Art deco houses were built without insulation, and typically still lack it. Adding insulation can be difficult.

The low or non-existent roof pitch and the lack of roof space means installing insulation is very difficult and may not have been done since construction.

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

Wall insulation installation is unlikely to have occurred unless the interior or exterior wall linings were removed for major renovation work.

Although there is no mandatory requirement to upgrade the insulation of existing owner-occupied houses, all new work is required to meet current standards. New work can easily be insulated to the required levels, but difficulties in interpretation of requirements can arise where renovated spaces comprise both existing and new construction, such as an extension of a room. Rental property owners should know that minimum levels of thermal insulation have been a legal requirement in privately-owned rental homes since 1 July 2019. You can find more information from Tenancy Services.

Roof insulation

Where there is reasonable access into the roof space, the installation of insulation does not pose a problem. In the case of many art deco houses however, access is limited due to a low roof pitch and it may be difficult to install insulation. A minimum clearance of 25 mm must be maintained between the underside of the roofing and the top face of the new insulation.

The insulation options available for roof spaces include:

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

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.
It should be installed with extra thickness to allow for settlement and can also give total coverage across joists. It can be blown into inaccessible corners of the roof space but must not be compressed. It is the only suitable option for very low-pitched roofs and should be installed by a professional installer.

Blanket insulation

Blanket or segment insulation is available in a range of thicknesses and R-values. Blankets are fitted between roof framing members and may also be draped over the framing to increase the effectiveness of the insulation. They must be fitted snugly between framing without gaps, tucks or folds and must not be compressed or packed tightly around electrical wiring. Blanket insulation may not be a suitable option where there is limited access to the roof space.

Board insulation

Expanded polystyrene boards must be fitted tightly between roof framing members and are only suitable where the access and roof space are generous.

Keeping insulation dry

Insulation generally must be kept clean and dry. It should be clear of water storage tank overflow trays and flues.

Insulation and recessed downlights

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.

Adding insulation during roof replacement

If a roof is being replaced, this is the ideal opportunity to install insulation before the underlay and new roof go on.
It is technically possible to lift existing roofing, install insulation, then replace the roofing, but great care needs to be taken to replace and reinstate flashings correctly.

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Wall insulation

The insulation options available for walls are generally:

  • segmented or sheet insulation materials such as glass wool, wool, polyester
  • rigid and semi-rigid board (expanded polystyrene).

It is difficult to install insulation in external walls if neither the exterior cladding nor the interior linings are being replaced.

If the external cladding is sound, removing the cladding to install insulation is unlikely to be cost-effective, particularly if the cladding is weatherboard. It is estimated that more than 50% of weatherboards are likely to be damaged during removal as old timber is usually dry and prone to splitting.

However, if the external cladding is stucco, it may be in poor condition and re-cladding may be a viable option. In this case, however, the current requirement for a cavity may mean that existing detailing will not comply.

Options for insulating walls include:

  • where the exterior cladding is in need of replacement, the insulation can readily be installed. New wall underlay can also be installed if it has not been installed or if it requires replacing
  • where walls are already plasterboard-lined, assess their condition. If in poor condition, it may be economical to remove the lining, install insulation and replace the wall lining. Be aware, however, that original skirtings and architraves will be dry and may split when removed. If the profile is unusual, you may need to get replacements specially made.

For weatherboard art deco houses, if insulation is installed from the inside it can be difficult to ensure that the drainage path on the back face of the weatherboard cladding is maintained as it is not visible. If a clear drainage path is not maintained, water may be trapped by the insulation, causing timber weatherboards and/or timber framing to remain wet. An option to avoid this is to install polystyrene insulation or a proprietary bulk insulation with a water-repellent face.

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. Currently none of the available systems hold a BRANZ Appraisal. 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.
  • 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

The insulation options available for under floors are generally:

  • rigid and semi-rigid board (expanded polystyrene)
  • bulk insulation materials such as glass wool, wool, polyester.

If the floor has had reflective foil installed underneath, the foil should be removed and replaced with a polystyrene or bulk insulation suitable for use in subfloor spaces. This will reduce the amount of air leakage through the floor as well as increasing the 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 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.

If there is any moisture in the subfloor space, it can be reduced by laying polythene over the ground. By keeping the air drier, this will effectively make the house feel warmer.

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Window insulation

Original window units are likely to be relatively inefficient. There are several options for improving thermal performance, from curtains to installing insulating glass units.

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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.