U-value (thermal transmittance)
U-value is heat transfer rate (W/m²·K) through a building element, lower is better. Used for windows in NatHERS modelling. Typically 2-6 for windows, 0.3-0.5 walls.
Ask Chalkline about this →U-value is the rate of heat transfer through a building element, expressed in W/m²·K (watts per square metre per Kelvin temperature difference). It is the mathematical inverse of R-value (U = 1/R), expressed as a transmission rather than a resistance. Lower U-value = better insulation. The term is used for window and door assemblies in NatHERS modelling and NCC compliance because R-value (resistance) becomes impractical to express for transparent assemblies with multiple panes, frames, and gaps. For opaque elements (walls, roofs, floors), R-value remains the standard.
Typical residential U-value ranges:
| Element | Typical U-value (W/m²·K) | Equivalent R-value |
|---|---|---|
| Single-glazed clear float, aluminium frame | 5.7-6.4 | R 0.16-0.17 |
| Single-glazed Low-E, aluminium frame | 4.0-5.0 | R 0.20-0.25 |
| Double-glazed clear, aluminium frame | 2.8-3.5 | R 0.29-0.36 |
| Double-glazed Low-E, thermally broken frame | 1.8-2.5 | R 0.40-0.56 |
| Triple-glazed Low-E, thermally broken | 0.8-1.4 | R 0.71-1.25 |
| Standard brick veneer wall (R2.0 insulation) | 0.45-0.55 | R 1.8-2.2 |
| Lightweight clad wall (R2.5 insulation) | 0.35-0.45 | R 2.2-2.8 |
| Roof / ceiling (R6.0 ceiling batts) | 0.15-0.18 | R 5.6-6.7 |
For windows, U-value is the dominant metric; for opaque elements, R-value is more commonly used.
Why builders care:
The NCC Volume Two Part H6 (Energy Efficiency) requires that the whole house meets a minimum NatHERS star rating. NatHERS modelling uses:
- Uw for windows (whole-assembly U-value including frame and glass).
- SHGC for windows (solar heat gain coefficient).
- R-value for opaque elements (walls, roofs, floors).
To pass NatHERS, the building envelope’s U-values and SHGCs need to align with the climate zone and the house design. In cold climate zones (zones 6-8), low U-values are essential; in hot dry climates (zones 1-3), SHGC matters more than U-value for windows.
Distinguishing U-value variants:
| Variant | What it covers |
|---|---|
| U-glass | Just the glass; ignores frame |
| Uw | Whole window (glass + frame + edge of glass) |
| U-factor | American term, equivalent to U-value |
For NatHERS and NCC compliance, Uw is the correct metric.
Climate zone targets (indicative):
| Climate zone | Typical Uw target (NCC compliance) |
|---|---|
| 1-3 (hot) | Uw ~5-6 acceptable if SHGC low |
| 4-5 (mixed) | Uw ~3-4 |
| 6 (mild) | Uw ~2.5-3.5 |
| 7-8 (cool) | Uw ~2.0-2.8 |
These are indicative; the actual NCC requirement comes through NatHERS modelling.
Common builder issues:
- Spec’ing standard aluminium-frame single-glazed in zone 7: NatHERS fail. Need double-glazed at minimum.
- Quoting U-glass instead of Uw: the frame adds significant heat loss; Uw is what matters.
- Confusing U-value and R-value direction: high R = good, low U = good. Easy to invert when comparing products.
- Ignoring frame: thermally broken frames give 0.5-1.0 lower Uw than non-broken; significant on whole-house modelling.
For builders:
- Get the Uw (whole window) and SHGC from the window supplier for every product.
- Run NatHERS at design lock-in: the model tells you which windows pass and which fail.
- In cold climates, default to double-glazed Low-E with thermally broken frames.
- In hot climates, prioritise SHGC over Uw: solar heat gain often the dominant load.
- Check the NatHERS report aligns with what’s installed: substituting different glazing post-modelling can fail compliance.
Also known as: thermal transmittance, Uw (window), U-factor.
Category: Energy efficiency / NCC / glazing.
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Last updated: 2026-05-16. Verified: 2026-05-16.