Steel-frame homes · Brisbane & SE Queensland
Insulating a steel-frame home, why batts lose half the benefit.
Steel is a thermal bridge. It carries heat straight through the frame, and in my experience a batt laid beside the steel loses about half the benefit. Cellulose pumps right over the steel by 50mm or more, with no gaps.
I’m Peter Johnson. After 40 years and 6,000-odd roofs, here’s the case for how a steel-frame home should actually be insulated, and why a slab of batts beside the steel is the wrong tool for the job.
Pumped over the steel, 50mm+
no gaps, the thermal bridge buried
Why a steel frame is uniquely hard to insulate
The steel is the problem, so the insulation has to cover it.
A steel-frame home is harder to insulate than a timber one because steel conducts heat readily. The whole frame becomes a thermal bridge, carrying heat straight from the hot side of the house to the cool side. Insulation that only sits beside the steel leaves the metal doing its worst. The answer is a seamless blanket pumped right over the steel.
Picture a steel pot and a wooden breadboard on the same stovetop. Grab the steel handle and you’ll burn your hand; the wooden board next to it stays cool enough to touch. That’s conduction, and it’s why a steel frame moves heat through your house in a way a timber frame never does. A batt laid in the bay next to the steel does nothing to slow the heat pouring through the metal itself. It’s like putting an oven mitt on the bench beside the hot pot instead of around the handle.
The Australian Government’s yourhome guide spells out the mechanism: thermal bridges reduce the effectiveness of insulation and can lead to condensation problems (yourhome.gov.au), and CSIRO’s building-technology note adds that the benefit of insulation is undermined by heat bridges unless you install to a high standard. So the goal on a steel frame isn’t just to fill the bays. It’s to bury the steel under a continuous layer that breaks the bridge.
One note up front: when I say a batt beside the steel loses about half the benefit, that’s my own field framing after 40 years on the tools, not a figure off a lab certificate. The mechanism is government-documented; the “about half” is what I’ve watched happen in real steel-frame roofs.
Our installer inside a steel-frame roof cavity, correct respirator on. Cellulose pumped over the steel, not cut into the bays beside it.
One-line version: a batt can only sit beside the steel. Cellulose pumps right over it. On a frame that conducts heat, that’s the whole game.
Before you buy batts, read this
5 reasons standard batts struggle on a steel frame.
I fit batts every week for the people who want them, and I'll fit them in a steel-frame home if that's what you've decided on. But here's the case for why they're the wrong tool for this particular job.
Steel conducts the heat. It's a thermal bridge
Steel conducts heat readily, so a steel frame is one big thermal bridge from the hot side of your house to the cool side. Think of it like the difference between a steel pot and a wooden breadboard: grab the handle of a pot that's been on the stove and you'll burn your hand; the wooden board next to it stays cool to touch. A batt laid beside the steel does nothing to slow the heat that's pouring straight through the metal itself, and the Australian Government's yourhome guide warns that these thermal bridges reduce the effectiveness of insulation and drive condensation.
The odd steel shape leaves gaps everywhere
Steel sections aren't a clean rectangle like a timber stud. They're C-shapes, top-hats and odd profiles, with brackets, screws and overlaps everywhere. A batt is a flat slab; it can't wrap around all that. So a batt cut to fit a steel bay leaves gaps down both edges and around every bracket, and an R-rating only counts where the insulation actually covers. Even small gaps greatly reduce the effective R-value, so a batt squeezed into a steel frame loses a big slice of what's printed on the bag before it's even finished.
Steel roofs cook, and a brutal roof means rushed jobs
A steel-framed, steel-roofed home gets ferociously hot in the roof space on a Queensland afternoon. The hotter and tighter the roof, the faster a batt installer wants to be out of there, and that's exactly when the corners get skipped and the slabs get dumped where they don't reach. I've seen it in plenty of roofs over 40 years. Cellulose is pumped through a hose, so the installer isn't dragging slabs around a 60-degree roof cavity, and the product still gets into every corner.
