Compare · Underfloor insulation · SE Queensland
Spray foam vs polyester under your floor: what foam installers won’t tell you.
Closed-cell foam seals the underside of your floorboards. In our humid climate that can trap moisture against the timber and drive rot you can’t see or easily remove. Polyester breathes, lifts out, and costs a fraction.
I get asked about spray foam under floors every week. I don’t sell it, so this isn’t “my product versus yours”. It’s the version the foam installers won’t put on their own site. I’ve been in thousands of SE-QLD subfloors, and here’s what I’d never do to my own house.
Polyester, stapled to the joists
vapour-open, non-itch, and it lifts straight back out
The lead argument
Moisture is the one I’d worry about first.
Closed-cell polyurethane foam sets like a hard skin and seals the underside of your floorboards airtight. The Australian Building Codes Board’s Condensation in Buildings Handbook spells out the mechanism: closed-cell insulation typically has a low vapour permeance, while open materials like mineral wool let moisture through. Under a Queenslander on stumps, that sealed skin can trap whatever damp is in the timber and stop it drying out, and that’s the documented road to hidden rot and surface mould.
You don’t have to take my word that crawlspaces and foam are a bad mix. Even a Brisbane cellulose installer, Roo Roofing, publishes that crawl spaces are prone to mould growth and need a material that lets the subfloor breathe. Polyester does exactly that: it’s hydrophobic and won’t absorb or retain moisture, so the floor keeps drying out the way it’s meant to.
“In a humid SE-QLD subfloor, I want the insulation to let the timber dry, not seal the damp in. That’s why I’d put polyester under my own floor and never foam.”Peter Johnson, Comfort Zone Insulation Team
The short version
Foam traps moisture against the timber. Polyester lets it breathe. In our climate, that’s the whole argument right there.
Side by side
Polyester vs closed-cell spray foam, under the floor.
Six things a homeowner actually cares about. I've flagged the winner on each one, including the single axis where foam beats polyester.
| What matters | Polyester batts | Closed-cell spray foam |
|---|---|---|
| Moisture in our climate | Vapour-open and hydrophobic. It lets the subfloor breathe and won't hold dampness against the timber. better | Closed-cell foam seals the underside airtight (low vapour permeance, per the ABCB). In a humid subfloor that can trap moisture against the wood and drive rot you can't see. |
| Removability | Stapled to the joists, lifts straight out in an afternoon for plumbing access, repairs, or resale. Can be reused. better | Permanent. It bonds to joists, pipes and wiring; removal is complex, costly and can damage the floor structure. Once it's on, it's on. |
| Health during install | Inert from day one: no isocyanates, no off-gassing, no skin-irritation warning on the bag. better | Sprayed isocyanates (a leading cause of occupational asthma in Australia). You vacate ~24h while it cures; cheap foam can off-gas VOCs for days. Cured, well-installed foam is largely inert. |
| Cost | Around $28–35/m² installed (Pricewise). better | Roughly 2–4× more: closed-cell $70–120/m² on Brisbane Spray Foam's own price list; batts cost about a quarter of spray foam (Pricewise). |
| R-value per inch | Lower per inch, but you've got plenty of depth between joists, so a thicker batt reaches the same total R with no gaps. | Higher R-per-inch, the one axis foam wins outright. It just doesn't matter once polyester is fitted full-depth with no gaps. better |
| Who carries the risk | You keep your options open. It comes out, gets reused, and doesn't lock you in. better | The homeowner, permanently. If the moisture, removal or resale concerns ever bite, you own them, not the installer who's long gone. |
That’s the scorecard. Polyester wins underfloor in SE-QLD on every axis a homeowner cares about (moisture, reversibility, health during install, and cost) and the one thing foam beats it on, R-value per inch, simply doesn’t matter once polyester is fitted full-depth between the joists with no gaps. Read the detail behind each line below.
Removability
Once foam’s on, it’s on for good.
This is the part that gets people stuck. Closed-cell foam doesn’t sit between your joists. It bonds to them, and to the pipes and wiring it was sprayed over. Australian coating specialists describe spray-foam removal as complex, labour-intensive and costly, often needing damaged timbers repaired or replaced afterwards. Taking it off means grinding it back off the structure, and you can damage the floor doing it.
Polyester is the opposite. It’s stapled up between the joists, so when a plumber needs to get at a pipe, or you’re selling and a building inspector wants to look, it lifts straight back out in an afternoon and goes straight back up. No grinding, no structural risk, no permanent commitment. If you ever change your mind, polyester lets you. Foam doesn’t give you that choice.
Health during the install
Polyester is inert from day one. Foam isn’t, while it goes in.
Spray polyurethane foam is made by reacting isocyanates on site. SafeWork NSW lists isocyanates as one of the leading causes of occupational asthma in Australia, and once you’re sensitised, even tiny later exposures can set it off. That’s why occupants are typically asked to leave the home for around 24 hours while it cures, and why cheap or badly-mixed foam can off-gas VOCs for days.
To be fair about it: fully cured, correctly-installed foam is largely inert. The risk is the spraying and curing window, and poorly-done jobs, not the foam sitting there years later. But that’s the point. Polyester carries none of that risk at any stage. It’s a non-itch plastic fibre, inert from the moment it’s fitted, and our installers go home in the same clothes they came in.
Cost
You pay multiples more, for the harder option.
