Spray foam under the floor
Spray foam underfloor insulation: what the sales rep doesn’t tell you
Toxic smoke, chemicals that keep coming off the foam, and what happens if it is mixed wrong. Here is what the published sources say.
I’m Peter Johnson. I’ve insulated Brisbane houses since 1986. The US Consumer Product Safety Commission advises you to leave the building for at least 24 hours after spray foam is installed, and it states there is no standard process for removing or fixing misapplied foam US Consumer Product Safety Commission, spray foam guidance for consumers ↗. Ask the company quoting you why their advice is different.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
What is TCPP, the flame retardant in spray foam?
TCPP is tris(1-chloro-2-propyl) phosphate. It is a flame retardant added to rigid polyurethane foam, and Health Canada lists it as an additive flame retardant used in spray foam insulation, with exposure through air, water and dust Health Canada, TCPP and TDCPP fact sheet ↗.
- Cancer. In March 2026 the International Agency for Research on Cancer classified TCPP as probably carcinogenic to humans (Group 2A). It found sufficient evidence in animals and strong mechanistic evidence in human cells, and inadequate evidence in humans IARC Monographs Volume 141, 27 March 2026 ↗. The US National Toxicology Program found clear evidence that TCPP caused liver cancer in female mice US National Toxicology Program, Technical Report 602 ↗. Australia’s industrial chemicals regulator wrote in 2016 that the long-term effect of carcinogenicity cannot be excluded, and listed possible reproductive and developmental toxicity NICNAS (now AICIS) human health assessment, 2016 ↗.
- Reproduction and development. Health Canada says reproductive and developmental effects were considered the important or critical effects for TCPP Health Canada ↗.
- How long it stays. In a US government test house, researchers at the National Institute of Standards and Technology still measured TCPP two years after open-cell foam was applied NIST Technical Note 1921, report to the Consumer Product Safety Commission ↗.
For a homeowner that means a chemical that international cancer researchers rate as probably carcinogenic was still measurable two years later in one unoccupied US test house, which had about 15 square metres of open-cell foam on basement rim joists. NIST says its conclusions do not necessarily apply to all foams, and that high-pressure closed-cell and field-sprayed foam were not tested in that phase. The cancer findings are from animals, and the human evidence is inadequate, so nobody can say TCPP causes cancer in people.
Spray foam off-gassing: what comes off after installation and for how long
Spray foam is mixed from two parts. One contains isocyanates, and the other contains polyols, amine catalysts, flame retardants, blowing agents and surfactants US Consumer Product Safety Commission ↗. Isocyanates are sensitisers that can cause occupational asthma SafeWork NSW, isocyanates technical fact sheet ↗. A Consumer Product Safety Commission document notes that exposure limits are not yet established for the majority of the amine catalysts US Consumer Product Safety Commission, 2006 contract task statement ↗.
The US Environmental Protection Agency says foam that is correctly applied and cured is usually considered relatively inert, and that foam that is not mixed or applied properly can leave contaminants on surfaces and residual odours US EPA, Potential chemical exposures from spray polyurethane foam ↗. It says cure time depends on the mixing of the two parts, ventilation, temperature and humidity US EPA, Ventilation guidance ↗. In laboratory chamber tests the amine catalysts fell away over time, and TCPP did not NIST Technical Note 1921 ↗.
What happens if spray foam is mixed wrong and does not cure?
Foam that is mixed wrong or sprayed wrong can fail to cure completely. A conference paper on spray foam odours says the foam may fail to completely cure when the contractor does not follow the specified procedures, and that tertiary amines from the catalysts are significant contributors to the odour ASTM STP 1589, 2017 ↗. A 2014 study of 13 adults in 10 homes with faulty spray polyurethane foam found that all of them reported fishy odours, and that in some cases the foams were sprayed using an improper mixing technique Huang and Tsuang, Environmental Research, 2014 ↗.
