Physics & Maths

1. Thermal Performance — NZBC Clause H1

The thermal resistance calculations in Buildup are based on NZS 4214:2006 — Methods of Determining the Total Thermal Resistance of Parts of Buildings, published by Standards New Zealand. This standard is the primary cited method for R-value calculation in the New Zealand Building Code's Acceptable Solution H1/AS1 (Energy Efficiency for Housing and Small Buildings) and Verification Method H1/VM1, and is co-cited in H1/AS2 and H1/VM2 for larger buildings. It has been current since 29 May 2006, superseding the interim NZS 4214(Int):2002.

NZBC Clause H1 requires that buildings are constructed with adequate thermal resistance to limit unwanted heat flow. NZS 4214:2006 is the normative method for calculating the construction R-value of any building element that contains thermal bridging — most commonly timber or steel framing — and its results are directly used to demonstrate compliance with the H1 schedule method.

The Combined Averaging Method

NZS 4214 specifies that the total R-value of an element containing thermal bridges shall be the arithmetic mean of two independently calculated bounds. This is known as the combined method:

The two bounds represent physically limiting cases of heat flow behaviour:

• Isothermal Planes Bound ($R_{iso}$): Assumes that temperature is uniform across each plane parallel to the wall surface — i.e., heat flows laterally to equalise at each interface. This produces the lower bound (less conservative) of the true R-value. $$R_{iso} = R_{si} + \sum_{layers} \left( \frac{1}{\dfrac{f}{R_f} + \dfrac{1-f}{R_i}} \right) + R_{se}$$ where $f$ is the framing fraction, $R_f$ is the resistance of the frame path through that layer, and $R_i$ is the resistance of the infill (insulation) path.
• Parallel Path Bound ($R_{par}$): Assumes heat flows in straight lines from inside to outside — either entirely through the frame or entirely through the insulation — with no lateral heat redistribution. This produces the upper bound. $$R_{par} = \frac{1}{\dfrac{f}{R_{path,\,f}} + \dfrac{1-f}{R_{path,\,i}}}$$ where $R_{path,\,f}$ and $R_{path,\,i}$ are the full inside-to-outside series resistances of each respective path including surface resistances and all non-framed layers.

The arithmetic mean of these two bounds is required by NZS 4214 because the true thermal behaviour of a framed wall lies between them. No simple one-dimensional calculation captures the full effect of thermal bridging, and the averaging method provides a practical, standardised approximation that has been the basis of NZ compliance calculations since the standard's introduction.

Why the Framing Fraction Matters — BRANZ ER53

The framing fraction $f$ is the single most consequential input in any framed-wall R-value calculation, and it is widely underestimated in practice. BRANZ External Research Report ER53 (2020) — Measuring the Extent of Thermal Bridging in External Timber-Framed Walls in New Zealand examined 1,103 wall panels across 47 newly constructed dwellings in Auckland, Christchurch, Wellington, and Hamilton. The findings were unambiguous:

The reasons for this discrepancy identified by ER53 include structural requirements from NZS 3604 and council wind zones, E2/AS1 cladding requirements introduced after the weathertightness crisis, double studs at panel junctions generated by frame-and-truss design software, and design trends such as double-height vestibules requiring increased stud depth. Critically, the standard framing percentage definitions in NZS 4218 allow lintels, trimming studs, and double studs to be excluded from the framing fraction calculation — meaning the code-compliant fraction can significantly understate the actual thermal bridge area in a finished wall.

A subsequent larger study by PlaceMakers, covering both residential and commercial construction, found framing percentages ranging from 23–64% with an average of 38%. This figure has been taken up in MBIE's proposals for the next update to NZBC H1, where a consistent value of 38% timber framing is proposed as the default assumption for residential walls.

Buildup allows the framing fraction to be set independently per layer, enabling accurate modelling of the actual framing content in a specific assembly rather than defaulting to the assumed 15% that understates real-world thermal bridging.

