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How Thermly calculates compliance
Sources and verification
This page describes a compliance method. The figures and rules are cited to H1/AS1 6th edition and NZS 4214:2006 by clause and table number so they can be checked against the current standards. Clause text is not reproduced (both standards are licensed). Verify against the standards and current MBIE guidance before relying on a result.
Thermly's compliance check has two layers. First it computes each element's construction R-value by the isothermal-planes method of NZS 4214:2006. Then it runs those R-values through the H1/AS1 6th edition Calculation Method whole-building heat-loss comparison. Both are built in: you provide areas and assemblies, Thermly does the arithmetic, and you review and certify the result.
Layer 1: construction R-value (NZS 4214:2006)
Surface resistances
Every element gets an internal and external surface resistance added. For compliance, NZS 4214 §5.2 mandates a single pair for all elements: Rsi = 0.09 and Rse = 0.03 m²·K/W. Thermly uses these throughout (it does not use the heat-flow-direction-varying ISO 6946 values, which would overstate R on non-cavity elements).
Simple layers
A simple layer contributes its own R-value (a product R, or a custom R you enter).
Bridged layers (isothermal planes)
Where insulation sits between framing, the two parallel paths are combined by the isothermal-planes method (NZS 4214 §5.7, Equations 5 and 6): the layer's resistance is the area-weighted combination of the framing path and the insulation path, using the framing fraction. This is why the framing fraction matters so much (see the 38% rule below).
Air cavities
An enclosed cavity contributes a resistance from NZS 4214 Table E3. A ventilated cavity is handled per §5.3.2 and the Appendix F4 worked example: a vented gap that bounds a bridged layer folds into that layer at its full enclosed value (under NZS 4214 §5.7 an air gap next to a bridged layer folds into it rather than standing as its own layer), and only the layers outboard of the gap are de-rated. The engine is regression-anchored to the standard's own worked examples so the cavity treatment reproduces the standard.
Total R
The element's construction R is the sum: Rsi + each layer + Rse (NZS 4214 §6.2, Equation 10).
Floors
A slab-on-ground floor is not a layer stack. Its R-value is looked up from the floor's area-to-perimeter ratio, edge insulation and wall cladding, per H1/AS1 6th edition Appendix E (the geometry method of E.1.1.1(a) and the performance tables). A proprietary raft uses a manufacturer-supplied R under Appendix E, Paragraph E.1.1.1(b) - see Supplied floors and rafts.
Steel framing
Light-gauge steel sections are transformed by the NZS 4214 metal-frame method (§5.7). That one-dimensional transform is not valid for every configuration: where a steel batten bridges continuous insulation (a top-hat or Z-girt over a rigid board), it over-states the result, so Thermly warns that a two-dimensional determination (THERM / ISO 10211) is required for a compliance figure rather than crediting an optimistic value.
Skylights
Skylights are not vertical windows: H1/AS1 Appendix D Table D.1.1.1 (the vertical-window construction R-values) excludes them, and they have no §2.1.2.11 minimum-R backstop. A skylight's construction R is taken from its U-value, R = 1/U_w (Appendix D §D.2.1, Equation D.1), with U_w determined to ISO 10077 / H1/VM1. Where a supplied skylight R states no such basis, Thermly uses it unchanged but flags it as unverified for you to confirm.
Downlights
Non-IC-rated recessed downlights must have ceiling insulation held clear of the fitting - NZS 4246:2016 (§4.2, Table 1) requires at least 100 mm clearance and prohibits covering the luminaire. Each cleared circle is a bare patch in the ceiling, and with many fittings this measurably reduces the ceiling's effective R-value. On a roof / ceiling element, enter the fittings' count and clearance radius (and whether they are IC-rated - IC fittings may be abutted and covered, so they need no derate). Thermly applies the published area-weighted derate: each cleared circle is assigned R0.25, and the ceiling's effective R is the area-weighted mean of the cleared and insulated regions - for example, 40 fittings at a 0.25 m clearance radius in a 100 m² R3.0 ceiling derate it to about R2.78. The derate and its working appear in the report's validation warnings.
Layer 2: whole-building heat loss (H1/AS1 6th edition Calculation Method)
The Calculation Method (§2.1.2) compares your proposed building against a reference building for the climate zone.
The comparison
For each element, heat loss = net area ÷ construction R-value. Thermly sums the proposed building's element heat losses (Equation 2.1) and compares them against the reference building's (the per-zone coefficients of Table 2.1.2.7). The building passes when proposed heat loss does not exceed reference heat loss (§2.1.2.6).
The reference building
The reference building treats the gross wall as 70% opaque wall + 30% glazing (doors are grouped with the glazing), and rolls skylights into the roof. Its per-element divisors come from Table 2.1.2.7 for the zone: roof-plus-skylight 6.6 and opaque wall 1.6 in every zone; the glazing/door and floor divisors vary by zone.
Where an element's construction R-value is not known, the §2.1.2.13 defaults are used in the proposed building's equation - R0.18 for an opaque element, R0.15 for a window or glazed door - and the report notes where a default was applied.
The 38% wall framing default
For a framed wall, §2.1.3.1(a)(i) requires a wall framing fraction of no less than 38% unless a lower value is demonstrated for the building. Thermly's default wall assemblies use 38%, the builder warns below it, and the report discloses the assumption. See Assemblies and products.
Hard rules that apply regardless
- Glazing cap (§2.1.2.5) - the Calculation Method applies while wall glazing is 40% or less of the gross wall area; above it, the Modelling Method (H1/VM1) is required.
- Minimum construction R-values (§2.1.2.11) - roof R2.6, wall R1.0, other (suspended) floor R1.3. There is no minimum for slab-on-ground floors, glazing, doors or skylights - those rely on the heat-loss comparison, which is why Thermly adds plausibility flags on supplied slab and skylight values.
- Heated-element minimums (§2.1.2.12) where an element serves a heated space.
How the method is validated
Thermly's engine is checked against authoritative references, not only its own tests:
- Construction R is regression-anchored to the NZS 4214 worked examples (F1 direct-fixed timber, F2 brick-veneer enclosed cavity, F3 steel frame, and F4 vented cavity).
- Whole-building heat loss is validated against the official BRANZ H1/AS1 6th edition Calculation Method spreadsheet: the reference coefficients, minimums, 70/30 split, glazing cap and skylight-in-roof treatment all match, and the proposed heat-loss figure reproduces the spreadsheet across a large oracle set.
- It has also been reproduced against council-approved reports prepared with other tools.
This is why you can trust the arithmetic, and why the professional-tool relationship still holds: Thermly computes to the method faithfully, and you review the inputs and certify that the result is right for the building. See How do I know the calculations are correct?