There is no single R-value for a house. The code sets a separate requirement for the roof or ceiling, the walls and the floor, and what you can actually reach in each one is capped by the depth available. A ceiling has a whole roof space to play with. A wall has a 90 mm stud. A suspended floor is limited less by depth than by how you hold the insulation up and keep it dry.
In short
- Ceilings carry the highest R-value because there is room for a thick batt.
- Walls are capped by the stud. In a 90 mm cavity you are choosing between roughly R1.5 and R2.7, not R5.
- Underfloor batts have their own widths and their own fixing system. A wall batt is not a substitute.
- The number on the pack is the material R-value. The number that counts is the Total R-value of the built system.
- Framing runs straight through the insulation. That is thermal bridging and it is why the built number is lower.
The question comes in every week in the same shape. Someone has an R-value in their head, usually one number, and wants to know if it is right. It is the wrong question, because the roof, the walls and the floor are three different problems with three different answers.
Why there is no single number
NCC 2022 sets thermal requirements element by element. In the Housing Provisions, Part 13.2 Building fabric deals with roofs and ceilings, walls, and floors separately, with different provisions for each. Volume One Part J4 does the same for the buildings it covers. There is no clause that says "insulate the house to R4".
On top of that, the requirement changes with climate zone, with the construction type and with what else the building is doing. We covered how that works in R-value by Australian climate zone, including why the neat single-number tables you find online are usually a simplification of something much more conditional.
So the practical order is: get the target for each element from your energy report or specification, then work out what will physically fit. This article is about the second half of that.
Material R-value and Total R-value
This distinction explains most of the confusion, and it is worth two minutes.
Material R-value is the thermal resistance of the insulation product on its own, declared to AS/NZS 4859.1. That is the R3.5 or the R2.0 printed on the pack.
Total R-value is the resistance of the whole assembly: the insulation, plus the lining, the cladding, any air cavity, the air films on each face, and the effect of the framing running through it. It is calculated to AS/NZS 4859.2, and it is the number the code and the energy report are actually talking about.
The two are not the same, and the Total R-value of a real wall is usually lower than the batt suggests, because the timber or steel studs are not insulated and they run right through. Buying a batt with the number from the report printed on it does not automatically deliver that number in the wall.
The ceiling
The ceiling gets the highest number of the three, and it is the easiest to satisfy. That is not a coincidence.
A pitched roof with a ceiling below it gives you a roof space, so depth is rarely the constraint. Batts of 175 mm and 240 mm sit between the joists without argument. The roof space is also where the biggest temperature swing happens, since the roof faces the sky all day and all night, so the insulation there earns more than the same money spent elsewhere.
Practical points that decide the choice:
- Joist centres set the width. 430 mm batts suit 450 mm centres, 580 mm suits 600 mm. Measure in a few places, because framing drifts.
- Depth is only free if the batt is not squashed. A 240 mm batt in a space with 200 mm of clearance is a 200 mm batt, at whatever R-value that gives.
- Downlights change the plan. Fittings that cannot be covered leave gaps, and gaps cost far more than the R-value difference between two products.
- Access matters more than people expect. A tight, trussed roof space is slow work and the batts still have to reach the eaves.
The walls, where the stud decides
Walls are the element people most often get wrong, because they carry the target from the report across without checking whether it fits.
A wall cavity is the stud depth, and in Australian housing that is usually 90 mm or 70 mm. There is no room to go thicker without changing the frame. What you can do is choose a denser, better performing product in the same depth, which is exactly what the high performance ranges exist for.
Reading our own catalogue by cavity depth makes the ceiling on this very clear:
| Cavity | Product | R-value |
|---|---|---|
| 90 mm | Bradford Polymax Wall Batts | R1.5 |
| 90 mm | Bradford Gold Wall Batts | R2.0 |
| 90 mm | Pink Soundbreak Batts, high density acoustic | R2.7 |
| 75 mm | Bradford Polymax Wall Batts | R2.0 |
| 75 mm | HP Bradford Gold Wall Batts | R2.0 and R2.2 |
| 75 mm | Pink Soundbreak Batts, high density acoustic | R2.0 |
Two things fall out of that table. A 90 mm cavity is roughly an R1.5 to R2.7 proposition, not an R4 one. And a thinner high performance batt can beat a thicker standard batt, which is how a 75 mm product reaches R2.2 while a 90 mm one sits at R1.5.
If the specification asks a wall for more than the cavity can deliver, the answer is not a bigger batt. It is a wall wrap, a cavity, external insulation, or a change to the frame, and that is a design decision rather than a purchasing one.
The floor
Underfloor is the element most often skipped, and the one with the most specific requirements.
On a suspended timber floor the batt sits up between the joists from below. Depth is usually available. The two real constraints are different:
- Holding it up. There is no ceiling lining underneath to keep the batt in place, so it needs a dedicated fixing system. Bradford supplies saddles and straps for exactly this, sold separately from the batts.
- Keeping it dry. A subfloor is a damp, ventilated space. Product choice and subfloor ventilation both matter, and wet insulation is worse than none.
Note also that underfloor batts come in their own widths. Our R2.5 Bradford Optimo Underfloor Batts are 415 mm and 565 mm, where the wall and ceiling batts are 430 mm and 580 mm. Ordering wall batts for a floor is a common and annoying mistake.
