Few houses built in the 1940s and 1950s were insulated. Insulation may be required as part of any renovation project, and in any case will provide benefits for occupants.

While roof and underfloor spaces can generally be easily accessed to install insulation, it is more difficult to install insulation in external walls where exterior cladding or interior linings are not being replaced. If the external cladding is sound, removing it to install insulation is unlikely to be economical.

See Remedies: windows for information about insulating glass units (IGUs).

Original insulation

Insulation was very rarely installed in houses built during the 1940s to 1960s, although it was available and its benefits were understood.

In the 1950s, insulation in the form of a spun or ‘silk’ glass was available (Figure 1) but very seldom installed.

In the 1960s, fibreglass and blown-in insulation were readily available, and aluminium foil was marketed for use behind internal linings to provide some insulation.

Cost was possibly a factor in the lack of use of insulation, but there was also a perception that it was unnecessary in the New Zealand climate.

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Retrofitted insulation

The high roof spaces and good ground clearance underneath the houses built during this time mean that roof space and possibly underfloor insulation may have been installed. It is unlikely that the levels of insulation comply with today’s Building Code requirements.

Houses of the later 1960s with less accessible roof spaces are more difficult to insulate and blown-in material was commonly used to overcome the difficult access. If the roof space was insulated with blown-in macerated paper it may have settled and may need to be topped up or replaced.

Where floors have had insulation installed, the insulation may be reflective foil fixed to the underside of floor joists or, if installed after 1990, it may be polystyrene boards fitted between floor joists.

Unless major renovation work has been carried out recently, it is unlikely that insulation has been installed in the walls of the existing house.

However, any additions built after 1978 should have insulated walls and ceilings.

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Insulation requirements

Although there is no mandatory requirement to upgrade the insulation of existing owner-occupied houses, all new construction 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 of interpretation of requirements can arise in situations such as a room extension, where renovated spaces comprise both existing and new construction. 

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. All private rental homes must comply by 1 July 2025. (The Government extended the original date of 1 July 2024.)

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Roof insulation

If there is access into the roof space, installing insulation is not likely to be 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 include:

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

Insulation must be kept clean and dry. It should be clear of water storage tank overflow trays and flues. Insulation must not sit over older recessed downlights or touch the sides unless the fitting manufacturer can verify that this is acceptable. Newer types of recessed downlights that are labelled IC and IC-F can be covered with insulation. Consult the drawings and details in NZS 4246:2016 Energy Efficiency – Installing bulk thermal insulation in residential buildings.

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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 between framing without gaps, tucks or folds and should not be compressed or packed tightly around electrical wiring.

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Rigid and semi-rigid boards

Expanded polystyrene boards must be fitted tightly between roof framing members. They are only suitable where the access into the roof space, as well as the roof space itself, is generous.

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Loose-fill insulation

Loose fill (glass wool, mineral wool, sheep’s wool or macerated paper) is blown into the roof space to the thickness and density required to achieve a particular R-value and can also give total coverage across joists. It can be blown into inaccessible corners of the roof space but must not be compressed, and it should be installed with extra thickness to allow for settlement. It is the only suitable option for very low pitched roofs and must be installed by a professional installer.

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Insulating skillion roofs

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

  • from above by replacing the roofing, or
  • from below by removing the ceiling lining.

The low pitch of skillion roofs tends to cause metal roofs to deteriorate more quickly than if more steeply pitched, so if the roof needs to be replaced, access from the outside for insulation installation may be a viable option and one which also allows roof underlay to be installed.

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

Alternatively, a skillion roof may be insulated from below by installing a foam-backed sheet material over the existing interior lining or by installing a rigid foam insulation product then overlaying a new plasterboard ceiling lining. The R-value achieved depends on the type of material used and its thickness and there may be difficulties in incorporating sufficient thickness into existing detailing.

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Wall insulation

Options to insulate walls include:

  • If the exterior cladding needs replacement, new insulation can easily be installed. New wall underlay can also be installed at the same time.
  • If internal plasterboard walls are in poor condition, it may be cost-effective to replace the linings and install insulation from the inside at the same time.

A problem with installing insulation from the inside is that it is more difficult to ensure that a drainage/drying path is maintained on the back face of the cladding to prevent the insulation material absorbing any water that might leak through the cladding and keeping the framing wet.

BRANZ conducted lab experiments on insulation retrofit options for timber-framed walls with direct-fixed weatherboard cladding and without existing wall underlay. The most successful option for keeping insulation and framing dry was found to be installing drainage plane mesh in conjunction with an underlay. You can find the details in study report SR484 Assessing retrofitted external wall insulation techniques.

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

Blown-in insulation for walls

There is a range of blown-in insulation options for retrofitting insulation to existing walls. Key considerations when retrofitting blown-in wall insulation are:

  • Only use this option if it is appropriate for the existing construction – for example, such products should not be installed into walls that do not have a flexible wall underlay (building paper or building wrap) installed behind the cladding. A 20 mm clear gap must be left between the underlay and the back of the cladding. This is to allow for some drainage and drying and prevent direct contact (and water transfer).
  • Injected or blown-in insulation must not be installed into any drained and ventilated cavity (especially brick veneer) – it will restrict the cavity drainage and drying.

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Floor insulation

Houses built during this period generally had good ground clearance, which makes underfloor insulation installation relatively easy. If the floor has had reflective foil installed underneath, it should be removed and replaced with polystyrene or bulk insulation suitable for use in subfloor spaces. (Make sure you switch off the power before you remove the old foil.) Insulation will reduce the amount of air leakage through a timber board floor as well as increasing the floor’s R-value.

Retrofitting or repairing foil insulation under floors was banned from 1 July 2016. This practice can be dangerous – the risk is that people using metal staples 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.

Expanded polystyrene

Expanded polystyrene panels are specifically designed for insertion between floor joists and are easy to retrofit into existing timber floors, especially when there is good underfloor access (Figure 2).

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

Bulk insulation

Before installing bulk insulation, the joists must be dry (lay polythene sheeting over the ground if the subfloor space is damp). Ensure that the selected insulation is recommended for subfloor use by the manufacturer and install according to their recommendations.

Some bulk insulation materials can be installed without lining material or tape to hold the material in place, but generally fixing a sheet material to the underside of joists is recommended for open subfloors.
If there is any moisture in the subfloor space, it should also be reduced as keeping the air drier will effectively make the house feel warmer (see Remedies: foundations and subfloors).

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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 1.4°C.

In a separate research project, BRANZ made one of its older test buildings more airtight. Simple techniques were used with an appropriate sealant and in some cases a skirting or scotia as well where the gap was large. The work included:

  • sealing the junction between ceiling and walls (top plate/walls) – this gave the single largest improvement
  • for strip flooring, inserting expanding foam between the bottom plate and the last floorboard before fitting the skirting board
  • addressing window reveal to plasterboard connections and the connection between the window frame and the reveal itself. Internal doorway jamb to plasterboard connections were filled with a bead of sealant
  • sealing attic hatches with a closed-cell EPDM strip.