Does cellulose settle? · here’s what actually happens
Does cellulose insulation settle, and does it matter?
Any loose-fill can settle a little. But we pump ours dense and quote the settled R-value, so there’s little left to settle out of, and because we install R3.0 over the ~R2.5 code minimum, a minor settle over decades still leaves you above what the NCC asks. The real enemy of ceiling insulation isn’t settling. It’s gaps.
I’m Peter Johnson. After 40 years and 6,000-plus roofs, here’s the straight story on settling: the bit that’s true, the scary numbers that don’t apply to a proper job, and the thing that actually wrecks ceiling insulation that nobody talks about.
Pumped to settled density
the R-value you’re quoted is the one you keep
Here’s the deal
Yes, loose-fill can settle, so the trick is how it’s installed.
I’m not going to tell you cellulose magically defies gravity. Any loose-fill insulation can settle a little over time. The Australian Government’s CSIRO says so plainly. What decides how much is the install density. We pump our cellulose to a specified settled density of around 35–50 kg/m³, so it goes in already packed to the depth it’ll hold, not fluffed up to look thick on day one.
That matters because the R-value we quote you is the settled R-value. It’s the figure the coverage charts are built around, and it’s what our transferable Life-of-House Guarantee is written against. In plain terms: there’s nothing left to settle out of, because we didn’t sell you day-one fluff in the first place.
The settling horror stories people hear about loose-fill all trace back to the same thing, an under-filled job. A cowboy blows it in light and airy so the bags go further, it slumps, and it opens up. Pack it to the right density and it stays put. The Australian Government’s yourhome guide tells you how to protect yourself in one line: ask your contractor for a guaranteed settled R-value, not a day-one number. That’s exactly what we install to.
Cellulose pumped to a settled density, full-depth and even. Packed right, it holds its depth. This is what “rated at settled density” looks like in a real roof.
Where the “20% settling” line comes from
Those scary numbers are from a different job entirely.
If you've read that cellulose settles 20%, or loses 6–10% of its R-value, that's real research, but read the fine print. It describes a deep, low-density North-American attic blow, not a dense Australian ceiling pump.
| The “cellulose settles a lot” studies | How we pump a ceiling | |
|---|---|---|
| Depth | Deep attic blow, ~250–300 mm | Shallow ceiling, ~100 mm, far less weight pressing down |
| Density | Low, ~24–34 kg/m³ (light open blow) | Dense, ~35–50 kg/m³, packed to hold |
| How “settling” was measured | Forced in a lab by striking the ceiling. The Oak Ridge researchers said it “may or may not” represent real in-service settling | Real roofs, packed once to settled density and left alone |
| R-value quoted as | Day-one thickness (so settling eats into it) | Settled density, the number you keep |
Industry research finds settling is virtually nil once cellulose is packed to around 3.5 lb/ft³, and that’s precisely why coverage charts are written at settled density. So the takeaway isn’t “ours never moves”. It’s “we install it dense and quote you the settled figure, so you’re not paying for fluff that disappears.”
You bought a buffer
Even if it settles a little, you’re still over what the code asks.
Here in South East Queensland (NCC climate zone 2) the minimum added ceiling insulation the building code asks for is about R2.5. We install around R3.0 as standard, roughly 20% over. So a small, gradual settle over many years still leaves you at or above the figure the code actually requires. You paid for a margin, and the margin does its job.
Settling isn’t a cliff. It’s slow and minor in a dense install, measured over decades, not something that happens to your power bill next winter. By the time any of it could matter, you’re still sitting on more insulation than Brisbane’s climate calls for.
Ceiling R-value, zone 2
That ~20% gap between what we install and what the code asks is the buffer. A minor settle over decades eats into the top of it, not into your compliance, and not into your comfort.
The thing nobody warns you about
The real enemy of ceiling insulation isn’t settling. It’s gaps.
Worry less about a few millimetres of settle and more about this: a rating only counts if the insulation actually covers the whole ceiling. That's where batts quietly fail, and where pumped cellulose wins.