Foam is not the cheap choice. Pricewise Insulation states plainly that insulation batts cost nearly a quarter of the price of spray foam. Their underfloor cost guide puts polyester around $28–35/m² installed against $40–60+/m² for closed-cell foam, and a Brisbane spray-foam installer’s own price list lands closed-cell at $70–120/m². Whichever set of numbers you use, you’re looking at roughly two to four times the cost of polyester.
So weigh it up. You’d be paying two-to-four times more for the option that can trap moisture against your timber, can’t be removed without grinding it off the structure, and carries a real health risk while it goes in, versus polyester, which is cheaper, inert, breathable, and lifts straight back out. The maths only works one way under a SE-QLD floor. If you want the full breakdown, here’s what underfloor insulation actually costs in QLD.
The one thing foam wins
Yes, foam has a higher R-per-inch. It just doesn’t matter here.
I’m not going to pretend otherwise. Inch for inch, closed-cell foam has a higher R-value than polyester. That’s a genuine advantage when you’re fighting for every millimetre, like inside a thin wall or a tight retrofit. But it’s the wrong fight under a floor. You’ve got the full depth of the joist to play with, so we just fit a thicker polyester batt and reach the same total R-value, and for most Brisbane homes that’s R2.0 to R2.5, which polyester hits comfortably. (The same R-per-inch story plays out in a ceiling too: here’s cellulose vs spray foam up top.)
And the R-rating on the bag only counts if the insulation is fitted with no gaps. Polyester cut and friction-fitted snugly between the joists covers the whole floor with no cold spots. So once it’s in, that R-per-inch number foam brags about buys you nothing the polyester didn’t already deliver, without the moisture, removal and health baggage that comes with it.
Who carries the risk
The homeowner does, permanently.
Here’s the bit that doesn’t fit in a price quote. With foam, every one of the concerns above lands on you, and it lands for good. If moisture gets trapped and the timber starts to go, that’s your floor. If you need to remove it, that’s your cost and your structural risk. The installer who sprayed it is long gone. Foam is a one-way door. You only get to walk through it once.
Polyester keeps every door open. It breathes, so the moisture risk barely arises; it lifts out, so removal isn’t a problem; it’s inert, so there’s no health question; and it costs a fraction. That’s why, after thousands of SE-QLD subfloors, it’s the product I’d put under my own house, and the one I’d recommend for yours.
Straight answers
Spray foam under the floor: the questions I get asked most.
Is spray foam under my floor a problem in a humid climate like SE Queensland?+
It can be. Closed-cell polyurethane foam has a low vapour permeance, so it seals the underside of your floorboards. The Australian Building Codes Board's Condensation in Buildings Handbook notes that closed-cell insulation typically has low vapour permeance while open materials like mineral wool let moisture through. In a damp subfloor that means moisture can be trapped against the timber instead of drying out, the documented pathway to hidden rot and mould. Polyester batts stay vapour-open, so the floor can keep breathing.
Can you actually remove spray foam from under a floor later?+
Not easily. Once it cures, closed-cell foam bonds to the joists, pipes and wiring it was sprayed over. Australian installers describe removal as complex, labour-intensive and costly, often needing damaged timbers repaired or replaced afterwards. Polyester batts are the opposite. They're stapled up and lift straight back out in an afternoon if a plumber needs access or you ever sell. With foam, what goes on stays on.
Is spray foam off-gassing something I should worry about?+
The real, documented risk is during spraying and curing, not for years afterwards. Spray polyurethane foam is made by reacting isocyanates on site, and SafeWork NSW lists isocyanates as one of the leading causes of occupational asthma in Australia. Occupants are typically asked to leave the home for around 24 hours while it cures, and cheap or badly-mixed foam can release VOCs for days. The fair caveat: fully cured, correctly-installed foam is largely inert. Polyester carries none of that. It's inert from day one.
Doesn't spray foam have a higher R-value than polyester?+
Per inch, yes. Closed-cell foam has a higher R-value than polyester. But under a floor that rarely matters. You've got plenty of depth between the joists, so we simply fit a thicker polyester batt (R2.0 to R2.5 suits most Brisbane homes) to reach the same total R-value with no gaps. The R-per-inch advantage only counts when space is tight, which a subfloor isn't.
Why don't you pump cellulose under floors instead?+
Because it's the wrong product for a crawlspace. Cellulose is the right answer in a ceiling, but under a floor, exposed to ground moisture and humidity, polyester is the material I'd use. It's hydrophobic, it doesn't waterlog, and it won't hold dampness against the timber. We use whichever product is right for the cavity, not whichever one we'd rather sell.
Is spray foam really more expensive than polyester underfloor?+
Yes, usually two to four times more. Pricewise Insulation states insulation batts cost nearly a quarter of the price of spray foam, with underfloor polyester landing around $28–35/m² installed against $40–60+/m² for closed-cell foam. Brisbane Spray Foam's own published price list puts closed-cell at $70–120/m². So you pay multiples more for the option that's harder to fix and can trap moisture.
Thinking about insulating under your floor?
I don’t sell spray foam and I won’t talk you into anything. If your floor is worth doing, we’ll fit polyester between the joists with no gaps: vapour-open, non-itch, removable, and a fraction of the price of foam. If it’s not worth doing in your home, I’ll tell you that too.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986
Want the bigger picture on which product goes where? Read cellulose vs polyester for ceilings versus floors, or our underfloor insulation service page.