Spray foam under the floor
Smell from under the floor after spray foam underfloor insulation?
A fishy or urine-like smell, and spray foam went under the floor. Here is what the published sources say about it, what to do first, and what your options are.
I’m Peter Johnson. I’ve insulated Brisbane houses since 1986, and I get this call.
Spray foam has only just arrived in Brisbane homes. In my experience it was used in farm sheds and industrial buildings, and it is only recently that some Brisbane companies have started spraying it in houses. The Australian Building Codes Board (ABCB), which publishes the National Construction Code (NCC), says spray foam is “not commonly used in Australian residential construction” ABCB, NCC 2025 Condensation in buildings handbook, p.59 ↗. Ask any installer whether their foam has been tested in Queensland’s climate, and ask for the test reports in writing.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
Reported causes of a fishy smell after spray foam underfloor insulation
A fishy, urine-like or ammonia-like smell can come off spray foam that was applied incorrectly. The published sources link a fishy smell to incorrect application. A 2014 study of 13 adults in 10 homes with faulty spray polyurethane foam found that all of them reported fishy odours Huang and Tsuang, Environmental Research, 2014 ↗. CBC News (Canada) reported that, when installed incorrectly, spray foam can give off “a strong, unpleasant fishy smell” CBC News (Canada), 25 October 2013 ↗. The foam is mixed from two parts, and one of them contains amine catalysts US Consumer Product Safety Commission, spray foam guidance for consumers ↗.
The published sources that discuss the smell tie it to how the foam was applied. Ask your installer which part of the job could have caused it in your house.
Damp timber, or animals living under the house, can smell too
The UK sources on foam and moisture are about roofs, and the worry carries over to timber under foam. The Royal Institution of Chartered Surveyors in the UK, writing about spray foam in roofs, warns that it will seal in moisture unless adequate precautions are taken Royal Institution of Chartered Surveyors (UK), Spray foam insulation consumer guide, March 2023 ↗, and the Property Care Association says closed-cell foam, particularly when applied directly to roof tiles, can create a barrier that traps moisture and lead to timber rot or decay Property Care Association, spray foam customer guidance ↗. For a foil-faced foam board fixed under floor joists, the ABCB says that if it is successfully sealed it “may also restrict downward drying”, which can increase the risk of trapping liquid moisture from leaks or spills above the floor. That is a rigid board, not spray foam ABCB, NCC 2025 Condensation in buildings handbook, s.5.8.2, p.61 ↗.
Animals can be the cause as well. Possums and rodents sometimes make a home under a house, and so does the odd much-loved cat or dog that has found a cosy spot. Have a look under there before you decide what the smell is.
What to do first about a spray foam smell under your floor
- Tell the installer who sprayed it, in writing, and ask them to check the foam and the cure.
- Ventilate the space under the floor. The US Consumer Product Safety Commission says to seek medical attention if anyone has breathing problems or other adverse health effects, and to work with the contractor or hire an independent indoor air quality expert to find the source CPSC, What should homeowners do if they have concerns? ↗.
- Check the timber for moisture (next section) before you decide what to do.
How to check for moisture in spray foam under a timber floor
Moisture under foam stays hidden, so the check is on the timber behind it. Where an edge of foam has already come away from the floorboards or a joist, look at the timber behind it with a torch for damp, dark or soft patches. Foam can hide wiring and pipes, so a building inspector or timber specialist is the safe way to take a small piece out and check. Damp timber under foam is a job for them to look at before anything else, and in my experience it often means the foam has to come off.
The Property Care Association, the UK damp and timber specialist trade body, explains how surveyors inspect foam-covered roof timbers in this talk, and moisture readings of the timbers are part of its guidance Property Care Association, New guidance on sprayed foam insulation inspections, 27 April 2023 (from 17:17) ↗. A short clip from a UK surveyor shows what he describes as the damp and rot that can occur with older-type spray foam in lofts Domestic Surveys Bristol, 9 November 2021 ↗.
Spray foam under a timber floor: what the National Construction Code says about moisture and ventilation
The ABCB handbook explains the principle with a wall. In its example of high-density spray foam in a brick veneer wall, the foam has a high resistance to water vapour, so the materials on each side are intended to dry away from it in one direction, which the handbook calls an intentional design trade-off ABCB handbook, p.59 ↗. On a timber floor the boards sit on one side of the foam and the room is above them, which is where mop water lands.
The 2022 National Construction Code requires the space under a timber floor to be ventilated through set openings and to “contain no dead air spaces” NCC 2022 Housing Provisions, 6.2.1(2)(c) ↗. Its minimum vapour permeance rules cover external wall layers in climate zones 4 to 8, and its roof space ventilation rule covers zones 6 to 8 NCC 2022 Housing Provisions, Part 10.8 ↗. Brisbane is climate zone 2, and those rules do not cover floors, so the code does not say how a floor with foam under it should dry. Ask the installer how their product meets the code’s performance requirements.
So the questions for any installer or sales rep are:
- What happens when I mop the floor, and what lets the water evaporate?
- Has the foam been tested with water from mopping, with a roof leak that would normally escape through the floor, or with flood water?
- Can you give me the test reports in writing?
To see for yourself before you commit a whole house, you can buy a small can of polyurethane foam from the hardware store, spray a patch on an offcut of floor timber outdoors (gloves on, can instructions followed), pour water over it, and see whether the water gets through and how many days or weeks it takes to dry. A hardware-store can is not the same as a professional two-part foam, so treat it as a rough guide and still ask the installer for their test reports. That costs a few dollars. Scraping foam off a whole floor and replacing the timber costs far more.
