R-value targets: the numbers your thermal envelope has to hit

R-value targets are the construction R-values each part of your thermal envelope must reach — the resistance of the whole built-up assembly once timber and gaps are accounted for, not just the insulation product on its own. Higher number = warmer, drier and cheaper to heat, so it pays to know what you’re actually building to before you order batts.

Targets by element

These are the H1/AS1 construction R-value targets by part of the envelope. Some are the same everywhere; others step up the further south (colder) you build. Zones run 1 (warm — upper North Island) to 6 (cold — Central Otago / inland South Island).

• Roof / ceiling: R6.6 — all zones.
• Wall: R2.0 — all zones.
• Slab on ground: R1.5 (zones 1–3) rising to R3.0 (zone 6) — and higher again if the slab is heated.
• Suspended floor: R1.5 (zones 1–3) rising to R2.5 (zones 4–6).
• Windows / glazing: R0.46 (zones 1–2) rising to R0.50 (zones 3–6).

Framing fraction — the catch on walls

Timber conducts heat around 3–4× faster than insulation, so the more framing in the wall, the lower the real R-value you end up with. H1/AS1 assumes a default framing fraction of roughly 38% for walls, but BRANZ has measured real walls anywhere from 24% to 57% — all the dwangs, trimmers and multi-stud corners push it up.

If your design is framing-heavy, use the actual fraction rather than the default. It’s more honest and it stops you signing off an overstated R-value that the wall never really achieves.

Hitting wall R2.0 in practice

Hitting the wall target on paper and hitting it in the real assembly aren’t the same thing. A bare 90×45 wall with R2.6 batts only reaches about R2.0 once thermal bridging is counted — and only if nothing’s compressed. Here’s the reliable build-up:

1. 90×45 framing.
2. An R2.2–2.6 batt between the studs.
3. A 45 mm service cavity — battens running across the studs, insulated — so wiring and plumbing don’t crush the insulation, and the cavity adds resistance over the bridges.