FAQ · The two questions people worry about most
Does cellulose settle or cause mould?
Loose-fill can settle, but the right install density controls it, so we pump to a specified settled density that holds. And cellulose doesn’t cause mould: full contact removes the cold gap where condensation forms.
These are the two worries I hear most once people get past “is it safe?”. They’ve read that loose-fill slumps, and they’ve seen mould creep across a ceiling. Both have a real answer, and I’ll give you the straight version of each, with the Australian Government’s own sources behind it, not a salesman’s “never happens.”
Settling
Density controls settling, and the settling stories come from under-filled jobs.
Here’s the plain fact, straight from the Australian Government: loose-fill insulation does settle a little, and that’s exactly why it has to be installed properly. The CSIRO says loose-fill cellulose may settle over time and that allowance should be made during installation, and that correct installation requires an appropriately high density that the installer should state. That last bit is the whole game. Pump it in light and airy and it looks thick on day one, then slumps. That’s where the settling horror stories come from. Pump it to its proper settled density and it’s already at the depth it will hold.
So the settling people hear about isn’t a cellulose problem. It’s an under-filled-job problem. A cowboy blows it in light so the bags go further and the quote looks cheaper, the customer never climbs up to check, and a year later it’s slumped and there are gaps. We pump to a specified density, so what goes in is what stays. The Australian Government tells you the single best way to protect yourself, and it’s the question to put to every quote you get: yourhome.gov.au says to ask your contractor for a guaranteed “settled R-value” , not a fluffed-up day-one number.
What you actually rely on
I lead with the guarantee, not a “never settles.”
I’m not going to give you a physics absolute and say it can never move. Nobody can actually promise that. What I’ll give you instead is a written commitment. In more than 6,000 jobs I’ve never seen our correctly-installed cellulose settle, blow around or open up gaps. The only time I’ve pulled it out is when someone was replacing their ceilings. The cellulose we pumped 10+ years ago still looks new.
That’s lived experience, and it’s worth telling you. But the thing you actually rely on is the paperwork: our cellulose carries a transferable Comfort Zone® Life-of-House Guarantee that passes to the next owner if you sell, and we install to the product’s specified settled density. So the answer to “will it slump?” isn’t a salesman’s promise. It’s a settled R-value installed to spec, in writing.
Mould
Mould grows on the cold gaps. Full contact takes the cold spot away.
First, let’s be clear what it does and doesn’t do: I won’t tell you cellulose kills mould. What it does is remove the place mould likes to grow. Mould on a ceiling comes from condensation, and condensation forms on cold spots. Where insulation is missing or gappy, that strip of ceiling stays cold while the rest is warm. Warm, moist house air hits the cold strip, condenses, and you get the mould. It’s why you sometimes see mould in dead-straight lines on a ceiling: those lines map the gaps between badly-laid batts.
This isn’t just my theory. The Australian Government joins the dots directly. The yourhome guide says thermal bridges, the cold pathways where insulation is missing, lead to condensation problems, and the ABCB’s Condensation in Buildings Handbook warns that condensation in concealed spaces breeds fungus and mould. Pumped cellulose covers the whole ceiling with full contact and no gaps, so there’s no cold strip for the moisture to condense on. That’s the mechanism. A thermal-gap fix, not a mould killer.
And if the roof leaks
Cellulose pins a leak to one spot. Batts hide it and spread it.
Here’s a difference most people don’t know about until they’ve had a leak. Cellulose is hygroscopic. It’s a paper fibre, so it absorbs a bit of incidental moisture and then dries back out. Fibreglass and mineral-wool batts are hydrophobic, so when water comes through a roof it runs straight underneath them and spreads across the whole ceiling, often hiding the leak until the plasterboard finally sags. With cellulose, a small leak tends to stay in one spot and leave a discoloured patch, which actually helps us find where the roof is leaking.
