Types of underfloor insulation

There are two main types of underfloor insulation: bulk and foil insulation products. We recommend using bulk insulation. Retrofitting or repairing foil insulation in residential buildings is now banned.

Bulk underfloor insulation

Bulk underfloor insulation can be made from polyester, wool, polystyrene, fibreglass and a range of other materials. Bulk products have a measurable R-value (foil products generally don't) and are easier to install.

How bulk insulation works

In most cases, bulk underfloor insulation products are friction-fitted between the floor joists. This means that they are held in place by one edge being folded down to spring against the joist. Otherwise clips, or other fixings such as staples and strapping are used.

Checklist

To get a suitable, effective bulk insulation product, choose one that is:

  • fit for purpose - intended for installation under suspended floors
  • an R-value of at least R1.4 - R-value is a measurement of the insulation's effectiveness, the higher the R-value the better
  • the right width - for your floor joist spacing (this can vary under older houses, so you may need to measure between all joists)
  • compliant with the Standard AS/NZS 4859 – so you know the insulation works as its says it does (look for the label on the insulation packaging)
  • installed hard against the underside of the floor - there should be no air gap between the bottom of the floor and the top of the insulation
  • able to perform in your location - if you live in a damp or windy area, you need a product that has been tested for performance and durability in these conditions.

You may also want to check:

  • performance guarantees offered by insulation manufacturers on their products
  • manufacturers’ instructions for safely and correctly handling and installing (if you’re thinking of doing it yourself).

Foil insulation

Foil used to be the most common material used for underfloor insulation in New Zealand. Because of safety concerns, retrofitting foil insulation and repairing foil insulation in residential buildings is now banned under section 26 of the Building Act 2004. There have been five reported deaths in New Zealand caused by electrocution associated with foil insulation in residential buildings.

Current warnings and bans issued under the Building Act - Building Performance website

Foil insulation is still allowed to be installed in new homes without an existing electrical installation but we strongly recommend using bulk insulation products which perform better and last longer.

Safety risks

If you install or repair foil insulation under an existing house you face serious risk of electrocution 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 The risks are heightened when you install foil under the floor because of low light levels and limited space.
When checking or removing existing foil insulation, always turn off the power supply to the house and follow the Electrical Code of Practice ECP 55 which provides guidance for managing electrical safety risks of foil insulation.

Electrical Code of Practice ECP 55 - Energy Safety website

Performance concerns

The performance of underfloor foil is dependent on how well it is installed. The air gap between the floor and the foil needs to be well sealed which is very difficult to achieve. Any air movement between the floor and the foil will reduce the foil's effectiveness, as will dust settling on the foil over time, which reduces its reflectivity. Foil insulation can also be damaged easily, e.g. by wind or cats. Damaged foil will need to be replaced with bulk insulation as you are not allowed to repair foil insulation in residential buildings.