Underfloor insulation

Research indicates that over one million New Zealand homes have no, or inadequate, underfloor insulation.[1] Some floors in New Zealand homes have particularly high heat loss, such as ill-fitted tongue and groove planking.

On this page:

How much insulation?

For all areas of New Zealand, bulk insulation with a minimum R value of R1.4 is recommended for existing homes.

Which insulation material?

Bulk underfloor insulation can be made from polyester, wool, fibreglass and a range of other materials. Not all insulation is created equal though, and some products do have definite drawbacks.

If you are comparing different types of insulation, remember to check:

  • what sort of performance guarantees the insulation manufacturers offer on their products
  • that the insulation you are looking at has been tested to the AS/NZS 4859.1 Standard. Look for a AS/NZS 4859.1 compliance statement on the insulation packaging
  • What's involved in safely installing the insulation if you are thinking of doing it yourself.

Once you know these things, make sure you are comparing products with similar R-values.

Foil or bulk insulation?

There are two main types of underfloor insulation: foil and bulk insulation products.

Foil

In the past, the most common material used for underfloor insulation was a reflective foil, stapled along the floor joists. Reflective foils do not have an R-value by themselves, but work in the following ways:

  • They trap air between the floor and the foil which serves as insulation
  • Escaping radiant heat is partially reflected back into the house.

The performance of underfloor foil is completely dependent on how well it is installed. Incorrect installation will significantly reduce effectiveness.

People who install the foil insulation 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.

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

Bulk products

Bulk underfloor insulation includes products such as polystyrene, wool, polyester or fibreglass.

Bulk products do have an R value, and are easier to install correctly than foil products. EECA recommends using bulk underfloor insulation with a minimum thermal resistance of R1.4.

These products fit between the floor joists. They either go hard against the underside of the floor or have a small air gap between the bottom of the floor and the top of the product.

EECA requires that products used in its funded insulation programmes have no air gaps as these can reduce the performance of the insulation.

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. For more information, refer to the instructions supplied by the manufacturer.

Take action under your floor

The easiest way to check if you need underfloor insulation is to have a quick look under your floor. See the underfloor insulation section of the page Is you insulation adequate?

You may be eligible for ENERGYWISETM funding to get insulation installed in your home.

Dealing with dampness

Any moisture or dampness in the area underneath or around the edge of your floor has the potential to make your house damp and musty. It may also reduce the effectiveness of your underfloor insulation.

If the area underneath your house is damp then you should get any drainage issues checked by a professional. Also consider installing an on-ground vapour barrier in enclosed underfloor areas. For more detail, see page 45 of the standard for installing insulation.

Download the New Zealand standard on installing insulation, NZS4246:2006.

Although dehumidifiers help fix the symptoms of damp problems, better drainage and/or an on-ground vapour barrier will fix the cause of the problem in many cases.

Installing underfloor insulation

It is easiest to install insulation with two to three people. Although it is possible to do it by yourself, it will be harder to get the quality of installation that you need for your insulation to work as well as possible.

Access under floors is often restricted. If you are unsure of how to access and safely work in an underfloor space, EECA suggests that you get your insulation professionally installed.

Standards New Zealand has published a standard on installing insulation. It covers a variety of products available and provides detailed instructions on how they should be installed.

Download the New Zealand standard on installing insulation, NZS4246:2006.

Semi-rigid underfloor insulation products aren't presently covered by the standard. Please refer to the manufacturer's installation instructions.

Keep safe when installing underfloor insulation

It's essential that you keep staples and any other fixings well clear of any power cables when installing underfloor thermal insulation.

For more information see the Installing insulation page.

More information

Apply for an insulation grant

Grants are available for ceiling and underfloor insulation, regardless of your income, if your house was built before 2000. Find out more about home insulation funding.

Seek professional advice

Notes:

[1]Clark, S.J., Jones, M. & Page, I.C. 2005. New Zealand 2005 House Condition Survey. Wellington: BRANZ

From the Resource Centre

Related documents and additional information: