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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).
Where the framed portion ends is set by H1/AS1 6th edition §2.1.3.2, which modifies NZS 4214 clause 5.7.1(a) with three exceptions Thermly implements in full: an adjacent continuous insulation layer (a rigid board or insulated rigid air barrier over the framing) is carried inside the framed portion - its R counted in both parallel paths, not added in series behind the frame; a layer sitting between two framed portions is split half into each; and a pliable membrane or underlay is treated as if it were not there. Each affected layer states on the report which part of the clause moved it. The same clause settles a ceiling where framing only part-penetrates the insulation (batts laid over truss bottom chords or joists): the batt carrying on over the member is the same layer, not a next one, so it stays in the bridged portion and is counted in both parallel paths, not added in series afterwards. Such a layer prints its working on the report.
The standard's refinements of the bridged case are supported too - partial chord penetration, a bounding enclosed cavity, compressed insulation (Appendix C2), and the §5.1.1(b) route for supplying a two-dimensional or measured determination where the one-dimensional method cannot certify a configuration. They are set per layer in the builder - see Advanced NZS 4214 inputs.
Air cavities
An enclosed cavity contributes a resistance from NZS 4214 Table E3. Thermly applies the table's non-reflective values, chosen by heat-flow direction (wall, roof or floor) and, through the presets, by gap width; there is no emittance input, and a reflective (foil-faced) cavity is not credited at a reflective value - H1/AS1 §1.1.2.1 places a building with foil insulation outside the Acceptable Solution, and NZS 4214 §5.3.1.4 never allows a bright-foil nameplate emittance in any case. A ventilated (drained) cavity is handled per §5.3.2: its air-space R is taken as 0.45 × the enclosed Table E3 value (0.45 × 0.17 = 0.0765 for a wall cavity), as the normative clause requires. Where the vented gap bounds a bridged layer it folds into that layer at that de-rated value rather than standing as its own layer (H1/AS1 §2.1.3.2 / NZS 4214 §5.7) - the result and the report print the folded amount on the cavity's own row, marked as carried inside the framed layer rather than summed separately - while a vented gap that does not bound a bridged layer carries the same 0.45 × value as a layer of its own. Only the layers exterior to the gap are de-rated, by 45% (each retains ×0.55 of its R, per the arithmetic of the Appendix F4 worked example: brick 0.10 becomes 0.06), and the external surface resistance Rse is not de-rated. An enclosed cavity declared against a bridged layer still folds at its full Table E3 value (§5.7, worked example F2) - only a ventilated one is de-rated. The informative F4 example itself keeps the full enclosed value inside the framed layer; Thermly follows the normative clause over the example, so its result for that wall sits deliberately below the example's published figure.
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, and the calculator refuses to total one from layers - a slab's R-value is dominated by ground coupling, which depends on the floor's geometry, and the Appendix E figures already include the surface films a layer stack would add again. 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), with the method confirmed by the preparer - see Supplied floors and rafts.
Steel framing
Light-gauge steel sections are transformed by the NZS 4214 metal-frame method (§5.7). A layer that declares a steel section resolves only through that transform - if a dimension is missing, the layer reports no R-value and the total stays blank, exactly like any other incomplete layer, rather than silently substituting something else. The 38% framed-wall floor applies to steel walls the same as timber (§2.1.3.1(a)(i) names framed walls, not a material). The one-dimensional transform is also 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.
A steel layer with strip thermal breaks - break material fixed over each member face, between the framing and the cladding - is calculated to NZS 4214 §5.7.4 and the Appendix F3 worked example: the metal path carries the strip's R plus the standard's two frame-facing contact resistances (0.03 m²K/W each, exactly F3's values), and where the strips cover more of the face than the member flanges, that extra coverage is a third parallel region (the insulation path with the strip in front of it). The engine reproduces F3's published R1.53 end-to-end; the layer's "Show working" panel prints the three-region arithmetic so the row can be checked from the page, and the report prints the same working under the layer's build-up table (and records the break in the layer's notes in short form). The contact resistances are claimed only when a break is declared: a bare steel frame keeps the plain equivalent-rectangle value, the conservative reading. The catalogue's break materials include the 10 mm EPS strip at R0.25 that E3/AS1 clause 1.1.4(d) requires at the outside face of every steel framing member in housing (the clause states that value for a 10 mm strip), so the mandated minimum is one pick away on a wall, roof or floor layer. A continuous board across the whole frame face can be declared as that break at 100% coverage, where it takes the same combination and the contact resistances, or entered as its own layer and folded under §2.1.3.2(i), which puts the same R on both paths but declines the contacts and so reads slightly lower. Thermly cautions when a strip break is declared on a layer that already folds such a sheet - the same sheath counted twice over-states the wall. Steel sections themselves come from published supplier documents: the library's steel build-ups carry the 89 × 41 × 0.75 mm standard stud and a 150 × 41 × 0.75 mm rafter-range section, and the picker offers fifteen sections in all, floor joists and a ceiling batten among them, each cited to its source in the catalogue, and a section picked from the catalogue keeps the record's identity on the layer, which the report cites (the build-up card names the section record and the supplier document it came from).
