Only the most practical opportunities have been presented here. The improvements are averages based on the careful computer modelling of many recently consented, stand-alone volume-built homes. Generally, the more money invested in thermal improvements early on, the better the return in terms of ongoing comfort.
These thermal efficiency improvements were explored:
- House orientation - facing the living areas to the north.
- Wall insulation - upgraded from R2.2 in 90 mm timber framing.
- Windows - upgraded from standard (non-thermally broken) double glazed, aluminium framed with standard glazing.
- Concrete slab floor - upgraded from standard (uninsulated) slab on ground.
- Combinations of several thermal improvements.
Approach
Eighteen 2012-consented group home builder specifications were sourced from three councils - six each from Auckland, Hamilton and Christchurch. The houses were then all built virtually to thermally examine using a specialist computer program (AccurateNZ). Different combinations of the original 18 houses were then computer modelled in the remaining six climate zones (Tauranga, Napier, Wellington, Nelson, Dunedin and Invercargill) to ensure appropriate representation.
The computer model assumed sensible and identical occupant behaviour to ensure that a fair comparison between buildings could be made. For example, the heating schedule was modelled using the following comfort considerations:
- In winter, the thermostat is set to 20°C from 7am to 11pm in living, kitchen and bedrooms, with the remainder of the house being left unheated.
- In summer, when indoor temperatures are near 25°C, the windows are opened for cooling.
All houses were assumed to be electrically heated with a 100% efficient space heater so that a fair and simple comparison could be made.
Each of the houses selected had the same thermal upgrades applied using available products and systems while keeping costs in mind. The upgraded versions were then compared with the original (as consented) version to see what impact the upgrades really made for that particular climate zone and house configuration.
The economic viability of each upgrade was then assessed to ensure that the final suggestions would fall within a reasonable budget and have a positive net present value (NPV). The benefits (specifically, reduced heating costs) were thus assessed against the liability (i.e. higher initial purchase and installation costs), based on an electricity charge of 30 c/kWh. A discount rate of 5% over 25 years was applied, as suggested by the BRANZ economist.
Key costings reference sources were QV costbuilder and personal communications with BRANZ economists. Key thermal materials and buildings references were BRANZ experts and the BRANZ House insulation guide (5th edition).
Figures are based on new, volume-built homes with an average floor area of 160 m², averaged for all New Zealand.
For location-specific costings that are thermally beneficial, refer to the relevant climate zone.
House orientation
Ideally, all living areas in homes should be north facing. Given that all the homes examined had at least one living area window facing in a northerly direction, many were close to their optimal thermal orientation already.
Wall insulation
The most commonly installed wall insulation only just meets Code minimums, using bulk insulation in 90 mm framing. The implications of increasing wall insulation were explored but were found to be not financially viable even though thermally beneficial.
The table below lists the complete set of upgrade options examined for wall insulation - whether they achieved a positive NPV or not.
| Thermal variation: wall insulation | Thermal benefit |
|---|---|
| R2.8 bulk insulation to walls, with 90 mm timber framing | Good |
| R3.2 bulk insulation to walls, with 140 mm timber framing | Good |
| R4.0 bulk insulation to walls, with 140 mm timber framing | Good |
Windows
Windows are a key thermal weak point, so there is a great opportunity to substantially reduce heat losses by upgrading the frames or the glazing.
The table below lists the complete set of upgrade options examined for windows - whether they achieved a positive NPV or not.
| Thermal variation: window upgrades | Thermal benefit |
|---|---|
| Standard (non-thermally broken) double glazing framing, with low-E coating | Very good |
| Thermally broken framing; glazing with low-E coating | Very good |
| Thermally broken framing; double glazing with low-E coating | Very good |
| Timber framing, with low-E glazing | Very good |
Concrete slab floor
The perimeter of the slab is where most flooring warmth leaks out, so it should be insulated. Underfloor insulation must be continuous to minimise thermal weak points. The pod-style flooring systems have a base of non-continuous segments of polystyrene. Exposing uninsulated slabs internally increases heating costs when compared to the traditional slab construction, so is not recommended.
The table below lists the complete set of upgrade options examined for concrete slabs - whether they achieved a positive NPV or not. Figures are based on new, volume-built homes with an average floor area of 160m2, averaged for all New Zealand.
| Thermal variation: concrete slabs | Thermal benefit |
|---|---|
| Generic pod-style polystyrene insulation under concrete slab, with either carpet or vinyl floor coverings in the usual places | Marginal |
| 50 mm expanded S-grade polystyrene insulation, continuous, under concrete slab (i.e. not pod style) with either carpet or vinyl floor coverings in the usual places | Very good |
| As above, but with 20 mm insulation around perimeter | Very good |
| Uninsulated (i.e. traditional) concrete slab with fully exposed and polished topping | None |
| Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas carpeted | None (except for Auckland) |
| Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas vinyl | None |
| Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas tiles | None (except for Auckland) |
Combinations of several thermal improvements
A combination of several thermal improvements was explored that consider the whole thermal envelope (i.e. floor insulation, wall/ceiling insulation plus glazing). These more comprehensive upgrades result in the most thermal benefit.
The table below lists the complete set of upgrade options examined for combinations of thermal improvements - whether they achieved a positive NPV or not.
| Thermal variation: combinations | Thermal benefit |
|---|---|
| 50 mm expanded polystyrene insulation, continuous, under concrete slab + exposed slab in living areas | Excellent |
| As above, but with 20 mm insulation around perimeter | Excellent |
| 50 mm expanded polystyrene insulation under slab + R2.8 walls + R4 ceiling + low-E glazing | Outstanding |
| As above but with thermally broken frames as well | Outstanding |
More information
- House insulation guide (6th edition) – this is the edition that supports the current Building Code H1 requirements, not the edition used for the calculations on this page
- BU568 Low-impact homes
- BU571 Thermal mass
- www.level.org.nz/passive-design
- www.qvcostbuilder.co.nz
Updated: 20 September 2023