Insulation

Villas were typically constructed without insulation. Even with modifications, most villas fall below current insulation standards.

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 wall insulation became a requirement, 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.

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Insulation requirements

While there is no mandatory requirement to upgrade the insulation of the existing building, all new work is required to meet current standards. Generally, new work can be insulated to the required levels quite easily, but difficulties in interpretation of requirements can arise where renovated spaces comprise both existing and new construction, such as the extension of a particular room.

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. 

Economic payback

For roof spaces that have insulation of R3.0 or more, it is not usually economic to add more. Doing so will reduce heat loss further, but the benefit in pay-back terms is relatively small. Adding more insulation when the level is below R3.0 is beneficial in all areas of the country.

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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.
  • For brick veneer construction the insulation must not fill the drainage and ventilation cavity between the bricks and the framing. It was common for brick veneer houses to be constructed without a flexible wall underlay fixed to the outer face of the framing.

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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. 

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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 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.

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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. You can find more information in BRANZ Bulletin 579 WEERS - Window Energy Efficiency Rating System.

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.)