Cost versus benefit of greater than Code insulation

By - , Build 119

Can going beyond the Building Code with insulation be justified financially? Some new BRANZ research aimed to find out.

The requirements for thermal insulation in the Building Code were updated in 2006 and came into force in stages across the country between October 2007 and September 2008. These changes aimed to save energy and/or improve comfort levels in a cost-effective manner.

Is more insulation worthwhile?

BRANZ recently examined whether a further increase in levels of insulation in the Building Code could be justified from a cost-benefit point of view. The work was based on a specific medium-sized house with an assumed heating regime of 21°C in the evenings only (5–10 pm). The main analysis involved the costs versus benefits of insulation combinations greater than Code (levels 2 and above from Table 1) against the base case (level 1) under various arrangements of:

Table 1: Insulation combinations used in the research.
  Floor typeWall insulation R-valueCeiling insulation R-value
Climate zones 1 and 2 insulation levels      
Level 1 (base case)(1) 90 mm stud wall plain slab 2.2 3.2
Level 2 1.2 m × 50 mm EPS perimeter insulation under slab 2.4 3.6
Level 3 1.2 m × 50 mm EPS perimeter insulation under slab 2.6 4
Level 4 50 mm EPS full cover 2.8 4.6
Level 5 100 mm EPS full cover 4.5 5
Climate zone 3 insulation levels      
Level 1 (base case)(1) 1.2 m × 50 mm EPS perimeter insulation under slab 2.4 3.6
Level 2 1.2 m × 50 mm EPS perimeter insulation under slab 2.6 4
Level 3 50 mm EPS full cover 2.8 4.6
Level 4 100 mm EPS full cover 4.5 5

Note:

(1) In climate zone 3, level 1 floor EPS insulation is not generally required to achieve the schedule method R-value, but it was included in the base case as most new houses in the South Island have under-slab polystyrene.

  • heater types (heat pump, electric resistance, gas and so on)
  • heating and cooling temperatures and time schedules
  • partial or whole-house heating
  • orientation
  • with and without a thermal mass wall
  • economic analysis factors.

Benefits from some…

In all regions, it is cost-effective to install:

  • polystyrene insulation around the slab perimeter for partial heating of the house, such as living areas only
  • polystyrene under the entire slab area for whole house heating.

In nearly all areas of the country, it is cost-effective to install a thermal mass wall for whole-house heating (a 17 m long thermal mass wall is $1,000 more than a timber-framed wall).

In climate zones 1 and 2, some extra insulation above Code levels in the ceiling and walls is cost-effective for whole-house heating up to level 2 in the table.

…but not from others

It is not cost-effective in climate zone 3 to increase thermal insulation in the wall and ceiling beyond level 1.

The Code currently requires double glazing in all regions, and an increase on this was not examined in the report. Any health benefits of increased comfort levels were also not considered.

Benefits for new builds

The findings suggest that greater than Code insulation could well be worthwhile in new housing and the use of thermal mass walls has cost advantages when moderate heating regimes are used.

For more

See BRANZ Study Report 230 Higher than NZBC thermal insulation in new housing cost-benefit analysis, available for download from the BRANZ Shop at www.branz.co.nz.

Download the PDF

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Articles are correct at the time of publication but may have since become outdated.

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