FAQ · R-value & climate
My architect said reflective foil insulation works better — does it?
Only with a clean 25mm air gap on at least one side. Once that gap is dusty or compressed — which happens within a few years under tiles — the Australian Government says its value diminishes towards zero.
It’s a question I get from homeowners who have spoken to their architect or builder and come away thinking foil is the smart option. Here’s what’s actually going on: foil is a radiant barrier, not a bulk insulator, and in South East Queensland you are dealing with both radiated and conducted heat. The Australian Government Your Home guide on insulation is clear on what happens to foil’s performance without that gap. For the ceiling, there is no substitute for pumped-in cellulose ceiling insulation. What foil can and can’t do on top of that — that’s what this page covers. There is more detail on the broader category at sarking, anticon and reflective foil.
The 25mm air gap condition
Foil reflects radiant heat. Without a gap, there is nothing to reflect into.
Reflective foil insulation works the same way a mirror works: it bounces radiant energy back in the direction it came from. For that to happen, the foil surface has to be facing an air space. The Australian Government Your Home guide on insulation states that foil’s insulating value diminishes towards zero if it becomes dusty or if it is in contact with a surface. That is not a small caveat — that is the entire operating condition for the product.
Under tiles, the gap rarely stays clean. Dust settles from above, condensation cycles leave residue, and tradies cutting through the sarking for electrical or solar work compress it. Within a few years, the foil under most tile roofs is sitting against the tile battens or the underside of the tiles themselves and doing very little. At that point it has the R-value of the physical material — a fraction of R0.2 — not the radiant performance it was sold on.
Anticon is different — but still not the ceiling insulation
In SE-QLD, the problem is conducted AND radiated heat. Foil addresses only one of them.
Anticon blanket under a metal roof is a different product from plain reflective foil sarking. It is a bulk-plus-foil hybrid: a fibreglass or polyester blanket with a foil face bonded to one side. The bulk component gives it a modest R-value that does not depend on an air gap. Correctly installed under iron, with proper ceiling insulation on top, anticon can contribute. On its own, it is not enough.
Here’s the part that gets left out of the architect’s spec. Radiant heat is the energy that comes off a hot surface like a heater element — you can feel it from across the room without touching anything. That is what foil reflects. Conducted heat is energy moving through a solid object by molecular contact — through the iron, through the roof timbers, through the ceiling plaster, into your room. Foil does nothing to stop conducted heat. In South East Queensland’s climate, a significant proportion of what is heating your ceiling is conduction through the roof structure itself. You need bulk insulation to address that.
Of the thousands of roofs I have been in over nearly 40 years, the ones with anticon blanket and no ceiling insulation are still hot. Some are hotter than roofs with neither, because the blanket closes off the air gap at the eaves that used to allow some convective cooling of the roof space. The right sequence is: pumped-in cellulose in the ceiling first, always. Foil or anticon under the iron is an add-on for specific situations — not a substitute, and certainly not a first choice over the ceiling.
For more on what how the R-value on the label compares to the R-value in your roof — for any product — that page explains the gap problem, the joist cold-bridge problem, and why installed performance is what matters, not what is printed on the bag.
“Sarking is not an insulator of any quality in my opinion. As far as stopping heat goes by reflecting radiation, unless you need sunscreen when you are inside your home, sarking is not reflecting any radiation. And the ones with anticon — of the thousands of roofs I have been in, those are actually hotter than the ones without it. The blanket blocks the air gap at the eaves.”
The difference with cellulose
Cellulose has no condition. R3.0 in year one is still R3.0 in year twenty.
Cellulose is a bulk insulator. It traps still air within millions of tiny fibres and slows heat moving by conduction and convection. It does not need an air gap. It does not need a clean surface. Dust, age, and the weight of the product itself do not change how it performs. What you order is what you get — no conditions attached.
Because we pump cellulose in, it fills around every joist, pipe penetration, and light fitting without gaps. The ICANZ 2024 ceiling insulation guidelines found that gaps of around 6% of the ceiling area can roughly halve the effective R-value of a batt install. With pumped-in cellulose there are no cut-to-fit gaps, so the rated R-value is the actual R-value. See how cellulose vs fibreglass batts compares across gaps, joists, vermin resistance, and long-term performance.
Peter’s recommendation is always the same: cellulose in the ceiling first and always. If you are building with iron and the spec includes anticon, have it installed correctly with the air gap maintained — it can contribute as a secondary layer. Foil sarking under tiles is not something I recommend spending money on. The ceiling is where the work gets done.
See what actually happens under a metal roof — and what anticon does and doesn’t do.
The featured video shows what the foil in anticon blanket actually does under a metal roof, and why ceiling insulation has to go on top. The second clip shows how we pump cellulose into a steel-frame roof with no manhole access. The third is our raked-roof tool for pumping under iron where there is almost no clearance.
