Insulation of walls, floors, and ceilings/roofs is a key element of passive design. It helps to even out temperature changes, making a home warmer, drier and more energy efficient.

In general, the aim should be to achieve as high a level of insulation as is practical. The benefits will eventually outweigh the increased initial cost. Remember that Building Code requirements are the minimum. On a scale of good–better–best practice, aim for the best. If you don’t put in the maximum insulation at construction stage, it will be difficult and in some cases practically impossible to increase it after completion.

Correct installation is essential in achieving the design R-value. Insulation material must be installed without gaps, and for blanket or mat material, without creases of folds. A small gap of 5 mm between the insulation and the framing can result in a 20% loss of performance.

How heat is lost

  

How thermal insulation works

Insulation can either work by trapping air which is the most effective method and/or reflecting heat.

Air is a poor conductor of heat. Materials that provide good heat insulation are lightweight because they contain large amounts of tiny pockets of still air. Examples of this type of ‘bulk’ insulation include:

• segments or rolls made from glass fibre, polyester fibre or wool
• loose fill materials such as macerated paper or cellulose fibre
• lightweight boards such as polystyrene or polyurethane foam.

Insulation value will be significantly reduced or lost when a bulk insulation material becomes wet

Materials that have high density such as concrete, brick or stone have a high thermal mass but are not good insulators. Thin metals such as profiled steel claddings and fibre-cement sheets are also poor insulators and provide little thermal mass.

Shiny aluminium foils, used in conjunction with still air spaces, reflect heat and this reduces the amount of heat transferred through them. Foil is commonly used to reduce downward heat loss through suspended timber ground floors. All underfloor foils in New Zealand should be perforated. However, foil by itself provides a lower level of performance compared to bulk insulation options.

R values

The ability of a material to resist the transfer of heat is its ‘thermal resistance’ (R). The higher the R value the greater the amount of insulation provided.

R-values can be calculated for whole building elements by adding the thermal resistance values of all the materials and the air gaps between them, plus an allowance for the layer of still air that clings to the outside and inside face of the wall cladding/lining. An allowance must be made for the heat loss through those parts of the wall where the R-value is not as great – e.g. at all timber framing positions. Therefore the R-value of the completed wall construction is almost always less than the R-value of the insulation material installed – the more timber framing in the wall the greater the difference.

Thermal bridges

Thermal bridges, sometimes referred to as ‘cold bridges’, are weak points in the insulation of the building envelope that allow heat to escape. Examples include:

• framing members in external walls which have a lower thermal resistance than the surrounding insulation – this is of particular concern with steel framing because of its high conductivity, and in walls containing a lot of timber; a thermal break must be installed on the outside face of steel framing before the cladding is installed
• aluminium window frames that do not incorporate a thermal break (these conduct heat readily and are liable to cause condensation)
• gaps in (poorly) installed insulation.

For your clients

To help your clients understand insulation requirements, refer them to www.smarterhomes.org.nz/design/insulation.