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Wall insulation

Wall insulation is not easy to check or install, but it makes a big difference to the comfort and warmth of your home.

Once you've insulated your ceiling and underfloor, installing wall insulation is the next most effective step for reducing heat loss in your home. It's difficult to check and install wall insulation in existing houses without taking the lining or cladding off, so take the opportunity to do it if you're renovating.

Use a trained installer

EECA recommends using a trained installer to install your wall insulation. Look for an installer who:

Which product?

Bulk insulation

Bulk insulation comes as segments or blankets. Segments, also known as biscuits, are pre-cut to small standard pieces of insulation whereas blanket products are available in rolls of insulation. Both require removal of either the wall lining or cladding when retrofitting into existing walls.

Choosing a bulk insulation product

Bulk insulation can be made from different materials, including polyester, wool, fibreglass and other materials. Whatever material you opt for, a good wall insulation product should meet the following criteria.

  • Fit for purpose. It should be intended for installation in walls.
  • The highest R-value possible for the thickness of your walls - see below.
  • The right width. You need the correct width for the stud spacing in your walls.
  • Compliant with The Standard AS/NZS 4859.1. Look for the AS/NZS 4859.1 Standard compliance statement on the insulation packaging.

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.

If you don't want to go to the trouble of removing wall linings or cladding you may want to consider injected insulation as an alternative.

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.
  • Look for insulation that is as thick as your wall cavity, with the highest R-value possible. If the insulation is thicker it becomes squashed, reducing its effectiveness.
  • For most wall insulation materials the highest R-values available for 90-100mm thick wall cavities are in the range of R2.5 to R2.8.
  • If you want to add more insulation than can be fitted into your wall cavities there are options for doing this, such as lining the inside of your walls with polystyrene insulation and then covering with internal wall board. EECA recommends you consult with a qualified builder about what might work in your house.

Some things to look at if you are thinking about getting your walls insulated.

  • Check local building consent requirements. Retrofitting wall insulation needs a building consent unless your local council has granted an exemption for this type of work. Either way, the work must comply with the Building Code. Find out about requirements through your local council.
  • Ensure your wall cavities are dry. Leaks or moisture accumulation issues in wall cavities can be made worse by installing insulation, and can lead to timber decay and toxic mould growth. To make sure that the outside cladding and flashings are in good repair and not leaking, and that there are no hidden plumbing leaks, EECA recommends consulting a registered building surveyor.
  • Check the building paper in your walls. Building paper prevents insulation from coming into contact with the outer cladding - this reduces the risk of the insulation getting damp, which would reduce its effectiveness. A building surveyor can check if there's intact building paper in the wall - if there's not, you can retrofit it provided your wall is dry - refer to page 20 of the NZS4246:2006 insulation installation standard.
  • Get your electrical wiring checked. If you have existing electrical wiring inside your external walls an electrical safety inspection by a registered electrician is recommended to ensure the wiring is in safe condition and adequately rated so it does not overheat when surrounded by insulation. Rewiring and/or installation of circuit breakers may be required in some houses with unsafe wiring.

Further technical information: Guide for retrofitting wall insulation 

EECA and the Ministry of Business, Innovation and Employment have developed this guide for retrofitting wall insulation. It is aimed at experienced insulation installers who are familiar with the principles of Standard NZS 4246:2006 (incorporating Amendment No. 1) Energy efficiency - Installing insulation in residential buildings.

This guide provides best practice guidelines for retrofitting the most common types of wall insulation (blanket, segment, rigid and semi-rigid sheet) into framed external walls of existing houses.