Compare · Cellulose vs fibreglass batts · SE Queensland
Cellulose vs fibreglass batts: an R5 batt isn’t an R5 in your roof.
Fibreglass is the cheapest, but batts have gaps from day one, and gaps wreck their R-value. Pumped cellulose fills every inch, deters rats and is shown to slow fire spread. You only insulate once, so get it right.
I sell all three insulation products, but after installing insulation since 1988 the pump-in cellulose fibre is the only one I’d use in my own home. This is the honest version of how cellulose stacks up against the itchy fibreglass batts: gaps, fire, rats, settling, the safety-sheet fine print, and the comparison table.
This is cellulose
one seamless blanket, no gaps, what an R-rating is supposed to mean
Start here
The two things that wreck batts: gaps and rats.
Most people who ring me have only ever heard of batts, so let me start with why you’re insulating at all. In summer your roof cavity can sit well over 60°C, and that heat soaks down through the ceiling and turns your upstairs into an oven. In winter the warmth you’re paying to make escapes straight back up through the same ceiling. Good ceiling insulation is the cheapest air-con and the cheapest heater you’ll ever buy. The Australian Government’s yourhome guide estimates roof and ceiling insulation can cut heating and cooling costs by up to 45%.
The question isn’t really “should I insulate?” It’s “which product, and who’s installing it?” I hope you realise by now that you are buying a service as much as a product. And when it comes to fibreglass batts, two problems do all the damage: gaps and rats. Everything else on this page comes back to one of those two.
“I sell all 3 but, after installing insulation since 1988, I only recommend the pumped-in cellulose fibre. It’s the only one I’d use in my own home.”Peter Johnson, Comfort Zone Insulation Team
The short version
Batts have gaps and rats love them. Cellulose has neither. In our climate, that’s most of the argument right there.
Try it yourself
The chair-and-hand test: feel the heat for yourself.
You don’t have to take my word for any of this. On the next hot afternoon, stand on a chair and hold your hand up near the ceiling in an uninsulated room, then do the same in a room that’s already insulated. The difference is night and day. The uninsulated ceiling radiates heat at you like a grill, because the roof cavity above it is sitting up over 60°C and there’s nothing stopping that heat coming straight through the plaster. That’s the heat your insulation has to stop, and it’s the heat a gap lets straight back in.
Then do the fan test. Take a fibreglass batt and hold it up against a fan. Air blows straight through it. Batts let air pass through, and they let air leak in from the edges under every batt, so even a perfectly-laid batt roof breathes hot air. Dense-packed cellulose is the opposite: it’s packed tight enough to resist air movement, so it cuts the draughts that whistle up through the gaps around your cornices, downlights and power points. Air leakage is a documented 15–25% of a home’s winter heat loss, so stopping that air movement matters as much as the R-value on the bag.
The evidence
Day one, a batt looks fine. The test is years later.
Why does cellulose still look almost new after 20 years while batts don’t? Two reasons. One: we don’t get rodents nesting in cellulose: no urine, no droppings, no dirt to make it smell or look grim. Two: cellulose is harder to disturb. When the sparky comes back to add a downlight, they can’t just lift a slab of cellulose aside and dump it on the aircon ducting like they do with a batt. The cellulose just stays the way we put it in.
Cellulose we pumped 10+ years ago still looks new. In 6,000 jobs I’ve never seen our correctly-installed cellulose settle, blow around, get mice nesting in it or start a fire. The only time I’ve removed it is when someone was replacing their ceilings. I’ve pulled out plenty of fibreglass batts, though, that settled and broke down as they aged. The settling problems people hear about with loose-fill come from under-filled jobs, not from cellulose packed to its proper density.
Three products, honestly
I sell all three. Here's the honest version of each one.
There are basically three types of insulation for houses in Queensland: fibreglass batts, polyester batts, and pumped-in cellulose fibre. I fit all three. Here's what I really think of each.
Fibreglass / glasswool batts: the cheap, itchy ones.
These are the itchy ones (Earthwool®, Gold Batts®, Pink Batts®) and they’re all fibreglass. They’re the cheapest insulation you can buy, and that’s the only thing they’ve got going for them. Fibreglass is a very cheap solution, but you really would be lucky if you’re just not buying trouble for the future. I do not recommend these, and only about 2% of my customers end up choosing them once they understand the trade-offs.
The trade-offs, in plain English: itchy install, gaps from day one, settles after about ten years, rodents love them, and no life-of-house guarantee. They’re hydrophobic, so when a leak comes through your roof the water just flows under the batts and spreads across the whole ceiling instead of staying in one spot. And the manufacturers’ own safety sheets tell you to wear a P2 dust mask and to wash your work clothes separately. Their fine print is more honest than their marketing brochure. I’ll come back to that one, because it matters.
