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Comfortable temperatures

Many opportunities are available for most new spec (volume-build) homes to improve their year-round comfort and reduce additional heating and cooling needs. 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.

The following options were explored:

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 programme (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 new 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.

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.

Ceiling insulation

Higher levels of ceiing insulation provide better comfort but there are decreasing returns in terms of heat loss, with even less opportunity in double-storey homes, which have proportionally smaller ceiling areas and therefore less thermal influence.

The table below lists the complete set of upgrade options examined for ceiling insulation - whether they achieved a positive NPV or not. Figures are based on new, volume-built homes with an average floor area of 160 m2, averaged for all New Zealand.

Thermal variation: ceiling insulationThermal benefit
R4.0 bulk ceiling insulationGood
R4.5 bulk ceiling insulationVery good
R5.0 bulk ceiling insulationVery good

For location-specific costings that are thermally beneficial, refer to the relevant climate zone.

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. Figures are based on new, volume-built homes with an average floor area of 160 m2, averaged for all New Zealand.

THERMAL variation: wall insulationthermal benefit
R2.8 bulk insulation to walls, with 90 mm timber framingGood
R3.2 bulk insulation to walls, with 140 mm timber framingGood
R4.0 bulk insulation to walls, with 140 mm timber framingGood

For location-specific costings that are thermally beneficial, refer to the relevant climate zone.

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. Figures are based on new, volume-built homes with an average floor area of 160 m2, averaged for all New Zealand.

 THERMAL variation: window upgradesthermal benefit
Standard (non-thermally broken) double glazing framing, with low-E coatingVery good 
Thermally broken framing; glazing with low-E coatingVery good
Thermally broken framing; double glazing with low-E coatingVery good
Timber framing, with low-E glazingVery good

For location-specific costings that are thermally beneficial, refer to the relevant climate zone.

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 160 m2, averaged for all New Zealand.

THERMAL variation: concrete slabsthermal benefit
Generic pod-style polystyrene insulation under concrete slab, with either carpet or vinyl floor coverings in the usual placesMarginal
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 placesVery good
As above, but with 20 mm insulation around perimeterVery good
Uninsulated (i.e. traditional) concrete slab with fully exposed and polished toppingNone
Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas carpetedNone*
Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas vinylNone
Uninsulated (i.e. traditional) concrete slab with only living areas having exposed, polished topping; other areas tilesNone*

Auckland is the exception.

For location-specific costings that are thermally beneficial, refer to the relevant climate zone.

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 imrpovements - whether they achieved a positive NPV or not. Figures are based on new, volume-built homes with an average floor area of 160 m2, averaged for all New Zealand.

THERMAL variation: combinationsthermal benefit
50 mm expanded polystyrene insulation, continuous, under concrete slab + exposed slab in living areasExcellent
As above, but with 20 mm insulation around perimeterExcellent
50 mm expanded polystyrene insulation under slab + R2.8 walls + R4 ceiling + low-E glazingOutstanding
As above but with thermally broken frames as wellOutstanding

For location-specific costings that are thermally beneficial, refer to the relevant climate zone.

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