2. Hygrothermal Screening — NZBC Clause E3

NZBC Clause E3 (Internal Moisture) requires that buildings be constructed to avoid the likelihood of fungal growth or the accumulation of contaminants on linings or other building elements, and to safeguard occupants against illness or loss of amenity from internal moisture accumulation (E3 Objective 3.1). The Acceptable Solution E3/AS1 requires, among other things, that wall assemblies achieve minimum thermal resistance values to prevent surface condensation, and notes explicitly that insulation satisfying NZBC H1 cannot automatically be assumed to satisfy E3 — they are separate requirements addressing separate failure modes.

To support analysis beyond minimum surface condensation checks, Buildup employs the Glaser Method (ISO 13788:2012) to screen for interstitial condensation risk within the wall assembly. This is a steady-state calculation that maps the temperature profile and vapour pressure profile through the build-up and identifies where — if anywhere — the actual vapour pressure exceeds the saturation pressure, indicating a condensation risk plane.

Saturation Vapour Pressure

The saturation vapour pressure $P_{sat}$ defines the maximum partial pressure water vapour can exert at a given temperature before condensing. Buildup uses the Buck equation (an accurate form of the Magnus–Tetens approximation), which is the same approximation family used in ISO 13788 for steady-state screening:

where $T$ is temperature in degrees Celsius and $P_{sat}$ is in Pascals. This is valid over the temperature range relevant to New Zealand building conditions (approximately −10°C to +40°C) and agrees with tabulated values to within 0.1% across that range.

Temperature Profile Through the Assembly

Under steady-state conditions, temperature varies linearly with thermal resistance across the assembly. The temperature at any interface is:

where $\sum R_{internal}$ is the cumulative thermal resistance from the internal surface to the interface in question, and $R_{total}$ is the total air-to-air resistance of the assembly including both surface resistances. This gives the saturation pressure profile when applied through $P_{sat}(T)$.

Vapour Diffusion and the $S_d$ Value

The resistance of a material to vapour diffusion is characterised by its vapour diffusion resistance factor $\mu$ (dimensionless), which expresses how many times more resistant the material is to vapour flow than an equivalent thickness of still air. The equivalent air layer thickness $S_d$ combines $\mu$ with the physical thickness $d$ of the layer:

Under steady-state conditions, vapour pressure varies linearly with $S_d$ across the assembly (analogous to how temperature varies with thermal resistance). The actual vapour pressure at any interface is:

where $P_{in}$ and $P_{out}$ are the actual vapour pressures of the internal and external air respectively, calculated from temperature and relative humidity as $P_v = P_{sat}(T) \times (\text{RH}/100)$.

A condensation risk plane exists wherever $P_{v,\,interface} \geq P_{sat}(T_{interface})$ — i.e., wherever the actual vapour pressure line intersects or exceeds the saturation pressure curve when plotted across the assembly depth.

Limitations of the Glaser Method

ISO 13788:2012 is explicit about the limitations of the steady-state Glaser approach, and Buildup's output should be interpreted with these in mind. The method assumes vapour transport is by diffusion only and does not account for: capillary suction and liquid moisture transfer within materials; variation of material properties with moisture content; air movement into the assembly through gaps or air spaces; or the hygroscopic buffering capacity of materials. ISO 13788 Section 1 notes that where these phenomena cannot be considered negligible, more advanced simulation methods such as WUFI (used in BRANZ SR344, described below) are required.

In practice, the Glaser method is a conservative screening tool. Where it predicts no condensation risk under the selected conditions, the assembly can be expected to perform well. Where it does predict an intersection, this indicates a zone warranting attention, but does not necessarily mean liquid moisture will accumulate — the hygroscopic capacity of typical NZ wall materials (discussed in SR344) provides meaningful buffering that the steady-state method does not capture.

