FAQ · Batts & rivals
Does insulation cover the joists, or just fill the gaps between them?
Batts fill the gaps between the joists — they do not go over the top of them. Every joist is left exposed, carrying only about R1.5 in a timber-frame home and near R0 in a steel-frame home. That leaves a stripe of low R-value running across your ceiling every 450 to 600mm.
This is called thermal bridging, and it’s one of the main reasons the R-value on the bag is not the R-value you end up with in your roof — something I go through in detail on why the R-value on the bag is not the R-value in your roof. Dense-packed cellulose is pumped over the whole ceiling — across the tops of the joists as well as between them — so it covers the frame instead of stopping beside it. One continuous blanket, no stripes.
Why batts leave stripes across your ceiling
The joist is not insulated. The batt sits beside it.
Insulation batts come in two standard widths — 430mm and 580mm — cut to fit between the ceiling joists, not over them. A typical timber joist is about 50mm wide, and the batt width is sized to sit snugly against it. The timber or steel of the joist itself is left uncovered, running from one end of your ceiling to the other.
A timber joist carries a thermal resistance of roughly R1.5. So even in a batt job with zero gaps — no compression, no missed corners, no downlight holes — your R5 batts come with R1.5 stripes running across the whole ceiling every 450 to 600mm. The Australian Government Your Home guide on insulation lists thermal bridging as one of the key factors that reduces the real R-value you receive from your insulation. In summer these stripes are hot bridges; in winter they are cold bridges. The heat moves both ways, through the joist, regardless of the batt in the bay beside it.
And where there are battens, the batt isn’t even touching your ceiling.
A lot of roofs have battens — either steel top-hat battens or timber battens — running underneath the frame. The batts lay acrossthose battens, which holds them up off the ceiling. Heat conducts through the joist, into the batten, and then travels across underneath the batt in the air gap the batten creates — an air gap that runs all the way to the edge of the roof and lets heat push around under the insulation. In a battened roof the batts can’t sit flush on the ceiling they’re meant to be insulating, so you get noticeably less R-value than the number on the pack.
Where it gets much worse
In a steel-frame home, the frame carries near R0 — and the batts leave side gaps too.
Steel conducts heat at roughly 50 times the rate of timber. A timber joist carries about R1.5 of resistance; a steel joist carries effectively near R0, behaving almost like a bare strip running across your ceiling every 450mm.
There’s a second problem in a steel-frame home that most people never hear about. Batts are cut to sit against a timber joist about 50mm wide. A steel frame member is only about 5mm thick. So a 430mm batt that fits neatly in a timber-frame home leaves roughly a 10 to 20mm gap down each side in a steel-frame home, between the batt and the frame and between the batt and the joist. That means a steel-frame home almost certainly has gaps beside the frame on top of the bridging — the two losses stack up.
Steel-frame construction became common in Queensland from the mid-1990s onwards, and a lot of homes built in that era now have owners wondering why their insulation isn’t doing what the bag promised. This is usually why. The batt between the members may be exactly what they paid for; it’s the frame, and the gaps beside it, bleeding the heat through.
| Frame type | Frame R-value | Typical batt fit | To rival R3 cellulose in the real world |
|---|---|---|---|
| Timber frame | ≈ R1.5 | Snug against ~50mm joist | R6 batts, perfectly installed |
| Steel frame | ≈ R0 | ~10–20mm side gaps on ~5mm steel | R6+ batts, perfect, and still bridging |
| Dense-packed cellulose (ours) | Frame covered over | No gaps, no stripes | Our R3 does it |
To rival our R3 cellulose in a steel-frame home you’d need R6 batts installed perfectly with no gaps — and unless they’re installed perfectly, they aren’t performing at R6, they’re performing at roughly R3 in the real world.
Source: ICANZ — Ceiling Insulation Guidelines: Existing Homes (2024) · Australian Government Your Home — Insulation
How we fix it
We pump over the frame — dense, and in contact with it.
When we pump in cellulose, we don’t stop between the joists. Our minimum standard is to pump to about 100mm above the ceiling — the depth that gives our R3 rating, measured from the ceiling, not from the top of the joists. In a lot of modern homes the joists are only about 90mm tall, so 100mm from the ceiling sits just over the tops of them and covers the frame. On an older home with deeper six-inch joists it may not reach the very top of every joist, but those bigger timbers carry more resistance of their own anyway.
The difference with a steel frame is the density. Because the cellulose is dense-packed, it sits tightly against and over the steel and stays in contact with it, so it can draw heat out of the frame and help pull the temperature of the steel down. A steel frame is usually only about 75mm tall, so pumping over the top of it to our 100mm minimum both covers it and lets the fibre hold the frame in check — as much as we can, anyway. A batt just lays beside the frame and can’t do that.
That’s how cellulose delivers roughly the R-value you paid for in real-world conditions instead of the R-value on the bag. No cuts to fit, no side gaps, no bare joist stripes, and the frame held in contact instead of left as a thermal highway the batts can’t touch.