What actually fits, by element
| Element | What limits you | Realistic range in our catalogue |
|---|---|---|
| Ceiling with a roof space | Roof space access and downlights, rarely depth | R2.5 through R7.0 |
| Cathedral or flat roof | Rafter depth, and the thermal break where the frame is steel | Set by the build up, not the batt alone |
| External wall, 90 mm stud | Stud depth | R1.5 to R2.7 |
| External wall, 70 or 75 mm stud | Stud depth, so a high performance batt earns its keep | R2.0 to R2.2 |
| Internal wall | Usually acoustic rather than thermal, so density matters more than R-value | Choose on the acoustic system, not the R number |
| Suspended timber floor | Fixing method and subfloor moisture | R2.5 in the standard underfloor batt |
| Slab on ground | Not a batt problem at all, it is edge insulation and slab detailing at design stage | Outside what a retrofit can change |
Thermal bridging
Insulation goes between the framing. The framing itself is not insulated, and it runs from the inside face to the outside face. Heat takes that path.
This is why Total R-value calculated to AS/NZS 4859.2 sits below the material R-value on the pack, and why the effect is larger in a wall than in a ceiling: a wall is full of studs, noggings, plates and lintels, while a ceiling is mostly uninterrupted batt between joists.
It bites hardest on steel framing, because steel conducts far better than timber. NCC 2022 Housing Provisions clause 13.2.3(3) sets out ways to deal with thermal bridging in metal framed roofs, including raising the R-value between the frames or running a continuous insulation layer, and clause 13.2.5(5) requires an R0.2 thermal break at the points of contact between lightweight cladding and a metal frame in the building envelope. Our guide to the R0.2 requirement covers what that means in practice.
What goes wrong
- One R-value applied to the whole house. Three elements, three targets, three products.
- Ordering the report's wall number without checking the stud. R4 does not fit in 90 mm.
- Wall batts ordered for the floor. Different widths, and no fixings in the box.
- Wrong width for the framing centres. 430 mm suits 450 centres, 580 mm suits 600. Measure before you order.
- Compressing to make it fit. A squashed batt loses the air that does the insulating, so you pay for R6 and install something less.
- Ignoring gaps. A small uninsulated gap costs more than the difference between two adjacent R-values. Fit matters more than the number.
We supply it and we install it
AWBM supplies ceiling, wall, underfloor, acoustic and commercial insulation across Australia, and we install batts, wall wrap and sarking. Material only if you have a crew, or the finished job with one price.
Or browse ceiling, wall and acoustic insulation.
Frequently asked questions
What R-value do I need for ceilings, walls and floors?
There is no single answer, because NCC 2022 sets the requirement separately for each element and it varies with climate zone and construction. Take the target for each from your energy report or specification, then check it against the depth available.
Why is the ceiling R-value so much higher than the wall?
Because there is room for it. A roof space takes a 240 mm batt without difficulty, while a wall cavity is the stud depth, usually 90 mm or 70 mm. The roof also faces the sky day and night, so the insulation there does more work.
What is the highest R-value that fits in a 90 mm wall?
In our range a 90 mm cavity runs from R1.5 in a standard batt to R2.0 in a Bradford Gold and R2.7 in a high density acoustic batt. Going beyond that means changing the wall build up, not the batt.
Can I use ceiling batts in a wall?
Only if the thickness matches the cavity. A thick ceiling batt compressed into a 90 mm stud does not deliver its rated R-value, because the air it relies on has been squeezed out. Use a batt made for the cavity depth.
Are underfloor batts different from wall batts?
Yes. They come in their own widths, 415 mm and 565 mm in our Bradford Optimo range against 430 mm and 580 mm for wall and ceiling, and they need a saddle and strap fixing system to hold them up, sold separately.
What is the difference between material R-value and Total R-value?
Material R-value is the product on its own, declared to AS/NZS 4859.1, and it is what is printed on the pack. Total R-value is the whole assembly including linings, cavities, air films and framing, calculated to AS/NZS 4859.2, and it is what the code and the energy report mean.
Does the timber frame reduce my insulation?
Yes. Studs, noggings and plates run through the insulation layer and are not insulated themselves, so heat takes that path. The effect is bigger in walls than ceilings, and bigger again with steel framing.
What batt width do I need?
430 mm for 450 mm framing centres and 580 mm for 600 mm centres, with some products also sold at 420, 450, 570 and 600 mm. Measure the centres in several places before ordering, because framing is rarely perfectly consistent.
Can AWBM quote the whole house?
Yes. Send the ceiling, wall and floor areas with the framing centres and the R-values from your report, and we will come back with products, pack counts and a price, delivered or installed.
Sources
- NCC 2022 Housing Provisions, Part 13.2 Building fabric, and clauses 13.2.3(3) and 13.2.5(5)
- NCC 2022 Volume One, Part J4 Building fabric
- AS/NZS 4859.1, Thermal insulation materials for buildings, materials and products
- AS/NZS 4859.2, Thermal insulation materials for buildings, calculation of Total R-value
- R-values, thicknesses and batt widths from the AWBM catalogue, taken from the manufacturers' product data
General information. R-value requirements vary by climate zone, building class and construction, so treat the ranges above as what physically fits rather than what your job requires. Take the target from your energy report or specification, and confirm the Total R-value calculation with your designer or assessor.