Do gaps matter? · the effective-R reality
Batts are rated in a factory. They get de-rated in your roof.
The number on the bag is a lab figure for a perfectly fitted product. By the recognised assessment classes, about a 6% gap roughly halves a ceiling’s effective R-value, so a rated R5 batt then performs like about R2.4. That’s why an R5 batt isn’t an R5 in your roof.
I’m Peter Johnson. After 40 years and 6,000-plus roofs, here’s the straight story on the one thing that decides whether your insulation actually works: coverage. Where batts gap, why steel frames make it worse, and why a seamless blown-cellulose blanket fixes it. With the Australian sources, not sales spin.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
This is what a gap looks like
pink batts with open seams and a bare downlight box, Tallai
Real reviews, real jobs
What our customers say
Genuine Google & hipages reviews from Comfort Zone customers across SE Queensland.
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Angela M.
SE Queensland
“The fact that I can't even tell it's 6 degrees outside when I wake up in the morning speaks for itself. Have wasted so much money attempting to heat and cool an uninsulated home. Worth every $.”
- Phipages
P Peter
Alstonvale, 2024
“Connected with Comfort Zone Insulation and would recommend them”
- J
Jessa B.
Brisbane
“It dropped about 4 degrees straight away, and we added another 3 with the second job. I appreciate Peter's honesty, and the team showed pictures before and after.”
- Nhipages
Nola M
Birtinya, 2024
“They were courteous and competent.”
- I
Iain V-B.
Brisbane
“Quick and polite service. Great follow-up advice and photos sent for our records. Above and beyond what we expected. Would highly recommend.”
- Jhipages
Jennifer's E
Upper Caboolture, 2024
“Excellent customer service. Highly recommended. Has a profound knowledge of insulation products and has the best interest of his customer.”
- G
Gerry S
Fitzgibbon, 2023
“I used Comfort Zone and they have a done an excellent job.”
- Jhipages
Jennifer
Upper Caboolture, 2024
“Excellent customer service. Highly recommend. Has a profound knowledge of insulation products and has the customer best interest.”
- J
Jung K
Riverhills, 2023
“An experienced family operation. Highly recommend. Thank you for the great job!”
- Dhipages
Diane A
Ormeau, 2024
“Peter and crew did a great job I would definitely recommend them”
- D
David H
Sunshine Coast, 2021
“Completed the job as quoted and to a high standard. Great personal service. Would highly recommend Comfort Zone for ceiling installation work.”
- Thipages
Timea
Highland Park, 2023
“I was extremely satisfied with the service they provided. They gave a very thourough explanation of the materials used, the way the work will be carried out and the price I had to pay was the exact amount quoted, no hidden costs included. They arrived on time, well prepared and workwas carried out exactly how they said it would be, they were super efficient, well prepared and were kind enough to even clean up after themselves. The services they provided was second to none! I don't hesitate to recommend them for any insulation job!”
- B
Benjamin H
Carseldine, 2019
“Very good explanation about their works. Advice of existing problems with the roof. Clean work. Very professional.”
- Mhipages
Mark
Pottsville, 2017
“Michelle, we are done - Peter from comfort zone insulation was very helpful. very honest with his recommendations - in fact he told me that the product my daughter had if installed correctly was superb. Thanks Peter you are a champion and i would recommend you to any person that was wanting professional advice and old school service.”
- I
Ian G
Burnside, 2019
“Good information, communication and professionalism.”
- Jhipages
Jessica
Pottsville, 2016
“This business offers a fantastic product that other businesses did not. Pump in ceiling insulation. Knowledge of the industry second to none.”
- D
Danny D
Boondall, 2018
“He explained everything he was going to do and the different types of insulation they used. He talked through the different options but made a recommendation for the one most people use, which is the one I chose. He was very understanding towards what I needed and not about himself.”
- Jhipages
Jack
Pottsville, 2023
“Excellent communication and informative. Professional.”
- G
Graham R
Riverhills, 2018
“Comfort Zone. Turned up ahead of time, completed in about 2 hours, cleaned up. All good. Very motivated installation team.”
- Thipages
Tony P
Redland Bay, 2023
“Very knowledgeable about insulation”
- A
Alex B
West Ipswich, 2018
“Fast, friendly, efficient.”