- How long does the smell last? The building-science writer Martin Holladay described a bad, usually fish-like odour that lasts for months in the cases reported to him Green Building Advisor, Spray foam jobs with lingering odor problems, 2 September 2011 ↗. The EPA says off-gassing is not fully understood US EPA ↗.
- How often does it happen?An industry manager told the same writer it appears to be less than one tenth of one percent of jobs. That figure is the industry’s own estimate, with no data given Green Building Advisor, 2011 ↗.
- Can it be fixed? The EPA says removal may not resolve the problem and may create other problems US EPA ↗, and the Consumer Product Safety Commission states there is no standard process for removing or fixing misapplied foam US Consumer Product Safety Commission ↗.
In my experience the smell from misapplied foam can be fishy, urine-like or ammonia-like. The full story is on my smell from under the floor guide.
Who is trained to mix and spray foam, and is a licence needed?
The Queensland Building and Construction Commission says the installation of home insulation in Queensland does not require a licence Queensland Building and Construction Commission, when you need a licence ↗. So ask the installer who trained the person on the spray rig, what certificate that person holds, and whether you can see it.
Spray foam fire: hydrogen cyanide and the most toxic insulation materials
A fire under a house with polyurethane foam on the floor makes more than timber smoke. A peer-reviewed 2011 laboratory study found hydrogen cyanide was the major toxicant for polyurethane foam in flaming fires. It ranked polyurethane and polyisocyanurate foams among the most toxic insulation materials it tested, while stone wool and glass wool failed to ignite Stec and Hull, Energy and Buildings, 2011 ↗. A US National Bureau of Standards review for the Consumer Product Safety Commission found carbon monoxide and hydrogen cyanide were the predominant toxicants among more than 100 gaseous products from burning rigid polyurethane foam Paabo and Levin, US National Bureau of Standards ↗. A US fire-service magazine article on spray foam lists, for burning polyurethane foam, hydrogen cyanide, nitrogen oxides, soot, carbon monoxide and aldehydes, and says burning foam makes “a lot of smoke” Fire Engineering, 21 July 2014 ↗.
How toxic is hydrogen cyanide (Wikipedia)? The US EPA says 100 mg/m³ or more will cause death in humans for acute, short-term inhalation, with no duration specified US EPA, Cyanide compounds ↗. The US safety body NIOSH sets the “immediately dangerous to life or health” level at 50 parts per million, based on 45 to 54 ppm being tolerated for 0.5 to 1 hour NIOSH, hydrogen cyanide IDLH ↗. Hydrogen cyanide is a poison that acts fast. It is not classified as a carcinogen: the EPA classifies cyanide as not classifiable as to human carcinogenicity.
Is hydrogen cyanide the same as cyanide gas?
Yes. Hydrogen cyanide(HCN) is the cyanide gas. “Cyanide” is also used for a wider family of compounds that includes solid salts such as sodium cyanide, and the US EPA page above covers that family US EPA, Cyanide compounds ↗.
How much cyanide gas could spray foam under a 150 m² house release in a fire?
This is an illustration, not a measurement. Assume spray foam 50 mm thick across a 150 m² floor. That is 7.5 cubic metres of foam. At 34 kg per cubic metre, the density of the polyurethane foam in the 2011 laboratory study and close to the density of closed-cell spray foam, that is about 255 kg of foam. In that study the yield of hydrogen cyanide from polyurethane foam in flaming fires reached about 0.01 g per gram of foam, read from the chart in the paper Stec and Hull, Energy and Buildings, 2011 (Figure 7) ↗. If every kilogram burned at that yield (it would not), that would be up to about 2.5 kg of hydrogen cyanide.
