Villas were not insulated when built, and while a number may have had roof space – and to a lesser degree – floor insulation added, most still fall well short of current insulation requirements.

Likely current insulation levels

• Ceilings: Where ceiling insulation has been fitted, unless this was done recently the insulation is likely to be only 3” (75 mm) thick fibreglass segments.
• Walls: Unless major work has been done since 1978, when minimum requirements for thermal insulation were introduced nationwide, where the match lining has been removed, it is unlikely that walls are insulated. 
• Floors: Floors may have had foil or polystyrene added – usually after 1990.

Back to top

Insulation requirements

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

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

While most roof spaces over the main living and sleeping areas and under the floor of villas can be accessed to install or improve insulation, it is a different story for: 

• external walls where the match lining remains in place or the walls have already been hard lined without installing insulation
• skillion roofs of lean-to additions, which are unlikely to have been insulated when built and which have limited space to achieve currently required insulation levels
• windows

Back to top

Insulating external walls with match lining or hard lining

For match lining, remove every second or third board and insert insulation. Where there is no underlay behind the cladding, install polystyrene insulation and leave a gap between the insulation and the back of the cladding or fold wall underlay into the framing before installing the insulation material. 

Where walls are already plasterboard lined, assess their condition. If in poor condition, it is economical to remove them and insert insulation and reline.

Where the exterior cladding is in need of replacement, the insulation (and new wall underlay) can readily be installed. Where weatherboard cladding is sound, removing the board to install insulation is unlikely to be economical – it is estimated that more than 50% of the boards will be damaged during removal as old native timbers are usually very dry and prone to splitting.

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. 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.

Back to top

Insulating skillion roofs

Installing the insulation to existing skillion roofs will depend on the condition of either the roofing or the internal lining as outlined for walls above. For skillion roofs, the lower slope of the roof usually causes a metal roof to deteriorate more quickly than a steeper sloped roof, so access from the outside may be a more viable option and it allows roof underlay to be installed. The amount of insulation able to be installed will depend on the rafter depth – there must be a gap (25 mm) between the insulation and the underside of flexible roof underlay. 

An alternative option is the installation of a foam-backed sheet material as a new wall or ceiling lining installed over the existing match lining. The insulation value achieved depends on the foam used and its thickness but R-values of around 0.7 for 50 mm EPS are possible. The difficulty is incorporating the thickness of the lining system into the existing detailing – therefore this would be an easier option where skirtings and architraves are being replaced. 

Back to top

Floors

If the floor is uninsulated, it will be beneficial to install polystyrene or bulk insulation suitable for use in subfloor spaces. For floors that have had foil installed, it will be beneficial to remove the foil and replace it with insulation. Installing the insulation will also reduce the amount of air leakage through the floor. 

Reducing subfloor moisture can also make the villa feel a little warmer by keeping the air dryer.

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.

Back to top

Windows

Original and any later window replacements are most likely to be single glazed and the window sections relatively air leaky. This makes them inefficient in terms of maintaining and retaining heat within the building.

Options that can be considered with villa renovation (in approximate increasing cost order) are:

• specifying heavy drapes that can be pulled across the existing windows with a Velcro seal down each side of the window
• adding removable secondary glazing (see below) that can be installed to reduce heat loss in winter and removed in summer to maintain ventilation
• removing existing glass and replacing with insulating glass units installed into the sash using a small aluminium section
• removing the existing sash and glass and modifying the sash to accept insulating glass units and reinstalling the sashes.

When replacing glazing, carefully consider thermal performance of the replacement glazing. One tool that can help with this is the Window Energy Efficiency Rating System (WEERS), a 6-star rating system for assessing the thermal performance of new residential windows.

Secondary glazing

Secondary glazing is a less expensive option than insulating glass units. With this, plastic film, magnetically attached plastic sheet, plain or low-E glass is installed inside the existing glass with a still air gap between them.

Research (largely carried out at BRANZ) found that secondary glazing gave R-values from 0.36 to 0.57 m² K/W. This confirms that secondary glazing can be used as a functional alternative to retrofitted insulating glass units (IGUs) in existing domestic single-glazed window frames. (In fact, the performance exceeded the expected performance of IGUs retrofitted into the existing framing due to the secondary glazing effectively insulating the framing.)

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 1.4°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.