We look at why you need insulation, how it works and explain 'R values'.

Where the heat goes

Heat is lost or gained from your home in 3 ways:

  • Conduction of heat through solid material. Insulation in the ceiling, walls and under the floor reduces heat transfer or loss.
  • Convection via air movement (both naturally through open doors, windows and unsealed downlights, and mechanically by fan systems).
  • Radiant transmission, typically through glass.

In an uninsulated home, 30-35 percent of the heat escapes through the roof. Another 18-25 percent escapes through the walls, 12-14 percent is lost through the floor and 20-30 percent heads out through the glass in windows. Between 6 and 9 percent is lost through draughts or when doors are opened and closed.

Insulating the ceiling/roof area is the top priority as this is where most heat is lost.

How insulation works

Bulk insulation can be made of a variety of materials such as glass fibre, polyester, polystyrene, paper and wool (see Home insulation materials). It acts like blankets or a duvet on your bed, trapping air and preventing heat transferring easily from the warm to the cold side. Bulk insulation may also act to deaden sound. It shouldn't be compressed below its design thickness or it will lose effectiveness.

Reflective insulation acts by reflecting radiant heat back where it came from.

If you insulate between ceiling joists more heat will be lost through the timber framing than the insulation. This is called thermal bridging. If thermal bridging occurs the effectiveness of ceiling insulation will be less than the stated thermal resistance for the insulation product.

To prevent heat loss it's best to insulate over the top of ceiling joists. However, this could make moving safety around the ceiling space difficult if you cannot easily see the joists. Using a plank to span several joists may be a safer option. Thermal bridging also occurs in walls, floors and window frames (especially aluminium ones).

R values

Heat travels from warm to colder areas. The resistance of a material or building structure to transferring or conducting heat is expressed as an 'R value'. Insulating materials with higher R values reduce the rate of heat loss (or gain in summer) from a building. This reduces the heating or cooling needed to maintain a comfortable indoor temperature.

The R value depends on the type of material, its density and thickness.

The R value is either given for the insulation alone (the product R value), or for a combination of building elements working together (a systems R value). For example:

  • foil insulation does not have an R value on its own, but combined with a 100mm air gap and timber flooring its R value is about 1.0, if properly installed
  • the insulation used in your ceiling may be R3.2, but the R value of the roof and ceiling structure or system might be higher or lower than this depending on the type of insulation used and the amount of thermal bridging.

The range of R values of insulation which are mandatory in new homes and additions varies depending upon where it is to be placed in a house, the location in New Zealand and the house construction type (see 'Building Code requirements').

Remember these are the minimum required values. It's better to aim for more than the minimum value. The extra will pay for itself in terms of comfort and long term energy savings.

R values in New Zealand are in metric units (m² °C /W). R values in the United States are in imperial units, and look much bigger. Divide US values by 5.68 to convert them to metric.