Underfloor foil insulation

Foil used to be the most common material used for underfloor insulation in New Zealand. Concerns with the safety and performance of foil, and the widespread availability of other products, means EECA does not recommend the use of underfloor foil insulation.

How foil insulation works

Reflective foil is draped over or stapled along the floor joists. Reflective foils do not have an R-value (a measure of insulation's effectiveness) by themselves, but work in the following way:

• Air is trapped between the floor and the foil, serving as insulation

• Escaping radiant heat is partially reflected back into the house.

Performance and safety concerns

EECA doesn't recommend the use of underfloor foil and this type of product is not used in EECA's insulation retrofit programme. This is because of the safety risks, challenging installation, unreliable performance and possible lack of durability.

Performance

The performance of underfloor foil is very dependent on how well it is installed as the air gap between the floor and the foil needs to be well-sealed which is very difficult to achieve in practice.

Any air movement between the floor and the foil will reduce the foil's effectiveness, and so will dust settling on the foil and reducing its reflectivity. Many foil insulation products can easily get damaged by cats or wind.

Safety

People who install the foil insulation under existing houses face a serious risk of electrocution. This can happen if the foil or staples used to fix it come into contact with live electric wires under the house. As foil is metal-based, it conducts electricity.

Messy electric wiring is very common under New Zealand houses. It can be hard to see during foil installation due to inadequate light and the limited mobility of installers under the house.

Alternatives to foil insulation

Bulk insulation is fitted between floorboard joists and is easier to install correctly than foil products.

A number of bulk underfloor insulation products are approved for use under the government's insulation programme.

>> Download the list of specific insulation products accepted for use under the Warm Up New Zealand: Heat Smart programme.