Guide · Underfloor insulation · Polyurethane spray foam
Spray foam under your floor: I put a flame to it, and there is a second problem.
Spray foam insulates well, and under a timber floor that is the problem. It is waterproof the wrong way round, so water through the boards sits against your joists, and when it burns it gives off black soot and fumes.
A new trend has turned up in Queensland: spraying polyurethane foam onto the underside of timber floors. I pulled a couple of pieces of it off a job site the other day, from under the house, and did what I do with every product people ask me about. I put a flame to it, then to one of our underfloor polyester batts, with the camera running. That is the video below. Then there is the second problem, the one you cannot see in a flame test, and it is the one that would stop me putting it under my own house.
Real reviews, real jobs
What our customers say
Genuine Google & hipages reviews from Comfort Zone customers across SE Queensland.
- A
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”
A flame on the foam, then on a polyester batt
Spray foam under your floor: a flame on the foam, then on a polyester batt
Peter puts a small flame to a piece of polyurethane spray foam he picked up under a house, then to one of our underfloor polyester batts, and explains why foam under a timber floor holds water against the joists.
Filmed 10 October 2026. A small torch flame on a piece of polyurethane spray foam, then on the edge of a polyester underfloor batt. What you see on the tile afterwards is in the photo further down.
“Now, both products aren’t, they’re not real good in the event of a fire. They’re both going to burn and cause toxic fumes. But if you put spray polyurethane under your home, you’re going to seal in any moisture.”
Neither product is fireproof, and I say so on camera. What the footage does show is the difference in what comes off them. The foam flares, keeps a bit of flame for a moment, and leaves a halo of black soot and char on the tile. The polyester batt melts back from the flame, goes crusty at the edge, and leaves the tile clean.
The sources say what the fumes are. The US National Bureau of Standards’ review of rigid polyurethane foam found that “carbon monoxide (CO) and hydrogen cyanide (HCN) were the predominant toxicants found among more than 100 other gaseous products” NBSIR 85-3224, 1985, p.1 ↗, and a 2011 study of building insulation in fire found that for polyurethane and polyisocyanurate foams “hydrogen cyanide is the major toxicant for all flaming fire conditions” Stec & Hull, Energy and Buildings 43, 2011, pp.498–506, Conclusions ↗. The US workplace safety regulator puts it plainly: rigid polyurethane foams “will, when ignited, burn rapidly and produce intense heat, dense smoke and gases which are irritating, flammable and/or toxic” OSHA Hazard Information Bulletin, 10 May 1989 ↗. The soot on the tile is what that smoke leaves behind.
The polyester batt is still combustible, and its manufacturer says so: “Low flame response”, and “product will melt and is ultimately combustible” Queensland Textiles, Polyester Insulation Material Safety Data Sheet ↗. Tested to AS 1530.3 it scores 0 for ignitability, spread of flame and heat evolved, and 0 to 3 for smoke developed Queensland Textiles, Polyester Insulation Material Safety Data Sheet, fire fighting measures ↗, on a smoke index that runs from 0 to 10 GreenStuf thermal ceiling insulation data sheet (Autex), July 2025 ↗. So both materials burn. The demonstration is mine, not a fire rating, and it shows the difference in what comes off them, not that one is safe.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Waterproof the wrong way round
“It’s very waterproof, and it locks moisture in. So if you’ve got a timber floor and you spray polyurethane foam underneath, now if you mop the floor, all the water sits on top of the polyurethane, trapped between the timber and the urethane. And so that can rot the timber out over time and cause mould and rot.”
Timber floors get wet from above, from a mopped floor, a spilled bucket, a leaking dishwasher or a bathroom that was never tiled properly. In an old Queenslander that water has always dripped through the boards and dried out in the breeze underneath. Spray a vapour-tight foam onto the underside and it cannot. It sits on the foam, against the timber, in the dark.
There are two kinds of spray foam. If what is under your floor is the rigid closed-cell kind, the US Department of Energy’s guide to spray foam says closed-cell foam “is hydrophobic and will not absorb water”, passes under 1 perm of water vapour at 2 inches thick, and “at typically applied thickness ccSPF is defined as a vapor-retarder, essentially moisture-vapor impermeable” Building America, Which Spray Foam Is Right For You?, 2009, p.3 ↗. Open-cell foam lets vapour through, but the same guide says it “will also absorb and hold liquid water” Building America, Which Spray Foam Is Right For You?, 2009, p.2 ↗. Either way, water that gets through the boards has nowhere to go.