I’ll be straight about the limits, because the government’s guidance is that any insulation performs poorly if it gets wet: cellulose doesn’t waterproof your roof and it won’t fix a leak. A serious or ongoing leak needs the roof repaired, and badly wet insulation of any kind has to be assessed. What cellulose does do is handle the odd bit of incidental moisture and localise the damage, instead of letting water run silently across your whole ceiling the way it does under batts.
One place we don’t use cellulose
Under the floor we use polyester, not cellulose.
To be clear: pumped cellulose is the right product for a ceiling, where it sits as a blanket on top of the plaster, but it’s the wrong product for a crawlspace, because there’s nothing under a suspended floor to hold a loose fibre up. So for underfloor we fit polyester batts, friction-fitted up between the joists. Polyester is locally made, non-itchy, doesn’t carry the glass-fibre handling warnings, and in 30-odd years I’ve never seen a polyester batt settle the way fibreglass does.
One more thing worth knowing: in a warm climate, underfloor insulation isn’t always worth doing. The Sustainability Victoria guidance notes that insulating a suspended floor can actually add to a home’s summer cooling load in some warmer climates, so I’ll tell you straight whether your floor is worth the money before you spend it.
More on settling, mould and moisture
Does cellulose insulation settle over time?+
Any loose-fill insulation can settle a little; the Australian Government's CSIRO says so plainly, and the reason it matters is that you have to allow for it when you install. The whole game is install density. We pump cellulose in to a specified settled density, so it's already packed to the depth it'll hold rather than fluffed up to look thick on day one. The settling horror stories people hear about loose-fill come from under-filled jobs, a cowboy who blows it in light and airy so the bags go further, then it slumps and opens gaps. Pack it to the right density and it stays put. yourhome.gov.au tells you exactly how to protect yourself: ask your contractor for a guaranteed settled R-value, not a day-one number. That's what we install to, and it's backed by our transferable Life-of-House Guarantee.
Does cellulose insulation cause mould?+
No, and done right it actually helps prevent the surface mould you get on a ceiling. Mould on a ceiling comes from condensation, and condensation forms on cold spots. The Australian Government ties this together directly: yourhome.gov.au says thermal bridges (the cold gaps where insulation is missing) lead to condensation problems, and the ABCB's Condensation in Buildings Handbook warns that condensation in concealed spaces breeds fungus and mould. Pumped cellulose covers the whole ceiling with full contact and no gaps, so you don't get the cold strips where warm, moist house air hits the plaster and condenses, which is why you sometimes see mould in straight lines that map the gaps between batts. So it's a thermal-gap mechanism, not a mould killer. It doesn't kill mould; removing the cold gaps removes the place mould likes to grow.
What happens to cellulose if my roof leaks?+
Cellulose is hygroscopic; it's a paper fibre, so it absorbs incidental moisture and then dries back out. Batts are hydrophobic, so when water comes through a roof it runs underneath and spreads across the whole ceiling, often hiding the leak until the plasterboard sags. With cellulose a small leak tends to stay localised in one spot and leaves a discoloured patch, which actually helps us find where the roof is leaking. There are limits: cellulose doesn't waterproof your roof and it won't fix a leak; a serious or ongoing leak needs the roof repaired, and badly wet insulation of any kind has to be assessed. What it does do is handle the odd bit of incidental moisture and pin the damage to one place instead of letting it run everywhere.
Why do you use polyester batts under the floor instead of cellulose?+
Because pumped cellulose is the wrong product for a crawlspace. It's a loose fibre that needs a ceiling to sit on top of, and there's nothing to hold it up under a suspended floor. For underfloor we fit polyester batts, friction-fitted up between the joists and held in place. Polyester is locally made, non-itchy, doesn't carry the glass-fibre handling warnings, and in 30-odd years I've never seen a polyester batt settle the way fibreglass does. One caveat: in a warm climate underfloor insulation isn't always worth doing. Australian guidance notes it can even add to summer cooling load in some homes, so I'll tell you straight whether your floor is worth doing before you spend the money on it.
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