3. BRANZ SR344 — Vapour Control in New Zealand Walls

SR344 is the most directly applicable BRANZ research to the type of steady-state condensation screening that Buildup performs. It is a two-year experimental study in which a series of wall specimens were installed into a BRANZ test building that was periodically humidified to simulate occupied New Zealand conditions. Measurements within the wall cavities were complemented by hygrothermal simulation using WUFI (Wärme Und Feuchte Instationär), the industry-standard transient simulation software.

Key Findings Relevant to Buildup's Output

• Humidity at sheathing reached 100% in most walls. SR344 found that relative humidity at the sheathing/underlay interface reached 100% in almost all of the tested wall configurations during the humidified periods. This means the Glaser method's identification of a high-risk zone near the outer structural layer is well-supported by experimental evidence — the outer cavity face of the framing is routinely exposed to saturated conditions in NZ climates.
• Liquid condensation only formed in a minority of walls. Despite reaching 100% RH, only a minority of the walls saw actual liquid moisture formation. This distinction — between reaching saturation humidity and accumulating damaging liquid water — is important context for interpreting a Glaser intersection. A flagged intersection in Buildup indicates a zone of elevated risk, not a certainty of liquid water accumulation.
• A dedicated vapour control layer is not necessary for most NZ assemblies. SR344's central conclusion is that it is not necessary to add a specific vapour control layer to prevent liquid moisture accumulation in typical New Zealand timber-framed walls. Acrylic paint systems and standard internal linings (e.g., plasterboard with paint) provide sufficient vapour resistance to prevent damaging condensation further into the wall, while still allowing adequate drying capacity. This finding is consistent with previous BRANZ guidance and with international practice for similar temperate climate zones.
• Smart vapour retarders were the exception. The only wall assemblies in SR344 that did not reach 100% RH at the sheathing were those fitted with a smart vapour retarder (a membrane whose vapour resistance varies with ambient RH) between the lining and the insulation. This is relevant for assemblies in colder NZ climate zones (Christchurch, Queenstown) where the vapour drive is more persistent.
• WUFI models validated the experimental results. The WUFI simulations in SR344 agreed well with measured wall performance, with the main exception being moisture levels in the cavity — where airflow effects (not modelled in WUFI's one-dimensional mode) appeared to play a role. SR344 concludes that airflow processes in walls warrant further investigation, which is consistent with the known limitation of both WUFI and the simpler Glaser method: neither accounts for advective moisture transport through gaps.

SR344 and the Glaser Method's Place in NZ Practice

SR344 was conducted using WUFI transient simulation, which is significantly more sophisticated than the Glaser steady-state method. However, the Glaser method's role is different: it is a screening tool, not a design verification tool. It identifies assemblies that warrant closer attention under worst-case steady-state conditions. Where a Glaser screen returns no intersection, the assembly is very unlikely to present a condensation problem in practice. Where it does return an intersection, SR344's findings suggest that the assembly's real-world performance will depend heavily on drying capacity, internal lining vapour resistance, and climate zone — factors that the tool surfaces through the vapour pressure profile visualisation, even if it cannot fully resolve them in steady-state alone.

SR344's finding that standard NZ wall assemblies without dedicated vapour barriers perform adequately in temperate zones (Auckland, Hamilton, Wellington) provides useful calibration: a Glaser intersection flagged in these regions, near the outer structural face, should be read as a managed risk rather than an assembly failure — particularly where a functional drained cavity is present. In colder zones (Christchurch and south), SR344's data and the more persistent inward vapour drive make intersections closer to the structural layer more significant.

4. Climate Inputs and NZ Region Data

The steady-state Glaser screen requires representative external temperature and relative humidity for the climate zone being assessed. Buildup's region presets are derived from the design winter conditions that appear in NZBC compliance documentation and BRANZ research for each major NZ centre. These represent characteristic cool-season conditions — not extreme minima — consistent with the steady-state screening intent of ISO 13788.

