H1/AS1 6th edition · Walls
Why R2.8 batts make an R1.67 wall
The R-value on the batt packet is the insulation's. The number H1/AS1 asks for is the construction R-value — the whole wall, surface to surface, with the timber that bridges the insulation. For a standard 90 mm wall they are not close.
The method
H1/AS1 6th edition, paragraph 2.1.3.1 a), says construction R-values for walls, roofs and floors other than slabs are determined by NZS 4214 as modified by paragraph 2.1.3.2. NZS 4214 uses isothermal planes: layers that run unbroken across the wall add up in series, and where framing interrupts the insulation, that part of the wall — the bridged portion — has two heat paths side by side, one through the timber and one through the insulation, averaged by area:
1/Rb = ftimber / Rtimber path + finsulation / Rinsulation path
Two numbers decide most of the answer: how much of the wall is timber, and what the timber path is worth.
The 38% framing fraction
For framed walls H1/AS1 2.1.3.1 a) i) uses a framing fraction of 38% unless a lower fraction is demonstrated for the building. Studs alone at 600 centres are 45 ÷ 600 = 7.5% — the default covers everything else in a real wall too. So 38% of the bridged portion is the timber path, and 62% is the insulation path.
A 90 mm stud is worth about R0.75 (0.090 m ÷ k 0.12 W/m·K for timber). Next to R2.8 insulation it is a thermal short circuit across more than a third of the wall.
Worked example: 90 × 45 @ 600, R2.8 batts
James Hardie Oblique weatherboard 14 mm on a drained and vented cavity, building wrap, 90 × 45 studs at 600 with Pink® Batts® R2.8 (90 mm), 10 mm plasterboard. Outside to inside:
| Layer | Own R | Counted | Why |
|---|---|---|---|
| Outside surface | 0.03 | 0.015 | Counted at half — outside the vented cavity |
| Cladding (Oblique 14 mm) | 0.056 | 0.028 | Counted at half — outside the vented cavity |
| Vented cavity | 0.08 | in Rb | Sits against the framing — part of the bridged portion |
| Building wrap | 0.01 | 0.01 | |
| Studs + insulation (bridged portion) | 2.80 | 1.49 | Rb below |
| Plasterboard 10 mm | 0.04 | 0.04 | |
| Inside surface | 0.09 | 0.09 | |
| Construction R-value | R1.67 |
The bridged portion, with the vented cavity on both paths:
timber path = 0.75 + 0.08 = 0.83
insulation path = 2.80 + 0.08 = 2.88
1/Rb = 0.38 / 0.83 + 0.62 / 2.88 → Rb = 1.49
That clears the 2.1.2.11 b) minimum for walls (R1.0) — but the number that goes into the calculation method is R1.67, not R2.8. Use R2.8 and the heat loss of every wall is understated by 40%.
Thicker batts help less than you think
| Wall | Batts | Construction R |
|---|---|---|
| 90 × 45 @ 600 | R2.8 | R1.67 |
| 140 × 45 @ 600 | R3.6 | R2.30 |
| 140 × 45 @ 600 | R4.3 | R2.42 |
Going from R3.6 to R4.3 batts in a 140 mm wall adds R0.7 of insulation and R0.13 of wall. The timber path doesn't change, and with 38% of the area on it, it sets the ceiling. To move the number meaningfully you change the timber path — a thermal break over the framing, internal strapping with a second layer of insulation, external insulation — or demonstrate a lower framing fraction.
Stud spacing on its own does not move it: at 400 centres the wall is still R1.67, because the 38% default already stands in for the timber.
The figures here use the calculator's default material values, which are provisional: confirm them against your own copy of NZS 4214 and the manufacturers' data before they go into a consent. The calculator does not decide compliance — the building consent authority does.
Check your own walls
The Construction R-value Calculator does this for walls, truss and skillion roofs, suspended floors and slabs on ground (Appendix E), shows every step, draws the section and exports a PDF calculation. It is free, open source and runs in your browser.