Spread evenly through the air of a 150 m² house with 2.4 m ceilings (360 cubic metres), 2.5 kg is about 7,000 mg/m³. The EPA says 100 mg/m³ or more will cause death in humans. The same laboratory study cites 165 ppm, roughly 180 mg/m³, as the 30 minute lethal level for half of exposed rats, and a fire-toxicology paper says hydrogen cyanide is often the most toxicologically significant component of fire smoke from nitrogen-containing materials Hansen-Bruhn, McKenna and Hull, Journal of Fire Sciences, 2023 ↗. A real fire burns only part of the foam, and smoke leaves through the roof and openings, so the real figure is lower. No one has measured this in a house fire, and the number is only to show the scale of the fuel, not what you would breathe.
Those were laboratory tests on rigid polyurethane foam panels, not on spray foam in a house fire, and cellulose and polyester were not in them. Here is what happened in our own flame test, with the foam burning on a white tile and giving off dense black smoke.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
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.
Read the transcript
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 to ask the spray foam sales rep
- The US Consumer Product Safety Commission advises leaving the building for at least 24 hours. Why is your advice different? One Australian installer’s website says homeowners typically vacate for a few hours while foam is applied and set, and can return once the area is ventilated and safe Installer FAQ, read 22 October 2026 ↗. The Commission notes the time can vary by product.
- What is the flame retardant in your foam, and can I see the safety data sheet?
- Who trained the person spraying it, and what certificate do they hold?
- What happens if the foam does not cure properly, and who pays to take it off?
- What happens when I mop the floor, and what lets the water evaporate? Can you give me the test reports in writing?
- What are the smoke and flame test results for your foam? Ask for the report.
The polyester underfloor insulation we fit instead of spray foam
Polyester is made of carbon, hydrogen and oxygen, according to the safety data sheet for the polyester insulation we fit. Hydrogen cyanide comes from nitrogen-containing materials Hansen-Bruhn, McKenna and Hull, 2023 ↗, and in the 2011 laboratory study the foams without nitrogen in the polymer (polystyrene and phenolic) gave hydrogen cyanide yields close to the limit of detection, while polyurethane and polyisocyanurate gave significant yields. In my experience polyester batts do not smell funny, and polyester fibre is the filling in plenty of pillows and quilts. The safety data sheet shows early fire hazard indices to AS 1530.3 (1999): ignitability index 0, spread of flame index 0, heat evolved 0 and smoke developed index 0 to 3. It says that on burning, like most organic materials, it gives off carbon monoxide and carbon dioxide Queensland Textiles, polyester insulation safety data sheet (the sheet does not name the testing laboratory) ↗. In our flame test the polyester batt melted back and left the tile clean. Neither product is fireproof.
Why take the risk with a product that is newer under Australian house floors and has had problems reported overseas, when there is a perfectly good Australian product that, in my experience, has been used under floors for around 20 years without the problems reported with spray foam? See the polyester underfloor batts guide, our underfloor insulation service and spray foam against polyester underfloor insulation.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Questions about spray foam fumes and toxic gases
How long should I stay out of the house after spray foam is installed?+
The US Consumer Product Safety Commission advises that residents vacate the premises during and after installation for at least 24 hours. Ask the installer why their advice is different if it is.
What is TCPP in spray foam?+
TCPP, tris(1-chloro-2-propyl) phosphate, is a flame retardant added to rigid polyurethane foam. In March 2026 the International Agency for Research on Cancer classified it as probably carcinogenic to humans (Group 2A), based on sufficient evidence in animals and strong mechanistic evidence in human cells, with inadequate evidence in humans. In a US government test house it was still measurable two years after open-cell foam was applied.
Is hydrogen cyanide from burning spray foam a carcinogen?+
No. The US EPA classifies cyanide as not classifiable as to human carcinogenicity. It is an acute poison. A 2011 laboratory study found hydrogen cyanide was the major toxicant when polyurethane foam burns.
How long does the smell last if spray foam does not cure properly?+
No regulator gives a set time. A building-science writer reported fish-like odours that last for months in the cases sent to him, and the US EPA says removing misapplied foam may not resolve the problem and may create other problems.
Does an installer need a licence to spray foam insulation in Queensland?+
The Queensland Building and Construction Commission says the installation of home insulation in Queensland does not require a licence.
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