The UK has the long track record, in roofs. The Royal Institution of Chartered Surveyors warns that spray foam “will also seal in moisture unless adequate precautions are taken” RICS, Spray foam insulation consumer guide, March 2023, p.10 ↗, and the Property Care Association says closed-cell foam against tiles “can create a barrier that traps moisture. This can lead to moisture build-up, potentially causing timber rot or decay” PCA, Spray Foam Insulation Consumer Guide, 2025, p.2 ↗. Those are roofs, and the UK’s own retrofit guide does allow spray foam between floor joists with the subfloor ventilated and the timber dry BEIS/TrustMark, Suspended Timber Floors Best Practice, 2020, pp.18 and 31 ↗, so I am not telling you the UK bans it. Here, the Australian Building Codes Board says of a sealed insulation layer under floor joists that it “may also restrict downward drying. This restriction can increase the risk of trapping liquid moisture resulting from leaks or spills above the floor” ABCB, Condensation in Buildings Handbook, NCC 2025 edition, s5.8.2, p.61 ↗. That sentence is about foil-faced board, and it is exactly my mop-water point.
The way I see it, under a floor it is the same problem as in those roofs: a vapour-tight layer on the wrong side of the timber, with no way for water to get out. The difference is that nobody looks under a floor until the boards go soft.
On your joists for good
“Your joists on an old home in Queensland, they might be 50 or 100 years old already, and they’re fine. Sitting underneath your house, they don’t have any rot in them yet. The timber’s good hardwood. But if you spray them with this stuff, it’s very hard to take off, very hard to scrape off.”
The surveyors agree with the scraping. RICS calls spray foam “a permanent installation” whose “removal could potentially damage the existing structure” RICS, Spray foam insulation consumer guide, March 2023, p.12 ↗, and the Property Care Association says removing it “is a time consuming and expensive process, often requiring specialised equipment and expertise” PCA, Spray Foam Insulation Consumer Guide, 2025, p.2 ↗. Historic England goes further for historic and traditional buildings: foamed plastics such as polyurethane “are therefore not usually appropriate for the insulation of suspended timber floors”, since their inability to absorb and release moisture may increase the risk of condensation Historic England, Insulating Suspended Timber Floors, 2016, s3.5, p.14 ↗. An old Queenslander on hardwood joists is that sort of building, as I see it.
That is the part that bothers me most. A set of hardwood joists that has done a century without trouble gets bonded to a plastic foam that holds water against it, and the foam does not come off when you change your mind.
What we fit under floors instead
“We use underfloor polyester batts, and ours are made specially for underfloor, so they’re very stiff, unlike the floor batts that are made by some of the other companies that are fibreglass. They’re very floppy, and they don’t just push in and hold in the bays. But these here will stay in the bays, but if you mop your floors, the water will run through this and out. It will dry out, no problem.”
Stiff polyester batts cut to the joist spacing, pushed up into the bays and held there. Water that gets through the boards runs through them and out. If you ever want them gone, you pull them down. The manufacturer’s technical documents are linked on our Documents & sources page. Our underfloor page has the sizes, the prices and the photos.
For the comparison laid out side by side, see spray foam vs polyester under a floor, and for the buyer’s view of every underfloor option, best underfloor insulation for SE QLD.
Is spray foam under a floor against the code?
No. Nothing in the National Construction Code prohibits it, and nothing approves it either. The code’s condensation provisions cover external walls, exhaust fans and roof spaces, not floors ABCB handbook, pp.13–14 ↗, and the ABCB notes that spray-applied foam “is not commonly used in Australian residential construction” and that with a foam layer “materials on each side of this layer are intended to dry away from the foam in a single direction” ABCB handbook, p.59 ↗. Under a floor, the only direction for mop water to dry is down, and the foam is in the way.
I am not telling you it is illegal. It is uncommon here, so we do not have the track record the UK now has. I am telling you what it does with water, what comes off it in a fire, and that I would not have it under my own house.
Read the transcript
Peter’s words on camera, as spoken. The sourced facts are in the sections above.
Peter here from Comfort Zone Insulation, and I just want to talk to you about the polyurethane foam. This is just a couple of pieces of polyurethane. I pulled off a job site the other day that was under the house, so I've got a photo of that. But now it's a new trend to spray the underfloor with polyurethane foam, and it's a very good insulator. It does a good job for that, but you might not want it under your floor, and there's a couple of reasons for it.
It's very waterproof, and it locks moisture in. So if you've got a timber floor and you spray polyurethane foam underneath, now if you mop the floor, all the water sits on top of the polyurethane, trapped between the timber and the urethane. And so that can rot the timber out over time and cause mould and rot. So it's not a good product.
We use underfloor polyester batts, and ours are made specially for underfloor, so they're very stiff, unlike the floor batts that are made by some of the other companies that are fibreglass. They're very floppy, and they don't just push in and hold in the bays. But these here will stay in the bays, but if you mop your floors, the water will run through this and out. It will dry out, no problem.
