Duct Insulation R-Value: Why Cheap Ducted Quotes Use R1.0
You've got three quotes for ducted reverse cycle. Same brand, same capacity, same number of outlets. One is thousands of dollars cheaper.
Before you assume you've found a good operator with low overheads, check one line: the R-value of the ductwork.
Key points
- For a typical Melbourne home with ducts in the roof space, ducted reverse cycle generally requires R1.5 flexible duct.
- R1.0 is roughly 45 mm of glasswool; R1.5 is roughly 63 mm — about 40% more material per metre and far harder to route.
- Under-insulated duct loses conditioned air to a 50–60°C roof space and can drop below dew point, causing condensation.
- Ask for the R-value to be stated in writing on your quote.
Because the most common reason a ducted quote comes in low isn't efficiency or a better buying price on the equipment. It's that the quote is built on R1.0 duct instead of R1.5.
What you're giving up
Your ducts run through the roof space. In a Melbourne summer that space sits between 50°C and 60°C. In winter it drops close to outside temperature. The air inside the duct is trying to be 22°C.
The insulation wrapped around that duct is the only thing separating the two. Its R-value measures how well it resists that heat transfer. Higher number, better resistance, thicker duct.
Under-insulate it and the air arriving at the far bedroom has already given up part of its heating or cooling to the roof space. You paid to condition that air and the roof got a share of it. The system runs longer to make up the difference — every cycle, every day, for the life of the system.
There's a second problem. In cooling mode the duct surface is cold. Insufficient insulation means the surface can drop below dew point, and you get condensation on the outside of the duct. That soaks the insulation, destroys whatever R-value it had left, and eventually finds its way onto a ceiling.
Why R1.5 is the number in Victoria
The required R-value is set by the National Construction Code, and it depends on your climate zone and where the ductwork runs.
For a typical Melbourne or Geelong home — climate zone 6, ducts in the roof space — a ducted reverse cycle system generally lands on R1.5. Parts of Victoria sit in a colder zone again.
It's worth knowing where that figure comes from, because it's the detail a cheaper quote will argue about. The Code treats a heating-only or cooling-only system — including evaporative cooling — differently from a combined heating and refrigerated cooling system. Reverse cycle is the combined case, and there's a specific concession that reduces the required R-value by 0.5 in certain climate zones where the ducts sit in a roof space with insulation beneath the roofing. That concession is what gets you to R1.5. It doesn't get you to R1.0.
Some of this genuinely does get specified wrong — an installer working off an interstate spec, or applying an evaporative figure to a reverse cycle job, lands on R1.0 without much thought. But for a lot of low quotes it isn't an accident. R1.0 is simply the cheaper way to price the job, and there's no realistic chance the customer will ever check.
Why R1.0 saves the installer real money — twice
Materials. Look at the Code's own reference figures for flexible duct. R1.0 is roughly 45 mm of glasswool. R1.5 is roughly 63 mm. That's about 40% more insulation on every single metre of duct in the house, and a ducted system uses a lot of metres.
Labour — and this is the bigger saving. R1.5 duct is fatter and considerably stiffer. It's harder to drag through a roof space, harder to get past trusses and bracing, harder to route around obstructions, and much harder to bend without kinking. Work that's straightforward in R1.0 becomes slow in R1.5, particularly in low-pitch roofs, tight roof spaces and double-storey homes.
So the saving isn't a few dollars of insulation. It's a materially faster job, on top of cheaper materials. That's how a quote comes in thousands under.
Why you'll never spot it afterwards
This is the uncomfortable part. Once the ceiling vents are on and the manhole is closed, R1.0 and R1.5 look identical. The system will heat and cool. It will pass the handover. You will have no idea.
What you'll notice instead, years later, is that the back rooms never quite get there on extreme days, and that your running costs are higher than the neighbour with the same system. Neither of those ever gets traced back to a line item on a quote from 2026.
The saving is real. It just isn't yours — you're funding it through running costs for the next fifteen years, and you've got non-compliant ductwork in your roof if you ever come to sell.
The other places ducted quotes differ
While you're comparing:
- Sizing. Was an actual heat load calculation done, or was capacity guessed from floor area? Undersized systems never recover on a 40°C day.
- Duct sizing and layout. Correctly sized duct per outlet, runs kept short and straight. Long, undersized or kinked duct destroys airflow regardless of how good the unit is.
- Return air. Undersized return air is one of the most common faults in residential ducted systems, and it caps the performance of everything downstream.
- Sealing. Leaking duct in a roof space is conditioned air you've paid for, delivered to your insulation.
- Zoning. How many zones, how they're controlled, and whether the system can handle zones closing without fighting itself.
Ask one question
Ask for the duct insulation R-value to be stated on your quote, in writing.
An installer specifying R1.5 will put it on the quote without hesitation — it's a selling point and they've priced for it. An installer who goes vague, tells you the rating doesn't really matter, or says R1.0 is "what everyone uses" has answered your question about why they're cheaper.