I’m not hiding fibreglass from you. If you decide on batts after reading this, my team will cut a piece to fit every gap, even the width of a pen. But if batts are what you want, I wouldn’t quote you fibreglass with a straight face. I’d quote you the polyester.
Polyester batts: if it has to be a batt, make it this one.
If you think you just must have batts, the polyester ones are the best of the batts. They’re the King of batts: locally made, non-itchy and safe for your health, with none of the glass fibre that makes fibreglass so unpleasant to handle. About 6% of my customers go this way, and I’m happy fitting them: in 30-odd years I’ve never seen a polyester batt settle over time the way fibreglass does. For under a floor, where pumped cellulose is the wrong product, polyester is exactly what I’d use.
But, and this is the honest bit, a batt is still a batt. It has to be cut into every bay and squeezed into every corner, so it still has gaps, and it still lets air pass through. So while the polyester is genuinely good, it’s still not what I’d put in my own roof. If batts are what you want, though, these are the only ones I’ll quote with a straight face.
Cellulose fibre: the only one I would use in my own home.
Pumped-in cellulose fibre is by far the best product to use, and the only one I’d put in my mother’s home. About 92% of my customers choose it once they understand why. I’ve done more than 6,000 houses with it, and it does the things batts can’t: it pumps in as a seamless blanket with full contact across every inch of the ceiling, so there are no gaps, which is the whole ballgame, because an R-rating only counts if there are no gaps.
It’s made from recycled waste paper and treated with borax. Insects won’t live in it, so there’s no food for rats and no food for snakes, and it’s been shown to slow fire spread. Borax melts at about 734°C, roughly 200° above the temperature of a house fire, and the cellulose gives off only CO₂ and steam rather than the poison gas fibreglass binders produce. It won’t blow around or settle when it’s installed to its proper density, and it carries a transferable Life-of-House guarantee we don’t know of any other insulation in Australia matching.
Worried about what’s in it? It’s just recycled paper, boric acid and borax: no ammonium sulfate, no asbestos, no formaldehyde. Borax is a natural mineral salt about as toxic to you as table salt: don’t eat the insulation, but unless you worry about the kids grabbing the salt shaker off the dinner table, you don’t need to worry about them touching this either.
The technical bit that matters most
“An R5 is an R5”, except it isn’t.
This is the single most important thing to understand before you buy batts. People assume that if you have R3.0 in one insulation and R3.0 in another, you get the same result. You don’t. The rating is the lab number for a perfectly-fitted product. The result is what actually ends up in your roof, and that depends entirely on the gaps.
According to Choice Magazine, leaving just 1% gaps in your insulation can reduce performance by 30%, and 5% gaps in batts reduce the efficiency by as much as 70%. When batts get cut into all the different size bays in your roof and squeezed into the tight corners, most batt installers take short cuts, especially in tight, hot roofs. So that R5 on the packet is not the R5 you end up with. You can see the mechanism in the Australian Government’s own guidance, too: yourhome.gov.au notes that even a small gap can greatly reduce the insulating value, and Sustainability Victoria’s Energy Smart Housing Manual shows the effective R-value collapsing as more of the ceiling is left uninsulated (p.63): even a 5% gap can drop an R3.5 batt’s effective R-value to R2.1, about 40%.
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.
And batts are easy to short you on
With cellulose there’s one way to short you. With batts there are a dozen.
Here’s another reason gaps are worse with batts than people realise: batts are easy to cheat with. With cellulose there’s only one way to short you, pump less in, and it’s measurable, because we can show you the depth across the whole ceiling. With batts there are a dozen ways to short you, and they rarely get caught. I’ve seen 100m of R3.0 batts stretched to cover 300m at R1.0, and the customer never noticed (he said the bloke was a real nice guy, too). I’ve found whole jobs where the batts were left in the roof still in the packets, because the installer gave up before reaching the tight corners.
Insulation hasn’t even been a licensed trade since 2006. Any bloke with a ute and a ladder can call himself an insulation contractor, and a lot of the big companies subcontract to the lowest bidder. That’s exactly why every job is photographed and the photos are checked before you’re invoiced. That’s our system, the same on every job, run by Comfort Zone franchise owner-operators who care about their own reputation in your area, not subbies paid by the job. Most customers never see inside their own roof, so we show them, which is hard to do honestly with batts laid full of gaps you can’t un-see.
Fire
Fibreglass melts away. Cellulose chars and holds.
Cellulose is treated with borax, which melts at about 734°C, roughly 200° above the temperature of a house fire. Hold a blowtorch to a handful and it chars and glows, but it won’t carry a flame. Sustainability Victoria states that cellulose treated with a borax fire retardant won’t spread flame, and the CSIRO describes cellulose as recycled paper combined with fire-retardant chemicals.