For more detailed hygrothermal analysis, MBIE released updated Typical Meteorological Year (TMY3) and Moisture Design Reference Year (MDRY) weather files for 18 New Zealand climate zones in October 2024. These were developed by NIWA and reviewed by BRANZ and Kāinga Ora. The MDRY files are specifically intended for hygrothermal analysis of moisture and mould risk (compatible with WUFI and ANSI/ASHRAE Standard 160), while the TMY3 files are used for energy modelling. These files supersede the outdated TMY2 files created by NIWA in 2008 and represent a significant improvement in the accuracy of NZ climate data available for building performance analysis.

Buildup's steady-state approach is appropriate for initial screening of any NZ wall assembly. Where analysis is required for a specific consent application or where the steady-state screen flags a risk in a complex assembly, the referenced MDRY files should be used as inputs to a transient WUFI model, which remains the gold standard for detailed NZ hygrothermal verification.

5. Loaded Dimension (Load Width)

The loaded dimension (also called load width or tributary width) is the plan distance over which a structural member collects load. For lintels and beams, this is typically the horizontal distance from the member to the ridge or nearest intermediate support — i.e., half the span of the roof or floor being carried. For bearers, it is the joist span divided by two (each bearer carries half of the joists spanning to it).

NZS 3604:2011 span tables are indexed by loaded dimension because it directly determines the load intensity on the member. A lintel carrying roof load from a 7 m span has a loaded dimension of 3.5 m — meaning each lintel supports 3.5 m of roof measured perpendicular to the lintel.

Getting the loaded dimension wrong is the most common source of incorrect span-table lookups. When in doubt, measure the plan distance from the member centreline to the ridge (or nearest parallel support) on the architectural plans.

6. Floor Imposed Loads

Floor joist and bearer span tables in NZS 3604:2011 are based on imposed (live) loads defined in AS/NZS 1170.1. For residential buildings the design imposed floor load is 1.5 kPa (equivalent to 150 kg/m²), which covers normal domestic occupancy including furniture and foot traffic. For areas of public assembly or commercial use, the load increases to 2.0 kPa or higher depending on the specific use category.

These loads are uniformly distributed loads (UDL) and do not include the self-weight of the floor structure (dead load), which is accounted for separately in the span tables. NZS 3604 Table 7.1 lists the joist spans for the 1.5 kPa residential case; higher imposed loads reduce allowable spans.

7. Roof Weight Categories

NZS 3604:2011 classifies roofs into two weight categories that determine rafter, purlin, and supporting member spans:

• Light roof (≤ 20 kg/m²): long-run profiled metal roofing (e.g., Colorsteel, Zincalume) on timber purlins. This is the most common residential roof type in New Zealand.
• Heavy roof (> 20 kg/m²): concrete or clay roof tiles, typically 40–55 kg/m². Heavy roofs impose significantly greater loads on rafters, lintels, and top-plate connections.

The weight category affects every span table in NZS 3604 that involves roof load — including lintels (Table 8.9), rafters (Table 10.1), underpurlins, and ridge beams. Using the wrong roof weight category is a common specification error that can result in undersized members.

8. Wind Pressure — AS/NZS 1170.2

AS/NZS 1170.2:2021 — Structural Design Actions, Part 2: Wind Actions is the joint Australia–New Zealand standard for determining wind loads on structures. It defines the regional wind speed $V_R$ for each wind region and return period, and the method for calculating site wind speed accounting for terrain category, shielding, topographic multipliers, and building orientation.

New Zealand is divided into wind regions (A1–A7, and W) defined in NZS 3604 Table 5.4 and AS/NZS 1170.2 Table 3.1(B). Each region has a characteristic regional wind speed for the design return period (typically 500-year for ultimate limit state, 25-year for serviceability).

