Types of wall 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. To put bulk insulation into existing walls you’ll need to remove the wall lining or cladding. When you remove the wall lining, do a thorough inspection of the wall for any leaks, presence and condition of any wall underlay (building paper) and electrical wiring.
Choosing a bulk insulation product
Bulk insulation can be made from various materials including polyester, wool, and fibreglass. Whatever material you opt for, a good wall insulation product should meet the following criteria:
- intended for installation in walls
- correct thickness
- highest R-value possible for the thickness of your walls
- right width for the stud spacing in your walls
- compliant with the New Zealand Standard AS/NZS 4859.1 (look for the compliance statement on the insulation packaging).
You should also check the performance guarantees and instructions for safely handling and installing insulation offered by manufacturers on their products.
How much wall insulation?
R-value is a measurement of 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 than the wall cavity, it will need to be squashed and will be less effective.
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.
Injected or blown-in insulation
Injected or blown-in insulation is pumped into existing walls through small holes in the cladding or lining. It can be made from various materials such as urea formaldehyde foam, wool or fibreglass fibres or polystyrene beads.
Before putting in any type of insulation into existing walls, it’s important to make sure the wall cladding is weather-tight and to check if there is a wall underlay behind the cladding. This is difficult to assess without opening the walls, so consult a registered or accredited building surveyor before you consider injected or blown-in insulation.
Some blown-in insulation products are approved for installation in walls without a wall underlay or for installation into a drained and ventilated wall cavity (like brick veneer). However, BRANZ recommends against this practice due to moisture risks.
There are two different ways of installing injected or blown-in insulation into walls:
- From the inside - preferred method, as it keeps the exterior cladding and the wall underlay intact. Redecoration may be needed.
- From the outside - less disruptive, but it creates a higher risk to the weathertightness of the cladding and damages the wall underlay.
Thermal imaging cameras can be used to check whether all wall cavities have been fully filled with insulation. It’s easy to miss cavities, as many older houses have irregular wall framing.
Performance of injected urea formaldehyde foam insulation (UFFI)
Urea formaldehyde foam insulation (UFFI) is injected as foam which then dries.
Research by BRANZ found that the actual R-value of UFFI insulation is about 50% lower in a house than if measured in a test lab. This is because the UFFI shrinks as it dries.
The BRANZ research found that a typical weatherboard wall with 100mm cavities and UFFI insulation installed has an R-value of about R1.6. An uninsulated weatherboard wall is about R0.5, so UFFI will make a difference. Taking the wall linings off and installing bulk insulation gives you an R-value of about R1.8 - R2.4.
There have been some concerns raised about injected foam insulation and the potential for it to transfer water in brick veneer houses. This was flagged as a potential issue in a report available on the BRANZ website.