08/11/2026
Yep, we’re picking a fight with every builder, architect and energy rater still putting foil under the roof, because it’s one of the biggest cons still being sold in Australian housing.
The foil itself has next to no insulating value. Any R value you see on the packaging depends on a perfectly clean, perfectly still air gap sitting right next to it, installed exactly as tested. That almost never exists on a real roof. The gap gets squashed, the foil sags, dust settles on it within a few years and the reflective surface that was meant to do all the work stops working.
Then there’s the moisture. Most foil is vapour closed, so it can’t breathe, and any moisture that gets into the roof has nowhere to go. It sits there, condensation builds up underneath, and the timber and insulation that are meant to last the life of the house start breaking down instead. Even the so called breathable foils give the game away, because the only way they let vapour out is by punching holes in them, and at least one of the big brands is officially classed as a non water barrier.
It isn’t built to handle the sun either. UV testing is optional under the Australian standard. And while we’re at it, the foil faced blanket under the roof is no better. It’s thin, it gets squashed flat over every batten and purlin, and it’s still got the same foil face trapping moisture against the roof. People swear it helps with sound, but a properly airtight house with thick insulation will do far more to keep the noise out than a thin blanket ever will.
We don’t use either, and we never will. A roof should be properly insulated, properly wrapped in a membrane that keeps the water out while letting moisture escape, and built so it can dry if something ever goes wrong.
So if you’re building and foil or blanket shows up in your spec, question it. Ask your builder what R value it’s actually delivering in your roof, how it handles condensation and how long it’ll keep working. If they can’t give you a straight answer, you’ve got yours.
A huge thanks to everyone who sent through photos of their foil sarking failing. You’ve made the case better than we ever could, so keep them coming.