Batts are awkward and itchy against the steel
Fibreglass batts are itchy to handle at the best of times, and a steel frame full of sharp edges and screw points makes it worse. The slabs catch and tear as you push them in. The manufacturers' own safety sheets tell you to wear a P2 dust mask and to wash your work clothes separately from the family laundry; the fine print says more than their marketing brochure does. An installer working that hard in that heat isn't going to spend extra time tucking a batt around every bracket, and that's how the gaps creep in.
Steel conducts electricity. The cutting knife is a real danger
Here's one most people never think about. Steel conducts electricity. Cutting batts to fit on a knife-edge steel joist means running a sharp blade across live-edge metal in a tight roof space. If a blade nicks a cable, the whole frame can become live. The less cutting an installer has to do up there, the safer the job is for everyone. A pumped product needs almost no cutting on the frame at all, so there's far less knife work happening over the steel.
Sources for the gap and thermal-bridge figures above: Australian Government, yourhome.gov.au (Insulation) and CSIRO, Thermal Insulation for thermal bridging and the “small gaps greatly reduce the insulating value” finding. The P2-mask and wash-separately instructions come from the batt manufacturers’ own safety data sheets.
Why cellulose on a steel frame
5 reasons pumped-in cellulose is the right answer.
The positive case, the way I'd put it to you standing in your roof. It's the only product I'd put in my own home, and on a steel frame the case is even stronger than usual.
No gaps: it fills around every bracket and screw
Cellulose is pumped in dry through a hose, so it flows around the C-sections, top-hats, brackets and screws that a flat batt could never wrap around. There are no cut edges and no joins, so there's nothing left bare for the heat to pour through. On a steel frame, where the awkward shapes are exactly where batts fail, a seamless pumped-in blanket is the difference between an R-rating that means something and one that's only true on paper.
It pumps right over the steel by 50mm or more
This is the big one. We don't just fill the bay beside the steel. We pump cellulose right over the top of the steel by 50mm or more, so there's a continuous blanket of insulation covering the metal itself. That's what breaks the thermal bridge: the heat coming through the steel now has to get through a layer of cellulose before it reaches your room. A batt can only ever sit beside the steel, leaving the metal exposed; cellulose buries it.
The bulk sits hard against the steel and slows the heat
Because it's dense-packed and sits in full contact with the steel rather than loosely beside it, the bulk of the cellulose absorbs and slows the heat coming off the metal instead of letting it radiate straight into the room. Insulation never stops conduction outright, nothing does, but full-contact cellulose resists it far better than an air-gapped batt that's barely touching the frame. On a steel-frame home that full contact is doing real work that a slab simply can't.
No cutting on the frame, so it's a safer job
Because cellulose is pumped rather than cut to fit, there's almost no knife work happening over the steel, and on a frame that conducts electricity, that's a genuine safety win, not just a convenience. One trained installer on the roof with a hose is safer than a crew dragging and cutting slabs across sharp, live-edge metal in a hot, tight space. Less cutting, less time crouched over a knife-edge joist, less chance of anything going wrong.
Borate-treated, and it handles a leak
Steel-framed homes still get the rest of the cellulose advantage. It's borate-treated, so the insects rats feed on can't survive in it (no insects, no food for rats) and the same treatment means it's been shown to slow fire spread rather than melting away and leaving your frame exposed. It's hygroscopic, too: if a roof leak finds its way in, cellulose holds the moisture in one spot and dries out, instead of letting water run across the ceiling the way it does under batts.
How we insulate your steel-frame home
One installer, a hose, and the steel buried under cellulose.
Our pump rig stays in the truck and feeds a 40-metre hose, so we don’t drag slabs through your house or across a 60-degree roof cavity. The installer pumps the cellulose in dry. It flows around the C-sections, top-hats, brackets and screws, then builds right over the top of the steel by 50mm or more as one continuous blanket. No cut edges, no joins, and almost no knife work on the frame.
