New Zealand studies have shown that thermal insulation delivers improved comfort and health for house occupants and/or reduced energy bills by reducing heat loss.
Figure 5. Typical heat loss through an uninsulated house.
Up to 30-35% of heat loss from a dwelling can occur through an uninsulated roof. Installing and topping up insulation in the roof space is therefore one of the most cost-effective solutions for improving thermal performance.
The 2015/16 House Condition Survey found 39% of homes had at least 120 mm insulation covering at least 80% of the accessible roof space area. 120 mm is the minimum recommended by the Energy Efficiency and Conservation Authority.
Just under half of houses surveyed (47%) had less than 80% coverage of insulation in the roof space or less than 120 mm-thick insulation (Figure 6).
Figure 6: Results on the depth and coverage of insulation in the roof space [Source: BRANZ House Condition Survey 2015/16].
Damaged or poorly fitted insulation can have a substantial impact on its efficacy in reducing heat loss. Around half of houses had at least one defect with insulation in the roof space that could reduce its effectiveness. Defects included gaps in insulation (31%) and insulation settling (22%), not fitted properly (16%) and displaced (11%).
The subfloor is another area of the house that can offer substantial benefits in home performance if properly insulated.
The survey found that in properties that had an accessible subfloor (around two-thirds of all houses surveyed) nearly one-fifth had no insulation or insufficient coverage, while 43% had good coverage of bulk insulation.
A further 18% of subfloors had foil insulation (12% of all houses surveyed). Foil performs far less effectively as an insulating layer. The install or repair of foil in existing homes was also banned in New Zealand in 2016 due to health and safety concerns associated with its electrically-conductive properties.
These findings highlight the potential opportunity for improving the thermal performance of a large proportion of the housing stock by fitting or upgrading insulation of the subfloor.
Figure 7. Results on the status of subfloor insulation [Source: BRANZ House Condition Survey 2015/16].
Overall retrofit opportunity of roofs and subfloors
The survey suggests that around 47% of houses (740,000 dwellings) could benefit from additional insulation in the roof space and 19% (290,000 dwellings) could benefit from additional insulation of the subfloor. Combined, the results suggest over half the housing stock (53%) could benefit from retrofit insulation in the roofspace and/or subfloor.
Figure 8. Potential for retrofit: (Figures may not sum due to rounding) [Source: BRANZ House Condition Survey 2015/16].
Insulation in rental properties
There was little difference overall in ceiling insulation levels between owned and rented houses, although a higher proportion of rentals had no insulation in the roof space (6% vs 2%).
The 2015/16 House Condition Survey findings suggest that around 35% of rentals (approximately 190,000 dwellings) may need to upgrade insulation in the roof space and/or subfloor to align with the new requirements. This includes:
Ventilation and moisture control
Mould and damp
Comparing 2015 results with 2010
2015 survey report and Warm, dry, healthy report
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