This polyurethane will seal the water in and cause a problem for you, or potentially. And even for your building envelope around the outside, very good at stopping moisture, but it doesn't breathe, so it doesn't allow moisture to get out. And so that can be a problem.
Now the other thing is, if I just show you, it's not the product that I'd put in underneath my house for a lot of reasons, but if you put a flame on it, that's just a little flame. See how much smoke comes off of that? If you have a fire in your house, the fumes from that are quite toxic. And just any little flame like that, and you've got black plumes of smoke. So there's black plumes of smoke coming out, and now this doesn't encourage or keep the flame. Right? Oh, hang on. It does a little bit. It's not supposed to keep the flame, but that's really quite toxic when I'm burning that.
Can you get a good shot of this, darling? Right? So this is, when you put a flame on that, all of that toxic fumes come off of there, and it burns.
But if we see what happens with the polyester, the polyester melts away from the flame. So that doesn't produce half as many toxic fumes. Have a look at this, darling. You've got a good shot of this. So it's not as toxic a fume that comes off of that. Not a big black smoke that comes off it. And you can see even on the tile below it there's nothing.
Now, both products aren't, they're not real good in the event of a fire. They're both going to burn and cause toxic fumes. But if you put spray polyurethane under your home, you're going to seal in any moisture. You spill a cup of coffee, you mop the floor. That's going to be there for 20 years, locking in all that moisture underneath your home, underneath your floorboards and attached to your joists.
Your joists on an old home in Queensland, they might be 50 or 100 years old already, and they're fine. Sitting underneath your house, they don't have any rot in them yet. The timber's good hardwood. But if you spray them with this stuff, it's very hard to take off, very hard to scrape off. And so, whereas these polyester batts, you ever want to take them out, you just pull them down, they come out, and they don't hold moisture against your home.
So that's my recommendation, is polyester batts perform better in a fire. They don't make as much toxic fumes, and they're not going to hold moisture against your timber. So like, comment, subscribe, share, and look us up, get a free quote. If you want your floors done, I definitely wouldn't do it with spray polyurethane. It's a good insulator, but not the product you want to put under your house.
And that spray foam that I just burned, just burned to nothing. That's probably why it's so toxic is that it literally just burns to nothing. But anyway, but you see here, all the soot and that, I wouldn't want to be inhaling that if it was underneath my house, and all of those fumes coming up through my floorboards into my home while I'm trying to escape a fire that started under the house. Anyway, or a bushfire that comes through underneath the house, if the bushfire comes through and it started burning at the bottom of your house, I'd much rather have this, it just melts away from the flame and doesn't have anywhere near the plumes of black soot and smoke that comes off it.
Questions people ask
Questions about spray foam under a timber floor
Is spray foam insulation under a timber floor a good idea?
It insulates well, and that is where the good news stops for a timber floor. Closed-cell foam is waterproof the wrong way round, so water that gets through the boards is held against the joists, and when it burns it gives off black soot and fumes. We fit stiff polyester batts instead.
Does spray foam cause timber rot?
It can seal moisture in against timber. UK surveyors warn that spray foam “will also seal in moisture unless adequate precautions are taken” (RICS, 2023), and the Property Care Association says closed-cell foam can trap moisture and lead to timber rot or decay. Under a floor the risk is mopping water, a spill or a leak coming through the boards with no way to dry out below, which the Australian Building Codes Board describes for a sealed foil-faced insulation board under floor joists.
What does polyurethane foam give off when it burns?
Burning polyurethane foam gives off carbon monoxide and hydrogen cyanide, which a US National Bureau of Standards review found were the predominant toxicants among more than 100 gaseous products, with dense smoke (NBS, 1985; OSHA, 1989). The foam in our test left a halo of black soot on the tile; the polyester batt melted back and left it clean. Neither product is fireproof.
Can spray foam be removed from floor joists?
Not easily. RICS calls it a permanent installation whose removal could damage the structure, and the Property Care Association calls removal time-consuming and expensive. Polyester batts pull down by hand.
What do you put under a floor?
Stiff polyester batts made for underfloor work, cut to the joist spacing and held in the bays. Water drains through them and they dry out. Prices and sizes are on our underfloor insulation page.
Keep reading
Did we do your roof? Leave us a review.
A quick honest review genuinely helps a small family business, and helps the next person decide. Thank you.
Got photos or a video from the job? Here’s where to send them →
Got a quote for spray foam under your floor? Get ours first.
Send me the quote and a photo under the house. I will tell you what polyester batts would cost for the same floor, and you can decide with both in front of you.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986