A lot of batt companies will tell you a steel frame, a wrap-over iron roof or a tight metal-framed cavity “can’t be done”, but usually that’s a sales rep talking, not an installer. We’ve got the rig and the training to do the roofs others knock back. Every job is photographed and the photos are checked before you’re invoiced. That’s our system, the same on every job, run by trained franchise owner-operators, so you can see the steel was actually covered even though you’ll never climb up there yourself.
Roofs others can’t do →Set on fitting batts yourself? Read the DIY guide →
“On a steel-frame home I’m not looking for some rocks to stand on to cool my feet. The frame conducts the heat, so you cover the steel, you don’t just lay something beside it.”
What you get
A steel-frame roof that’s actually covered.
Send me your address and I’ll work out a proper fixed-price quote, no deposit, and no price increases on the day. The price I quote is the price you pay; if I’ve under-measured something, the extra is on me. Steel frame, wrap-over iron, tight metal cavity. If it can be pumped, we’ll do it.
See it on a real job
Watch: why batts fail on a steel frame
Batts laid beside the steel leave the frame as a thermal bridge. Here's how pump-in cellulose covers it as one seamless blanket.
Filmed on real jobs over the years — our methods, safety standards and products have moved on since. Subscribe to the channel for more.
Straight answers
Steel-frame insulation: the questions I get asked most.
How should you insulate a steel-frame home?+
With a product that covers the steel, not just the bay beside it. Steel is a thermal bridge (it conducts heat readily from the hot side of the house to the cool side) so insulation that only sits next to the frame leaves the metal doing its worst. We pump cellulose right over the top of the steel by 50mm or more as one seamless blanket, so the heat coming through the metal has to get through a layer of insulation before it reaches your room. Batts can only ever sit beside the steel and leave gaps around all the brackets and odd shapes, which is why I don't recommend them for a steel frame.
Why do batts lose so much of their benefit in a steel frame?+
Two reasons. First, steel conducts heat straight through the frame, and a flat batt laid beside it does nothing to slow the heat moving through the metal itself. Second, steel sections are C-shapes and top-hats with brackets and screws everywhere, so a slab batt can't wrap around them. It leaves gaps down both edges and around every fitting, and even small gaps greatly cut the effective R-value. In my 40 years on the tools, a batt beside a steel frame loses a big slice of what's on the bag before it's even finished. The fix is a pumped product that covers the steel rather than sitting next to it.
Can you pump cellulose over the steel in a steel-frame ceiling?+
Yes, that's exactly what we do, and it's the whole point. We pump the cellulose right over the top of the steel by 50mm or more so there's a continuous blanket covering the metal, breaking the thermal bridge. It flows around the C-sections, brackets and screws that a flat batt could never wrap around, with no cut edges and no joins. We've got the rig and the training to do it; a lot of batt companies say a steel frame is too hard, but usually that's a sales rep talking, not an installer.
Is it safer to pump cellulose than to cut batts on a steel frame?+
There's a real safety angle, yes. Steel conducts electricity, so cutting batts to fit on a knife-edge steel joist means running a sharp blade across live-edge metal in a tight roof space. Nick a cable and the whole frame can become live. Because cellulose is pumped rather than cut to fit, there's almost no knife work over the steel. One trained installer on the roof with a hose is a safer setup than a crew dragging and cutting slabs across sharp, hot metal.
Does the 'half the benefit' figure come from a lab test?+
No. The roughly 'half the benefit' framing is my own field experience after 40 years and 6,000-odd roofs, not a figure off a test certificate. What's solidly documented is the mechanism: the Australian Government's yourhome guide and CSIRO both state that thermal bridges like a steel frame reduce the effectiveness of insulation and that even small gaps greatly cut the effective R-value. So the direction is rock-solid; I just won't dress up a field observation as a lab number.
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Fill in the simple online form and you’ll have a detailed, fixed quote within 48 hours for most houses. Cellulose pumped right over the steel, no gaps, the way a steel frame should be insulated. Servicing Brisbane & SE QLD.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986
In the trade and want to install it yourself? We make cellulose in Tiaro and run exclusive territories, franchise with the family.