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NZ Building Code · Energy & H1Glazing R-values: it's the whole window, not just the glass
A window's thermal performance comes from the frame and glass together, so two “double glazed” units can perform very differently.
A window’s thermal performance comes from the whole unit — the glass and the frame together — not just “double glazed”. A standard aluminium frame is a heat highway, so two windows that are both “double glazed” can perform very differently. The R-value (or its inverse, the U-value) is stated on the product’s BPIR sheet.
Whole-window R-values: what the frame does
The frame matters as much as the glazing. Here’s how typical whole-window R-values stack up from worst to best:
- Aluminium + single glaze — around R0.16. Not H1-compliant, and a condensation magnet.
- Aluminium + double (IGU) — around R0.30. Below current targets in most zones.
- Thermally broken aluminium + IGU — R0.45–0.55. The current NZ workhorse; meets the R0.46/0.50 targets.
- uPVC + IGU — R0.55–0.65. The frame itself insulates, so it’s strong value.
- Thermally broken + triple — R0.7+. High-performance and cold-zone homes.
R-value, U-value, SHGC, VLT — what they mean
- R-value — resistance to heat flow. Higher means warmer.
- U-value — 1/R, so lower is better. Windows are often quoted in U.
- SHGC (solar heat gain) — how much sun-heat comes through. You want it higher on south and cold rooms, lower on hot west glass.
- VLT (visible light transmittance) — how much daylight gets through.
- Thermal break — an insulating barrier in the aluminium frame that stops the cold outer face conducting straight to the warm inner face. It cuts heat loss and frame condensation.
BPIR — Building Product Information Requirements
Since 11 December 2023, building products (including windows) must carry a BPIR datasheet stating the R-value/U-value, SHGC and VLT.
- Spec the system — frame, glazing, spacer and coating — to a target R-value and get the BPIR. Don’t accept just “double glazed”.
- If a supplier can’t produce a BPIR, don’t buy.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
Does “double glazed” mean a window is H1-compliant?
Not on its own. A window’s thermal performance comes from the whole unit — frame and glass together. A standard aluminium frame is a heat highway, so an aluminium + double (IGU) window sits at around R0.30, which is below current targets in most zones. Thermally broken aluminium + IGU (R0.45–0.55) is the current NZ workhorse that meets the R0.46/0.50 targets.
What’s the difference between R-value and U-value?
R-value is resistance to heat flow — higher means warmer. U-value is its inverse (1/R), so lower is better. Windows are often quoted in U rather than R.
What are SHGC and VLT?
SHGC is solar heat gain — how much sun-heat comes through the glass. You want it higher on south and cold rooms and lower on hot west glass. VLT is visible light transmittance — how much daylight gets through.
What is a BPIR and why does it matter for windows?
BPIR stands for Building Product Information Requirements. Since 11 December 2023, building products including windows must carry a BPIR datasheet stating the R-value/U-value, SHGC and VLT. Spec the system to a target R-value and get the BPIR — if a supplier can’t produce one, don’t buy.
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