Batts: cut to fit, shoved around, gappy
Batts are cut to fit and nearly always leave gaps over wires, battens and around fittings. Worse, over the years the sparky, the plumber and the air-con installer climb up there, shove them aside and leave them open. Even a small uninsulated percentage of a ceiling drags the effective R-value down sharply, per Sustainability Victoria’s government nomogram. That’s why an R5 batt often isn’t an R5 once it’s lived in your roof a few years.
Cellulose: seamless, dense, stays put
Pumped cellulose fills right into every corner as one seamless blanket, no cut lines, no gaps over the wiring. And because it’s dense, it’s hard for other trades to push out of the way, so the job you paid for stays in good condition for decades. A gap-free R3.0 beats a gappy R5 every single time, because the rating you bought is the rating you actually get.
So when someone tries to scare you about cellulose settling a few millimetres, ask them what their batts look like after ten years of tradesmen walking through them. Mine still look like the day we pumped them. See cellulose vs fibreglass batts →
One exception
If you ever see settling, look at the roof, not the insulation.
There’s one situation where a properly-packed cellulose ceiling can compact a little, and it’s not the product’s fault. On an older tile roof that lets a fine mist of water in during heavy rain (through gaps between tiles, a tired whirlybird or a small leak), that incidental moisture adds a bit of weight on top of the cellulose. Over many seasons of wetting and drying, that can press it down slightly.
That’s a roof problem, not an insulation problem. Cellulose is hygroscopic: it handles incidental moisture and dries back out, regaining its dry performance once the water source is fixed. The fix is the roof, not ripping out good insulation. If we ever see it on a job, we’ll tell you straight where the water’s really coming from.
Where a roof leaks, cellulose tends to hold the damage to one spot and dry back out, which actually helps us find the leak. The insulation isn’t the problem; the roof is.
Straight answers
Settling questions I get asked most.
Does cellulose insulation settle?+
Any loose-fill insulation can settle a little over time. The Australian Government's CSIRO says so plainly, and we won't pretend otherwise. But how much comes down to how densely it's installed. We pump our cellulose to a specified settled density (around 35–50 kg/m³), so it goes in already packed to the depth it will hold, not fluffed up to look thick on day one. At the dense end of that range settling is minimal, and the R-value we quote is the settled R-value. There's nothing left to settle out of. The settling horror stories come from under-filled jobs blown in light and airy, not from a roof packed to the right density.
Doesn't cellulose settle by 20%? I read that somewhere.+
That figure, and the lab number of a 6–10% R-value drop from a 9% settle, comes from deep North-American attic blows: loose-fill piled 250–300 mm thick at a low density of about 24–34 kg/m³, and in the lab study the settling was forced by hitting the ceiling, which the US Government's Oak Ridge researchers themselves said 'may or may not' represent real in-service settling. A dense, shallow ~100 mm Australian ceiling install is a completely different animal, far less depth pressing down, far higher density. Industry research finds settling is virtually nil once cellulose is packed to around 3.5 lb/ft³, and the very reason coverage figures are quoted at settled density is so the number you're sold is the number you keep.
If it settles a bit, do I lose R-value?+
If a layer loses thickness, it loses some R-value with it. We're not going to tell you physics works differently for us. The point is the size and the timeframe. Pumped dense, any settling is small and happens slowly over years, not overnight. And we deliberately install around R3.0 when the NCC minimum for our climate zone is about R2.5 (roughly 20% over), so even a minor settle over decades still leaves you at or above what the building code actually asks for. You bought a buffer.
Will cellulose leave gaps the way batts do?+
No, and this is the part that actually matters for your power bill. Batts are cut to fit and nearly always leave gaps over wires, battens and around fittings, and over the years electricians, plumbers and air-con installers shove them out of place and leave them open. Even a small uninsulated percentage of a ceiling drags the effective R-value down sharply (per Sustainability Victoria's government nomogram). Pumped cellulose fills in as one seamless blanket with no gaps, and because it's dense it's hard for other trades to move, so your ceiling stays in good condition far longer. A gap-free R3.0 beats a gappy R5 every time.
Does your guarantee cover settling?+
Yes. We install to a specified settled R-value and back it with our transferable Life-of-House Guarantee. The Australian Government's yourhome guide tells you to ask your contractor for a guaranteed settled R-value rather than a fluffy day-one number. That's exactly what we install to, and what we stand behind.
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