- Shipages
Steve
Redland Bay, 2017
“Excellent job and reasonable price.”
- L
Luke D
Mcdowall, 2017
“Peter did a good job. It was a quick and clean service. I'm happy to recommend!”
- Bhipages
Bruce H
Kuluin, 2023
“Prompt and efficient quoting.”
- B
Brendon
Brays Creek, 2016
“Peter supplied and installed roof insulation for me. He was very informative and provided good advice.”
- Ghipages
Gerry S
Fitzgibbon, 2023
“I used Comfort Zone and they have a done an excellent job.”
- T
Trevor G
Brookside Centre, 2016
“Excellent tradesmen from Comfort Zone Insulation. They were punctual and cleaned up after. Highly recommended.”
- Thipages
Tamara
Underwood, 2023
“Peter is honest, hard-working and came on time. Knew excally what he was talking about and answered my questions. Would 100% recommend”
- J
John G
Beaudesert, 2019
“Peter is an honest person who provided me with the information I wanted then performed a good job with great results for the benefit of myself and my family.”
- Shipages
Sterling G
Ashgrove, 2023
“Comfort Zone were very knowledge with great communication and follow up”
- Ghipages
Graham R
Riverhills, 2018
“Comfort Zone. Turned up ahead of time completed in +- 2 hours cleaned. All good. Very motivated installation team”
- Jhipages
Jung K
Riverhills, 2023
“An experienced family operation. Highly recommend. Thank you for the great job!”
- Khipages
Kathy A
North Lakes, 2023
“We connected with Peter through HiPages and he was prompt, professional and even came back after the job was complete to assist with a question we had. We would highly recommend Peter for further insulation works.”
- Dhipages
David H
Sunshine Coast, 2021
“Completed the job as quoted and to a high standard. Great personal service. Would highly recommend Comfort Zone for ceiling installationn work.”
- Shipages
Sue H
Sunshine Coast, 2021
“Incredible customer service”
- Ehipages
Eileen C
Cedar Vale, 2021
“Quality work, good customer service, prompt”
- Chipages
Craig M
Woody Point, 2021
“Called within 5 minutes of request. Very knowledgeable and explained job in great detail, provide great advice in prior preparation for works required. Very friendly and helpful.”
- Jhipages
Jenny C
Plainland, 2021
“Although I did not hire Peter I was impressed with the initial contact and the knowledge he was willing to impart. I was treated with respect which I appreciated. I would have hired but I received a lower quote.”
- Qhipages
Quinton
Coomera, 2020
“Professional installation without any short cuts. True to their word with high integrity. Response from Comfort Zone Insulation”
- Ghipages
Gary P
West Kempsey, 2020
“Came & Gave a free quote”
What the Australian industry actually measures
The recognised gap classes, and the figure that should scare you.
The insulation industry's own assessment guide doesn't just say 'gaps are bad'. It puts numbers on it, using the same CSIRO calculation engine that rates every new home in the country.
The ICANZ Guide to assessing ceiling insulation R-values in existing homes (2024) defines four gap classes for a ceiling, and works out each one using CSIRO’s AccuRate / NatHERS calculation method together with the Australian standard AS/NZS 4859.2:2018. These aren’t my numbers. They’re the recognised way insulation in this country is assessed.
| Gap class | Share of ceiling left as a gap |
|---|---|
| No Gap | 0% — what a seamless blown blanket gives you |
| Slight | 3% |
| Obvious | 6% — at about here, effective R is roughly halved |
| Large | 12% |
Here’s the line that matters. At around the Obvious (6%) class, the effective R-value of a ceiling is roughly halved. A batt rated R5 then performs like about R2.4. Not a little worse. Half. And 6% gaps isn’t some worst-case horror, it’s what the guide flatly calls “obvious”, the kind you’d spot if you put your head up in the roof.
One point worth being clear on. ICANZ applies the samegap penalty to cellulose too. So this isn’t “our material magically beats batts at the same gap.” It’s about coverage: blown cellulose installs as a seamless, gap-free blanket that sits at the No-Gap (0%) end, while hand-cut batts almost always leave gaps. The 3% / 6% / 12% figures are the recognised assessment classes, not a measured count of how gappy the average roof is.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Brown Earthwool batts laid with open seams in a real roof. Even “slight” gaps like these start dragging the effective R-value down, and they only get worse with age.
