Insulation options for existing homes

Existing buildings will often benefit from insulation retrofits.

On this page:

• 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. It is advisable to add insulation during any renovation, and it can also be cost-effective – especially in colder parts of the country – to remove internal linings in order to add wall insulation.

 

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.

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 insulating plaster or an EIFS cladding system externally
• battening with 90 mm framing and adding insulation and new plasterboard linings
  internally.

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.

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.