Construction R-values: the number H1 actually checks

The single biggest H1 mistake is confusing the number on the insulation bag with the number the Code asks for. H1 targets the construction R-value — the resistance of the whole assembly — which is always lower than the bag’s material R-value, because heat takes the easy path around the insulation: through the timber, the gaps and any compressed spots.

The two R-values, and why they differ

Get these two straight and most H1 confusion disappears:

• Material R-value — printed on the product. An R2.6 batt resists R2.6 through the batt itself.
• Construction R-value — the assembly’s real-world resistance, after the timber framing, dwangs, gaps and services are blended in. This is what H1 checks.

Worked example: a 90×45 wall with R2.6 batts, at the default 38% framing fraction, lands at only ≈R2.0 construction — the timber bridges drag it down.

Beating thermal bridging

To lift the construction R-value back up, you have to deal with the heat leaking through the framing. A few proven moves:

• Insulated service cavity — 45 mm battens across the studs, filled with an R1.2–1.4 batt. This adds resistance over the bridges and keeps wiring and plumbing out of the main batt so it isn’t crushed. It’s the standard way to genuinely reach wall R2.0+.
• Don’t compress the insulation — a batt squashed behind a pipe loses R-value fast, and a gap loses even more.
• Deeper framing or continuous external insulation — 140 mm framing, or continuous external insulation, lifts the construction value where you need more.
• Use the real framing fraction — multi-stud corners, trimmers and dwangs push it well above 38% in some walls, so don’t just assume the default.

Let the spreadsheet do the maths

The BRANZ House Insulation Guide spreadsheet does this maths for you — enter the build-up and it returns the construction R-value to check against the H1 R-value targets.