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More information on underfloor insulation

Using a qualified professional

 EECA recommends using a trained professional. Look for an installer who:

  • works to the New Zealand insulation installation standard NZS 4246; and
  • has completed the insulation installer training of the Insulation Association of New Zealand (IAONZ) - see a list of graduates on the IAONZ website

Doing it yourself

If you are going to install your underfloor insulation yourself then there is a lot to think about. It can be a difficult, dirty and time consuming job and the quality of the job has a big impact on how well the insulation works.

Before you start:

  • Safety. As with all DIY projects, there are safety considerations. Make sure you read and follow the Health and Safety advice in Appendix B of New Zealand standard NZS 4246:2006. If you're not sure, get a professional to install the insulation for you.
  • Check your insulation. A quick look under your house will tell you if you have any. If there is, you'll still want to check what state it's in. View our video on how to check your underfloor insulation.
  • Check that your underfloor is accessible. Your underfloor must be more than half a metre off the ground to be able to put in new insulation. Otherwise you are going to have trouble getting under the joists and bearers that hold the floor up
  • Check what repairs are needed. It's best to get your underfloor repairs sorted before your insulation is installed
    Look for:
    - borer and other pest infestations - find out more about these on the ConsumerBuild website
    - rotten piles or subfloor framing and any corroded fixings
    - electrical wiring issues
    - drainage, guttering, downpipe or plumbing problems (i.e. water flowing under your house after rain or from leaking pipes
    - obstructions (e.g. stored timber, rubbish)
    - dampness (mould or mildew, or dirt that stains like mud when you rub it in your hand).Find out more about tackling dampness
  • Is your underfloor space open? Exposure to wind and weather can impact on your insulation's effectiveness and durability. Consider enclosing the sub-floor perimeter (ensuring you have Building Code compliant vents), or installing plywood or fibre-cement sheets to the underside of the floor once your insulation is in - it's worth consulting a qualified builder to see what might work best for your house

Building a new house or addition?

It's much easier to get your insulation done properly at the outset than fix it up afterwards. There are some specific considerations - find out more about designing to keep in the heat.

How much insulation?

The R-value is a measurement of the insulation's effectiveness - the higher the R-value, the more effective it is at preventing heat loss.

Bulk underfloor insulation with a minimum R-value of R1.4 is recommended for existing homes in all areas of New Zealand.

Which insulation product?

There are two main types of underfloor insulation: bulk and foil insulation products. EECA recommends using bulk insulation and strongly advises against the use of foil insulation as there are serious concerns regarding the safety and performance of foil insulation.

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 than foil products. 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.

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

  • Fit for purpose. It should be 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. You need the correct 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 4859.1. Look for the compliance statement on the insulation packaging
  • installed hard against the underside of the floor. To be effective there should be no air gap between the bottom of the floor and the top of the insulation, so avoid products with ridges or similar
  • Able to perform in your location. If you live in a damp or windy location, you need a product that has been tested for performance and durability in such conditions - check with your supplier or the manufacturer

You may also want to check:

  • Performance guarantees offered by insulation manufacturers on their products
  • The manufacturer's instructions for safely and correctly handling and installing the insulation, if you are thinking of doing it yourself

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 strongly advises against 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

Safety risks

People who install 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.

Performance concerns

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.

Installing underfloor insulation

Correct installation is really important if underfloor insulation is to perform properly over its life.

Everything you need to know about installing insulation is in the New Zealand Standard NZS 4246:2006 Installing insulation in residential buildings. It's easy to follow with lots of colour pictures, and it's free to download.

As with all DIY projects, there are safety considerations. Make sure you read and follow the Health and Safety advice in Appendix B of New Zealand standard NZS 4246:2006. If you're not sure, get a professional to install the insulation for you.