Underfloor insulation

Warm your house from top to toe – once you’ve insulated your ceilings, do the accessible spaces between the ground and your floor.

Check

If you can access under your house, see what you’ve got – or get an insulation professional to do it for you. They often do it for free.

  1. 1

    Bare floorboards

    If your underfloor is more than half a metre off the ground, great news – you can insulate.
  2. 2

    Foil-based product

    Turn off the power supply before you inspect. If the foil is held in with metal staples – don’t touch it! There’s a risk of electrocution. If the foil is ripped or patchy you must not try to repair it – it’s banned. Leaving the power off, you can remove the foil. Replace with more effective bulk insulation. If the foil is well fitted and in good condition, it’s probably working okay for now. Call an electrician if you’re in any doubt.

    How to manage the electrical safety risk – ECP 55(external link)

  3. 3

    Bulk insulation

    Could be polystyrene sheets, polyester, wool or fibreglass. Check it fits hard against the underfloor with no gaps or missing pieces. You’ll need to replace any that’s slipped or missing.

Need to sort it out?

  1. 1

    See if you can get a grant

    Warmer Kiwi Homes is a Government programme offering insulation and heating grants to low-income home owners.

    Warmer Kiwi Homes(external link)

  2. 2

    Call the professionals

    Get quotes from at least two trained installation installers. We recommend choosing an Insulation Association of New Zealand (IAONZ) member.

    IAONZ(external link)

  3. 3

    Install it yourself

    Everything you need to know is in the New Zealand Standard. There’s a lot to know to do a good job – and it’s got to be done right to be effective. Take care to follow the health and safety advice in Appendix B. Fix any problems first – rot, infestations, damp, wiring or drainage issues.

    Insulation Standard(external link)

Choose an effective product

The R-value is a measurement of insulation effectiveness - the higher the R-value, the better it prevents heat loss. Go for at least R1.4 – or higher if you can. A professional installer will help you choose the right product. If you want to go DIY, choose a product intended as underfloor insulation and check for these features:

  • width – joist spacing can vary – check measurements between them all
  • meets Standard AS/NZS 4859.1
  • fits hard against the underfloor with no gaps
  • safe handling and installation instructions
  • performance guarantees
  • performs in windy conditions (if your underfloor space isn’t fully enclosed).

Insulating an exposed underfloor space

Wind and weather can reduce your insulation’s effectiveness and lifespan. You might want to install plywood or fibre-cement sheets to the underside of the floor once your insulation is in. Or you could enclose the area, as long as you create vents that comply with the building code. Ask a qualified builder what will work for your house.

If you can’t enclose your sub-floor space, choose insulation that has been tested for performance and durability in windy conditions.