The short version
A rating only counts if the insulation actually covers the whole ceiling. Gaps are where that rating leaks away.
The independent science, not the industry body
CSIRO's insulation researcher put numbers on this back in 1992.
Everything above comes from the batt manufacturers' own trade association. This next bit doesn't. CSIRO is Australia's national science agency — government-funded, no product to sell, and the body that runs the thermal testing the whole industry relies on. In 1992 a member of the public wrote and asked them what gaps actually cost. We hold a copy of the reply.
The original letter, 18 September 1992. Two things have been taken off it: the name and address of the person who wrote in, and CSIRO’s logo. CSIRO don’t allow their logo on a commercial site without their permission and an agreement, and I’m not paying for the privilege of showing you a letter, so it’s gone. Not a word of the letter itself has been touched, and I’m happy to show anyone the full copy.
It’s written on CSIRO Division of Building, Construction and Engineering letterhead and signed by their Technical Information Officer, Karl Armstrong, passing on the comments of their insulation researcher, Robin Clarke. Clarke ran the numbers on a rated R3.0 batt.
| Gaps in the ceiling | What the R3.0 batt performs at | What you’d have to buy to still get R3.0 |
|---|---|---|
| 1% | R2.5 | R3.7 |
| 5% | R1.5 | “infinite!” |
Read that bottom row again. At a 5% gap, a rated R3.0 batt is doing the work of an R1.5. That’s half your insulation gone. And Clarke’s answer to “what would you have to buy to still end up at an effective R3.0?” is that no batt exists that could do it. You can’t buy your way out of a gap.
What this letter is, and what it isn’t
This is private correspondence, not a published CSIRO report. And I won’t dress it up as one. Clarke says so himself in the letter: the figures come from “an inexact calculation method” using “fairly pessimistic assumptions”. The industry assessment classes further up this page are the proper modern source, and they’re the numbers I hang my hat on. What the letter gives you is corroboration across 32 years: a CSIRO researcher writing privately to a member of the public, with nothing to sell, landed in the same place.
Worth knowing who’s who here. The gap classes at the top of this page come from ICANZ, which is the batt manufacturers’ own council. I quote them because it’s their own product they’re marking down, and that makes it hard to argue with. This letter is the other kind of source: the national science agency, no product in the game. When both of them land on the same answer, from opposite sides of the fence, that’s about as settled as this trade gets.
Day one, and ten years later
Why batts gap in the real world, every time.
A batt is cut to fit. That single fact is the whole problem. Here's where the gaps come from, and why the CSIRO calls incomplete coverage a heat bridge.
Batts have to be cut to fit every bay, squeezed past wiring and battens, and trimmed around every fitting. So they leave gaps over the joists, around the downlights and along the edges from day one. Then it gets worse: over the years the sparky, the plumber and the air-con installer climb up there, shove batts aside and leave them open. The Australian Government’s CSIRO says incomplete coverage that leaves air gaps acts as a “heat bridge… drastically reducing the overall thermal resistance”, and that it’s preventable “by ensuring a high standard of installation.”
And a high standard of batt install is rare. Overseas, the California Energy Commission inspected homes and found essentially none had their batts installed without defects (a US figure, but the lesson travels). It matches what I see in Queensland roofs every week. The takeaway across the research: defect-free batt installs are the exception, not the rule.
“An R5 batt is the lab number for a perfect fit. I’ve never once seen a perfect fit in 40 years of climbing into roofs. The gaps are where your power bill leaks out.”Peter Johnson, Comfort Zone Insulation Team
The worst offender: downlights
Downlights force a bare patch in the ceiling, and they add up fast. Sustainability Victoria’s Energy Smart Housing Manual states that “leaving out half a batt around downlights can… reduce the effective R value of ceiling insulation by over 60%.” That’s a single common fitting, eating more than half your insulation.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
The same product years later: shrunken, sagging and gapped between the joists, with the plasterboard showing through. The rating on the bag is long gone.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Steel frames make it worse, twice over
Steel bridges heat, and batts gap even harder around it.
Steel-frame homes have a second problem on top of the gap problem. Steel conducts heat, so the frame itself carries heat straight past the insulation, a thermal bridge. The ABCB Energy Efficiency Handbook for the NCC says that without a thermal break, “the insulation’s effectiveness can be reduced by as much as half.” (p.92.) Half again, just from the metal.