The report discloses all of this under every steel layer's build-up table: the section modelled ("89 × 41 × 0.75 mm light-steel section", with the catalogue record and the supplier document it cites when one was picked), the method ("metal-frame equivalent conductivity, NZS 4214 §5.7.4 Eq 7, km 50 W/m·K (Table E1(d))" with the equivalent frame R actually used), the thermal break - product, R and face coverage, or "none declared" - and, for a layer with a break, the Appendix F3 three-region working so the row can be checked from the page. References & sources lists the NZS 4214 clauses behind the steel method (§5.7.3-5.7.4, Table E1(d), Appendix F3) whenever a steel layer is present, and cites the break material's own source alongside the insulation product's - the 10 mm strip cites E3/AS1 itself. On a house, a steel wall or roof layer with no break declared (or one below R0.25) raises an E3/AS1 clause 1.1.4(d) issue on the review rail and an optional review item on the report; Thermly assesses H1, so this is a prompt to resolve E3, not a determination.
Windows and doors
A vertical window's construction R comes from H1/AS1 Appendix D, Table D.1.1.1, keyed by the glazing's centre-of-glass Ug and the frame type. Each window row's Window table lookup… lets you pick the make-up by name - glazing (double / triple), the make-up ("Low E3 / Clear · Argon · thermally-improved spacer") and the frame - and fills the normative table value for you; a make-up whose glazing beats the lowest tabulated row is credited at the nearest tabulated Ug, the standard's own conservative round-up, and the lookup notes when it has done so. Experts can still enter a measured Ug directly. The lookup remembers the spec applied to each window, and can apply one spec to every window in a wall.
Opaque doors are outside that table (Appendix D excludes doors), so a bare typed door R-value has no stated basis and is raised for review. The Catalogue door… picker on a door row gives it one: a branded door carries its manufacturer's published whole-door construction R with its provenance (range, core, leaf size and calculation method), which the openings schedule and References & sources then cite. Editing the R afterwards detaches the product. Garage doors are a different case: their makers publish a figure for the insulated panel only, never for the whole door with its joints, guides and frame. A catalogue garage door therefore names the product and labels its figure "panel R", and picking one deliberately leaves the opening's construction R unset, so the §2.1.2.13 default for an opaque element applies and the report says so. Where a job needs better than the default, the route is a determination for the actual door, entered with its basis.
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 - or, for a catalogue skylight (VELUX, Adlux, Keylite, and Solatube tubular skylights), the manufacturer's published whole-unit R-value, which includes the frame; picking one in the skylight's row fills its R, dimensions and area, and cites the manufacturer's datasheet on the report. Where a supplied skylight R states no basis, Thermly uses it unchanged but flags it as unverified for you to confirm. In the reference building, skylights carry no reference R-value of their own - their area is combined with the roof at the roof reference (Table 2.1.2.7) - and the report says "Combined with roof" rather than inventing a divisor.
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. Tick Non-IC downlights on the roof row, beside its name, and enter the fittings' count and clearance radius in the detail line that appears under the row (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 show on the H1 page and in AI Review, and print in the report's Review items section when you switch that on.
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).
Areas are measured to overall internal dimensions (§2.1.2.10). A roof's area is the area of the insulated surface: the flat ceiling (plan) area under a pitched roof with a flat ceiling, which is the default, or the sloped area where the roof row's Ceiling follows pitch box is ticked for a skillion or cathedral ceiling. A storey is not a term in the comparison: a second storey is simply more rows on the same elements, and the glazing cap and the minimums read the whole building.
Alterations and additions
H1/AS1 6th edition says nothing about work on an existing building, so Thermly follows MBIE's guidance and Building Act 2004 section 112(1)(b). The comparison above is built from the new building elements only; existing fabric the work does not touch is in neither equation and claims no R-value; and existing fabric the work alters is checked separately, heat loss after against heat loss before, so that it is not diminished. The glazing cap may be read over the entire altered building or over the new work only. See Alterations and additions.
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. An opening entered with a sub-physical R (below R0.01, a typo like 0.005 for 0.5) is treated the same way, with a plausibility warning, rather than silently dropping out of the heat loss.
The 38% wall framing default
For a framed wall - timber or steel - §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 (including when a sub-38% figure came from a bare single-bay calculation, which excludes the timber around openings, corners and junctions), and every framed layer on the report declares its fraction and the basis it came from - the 38% default, declared geometry, or a recorded demonstration whose evidence is quoted verbatim. 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, and floors other than slab-on-ground R1.3. The clause is absolute: it binds identically in every climate zone and sits outside the heat-loss comparison, so Thermly raises a construction below its minimum as a blocking finding (checked even before a zone is set) that no surplus elsewhere can trade away. There is no minimum for slab-on-ground floors (the clause expressly excludes them), 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 contains embedded heating - heating pipes or cables within the element or its finishes.
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) and F3 (steel frame) reproduce the standard's published results; F4 (the same brick-veneer wall with its cavity ventilated) is pinned at the value the normative §5.3.2 rule gives, deliberately below the informative example's published figure, which folds the gap at its full enclosed value - see Air cavities.
- 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.
- The reference documentation behind the engine has been re-verified against the licensed source standards - NZS 4214:2006 and H1/AS1 6th edition - in a complete front-to-back pass: every worked-example anchor, surface resistance, reference coefficient, per-element minimum and default matched the standards exactly.
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?