Does cellulose settle or cause mould?

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.

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. I won't tell you it kills mould; I'll tell you that removing the cold gaps removes the place mould likes to grow.

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. I want to be straight with you about the 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.

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 honest 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.