“The joist bridging thing is probably the most under-explained problem in the whole insulation industry. Batts work well between the joists. But the joists themselves are always the weak link — and in a steel-frame home they’re nearly worthless as insulation. Dense-packed cellulose fixes it by covering the frame and sitting on it, not beside it.”
See it for yourself — joists, depth, and what a real ceiling looks like going in.
The first clip shows exactly how deep we pump the cellulose and how the roof structure changes that depth — covering the frame is the whole point. The second shows a batt install in progress, so you can see exactly where the joists get left exposed. The third is the guarantee that covers the whole ceiling, not just the bays.
Read the transcript
Another thing to note with the construction of this roof is that these beams don't have the plaster attached on the bottom of them — they've actually got a batten that runs through underneath. So it's about a two-inch-high batten, 40 or 50 millimetres, and then you've got 100 millimetres of beam, so you've got nearly 150 millimetres there. When we fill it up with insulation, we only put in 100 millimetres, so the beams will stick out the top of the insulation in this roof. But if the beams were only 100 millimetres, it would be flush with the top of them. That's important to note, because sometimes people hop up and have a look and go, "Oh, you said it would be to the top of the joists" — but their joists have got a batten underneath, so our height is actually a bit higher.
Read the transcript
Okay, so this is a job that we've just done in Earthwool fibreglass batts. You can see those all nicely laid in the bays. Earthwool's thing is that they don't dye the batts, so they're just the natural colour that they come out of the process — that way they're better for the environment. Anyway, so that's what it all looks like when it's done.
The clips play right here on the page, or open the playlist to watch them all on YouTube and subscribe.
Some of these were filmed a while back. Our methods, safety standards and products have moved on since. For how we work today, see the rest of this page.
Some of these were filmed a while back. Our methods and products have moved on since then.
More on joists, bridging, and what actually covers them
Do batts cover the joists or just fill the gaps between them?+
Batts fill the gaps between the joists — they don't go over the top of them. The joists themselves are left exposed. In a standard tile-roof home there is a joist every 450 to 600mm running the full length of your ceiling, and each of those strips has only the thermal resistance of the timber itself — roughly R1.5. Dense-packed cellulose is pumped over the whole ceiling instead, so it sits across the tops of the joists as well as between them, and the ceiling becomes one continuous blanket rather than insulated bays with bare joist stripes.
What is thermal bridging and why does it matter?+
Thermal bridging is where heat moves freely through the ceiling structure, bypassing the insulation. A timber joist has a thermal resistance of roughly R1.5. In a steel-frame home the steel is near R0 — almost no resistance at all. Every joist is a stripe where heat flows into your home regardless of what R-value the batt in the bay beside it carries. The Australian Government Your Home guide lists thermal bridging as one of the key factors that reduces the real R-value you get from your insulation.
How much does a steel frame change things?+
A lot, in two ways. First, steel conducts heat at roughly 50 times the rate of timber, so a steel joist carries near R0 and each joist line behaves almost like a bare strip across your ceiling. Second, the steel itself is only about 5mm thick, where a timber joist is about 50mm wide — and batts are cut to sit snugly against a 50mm timber joist. Against thin steel, a 430mm batt leaves roughly a 10 to 20mm gap down each side, so steel-frame homes almost always end up with gaps between the batt and the frame as well. To get anywhere near the real-world performance of our R3 cellulose in a steel-frame home, you'd be looking at R6 batts installed perfectly with no gaps at all — and the frame would still be bridging.
How deep do you pump the cellulose?+
As a minimum standard we pump to about 100mm above the ceiling, which is the depth that gives our R3 rating — measured from the ceiling, not from the top of the joists. In a lot of modern homes the joists are only about 90mm tall, so 100mm from the ceiling sits just over the tops of them and covers the frame. In an older home with deeper six-inch joists the cellulose may not reach the very top of every joist, but those bigger timbers carry more resistance of their own anyway. Because it's dense-packed, the cellulose sits tightly across the frame rather than draping loosely like a batt.
Why does dense-packed cellulose help with a steel frame specifically?+
Because it's pumped in at density, the cellulose packs tightly against and over the steel and stays in full contact with it. That contact lets it draw heat out of the frame and help pull the temperature of the steel down, instead of leaving the frame exposed to bridge heat straight through. A batt just lays beside the frame and can't do that — and where the batt is held up off the ceiling by battens, it isn't even in contact with the ceiling it's meant to be insulating.
Can I get a batt job that covers the joists?+
Not in a standard ceiling. Batts are pre-cut to fit between the joists, and the joist spacing sets the batt width you buy. You could lay a second layer crossways over the top — a cross-hatch or double layer — but it adds real cost, needs more roof space than most Queensland homes have, and still relies on two perfectly installed layers with no gaps. In most roofs a double batt layer isn't practical, which is one of the reasons dense-packed cellulose is the straightforward way to actually cover the frame.
Comfort Zone Franchise
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