Read the transcript
Okay, so we're just up on this roof lift here, lifting some sheets — we're going to pump it with cellulose. We've just lifted up, and it's all insulated with polyester batts underneath the Anticon blanket, so there's not much we can do there with pump-in insulation. But they've actually missed this one little bay on the end here, so we'll pump this bay.
Read the transcript
You can see I've got about half the flat pipe left underneath there. I've been pulling it back, and you can see how it comes back out onto the top of the old stuff as I pull it out. I can slide it across sideways because it's full — see how that's pushing it back out from underneath there now? That's how I know that cavity's all filled up. That's the difference between a good contractor and one who just does a bit of a dirty job. Okay, in the next bay — I'm going to blow a bit of air through first, just to make sure it's all clear, and then pump it. Feed the hose in a bit further so it gets all the way up there, and you should see some little wisps of dust coming out up on the eaves. So that's how we do it — filling it up to that height, as opposed to over here where he's only put in 30 or 40 millimetres. On a flat roof like this where you can put more in, there's no reason why the contractor shouldn't have, except that he's just not paid well enough to. So that's again the difference between a cheap job and a good job.
Read the transcript
Okay, we're just on a cathedral roof here — sorry about the chainsaw in the background — but we've got our flat pipe here. We just lifted that sheet there; you can see it's got a bit of cellulose on it still. We've pumped back this way, the full length of the pipe, so we don't have to lift any of those sheets. Now we've just lifted this sheet here, which is another two or three sheets past the end of that pipe where it got to. I can lift this up and have a look.
The clips play right here on the page, or open the playlist to watch them all on YouTube and subscribe.
Some of these were filmed a while back. Our methods, safety standards and products have moved on since. For how we work today, see the rest of this page.
More questions about foil, anticon and sarking
Does reflective foil insulation actually work?+
Reflective foil works by reflecting radiant heat — but only if it has a clean, unobstructed 25mm air gap on at least one side to reflect into. The Australian Government Your Home guide says foil insulation's value diminishes towards zero once it becomes dusty or comes into contact with a surface. In a typical Queensland roof, that degradation often happens within a few years. Foil under tiles in particular gets compressed and dusty quickly. Anticon under iron is a different product — it is a bulk-plus-foil hybrid blanket, and it can contribute meaningfully when ceiling insulation is also installed on top.
What is the difference between sarking, anticon and reflective foil?+
Sarking is a membrane laid under roof tiles or iron during construction. Its main purpose is to catch condensation and stray water — it is not a meaningful insulator. Anticon is a thicker blanket with a foil face used under iron roofs; it has a small bulk component plus the reflective layer, so it does something when combined with proper ceiling insulation. Reflective foil (or reflective foil batts) is a pure radiant barrier — no bulk, no R-value unless that 25mm air gap is maintained and clean. The three are often grouped together by architects and builders, but they behave very differently.
If my architect specified foil, can I just leave the ceiling uninsulated?+
No. Even when foil performs at its best, it only addresses radiant heat — the heat that radiates from a hot roof surface like a heater element. In South East Queensland you are dealing with both radiated and conducted heat coming through the roof structure. Conducted heat passes right through foil. The cellulose in the ceiling is the product that stops both. Foil under the roof is an add-on, not a substitute, and without ceiling insulation you are leaving the conducted heat problem completely unaddressed.
When does anticon under an iron roof actually make sense?+
Anticon under an iron roof makes sense when it is installed correctly during construction with the air gap maintained, and when bulk ceiling insulation is also installed. It can help reduce noise from rain on the iron and provides a small additional R-value contribution. What it does not do is replace the ceiling insulation. Of the thousands of roofs I have been in, the ones with anticon and no ceiling insulation are still hot — sometimes hotter than roofs with neither, because the blanket closes off the air gap at the eaves that used to let some heat out.
Does cellulose lose its R-value over time the same way foil does?+
No. Cellulose delivers its rated R-value regardless of dust, age, or whether it is sitting in contact with a surface — because it is a bulk insulator, not a radiant one. It does not depend on an air gap. A cellulose ceiling pumped to R3.0 in year one is still performing at R3.0 in year twenty, assuming it was installed at the right depth and the roof has no ongoing water leak. That is why we give it a life-of-house guarantee. Foil degrades because conditions change. Cellulose does not have that condition.
Why would an architect specify foil if it has these limitations?+
Architects are not always insulation specialists. The spec often comes from a standard detail that was accurate at the time it was written, or from a relationship the builder has with a particular supplier. Foil-sarking products are cheap to supply and easy for a builder to tick off during construction. The homeowner rarely knows to ask about the air gap condition, and by the time the roof is on and the house is hot, the architect is long gone. I am not saying architects are dishonest — most are just specifying what the product category is supposed to do, not what it actually does in a decade-old roof in South East Queensland.
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