Bulk insulation is effective – but you need to open the wall
R-value is a measurement of insulation's effectiveness. An uninsulated weatherboard wall is about R0.5. If you take off the wall linings to install bulk insulation, you can lift this to about R1.8 – R2.4.
Hire a professional or install it yourself?
We recommend hiring a trained professional to install your wall insulation. Members of the Insulation Association of New Zealand (IAONZ) have completed a training qualification and work to the New Zealand Standard for installing insulation.
If you decide to install it yourself, everything you need to know is in the New Zealand Standard. Take care to follow the health and safety advice in Appendix B. You’ll still need to hire experts to check your timber and wiring.
Retrofitting wall insulation needs a building consent unless your local council grants you a discretionary exemption. Either way, it must comply with the Building Code. Find out about requirements through your local council.
Choose your product
You can buy bulk insulation as pre-cut segments or blankets made from materials such as polyester, wool and fibreglass. Choose a product intended as wall insulation and check for these features:
- as thick as your wall cavity but no thicker – squashing it makes it less effective
- highest R-value that will fit
- meets Standard AS/NZS 4859.1
- safe handling and installation instructions
- performance guarantees.
Hire a building expert
Insulation can contribute to leaks or moisture issues inside wall cavities, leading to timber decay and toxic mould.
Hire a qualified builder who is a licensed building practitioner, and/or a registered or accredited building surveyor to make sure:
- the outside cladding and flashings are in good condition
- there are no hidden leaks
- the building paper in your walls is intact – if not you can retrofit it, provided your wall is dry.
Get your electrical wiring checked
If there’s electrical wiring in your external walls, hire a registered electrician to check it’s in safe condition and won’t overheat when surrounded by insulation. If it’s not safe, rewiring or circuit breakers may be required.
Injected or blown-in insulation
This type of insulation is pumped into existing walls through small holes in the wall. It can be made from urea formaldehyde foam, wool or glass wool fibres, or polystyrene beads.
It can be injected or blown in from the inside (which keeps the exterior cladding and building paper intact) or the outside (which has a higher risk that the wall may not stay weather-tight).
Just like bulk insulation, injected or blown-in insulation can contribute to leaks or moisture issues inside wall cavities, leading to timber decay and toxic mould.
Hire a registered or accredited building surveyor to make sure the wall cladding is weather-tight and check if there’s a wall underlay (building paper) behind the cladding.
Independent building research organisation BRANZ has published two investigations into urea formaldehyde foam insulation (UFFI) and does not recommend its use.