FAQ · The #1 technical question I get
Why isn’t an R5 batt an R5 in my roof?
Because an R-rating only counts with no gaps, and batts always have gaps. The R5 on the packet is the lab number for a perfectly-fitted batt, not the result in your gappy roof. Pumped cellulose fills every inch, so what you pay for is what you get.
This is the single most important thing to understand before you buy batts, and it’s the question I answer more than any other. People assume R3.0 in one product equals R3.0 in another. It doesn’t. The rating is what the product does in a lab; the result is what ends up in your roof, and that depends entirely on the gaps.
The rating is a lab number. The result is what’s in your roof.
The R-value printed on a batt is measured on a single sample, fitted perfectly flat with no gaps, in a lab. That’s a fair test of the material, but it’s not what happens in your ceiling. Your roof is full of odd-sized bays, downlights, pipes, cornices, hips and tight hot corners, and a batt has to be cut to fit every one of them. Every cut and every join is a chance for a gap, and gaps are where the heat sneaks through. So the R5 on the packet is the best-case number for a perfect install, not the number you actually live with.
Here’s the part that surprises people: gaps don’t just cost you the bit that’s missing. A 5% gap doesn’t cost you 5%. Heat takes the easy path, so even a small gap short-circuits the insulation around it and drags the whole ceiling’s effective R-value down. The Australian Government’s yourhome guide puts it plainly: even a small gap can greatly reduce the insulating value, and tells you to fit batts snugly with no gaps around ducts and pipes.
How big is the loss?
1% gaps, about 30% gone. A 5% gap drops an R3.5 batt to R2.1.
Choice Magazine’s rule of thumb is that leaving just 1% gaps in your insulation can reduce its performance by around 30%. That sounds extreme until you understand that heat behaves like water finding a crack. It pours through the gap and ignores the insulation either side of it. The mechanism is documented by the Australian Government, not just by us: Sustainability Victoria’s Energy Smart Housing Manual shows the effective R-value collapsing as more of the ceiling is left uninsulated.
The single starkest number in that government manual is about gaps: leaving gaps of just 5% of the ceiling area drops an R3.5 batt to an effective R2.1, about a 40% loss. Think about how many downlights, cornices and pipes are in a modern house, and how easy it is for a rushed installer to leave a fist-sized gap around each one. That R6 you paid for can quietly be performing like an R3.5, and you’d never know, because once the manhole’s shut you can’t see it. (Worth being precise here: the 1% figure is a Choice / building-science rule of thumb, not a number out of the NCC. The government source for the 5% gap figure is the Sustainability Victoria Energy Smart Housing Manual, p.63.)
An R-rating only counts if there are no gaps, and batts always have gaps. So an R5 batt is not really an R5 in your roof. Pumped cellulose has no gaps, so what you pay for is what you get.
The fix
Pump-in cellulose fills every gap a batt leaves.
This is the whole reason I switched to cellulose. Instead of cutting slabs to fit, our pump-in ceiling insulation goes in as one seamless blanket, so it flows into every bay, every corner, around every pipe and downlight, and right out to the eaves. There’s no seam to leave a gap, because there are no seams. Full contact across every inch of the ceiling means the R-value you bought is the R-value working in your roof, not a best-case lab number you never actually receive.
And there’s only one way to short you on cellulose, pump less in, and it’s measurable, because the depth shows across the whole ceiling. With batts there are a dozen ways to leave gaps and short the R-value, and they rarely get caught once the manhole’s shut. That’s why every job is photographed to the same system and the photos are checked before you’re invoiced, so you can see the finished, edge-to-edge coverage you paid for, including the spots you can never climb up to check yourself. Every job is run by a Comfort Zone franchise owner-operator, trained to one standard and held to a quality standard.
The honest part
You only insulate once, so you can’t compare.
Here’s what makes this decision different from most. You only insulate your roof once. You’ll never have the same house with cellulose in it one year and fibreglass the next, so you can’t stand in your living room and feel the difference. You just pick, and live with it for the life of the house. That’s exactly why the gaps argument matters so much. You can’t go back and fix a batt job full of gaps once it’s in and the ceiling’s shut.
So my advice is simple: get the product that hits its number without depending on a perfect day from a perfect installer in a hot, tight roof, and on who actually turns up to do it. Pumped cellulose fills every gap regardless, and we show you the finished depth in the photos. Whatever R-value is right for your roof, the question isn’t just “what does the packet say?”. It’s “what’s actually going to be in my ceiling once it’s done?” Get the R-value right for South-East Queensland, then make sure it’s installed with no gaps so you actually receive it.
More on gaps and effective R-value
Why isn't an R5 batt really an R5 once it's in my roof?+
Because an R-rating is the lab number for insulation fitted with no gaps, and batts always have gaps. The R5 printed on the packet is measured on a single, perfectly-fitted sample. Once that batt is cut into every odd-sized bay and squeezed into the tight corners of your roof, you get gaps along the seams from day one, and a few percent of gaps wrecks the effective R-value. Choice's rule of thumb is that even 1% gaps can cut performance by around 30%, and Sustainability Victoria's own government nomogram shows the effective R-value collapsing as more of the ceiling is left uninsulated. Pumped-in cellulose lays as one seamless blanket with no gaps, so what you pay for is what's actually in your roof.
How much do small gaps really reduce insulation performance?+
A lot more than people expect. Gaps don't average out, they short-circuit the whole ceiling. Choice Magazine's rule of thumb is that just 1% of gaps can reduce insulation performance by about 30%. The Australian government's guidance agrees with the mechanism: yourhome.gov.au says even a small gap can greatly reduce the insulating value, and Sustainability Victoria's Energy Smart Housing Manual nomogram shows the effective R-value falling steeply once even a few percent of the ceiling is left uninsulated. Their figures put leaving gaps of just 5% of the ceiling area at dropping an R3.5 batt to an effective R2.1, about a 40% reduction. So a batt roof full of little gaps around downlights, cornices and pipes never delivers the number on the bag.
If cellulose and batts are both R3, why pick cellulose?+
Because the rating only tells you half the story. An R3 batt and R3 of cellulose are the same in a lab, but in a real roof the batt has gaps and the pumped cellulose doesn't, so the cellulose delivers far closer to its rated number. Batts are cut to fit, so they leave gaps along every seam and get short-changed in the tight, hot corners where installers take shortcuts. Cellulose is pumped in as one continuous blanket with full contact across every inch of the ceiling, including the eaves and the awkward spots you can never climb up to check. Same R-rating on paper, very different result in your roof. After installing insulation since 1986 it's the only product I'd use in my own home.
I can't compare the two side by side, so how do I decide?+
That's the honest catch: you only insulate your roof once. You'll never have the same house with cellulose one year and batts the next, so you can't stand in your living room and feel the difference between them. You just have to pick, and live with it for the life of the house. That's exactly why the gaps argument matters. You can't go back and fix a batt job full of gaps once it's in. So get the product that doesn't depend on a perfect day from a perfect installer to hit its number. Pumped cellulose fills every gap regardless, every job is photographed to the same system, and the photos are checked before you're invoiced so you can see the finished coverage you paid for.
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