil facing blocks vapour from reaching the underside of the sheet and the bulk layer keeps that facing warmer than the steel. Full detail is in condensation in metal roofs.
What to put in
| Option | What you get | Best for |
|---|---|---|
| Roofing blanket, foil faced | Real R-value plus radiant control, vapour barrier and noise reduction | Any shed you actually spend time in |
| Reflective sarking only | Radiant heat and vapour control, very little R-value | Carports, storage sheds, tight budgets |
| Blanket plus a lining | All of the above, plus a finished interior and better acoustics | Workshops, studios, home gyms, anything heated or cooled |
For most sheds the answer is a foil faced blanket. It is one product, it goes on with the sheets, and it does the thermal, acoustic and vapour job in a single pass.
How thick
Sheds are usually done in 55 mm at R1.3 or 75 mm at R1.8. That is enough to take the edge off the radiant heat and the rain noise, which is what most people actually want.
Go thicker, to 100 mm at R2.3 or beyond, when the shed is being lined and used as a workshop, studio or gym, or when it is going to be heated or cooled. At that point you are conditioning the space and every step of R-value earns its keep.
One constraint. The blanket has to sit at full thickness. If the sheeting is fixed straight down onto it, it compresses and you lose the R-value you paid for. On a steel framed shed that means either battens or an insulation spacer to hold the sheet clear. The Roof Razor Insulation Spacer does that job in 60, 85 and 115 mm depths.
If the shed is a class of building the code covers and the sheeting is fixed straight to steel purlins, clause 13.2.3(7) also calls for a thermal break of at least R0.2 between the sheeting and the purlin. Worth checking with your builder or certifier rather than assuming a shed is exempt.
Walls, doors and the floor
The roof is where most of the heat comes in, so it is the first and biggest win. After that:
- Walls. Blanket products such as EcoRoof are rated for metal wall applications as well, installed behind a lining. West facing walls are the ones that matter in the afternoon.
- Roller doors. A large uninsulated steel door undoes some of the roof work. Insulating or shading it helps more than people expect.
- Ventilation. Hot air needs somewhere to go. A vent high up with an inlet low down moves far more air than a single extractor with nothing feeding it.
- Slab. A concrete slab is a moisture source. It is also thermal mass, which helps in some climates and not in others.
Insulating a shed that is already up
Be straight about this: the good options need the sheets off.
If the shed is being reroofed, that is the moment. Blanket goes on with the new sheets and costs very little extra on top of a job you are already doing.
If the roof is staying, the realistic options are lining the underside of the frame from inside, which is fiddly and never as good, or adding ventilation and shading to reduce the load. Neither matches doing it properly with the sheets off, so if a reroof is anywhere on the horizon, wait for it.
Working out how much
Measure on the roof plane, not the floor. A pitched shed roof has more area than its footprint: multiply the plan area by about 1.02 at 10 degrees, 1.08 at 22.5 degrees and 1.16 at 30 degrees. Then divide by the roll coverage and add about 10 per cent for laps and trimming.
Example. A 12 m x 7 m shed, 10 degree pitch. Plan area 84 m², times 1.02 is about 86 m², plus 10 per cent is 95 m². In 75 mm blanket at 18 m² per roll, that is 6 rolls.
The full method for both packs and rolls is in how much insulation do I need.
Building or reroofing a shed? Send us the size, the pitch and whether it will be lined, and we will tell you what thickness suits and how many rolls it takes.
Get in touch or browse roofing blankets.
Frequently asked questions
How do you insulate a metal shed roof?
Lay a foil faced roofing blanket over the battens or purlins, foil side down towards the inside, then fix the sheeting over it with battens or spacers holding the blanket at full thickness.
What R-value insulation does a shed need?
Most sheds are done in 55 mm at R1.3 or 75 mm at R1.8. Go to 100 mm at R2.3 or higher if the shed is lined and used as a workshop or is heated and cooled.
Can I insulate a shed without removing the roof sheets?
Not properly. Blanket belongs between the sheeting and the frame. From inside you can line the underside of the frame, but it is fiddly and performs worse. If a reroof is coming, do it then.
Will insulation stop my shed roof dripping?
It reduces it a lot. The foil facing keeps vapour away from the cold steel and the bulk layer keeps that facing warmer. Ventilation still matters, because insulation does not remove moisture from the building.
Does shed insulation reduce rain noise?
Yes. Blanket under the sheeting absorbs impact noise, and it also removes the friction between sheet and batten that causes the popping as a metal roof heats and cools.
Does a lighter roof colour help?
It does. A lighter roof absorbs less solar radiation, so the sheet runs cooler. It is not a substitute for insulation, but the two work together.
How much blanket do I need for a 12 by 7 shed?
About 95 m² allowing for pitch and waste, which is 6 rolls of 75 mm blanket at 18 m² per roll. Measure your own roof plane rather than relying on the example.
Do I need a thermal break in a shed?
If the building is one the code covers and the metal sheeting is fixed directly to metal purlins, clause 13.2.3(7) calls for a thermal break of at least R0.2. Check it with your builder or certifier rather than assuming a shed is outside the rules.
Sources
- 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
- NCC 2022 Housing Provisions, clauses 13.2.3(7) and 13.2.3(10)
- Product data from the manufacturers of the products listed above
General information for owner builders and trade. Whether a particular shed is covered by the energy efficiency provisions depends on its classification and use, so confirm with your builder or certifier.