Now stack the two on top of each other. Batts are even harder to fit gap-free around steel members than around timber, so a steel-frame roof tends to combine the worst bridging with the worst coverage. A seamless blown blanket that fills tight around every steel member is exactly what these homes need.
The fix is coverage, not a magic material
Seamless blown cellulose solves the one thing that matters.
Cellulose isn't exempt from the gap penalty. It just doesn't leave gaps in the first place, which is the whole game.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Batts: cut to fit, so they gap
Cut into every bay, squeezed past wiring, trimmed around fittings, and shoved aside by every trade that follows. So they live somewhere down the “slight”-to-“large” gap scale. That’s where the effective R-value bleeds away. The rating you paid for never makes it into the room below.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Cellulose: blown in as one seamless blanket
Pumped in, it flows into every corner and around every fitting with full contact across the whole ceiling, so in practice it sits at the No-Gap (0%) end of the scale. No cut lines, no open seams, no bare patches. Same physics as everyone else, but the coverage that lets the rating actually count.
So a gap-free R3.0 actually beats a gappy R5. The rating you bought is the rating you actually get. When someone sells you a big number on a bag, ask them what it’ll look like once it’s cut into your roof and lived in for ten years. See cellulose vs fibreglass batts →
Straight answers
Gaps & effective R-value: the questions I get asked most.
Do gaps in insulation really matter that much?+
Yes, far more than most people realise. The insulation industry's own assessment guide (ICANZ, 2024) defines recognised gap classes for a ceiling: No Gap (0%), Slight (3%), Obvious (6%) and Large (12%), worked out using CSIRO's AccuRate/NatHERS calculation method and the Australian standard AS/NZS 4859.2:2018. At about 6% gaps, the effective R-value of a ceiling is roughly halved, so a batt rated R5 ends up performing like about R2.4. A rating only counts if the insulation actually covers the whole ceiling.
If batts are rated R5, why don't I get R5 in my roof?+
Because the R-value printed on the bag is the lab number for a perfectly fitted product with no gaps. In a real roof, batts are cut to fit and almost always leave gaps over joists, around downlights and along the edges, and over the years other trades shove them aside. The CSIRO says incomplete coverage that leaves air gaps acts as a heat bridge that drastically reduces the overall thermal resistance, and that it's preventable by ensuring a high standard of installation. Studies repeatedly find defect-free batt installs are rare. So the R5 on the packet isn't the R5 you live with.
Why are downlights such a problem for insulation?+
Downlights force a bare, uninsulated patch in the ceiling for clearance, and a lot of patches add up fast. Sustainability Victoria's Energy Smart Housing Manual states that leaving out half a batt around downlights can reduce the effective R-value of ceiling insulation by over 60%. That's why we keep the legal clearance to fittings and pump cellulose right up to it as a seamless blanket, rather than leaving the open gaps batts leave behind.
Does a steel-frame home make insulation gaps worse?+
It does two things. First, steel conducts heat, so the frame itself bridges heat straight past the insulation. The ABCB's Energy Efficiency Handbook for the NCC says that without a thermal break, the insulation's effectiveness can be reduced by as much as half. Second, batts are even harder to fit gap-free around steel members, so you tend to get more gaps on top of the bridging. Blown cellulose fills tight around the steel as one continuous blanket, which is exactly the seamless coverage steel frames need.
Is blown cellulose really gap-free, or is that just marketing?+
Here's the straight version. The same assessment guide applies the very same gap penalty to cellulose, so the material isn't magically better at an equal gap. The win is coverage. Blown cellulose installs as one seamless, gap-free blanket that fills into every corner and around every fitting, so in practice it sits at the No-Gap (0%) end of the scale, while hand-cut batts almost always leave gaps. Fix the coverage and you finally get the rating you paid for.
See the gap problem in real roofs
Watch: gaps, settling, and what 30-year-old cellulose actually looks like.
Peter's footage from real Queensland roofs — batt gaps in action, cellulose that hasn't moved in 30 years, and the comparison that makes the coverage argument.
Filmed on real jobs over the years. Subscribe to the channel for more.
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Want the R-value you pay for, with no gaps to start with?
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Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986