Glossary

Terms you will meet in Thermly and in H1 compliance.

R-value (thermal resistance). How well a material or assembly resists heat flow, in m²·K/W. Higher is better. R-values in series add together.

Construction R-value. The R-value of a complete building element (wall, roof or floor), including its layers and the internal and external surface resistances, computed here by NZS 4214:2006. This is the figure H1 works with.

U-value (thermal transmittance). The inverse of R-value (U = 1/R), in W/m²·K. Lower is better. Window and skylight performance is often published as a U-value; a skylight's construction R is taken as R = 1/U_w.

Surface resistance (Rsi / Rse). The thin insulating film of air at an element's internal (Rsi) and external (Rse) faces. For H1 compliance, NZS 4214 §5.2 fixes these at 0.09 and 0.03 m²·K/W.

Isothermal planes method. The method (NZS 4214 §5.7) for an element with parallel paths of different resistance - insulation beside framing - that treats each layer's faces as an even temperature and combines the paths by area. It is the standard way to account for framing in a wall or roof.

Thermal bridging. Heat taking an easier path through a more conductive element, typically timber or steel framing bridging the insulation. Ignoring it overstates an element's R-value; the isothermal-planes method accounts for it.

Bridged layer. In Thermly, a layer that contains insulation between framing, calculated by the isothermal-planes method from the two products and the framing fraction.

Framing fraction. The proportion of a framed layer's face taken up by framing rather than insulation. For framed walls, H1/AS1 6th edition assumes at least 38% unless a lower value is demonstrated.

Vented (drained) cavity. An air gap behind cladding that is open to the outside. Handled per NZS 4214 §5.3.2 and the Appendix F4 worked example.

Downlight derate. The reduction in a ceiling's effective R-value caused by non-IC recessed downlights, which must have insulation held clear of the fitting (NZS 4246:2016 §4.2). Thermly derates the roof / ceiling element by area-weighting the cleared circles against the insulated ceiling. See Downlights.

A/P ratio (area-to-perimeter). A slab or raft floor's area divided by its exposed perimeter. Ground-floor heat loss depends on it, so Appendix E and manufacturer raft calculators are keyed to it.

Calculation Method. The H1/AS1 pathway Thermly implements: compute each element's construction R, then compare the whole building's heat loss against a reference building for the climate zone.

Schedule Method. The former prescriptive R-value look-up pathway, removed in the 6th edition. Not usable from 27 November 2026.

Modelling Method. The H1/VM1 energy-modelling pathway, for cases the Calculation Method does not cover.

Reference building. The notional building, at the zone's coefficients (Table 2.1.2.7), that your proposed building's heat loss is compared against.

Headroom. How far your proposed building's heat loss sits below the reference building's. Positive headroom passes.

Climate zone. One of the six H1/AS1 6th edition zones (Appendix C) that sets the reference building. Chosen by postcode or directly.

Net vs gross area. Gross wall area is the whole face; net wall area is gross minus windows and doors. H1 nets openings out of the opaque wall and counts them separately.

NZS 4214:2006. The New Zealand standard for determining the total thermal resistance of parts of buildings. Governs construction R-values.

H1/AS1. The Acceptable Solution for NZBC Clause H1 (Energy Efficiency) for housing. The 6th edition (2025) is current.