Spray foam insulation fire and smoke rules for houses
For a house, the fire hazard clause in Volume Two of the 2022 National Construction Code is H3D2. It sets limits for roof sarking-type materials and for flexible ductwork carrying products from a flame-fired heat source, and the clause does not list insulation NCC 2022 Volume Two, Part H3 Fire safety (Internet Archive copy) ↗. The flame-spread and smoke test it builds on is AS/NZS 1530.3. That leaves one useful question for your installer: what can you show me about how this foam behaves in a fire?
Here is what happened in our own flame test, with the foam burning on a white tile and giving off dense black smoke. The toxic gases from burning foam, the flame retardant TCPP and what comes off the foam after spraying are on a separate page: spray foam underfloor insulation: what the sales rep doesn’t tell you.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
How long has spray foam been used in Australian houses?
From what I have seen, spray foam has been sold in Brisbane for about four or five years, and it reached Sydney slightly earlier. The ABCB says it is “not commonly used in Australian residential construction” ABCB handbook, p.59 ↗, and the Australian Modern Building Alliance, a polymer-industry body, says spray foam in roofs is “not yet common practice in Australia”, though the same sheet says spraying timber floors is a common application in Canberra Australian Modern Building Alliance, Polyurethane insulation and fire safety ↗.
Is spray foam underfloor insulation proven in Queensland homes?
Plenty of products on the market, including ones you can buy online, are made for one purpose. The seller tells you it works for that purpose, but it can turn out to be unsafe, ineffective, or not approved for use in your home. Be wary of products that are not well tested and have not been in the market for long. Look at what has happened in other countries first. In Canada, CBC reported spray foam installed incorrectly driving people from their homes CBC News, 25 October 2013 ↗. In the UK, BBC News reported in November 2024 that a quarter of the biggest mortgage providers would not lend on homes with spray foam in the roof, and others only case by case BBC News, 18 November 2024 ↗.
Why we advise against spray foam underfloor insulation
We don’t sell it and we don’t install it, and we strongly advise against it under a timber floor. The reasons:
- It is very hard to get off once it is on. The Royal Institution of Chartered Surveyors in the UK, writing about roof foam and listed buildings, says spray foam is “a permanent installation” whose “removal could potentially damage the existing structure” Royal Institution of Chartered Surveyors, Spray foam insulation consumer guide, March 2023, p.12 ↗.
- Foam can hold moisture against timber (see above), and a floor that gets wet from mopping, a leak or a flood has to dry out on one side only.
- In our own flame test it burned with dense black smoke.
- A smell can come with it if it was applied incorrectly, and there is no easy fix once it is there.
- Lenders overseas have noticed. BBC News reported in November 2024 that a quarter of the biggest UK mortgage providers would not lend on homes with spray foam in the roof BBC News, 18 November 2024 ↗.
Can spray foam underfloor insulation be removed?
We remove old fibreglass batts and vacuum ceilings. Spray foam is different. If you want a quote to remove it, that is done job by job. It can’t be quoted easily, because it is a very hard product to remove, and once it is there it is very hard to get out fully. That is part of the reason we don’t recommend or sell it. One poster on an Australian forum put it this way: spray foam “is a pain to remove to fix properly later” Whirlpool forum, 10 June 2021 ↗.
The polyester underfloor insulation we fit instead of spray foam
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?
Stiff R3.0 polyester batts, cut to the joist spacing and held in the bays. Water drains through them and they dry out, and they pull down by hand if you ever need to get at the floor. See our underfloor insulation service and the polyester underfloor batts guide.
Comfort Zone insulation.team
Family-owned insulation manufacturer, making comfy homes since 1986
In this video I put a flame to spray foam and to a polyester batt, side by side. The full write-up is in my spray foam under your floor guide, and the two products are compared in spray foam against polyester underfloor insulation.
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 about the smell from spray foam underfloor insulation
Why does my house smell fishy or like urine after spray foam went under the floor?+
The published sources link a fishy smell to spray foam that was applied incorrectly. A 2014 study in Environmental Research looked at 13 adults in 10 homes with faulty spray polyurethane foam, and all of them reported fishy odours (Huang and Tsuang, Environmental Research, 2014). CBC News in Canada reported that foam installed incorrectly can give off a strong, unpleasant fishy smell. Both sources tie the smell to how the foam was applied, so ask the installer to check the foam and the cure in your house.
Could it be something other than the foam?+
Yes. Under a floor, a smell can also come from damp timber or from an animal. UK surveyors, writing about spray foam in roofs, warn it will seal in moisture unless adequate precautions are taken (Royal Institution of Chartered Surveyors, 2023). Possums, rodents and the odd much-loved pet can also make a home under a house. A urine-like smell could be any of these, so look before you decide.
What should I do first?+
Tell the installer who sprayed it, in writing, and ask them to check the foam and the cure. The US Consumer Product Safety Commission advises people to seek medical attention for breathing problems or other adverse health effects, and to work with the contractor or hire an independent indoor air quality expert to find the source. Ventilate the space under the floor while you wait.
Can spray foam underfloor insulation be taken off?+
It is done job by job, and it can't be quoted easily. Spray foam is a very hard product to remove, and once it is there it is very hard to get out fully. The Royal Institution of Chartered Surveyors in the UK, writing about roof foam and listed buildings, calls it a permanent installation whose removal could potentially damage the structure.
What underfloor insulation do you fit instead of spray foam?+
Stiff R3.0 polyester underfloor batts, cut to the joist spacing and held in the bays. Water drains through them and they dry out, and they can be taken down if a floor ever has to be reached.
Did we sort your underfloor? 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 →
All of the comfort. None of the gaps, pests, mould or itch.
Make your home a Comfort Zone today. Ask us how.