Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Insulation options for existing homes

Existing buildings will often benefit from insulation retrofits.

On this page:

  • Code requirements
  • roofs – timber frame
  • ground floor – timber frame
  • ground floor – concrete slab
  • exterior walls – timber and steel frame
  • exterior walls – concrete masonry or in situ concrete walls
  • glazing.

It is easier and less costly to fit insulation in a new home than to retrofit. However, many existing houses in New Zealand have little or no insulation, and they will benefit from any improvements that can be made to the levels of insulation.

In most houses, insulation can be reasonably easily added to roof spaces and under timber framed floors. It is more difficult to retrofit insulation to walls. Retrofitting wall insulation also requires a building consent unless the local council has made an exemption for this work.

Code requirements

When a building is being altered, the Building Act requires that it comply with the Building Code ‘to at least to the same extent as before the alteration’. So in any alteration that affects thermal performance, therefore, the part of the building being altered will have to comply with clause H1 Energy Efficiency at least to the same extent as before.

This means, for example, that if a window is enlarged, additional insulation will be required to offset the resulting reduction in thermal performance.

Roofs – timber frame
As most heat is lost through the roof of uninsulated homes, ceiling insulation is highly effective and should be the first priority in retrofit situations.

If the ceiling space is:

  • fully accessible, fit bulk insulation between and over ceiling joists.
  • partially accessible, install blown-in, loose-fill insulation.

If the roof is a skillion roof, options include:

  • fitting battens under the existing ceiling, installing insulation and a new ceiling lining
  • building a suspended or dropped ceiling and including insulation (can only be done if there is sufficient height)
  • removing the existing ceiling lining and installing insulation – the most cost-effective time to do this is when the lining needs to be replaced
  • removing the roofing and installing insulation, then replacing or reinstalling the roofing.
Adding insulation and a new ceiling to a skillion roof 
Adding insulation and a new ceiling to a skillion roof

Fitting battens under the existing ceiling, insulation and new ceiling lining is expensive but will provide improved insulation. The performance is limited by the thickness of insulation that can be installed.

Adding a suspended ceiling and insulation to a skillion roof 
Adding a suspended ceiling and insulation to a skillion roof

Installing a suspended or dropped ceiling is expensive but will provide very effective insulation.

Ground floor – timber frame

Existing houses may have no subfloor insulation, or foil. Foil can get dusty, torn, or otherwise damaged, and lose its effectiveness, so should be removed before new insulation is added.

Select an insulation product specifically designed for use under floors. Proprietary products include polystyrene friction-fitted between the joists, and segments such as polyester, glass wool or sheep’s wool that come with tabs for fixing, or are held in place by strapping. Make sure the insulation is pressed firmly against the floor so there is no air movement between insulation and floor. Exposed subfloors may require sheet material fixed under the insulation to hold it firmly in place.

If the material is designed to be fixed by stapling to joists, take great care to avoid electrical cables. Some installers have been electrocuted after they put a steel staple through a live cable.

While you are working under a house, it is a good idea to also put insulation around any hot water pipes that run under the floor. Insulating foam tubes with a slit along one side can be pressed over the piping.

If the ground under the house is damp, lay polythene sheeting on the ground, and improve subfloor ventilation if possible.

Ground floor – concrete slab

Improving the thermal resistance of an existing concrete slab on the ground is not usually a practical option. If renovations are to be carried out (provided there is sufficient ceiling height within the space), one option is to cover the existing slab with a polythene membrane, 25 mm thick polystyrene board and a 75 mm (minimum) thick topping slab. The new concrete must be isolated by a damp-proof membrane from existing timber framing to prevent moisture from the concrete being absorbed by the timber.

Alternatively, installing carpet and underlay will reduce the heat loss through an existing floor (although this cannot be used to meet minimum requirements of Building Code clause H1 Energy Efficiency).

Exterior walls – timber and steel frame

Although walls account for a significant proportion of total heat loss from a home, it is usually too difficult and expensive to retrofit insulation. The most cost-effective option is to wait until wall linings or claddings need to be replaced and to fit insulation at that time. Otherwise, a more cost-effective alternative is to further reduce heat loss through the roof by fitting rolltype insulation over the top of insulation segments between the joists.

Insulation options include:

  • removing external cladding or internal lining, installing insulation and fitting new cladding or lining
  • installing an EIFS cladding system over the existing cladding
  • battening existing interior linings, adding insulation and fitting new interior linings

Exterior walls – concrete masonry or in situ concrete walls

Insulation options include:

  • application of an EIFS cladding system externally
  • battening with 90 mm framing and adding insulation and new plasterboard linings internally
  • pumping loose fill or foam into the wall space through holes in the lining. Make sure the drainage cavity is not compromised.

By applying the insulation externally, the benefit of the thermal mass walls is maintained.

Glazing

For aluminium windows, insulating glazing units (IGUs) can be retrofitted. There are a number of options for improving the thermal performance of existing timber-framed windows. See BRANZ Bulletin BU 507 Timber windows – retrofit glazing options for thermal improvement.

A more cost-effective option, particularly for timber windows, is to increase the R-values of other building elements to compensate for heat loss through windows. Night-time heat loss can be reduced by installing curtains or blinds.

See glazing and glazing units for details.