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: its air-space R is 0.45 × the enclosed Table E3 value, folded into the framed layer it bounds (or carried as its own layer where it bounds none), and the layers exterior to it are de-rated by 45% - each retains ×0.55 of its R, per the Appendix F4 worked example. The external surface resistance is not de-rated. See Air cavities.

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.

Blocking finding. A finding stronger than a caution. Raised when a construction sits below its H1/AS1 §2.1.2.11 minimum R-value - the figure is trustworthy and still not good enough, since the minimum binds in every climate zone and cannot be traded against surplus elsewhere - and when a construction's claimed R-value is more than its own layer build-up supports. Leads the calculator's warning rail, and is what the report's Critical review items section is reserved for.

Review items. The report's optional section for cautions: method cautions, plausibility checks, values with no stated basis, a downlight derate's working. Off by default (Settings → Report), because those cautions are shown on the H1 page and in AI Review, where you can act on them.

Report credit. What a project's first consent-ready report spends, once payments are switched on. One credit covers that project for good: every later report for it is free. The insulation take-off is always free, and building projects, watermarked previews and sample reports never spend a credit.

Existing fabric. On an alteration or addition, a roof, wall, floor or opening that was there before the work. Unchanged existing fabric is left out of the heat-loss comparison and claims no R-value; altered existing fabric is checked against its before-state.

Not-diminished test. The check on altered existing fabric under Building Act 2004 s 112(1)(b): summed over the altered elements, heat loss (area ÷ R) after the work must not exceed heat loss before it. Upgrades elsewhere in the altered set count as offsets.

Ceiling follows pitch. The roof row setting that decides which area counts for heat loss. Off, the default, is a flat ceiling under a pitched roof, so the plan (ceiling) area is used and the pitch is only recorded. On is a skillion or cathedral ceiling, so the larger sloped area is used.

Take-off only. An element insulated outside the declared thermal envelope, such as a garage's own ceiling and walls. It is measured, shown on the plan and priced on the insulation take-off, and excluded from the H1 calculation and report.

Thermal break (steel framing). Material at the outside face of a steel framing member that interrupts the path through the metal: strips over each member, or a continuous board across the whole frame. E3/AS1 clause 1.1.4(d) asks for at least R0.25 on steel framing in housing.

Structural insulated panel (SIP). A factory-made panel of insulation between structural skins, bridged only by the timber that joins and trims it. Not a framed wall, so the 38% wall framing default does not apply to it; the timber is counted from the wall's own geometry instead.

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

Element tag. The automatic identifier each element carries - WA1 for walls, RO1 for roofs, FL1 for slab-on-ground floors, SF1 for suspended floors - with the number editable to match your drawings. The two-letter prefixes cannot collide with the joinery tags (W1, D1) an architect writes on a drawing. Openings are tagged by the element they sit in - WA1-W2 is window W2 on wall WA1, your own plan tag carried through after the hyphen. The same tags appear on the form row, the plan caption, the legend and the report's schedule of building elements.

Wall to an unconditioned space. A thermal-envelope wall that faces a garage, an unheated store or similar rather than the outside. It is a wall like any other - same tag class (WA), same reference R-value, same surface resistances - and its construction description is what records the adjacency on the report. (A wall between two conditioned spaces, such as a party wall between household units, is outside the thermal envelope under H1/AS1 §2.1.2.4 and belongs in no envelope row.)

Plan colour. The colour an element is drawn with on the plan. Each wall, roof and floor has its own, from a palette you manage under Settings → Plan markup; the legend, the form row's colour dot and the report's build-up colour bars all use it, so drawing, form and report cross-reference at a glance.

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.