Installing insulation so it actually hits its R-value

Insulation only delivers its rated R-value if it is installed correctly — fitted snugly, no gaps, no compression, and a vapour barrier where it is required. Poor installation is the number-one reason houses underperform their H1/AS1 targets in practice, so how you fit the batt matters as much as which batt you buy.

Wall insulation (bulk fibre — Pink Batts, Earthwool)

Bulk fibre batts friction-fit into the stud bays. The aim is a full, snug cavity with no bridging paths for heat.

• Friction-fit into stud bays — the batt must spring back to fill the cavity completely.
• Corner-to-corner — no gaps top, bottom or sides; cut to fit irregular shapes.
• No compression — R-value drops linearly with compression. Don’t squeeze an R2.8 into an R2.2-rated cavity.
• Trim around services — at power outlets and switches, cut the batt around the box so it still fills the full depth.
• Stud snug — flush with the face of the stud, not bulging past it.
• Vapour control — H1/AS1 doesn’t mandate it, but in colder zones (Z5–6) use a vapour-permeable wrap on the outside of the frame, not the inside.

Ceiling insulation

Ceiling batts (or loose-fill) sit between the joists or trusses. The detailing around lights, flues and eaves is where installs go wrong.

• Trim to fit — between joists/trusses, with no overlapping near soffits (which creates a moisture pocket).
• Clearance from downlights — 100mm minimum from any non-IC-rated downlight; IC-rated lights can be covered.
• Clearance from flues — 50mm minimum from a cold flue, 100mm from a hot one.
• Eaves baffles — keep insulation back from the eaves so soffit ventilation isn’t blocked.
• Top of plate — insulation should sit over the wall top plate to eliminate the thermal bridge.
• Loose-fill (Mason, Spaceloft) — blow it in to the depth marked on the rafters with a rule.

Underfloor and slab edge

Under the floor and around the slab, the job is keeping the insulation dry and held in place, and getting the perimeter R-value the calculation method needs.

• Polystyrene boards (Expol, Pink) — friction-fit between joists from below; most common for old houses.
• Encapsulated batts (Pink Batts Floor, Earthwool Underfloor) — wrapped in polythene to keep them dry.
• Hold it up — fix with retention straps or staples so it stays against the floor over time.
• DPM first — lay DPM polythene on bare earth ground first (Healthy Homes Standard 4).
• Underfloor minimums — R1.5 minimum under H1/AS1 6th edition; R1.3 minimum under the Healthy Homes Standards for rentals.
• Slab edge — R1.0 minimum around the slab perimeter for the H1/AS1 calculation method, using Expol Underslab 50mm or 75mm board between the formwork and slab edge. BRANZ research shows limited additional benefit beyond an R1.0 edge — most thermal performance comes from underslab insulation and thickness.
• Heated slab — R2.5–3.0 minimum depending on zone, and it cannot be reduced via calculation.

The 90+45 service cavity

This is best-practice NZ residential build-up since 2023. It keeps the services out of the structural insulation so the batt stays intact.

• 90×45 framing with an R2.6 batt in the main cavity.
• A 45mm horizontal service cavity on the inside of the frame, filled with an R1.4 batt.
• Total construction R-value roughly 2.6, which matches the H1 target.
• Services run in the service cavity, not through the structural batt — cleaner installation, no holes through the insulation, and easier modifications later.

Common installation defects

Most real-world underperformance comes down to how the batt was fitted, not the batt itself. A classic one to watch for:

• Gaps at studs — under-cut batts leave heat-loss bridges.