Fibreglass batts melt away in a fire and leave your timber exposed, and the binders give off poison gas. Cellulose has been shown to slow fire spread, and gives off only CO₂ and steam. That’s a demonstration you can watch, not a marketing line, and it’s one more thing the cheapest product simply can’t do.
The smell test
Rats love batts. They leave cellulose alone.
Cellulose is treated with borax, a natural mineral salt about as toxic to you as table salt, and insects won’t live in it. No insects means no food for rats, and no food for rats means no food for snakes. So a cellulose roof after 40 years is far cleaner than a batt roof. Batts are the opposite: they sit so loosely that mice push them up and nest underneath, soiling the insulation and leaving urine and droppings to soak straight into your ceiling sheets, like the rodent-fouled batts in the photo here.
“I pull rat nests out of batt-insulated roofs every week. I have NEVER pulled a rat nest out of a roof that was pumped with cellulose.”
This is also why a 20-year-old cellulose roof smells nicer today than the day we installed it, while an old batt roof just traps the smell. The borax slowly disinfects whatever ends up in the cavity instead of letting it sit there and stink. I’m confident enough about the rats leaving cellulose alone that I back it with a $1,000 reward.
Read the fine print before you buy batts
Their fine print is more honest than their brochure.
Here’s the argument I most want you to understand, in both halves. On the front of the brochure, the fibreglass batt makers market a soft, safe, comfortable product. But read the fine print. The manufacturers’ own Safety Data Sheets tell you to wear a P2 dust mask and to wash your work clothes separately from the family laundry. Their fine print is more honest than their marketing brochure: it’s telling you exactly what it is to handle. You can check it yourself on their own paperwork, for example, the Knauf Earthwool® glasswool Safety Data Sheet sets out the dust-mask and handling precautions in black and white.
Now the other half, so this stays fair. Cellulose isn’t dust-free either, like any loose-fill, we wear dust masks when we pump it. The difference is what the fibre is. Fibreglass is a glass fibre, and the CSIRO notes glass-fibre and rockwool can cause temporary skin, nose and eye irritation and that precautions should be taken when handling them. Cellulose is a recycled-paper plant fibre, gentler to handle, and there’s no SDS skin warning to wash off afterwards. The plain test of it: our installers go home in the same clothes they came in.
“An installer who has to wash their work clothes separately from the family laundry isn’t going to spend extra time dragging insulation into your tight corners, and that’s how gaps happen.”Peter Johnson, Comfort Zone Insulation Team
Side by side, honestly
Cellulose vs fibreglass batts: the clean comparison.
Nine things a homeowner actually cares about. I've flagged the honest winner on each one, including the single axis where fibreglass beats cellulose (price to buy).
| What matters | Cellulose fibre (pump-in) ★ Recommended | Fibreglass batts Not recommended |
|---|---|---|
| Install method & gaps | Pumped in as one seamless blanket: full contact across every inch of the ceiling, with no gaps. better | Cut to fit every bay and squeezed into corners, so it has gaps from day one, and the tight, hot corners get short-changed. |
| What an R-rating actually delivers | No gaps means the rating counts: what you pay for is what's in your roof, not just the lab number on the bag. better | An R5 batt is only an R5 with no gaps. Choice says 1% gaps cut performance 30%, 5% gaps up to 70%, so the packet number isn't the result. |
| Fire | Treated with borax (melts ≈734°C): shown to slow fire spread, and gives off only CO₂ and steam. better | Melts away in a fire and leaves your timber exposed, and the binders give off poison gas. |
| Rats, mice & insects | Borax keeps insects out, so there's no food for rats and no food for snakes. I've never pulled a rat nest out of pumped cellulose. better | Rodents love them. Mice push the loose batts up and nest underneath, leaving urine and droppings in your ceiling. |
| Settling over time | Installed to a specified density so it holds its thickness. In 6,000 jobs I've never seen our cellulose settle or blow around. better | I've removed plenty of fibreglass batts that settled and broke down as they aged, opening gaps where there used to be cover. |
| Health & handling during install | Recycled-paper plant fibre: no itch, no SDS skin warning. Like any loose-fill we wear dust masks; our installers go home in the same clothes they came in. better | The maker's own SDS tells you to wear a P2 dust mask and wash work clothes separately. Itchy glass fibre that can irritate skin, nose and eyes. |
| Air movement & draughts | Dense-packed, so it resists air movement and cuts the draughts that whistle up through gaps around cornices, downlights and power points. better | Hold a batt against a fan and air blows straight through, and air leaks in from the edges under every loose batt. |
| Guarantee | Carries a transferable Comfort Zone® Life-of-House Guarantee. We don't know of another insulation in Australia that does. better | No life-of-house guarantee. You're left with the manufacturer's product warranty only. |
| Price to buy | Dearer to buy, but for a little extra cost you get a far better result over the life of your house. | Cheapest to buy. A very cheap solution, but you really would be lucky if you're just not buying trouble for the future. better |
That’s the honest scorecard. Cellulose wins on every axis a homeowner cares about (gaps, effective R-value, fire, rats, settling, health, draughts and the guarantee) and the one thing fibreglass beats it on is the price to buy. But for a little extra cost the cellulose gives you a far better result over the life of your house. Want all three products side by side, including the polyester? See the full three-way comparison table.
