This guidance provides information on the Building Code and Building Act 2004 requirements that relate to retrofitting insulation in external walls of buildings. It highlights the main considerations when assessing work for compliance and lists the relevant Building Code performance criteria for common types of insulation retrofits, such as retrofitting rigid or semi rigid segments and sheet insulation, injected loose fill insulation or injected foam insulation.

Decisions on this work can be complex. Insulation is usually retrofitted into older houses that have stood the test of time but are now too cold and draughty for modern lifestyles. Old houses often have simpler designs and are built of more rot-resistant timber than houses built during the 1990’s, which are now stigmatised by moisture problems. Insulation retrofits deliver tangible benefits, but there can be unintended problems if the insulation or the installation is unsuitable.

This guidance does not provide solutions or recommend installation methods for particular types of insulation or house construction. Insulation specialists should be consulted if you have doubts about how best to retrofit insulation.

Who is it for?

The guidance is intended for building consent authorities (BCAs), to help them decide whether to exempt wall insulation from a building consent or to approve/decline building consent applications for retrofitting wall insulation. Designers and installers may also find the document a useful guide to the relevant Building Code requirements for retrofitting wall insulation.

Is a building consent required?

Schedule 1 of the Building Act now exempts underfloor and roof insulation retrofits from building consent2. However, retrofitting insulation into a wall cavity is not exempt; it either requires a building consent or specific approval from a BCA that a building consent is not required3.

Building consent applications for retrofitting wall insulation will need to cover all the associated building work, show that it will comply with the Building Code and that it will not adversely affect the performance of the existing building.

Why is retrofitting wall insulation not included in Schedule 1?

Retrofitting insulation to walls involves more work than simply placing or injecting insulation in walls. All the associated work must be assessed to see if it complies with the Building Code, whether it is as small as drilling and repairing holes in interior linings or as significant as removing and reinstating exterior claddings. Even if the building work is relatively simple, the effects on the existing building may be complex and potentially problematic.

Retrofitted wall insulation may affect moisture transfer inside timber framed walls and change drying rates, which in turn may cause moisture to accumulate in the wall and affect the durability of timber framing and cladding. Poorly installed insulation may also affect the fire and electrical safety of houses. Houses must be assessed on a case-by-case basis to determine if they are suitable for retrofitting wall insulation and if the proposed methods and materials are appropriate.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: