Houses built before 1978 were generally not insulated. Insulation may be required as part of any renovation project, and in any case will provide benefits for occupants.

Although insulation was available and its benefits understood, houses built during the early 1970s were generally not insulated as minimum levels of thermal insulation only became mandatory in 1978.

There were a few exceptions. Waimairi County, on the northern edge of Christchurch, was the first local authority to introduce a by-law requiring thermal insulation in 1971, followed by Christchurch City in 1972. In 1975 the government offered interest-free loans for house insulation, and 50,000 households took up the offer. In 1978 a nationwide law requiring minimum levels of insulation in new homes came into effect. The minimum levels required have been increased three times since then.

The lack of insulation through most of the 1970s – combined with large areas of single-glazed windows – meant that many houses were cold in winter. They could also be noisy as sound was readily transmitted due to the lightweight construction and lack of insulation.

Many houses may have had roof space insulation added later (Figure 1), and some have also had underfloor insulation added. But the level of insulation is likely to fall well short of current insulation requirements.


Retrofitted insulation

While most roof and underfloor spaces can be accessed to install insulation, it is more difficult to install insulation in external walls and skillion roofs unless exterior cladding or interior linings are being replaced. If the external claddings or internal linings are sound, removing them to install insulation is unlikely to be economical. Original concrete floor slabs are unlikely to have been insulated and the options for retrofitting are very limited. 

See Remedies: windows for information about insulating glass units.

Back to top

Insulation requirements

Although there is no mandatory requirement to upgrade the insulation of existing owner-occupied houses, all new building work must comply with the Building Code to the extent required by the Building Act. New work can easily be insulated to the required level, but difficulties with interpretation of requirements can arise in situations such as a room extension, where renovated spaces comprise both existing and new construction.

Note that the minimum insulation levels in new building work are due to increase. Building consent applications for housing submitted from 1 May 2023 onwards must meet new R-level requirements for roof, wall and floor insulation. The requirements for windows and doors and skylights increase in stages from 3 November 2022.

Rental property owners should know that thermal insulation (where it is reasonably practical to install it) is a legal requirement in privately-owned rental homes. The healthy homes standards came into force for privately-owned rental properties and boarding houses on 1 July 2021. Owners of rental homes must ensure that their properties comply with the standards within 90 days of any tenancy that starts or is renewed after 1 July 2021. The rules apply to boarding houses immediately. The standards apply to all rental homes from 1 July 2024.

Back to top

Roof insulation

If there is reasonable access into the roof space, installing insulation is generally not a problem. Where access is limited due to a low roof pitch, it may be more difficult to install. The options for roof space insulation are:

  • blanket or segment – glass wool, wool or polyester
  • rigid and semi-rigid board – polystyrene
  • loose fill – glass wool, sheep’s wool, mineral wool or macerated paper.

General requirements include:

  • insulation must be kept clean and dry
  • it should be clear of water storage tank overflow trays, ceiling extracts and flues
  • where there are older types of recessed downlights, insulation must be kept 100 mm away with a fixed guard to prevent the lights, ceiling materials or insulation from overheating. Recessed downlights labelled “CA80” or ”CA90” can have insulation abutted against them. BRANZ recommends replacing older downlights with surface-mounted lights or with IC or IC-F recessed downlights that can have insulation placed over them.

Back to top

Blanket or segment insulation

Blanket or segment insulation is available in a range of thicknesses and R-values. Segments are fitted between roof framing members and blankets may be draped over the framing to increase the effectiveness of the insulation. The insulation must be fitted snugly without gaps, tucks or folds and should not be compressed or packed tightly around electrical wiring.

New blanket insulation can be easily laid over the top of existing material that is in sound condition - that is it has not been compressed to reduce the original insulation thickness.

Back to top

Rigid and semi-rigid boards

Expanded polystyrene boards must be fitted tightly between roof framing members. They are only suitable to use where there is adequate access into the roof space, as well as enough room in the roof space for installation.

Back to top

Loose-fill insulation

Loose-fill, macerated paper, brown fibreglass, rockwool or wool is blown into the roof space to the thickness and density required to achieve a particular R-value and it can also give total coverage across joists. It is the only suitable option for very low pitched roofs with limited access and must be installed by a qualified installer.

It can be blown into inaccessible corners of the roof space, but must not be compressed, and should be installed with extra thickness to allow for settlement.

Back to top

Insulating skillion roofs

It is more difficult to install insulation in a skillion roof with a sloping ceiling. The options available are to install insulation:

  • from above when replacing the roofing, or
  • from below by removing the ceiling lining, or
  • by packing down the existing ceiling using battens to provide space for insulation material and then fixing new ceiling linings between the rafters and fixing them to the battens, or
  • for an exposed rafter installation, inserting insulation between the rafters and fixing a new ceiling lining under the rafter.

The low pitch of skillion roofs tends to cause metal roofing to deteriorate faster than that on more steeply pitched roofs, so if the roof needs to be replaced, access from the outside for insulation installation may be a viable option.

The amount of insulation able to be installed depends on the rafter depth as there must be a 25 mm gap between the insulation and the underside of any flexible roof underlay.

Back to top

Wall insulation

Insulating framed walls

As external wall claddings and internal wall linings are likely to be in good condition, taking them down to add insulation into existing walls may not be economically justifiable. The options to insulate walls are:

  • If the exterior cladding needs replacement, the insulation can be installed at the same time from the outside (the wall underlay will also require replacement).
  • If internal plasterboard walls are in poor condition, it may be cost effective to replace the linings and install insulation at the same time. It is likely that buildings built during the 1970s will have building paper behind the cladding, and provided it is sound the insulation can be fitted easily into the framing. Where building paper is damaged, determine and remedy the cause of the damage, then fold in new building paper from the inside. For brick veneer claddings, install mat or segment insulation carefully to prevent the building paper being pushed into the cavity (consider using a rigid sheet insulation which will minimise the risk of the building paper being pushed out into the cavity).

In most cases, retrofitting insulation into walls requires a building consent.

Another option is to strap and reline with insulation in the void created (this reduces available floor space and will require the removal and replacement of architraves and skirtings as well as modification to the window reveal).

Insulating concrete block walls

The options for insulating concrete block walls that have no insulation include:

  • strapping and lining the interior walls and installing insulation between the single skin masonry walls and the lining material
  • direct fixing EPS or XPS polystyrene to the inside face of masonry walls and installing sheet lining (a mandatory requirement under the fire safety requirements of the Building Code) such as plasterboard, over the polystyrene
  • applying an EIFS system to the outside of the building which retains the thermal mass benefits of the masonry
  • applying a proprietary insulating plaster containing polystyrene aggregate to the exterior and/or interior walls of the masonry walls to give a minimal increase in performance.

Strapping and lining (depending on the particular type of insulation used and the wall thickness) will provide an R-value of between:

  • R1.1 - 1.4 for 45 mm insulation
  • R1.7 - 2.2 for 90 mm insulation.

The disadvantages of strapping and lining or applying polystyrene to the inside are that the internal space is reduced and extensive cosmetic surgery is required, including new plasterboard, stopping and painting, and finishing around windows and doors, at ceiling and floor junctions, and possibly new floor finishes. Also, as both these options provide no thermal mass benefit to the interior, 90 mm framing is required for the strapping to achieve the required current minimum level of insulation performance.

Fixing a proprietary EIFS system to the outside will require the outside of the polystyrene to be plastered and painted as well as the development of a solution to maintain weathertightness performance at the top of the wall and around windows and doors.

Back to top

Floor insulation

Underfloor insulation will reduce the amount of air leakage through the floor as well as increasing the R-value of the floor (Figure 2).

Retrofitting or repairing foil insulation under floors was banned from 1 July 2016. This practice can be dangerous – the risk is that people using staples or nails to attach the foil to timber members accidentally pierce a live electrical cable. There have been five electrocution deaths and one non-fatal shock reported in New Zealand from installation of foil insulation under houses.

Houses built with a suspended timber floor during the 1970s generally had good ground clearance, making retrospective underfloor insulation installation possible.

If the floor has had reflective foil installed underneath, it should be removed (as it is likely to be ineffective) and replaced with polystyrene or bulk insulation suitable for use in subfloor spaces. Disconnect the power before removing foil – there are instances of the foil being live due to an electrical fault – and work with a torch. Always let someone know when you are working under a house. Before installing insulation, the subfloor space and joists must be dry – see Remedies: foundations and subfloors.


Expanded or extruded polystyrene panels specifically designed for insertion between floor joists can be retrofitted under existing timber floors, especially when there is good underfloor access.

They should be fitted snugly between the joists and as close to the underside of the flooring as possible. If the subfloor is exposed, a lining such as fibre-cement sheets should be fixed to the underside of the joists to provide protection to the insulation.

Bulk insulation

The selected bulk insulation must be recommended for subfloor use by the manufacturer and installed according to their recommendations. Some bulk insulation materials can be installed without lining material or tape to hold the material in place in a protected location. A sheet material fixed to the underside of joists is a requirement for open or exposed subfloors.

Back to top

Draughts and heat loss

While insulation has the key role in keeping homes warm, reducing draughts is also important.

Research by the Department of Public Health at the University of Otago in Wellington shows that fixing sealing strips to doors can help to eliminate draughts and lift indoor temperatures by an average of 1.36°C.