Terrain categories describe the surface roughness surrounding the site and affect how wind speed varies with height:

• TC1: Open water, mudflats — very exposed
• TC2: Open terrain with few obstructions — grassland, airfields
• TC2.5: Intermediate — scattered obstructions 3–5 m high
• TC3: Suburban terrain with numerous obstructions 3–10 m high
• TC4: Urban terrain with closely spaced buildings ≥ 10 m high

The site wind speed $V_{sit}$ determines the wind pressure $q$ used for structural design: $q = 0.5 \times \rho_{air} \times V_{sit}^2$, where $\rho_{air} = 1.2\,\text{kg/m}^3$. External and internal pressure coefficients are then applied to determine the net wind actions on each building surface.

9. Window U-Value

The U-value (thermal transmittance, W/m²·K) of a window describes how much heat flows through it per unit area per degree of temperature difference. Lower U-values indicate better insulating performance. NZBC H1 sets maximum allowable window U-values that vary by climate zone.

Window U-value depends on three main components:

• Glazing: Single glazing has a centre-of-pane U-value around 5.8 W/m²·K. Double glazing with a 12–16 mm air gap reduces this to approximately 2.7 W/m²·K. Low-E coatings and argon fill can reduce it further to around 1.3–1.6 W/m²·K.
• Frame: Aluminium frames without thermal breaks have high conductivity (U ≈ 7–10 W/m²·K). Thermally broken aluminium frames reduce this significantly. Timber and uPVC frames perform better (U ≈ 2–3 W/m²·K).
• Spacer bar: The edge-of-glass region where the spacer bar separates the panes creates additional thermal bridging. Warm-edge spacers reduce this effect.

The whole-window U-value used for H1 compliance combines all three components and accounts for the frame-to-glass area ratio. It is always higher than the centre-of-pane value alone because the frame and spacer introduce thermal bridges. Window U-values should be sourced from manufacturer test data (to NZS 4218 or ISO 10077) or from the WERS (Window Energy Rating Scheme) database.

10. Timber Treatment — NZS 3602

NZS 3602:2003 — Timber and Wood-based Products for Use in Building specifies the preservative treatment requirements for timber based on its hazard class (H1 through H6), which is determined by the exposure conditions and biological hazards the timber will face in service.

• H1.2: Interior framing, not in ground contact, protected from weather — general framing including wall studs, top/bottom plates, ceiling joists
• H3.1: Exterior above ground, coated — fascia boards, weatherboards, window joinery
• H3.2: Exterior above ground, uncoated — decking, pergola beams, exposed rafters
• H4: Ground contact or fresh water — fence posts, retaining wall timber, piles in non-aggressive ground
• H5: Ground contact in aggressive conditions — piles in aggressive soils, critical structural ground contact

NZS 3604:2011 references NZS 3602 for all structural timber treatment requirements. The correct hazard class depends on the element's location and exposure zone (as defined in E2/AS1). Common errors include using H1.2 framing in locations that require H3.1 treatment (e.g., bottom plates within 150 mm of ground level) or specifying insufficient treatment for timber in enclosed but damp environments such as subfloor framing.

11. Scope and Limitations of This Tool

Buildup performs two distinct calculations: an R-value calculation per NZS 4214:2006, and a steady-state Glaser condensation screen per ISO 13788:2012. Both are well-established, standardised methods with specific scopes of applicability.

What this tool is appropriate for: Initial screening of timber-framed NZ wall assemblies for thermal performance and condensation risk; comparison of alternative assembly configurations; preliminary compliance checking against NZBC H1 R-value schedules; identification of assemblies that warrant more detailed hygrothermal analysis.

What this tool does not replace: Project-specific hygrothermal modelling (WUFI) for complex assemblies, unusual climate exposures, or consent applications requiring formal verification; structural design to NZS 3604; specific design for assemblies outside NZS 3604 scope; steel stud frame calculations (which require a dedicated method not covered by the simple isothermal/parallel path averaging); or any calculation required to be signed off by a Chartered Professional Engineer.

All material property data (thermal conductivity $\lambda$ and vapour resistance factor $\mu$) used in the standard material library are drawn from BRANZ-referenced sources and New Zealand product testing data. Custom material inputs are the responsibility of the user to verify against tested data.