The honest last word
You only insulate once, so you can’t compare.
Here’s the thing that 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 in it the next, so you can’t stand in your living room and compare the two. You just have to pick, and live with it for the life of the house.
That’s why I won’t sell you the cheapest thing just because I can. For a little extra cost the cellulose will give you a much better-performing insulation, and it really is the only one I recommend you use. If you can afford a bit extra now, you’ll get a far better result over the life of your house: no gaps, borax for fire and pests, and a transferable Life-of-House guarantee you can pass on to the next owner. Whatever you choose, I’ll give you an honest quote and an honest answer.
Honest answers
Cellulose vs fibreglass batts: the questions I get asked most.
Is cellulose better than fibreglass batts in a Queensland roof?+
In my opinion, yes, and after installing insulation since 1988 it's the only product I'd use in my own home. The big reason is gaps. Pumped-in cellulose lays as one seamless blanket with no gaps, while batts have to be cut into every bay and always leave gaps from day one. Cellulose is also treated with borax so insects won't live in it and has been shown to slow fire spread; fibreglass batts do neither. The fibreglass is cheaper to buy, but you only insulate once, so I'd spend the little extra and get it right the first time.
Why isn't an R5 batt really an R5 once it's in my roof?+
An R-rating only counts if the insulation is fitted with no gaps, and batts always have gaps. According to Choice Magazine, leaving just 1% gaps in your insulation can reduce performance by 30%, and 5% gaps can reduce efficiency by as much as 70%. When batts get cut into all the different size bays and squeezed into the tight corners, most batt installers take short cuts, especially in tight, hot roofs. So the R5 printed on the packet is the lab number for a perfectly-fitted batt, not the result you actually get in your roof. Pumped cellulose has no gaps, so what you pay for is what you get.
Do rats and mice nest in cellulose like they do in batts?+
I pull rat nests out of batt-insulated roofs every week. I have never pulled a rat nest out of a roof that was pumped with cellulose. Cellulose is treated with borax, a natural mineral salt about as toxic to you as table salt, and insects won't live in it. No insects means no food for rats, and no food for rats means no food for snakes. So a cellulose roof after 40 years is far cleaner than a batt roof: no nests, no urine, no droppings soaking into your ceiling sheets.
Do fibreglass batts really need a P2 mask, and is cellulose any safer to install?+
Read the fine print before you buy batts. The manufacturers' own safety data sheets tell you to wear a P2 dust mask and to wash your work clothes separately. Their fine print is more honest than their marketing brochure. Our installers go home in the same clothes they came in. To be fair, like any loose-fill, we wear dust masks when we pump cellulose too, but cellulose is a recycled-paper plant fibre, not a glass fibre, and the Australian Government's CSIRO notes glass-fibre and rockwool can cause temporary skin, nose and eye irritation.
Do fibreglass batts settle and lose performance over time?+
They can. I've removed plenty of fibreglass batts that settled and broke down as they aged, and once they sag or get pushed aside you get gaps where there used to be cover. Cellulose we pumped 10+ years ago still looks new. In 6,000 jobs I've never seen our correctly-installed cellulose settle, blow around, get mice nesting in it or start a fire. The settling problems people hear about with loose-fill come from under-filled jobs, not from cellulose installed to its proper density.
Fibreglass is cheaper. Why would I pay more for cellulose?+
Fibreglass is the cheapest to buy, but you really would be lucky if you're just not buying trouble for the future. For a little extra cost the cellulose gives you a much better-performing insulation: no gaps, full contact across every inch of the ceiling, borax for fire and pests, and a transferable Life-of-House guarantee. It really is the only insulation I recommend: if you can afford a bit extra now, you'll get a far better result over the life of your house. And you only insulate once, so you can't compare the two side by side later. You have to get it right the first time.
Ready to insulate it once, and properly?
Thanks for reading this far. I’ve shown you why a batt’s gaps make an R5 something less than an R5, why rats leave cellulose alone, what the batt makers print in their own safety sheets, and the clean comparison. The rest is up to you, and whatever you decide, I’ll give you an honest quote within 48 hours for most houses.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986
Want the bigger picture? Read why cellulose wins, compare cellulose vs polyester, or just get your quote started.
We make our cellulose in Tiaro and we’re looking for installers to join the family. Ask about a Comfort Zone franchise territory.