Retrofitting wall insulation
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Christian Hoerning
Retrofitting wall insulation
Blog, Insulation, Wall insulation

insulation-for-webpageApproximately half of all New Zealand houses have no wall insulation*. This results in space heating energy being wasted, makes these houses difficult to heat to healthy and comfortable temperatures and contributes to mould-problems with associated health impacts on occupants.

In houses that have ceiling and under-floor insulation and draught-proofing retrofitted already (for example through the government-funded Warm Up New Zealand: Heat Smart programme) insulating external walls typically reduces heat loss by approximately 40%. This makes wall insulation the most effective next step in upgrading the thermal efficiency of such houses.

Current practice for retrofitting wall insulation requires removing the wall lining or cladding and is usually only done when major renovation work is undertaken. However, even when the opportunity arises to open wall cavities, retrofitting wall insulation is not always straightforward.

Getting a building consent and managing risks associated with the walls’ weathertightness and the safety of electrical wiring are just some of the issues to be considered when retrofitting wall insulation.

Guide for Retrofitting Wall Insulation

To help with this, the Energy Efficiency and Conservation Authority (EECA) and the Ministry of Business, Innovation and Employment (MBIE) have jointly produced a Guide for Retrofitting Wall Insulation. 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.

The guide covers the following aspects:

  • Choosing a wall insulation product;
  • Obtaining a building consent;
  • Necessary inspections before wall insulation installation can start; and
  • Detailed guidelines for fitting insulation from the inside or outside, in walls with different cladding types, and with or without existing wall underlay.

Installer training

The Insulation Association of New Zealand Inc. (IAoNZ), with support from EECA, has developed a training programme for insulation installers based on this guide. The first wall insulation training course was held in Christchurch in February 2013, further training courses will be offered depending on demand. If you are interested in this training, or want to see the list of trained installers, visit the IAoNZ website.

EQC Christchurch repairs to accommodate insulation

Today, 4 March 2013, the Earthquake Commission (EQC) announced that their customers will be given the opportunity to install insulation where wall cavities and otherwise inaccessible roof and underfloor areas are exposed as part of earthquake repair work. EECA contributed technical advice to help support EQC’s move.

EQC requires that insulation installation during EQC repairs is done by trained installers and refers homeowners to installers who have received the above-mentioned IAoNZ wall insulation training. For more information refer to EQC’s media release.

Insulating existing walls – key differences to new construction

Installing wall insulation in existing walls can present some unique challenges which are not found when insulating new walls. This starts with managing health and safety risks from potential contamination like toxic moulds or unsafe old electrical wiring.


Thoroughly assessing the walls’ weathertightness and selecting an insulation material and installation method which will not cause moisture problems in the future is absolutely crucial. Many of the houses built without wall insulation (generally before 1978) have cladding systems which are not as weathertight as today’s claddings. Houses built before 1964 often do not have a wall underlay; for masonry veneer walls wall underlay was not commonly installed until 1977.

Older weatherboard walls rely on air movement through the cladding to dry out small amounts of moisture entering the wall cavity in wet weather. Inappropriately retrofitted wall insulation can substantially reduce the drying rate and at the same time form a bridge which allows moisture on the inside of the cladding to penetrate further into the wall cavity.

Masonry veneer claddings rely on a vented cavity behind the veneer to dry out moisture penetrating the cladding. If this cavity is filled with insulation moisture is likely to accumulate in the wall cavity, causing rot, mould growth and structural damage.

For walls without wall underlay the Guide for Retrofitting Wall Insulation requires that only semi-rigid or rigid insulation products be used; installed with a 20mm minimum gap to the external cladding.  The gap is to ensure that the drainage path on the back of the cladding is maintained and that insulation does not allow moisture to bridge across. For walls with direct fixed claddings (e.g. direct fixed weatherboards) this means that the insulation product must be thinner than the wall framing, e.g. the insulation cannot be thicker than 70mm where the wall framing is 90mm thick.

In walls with a drained or veneer cavity, insulation must not protrude beyond the outside of the wall framing into the drained/veneer cavity.

Note that whilst Standard NZS 4246: 2006 (incorporating Amendment No. 1) Energy efficiency – Installing insulation in residential buildings specifies that pieces of wall underlay be installed from the inside before retrofitting insulation to walls without existing wall underlay, this practice was not included in the Guide for Retrofitting Wall Insulation. This is due to the risk of water being guided onto dwangs and the bottom plate by the wall underlay pieces.  Based on the (currently, very limited) research knowledge on wall insulation retrofit practices, maintaining a gap between the insulation and the cladding is considered a safer practice by EECA and MBIE.

Electrical safety

An electrical safety check is another essential part of the installation process regardless of the method for retrofitting wall insulation. The electrical wiring in older houses may have deteriorated over time and be unsafe now. If the wiring is not adequately rated to be surrounded by insulation it could overheat as the insulation makes it harder for the wiring to dissipate heat. Re-wiring or installation of mini circuit breakers may be required in some cases.


Retrofitting wall insulation can offer significant comfort, health and energy benefits to occupants and is an important step in upgrading our older housing stock. The Guide for Retrofitting Wall Insulation published by EECA and MBIE provides guidance to installers to ensure the risks associated with retrofitting wall insulation in older houses can be adequately managed. The associated IAoNZ training on wall insulation installation offers installers the opportunity to upskill and retrofit with confidence and enables homeowners to identify qualified insulation installers. Both the guide and the IAoNZ training also underpin EQC’s offer to accommodate insulation retrofits during earthquake repairs.

Christian Hoerning, Senior Technical Advisor Buildings, Energy Efficiency and Conservation Authority (EECA)

* BRANZ report E466, “Insulation in existing houses”, July 2007












[1] BRANZ report E466, “Insulation in existing houses”, July 2007

  1. Hi Christian,
    This is a wonderful post with a helpful document link. I read it, and spent some time thinking about it, as there’s something here that shouted out to me. What it is, I think, is that the approach you outline so well is very rational and thorough. The thing that is shouting out to me is that few people are that rational or thorough, and those who are on a low income can often just not afford to be thorough. I can give plenty of examples from in my own community. For example, get a building consent to put insulation in?! Already, many people will either not know that they should get a consent, and if they do, ignore it, because building consents cost money, and who will notice if you do the work without one? For us, as community advisors, it can also be difficult, because there’s little point in recommending a course of action that will not be followed, because the homeowner just doesn’t have the resource to follow the thorough course of action – and so may do nothing. Then too, we move into a more difficult area – that of retrofitting houses that are very poor quality. In that instance, should we adhere to best standards, or accept that sometimes just acceptable solutions are sufficient – to provide cheap comfort in the remaining 10-15 years of a buildings life?

    • Hi Scott, thank you for taking the time to comment on my post. I personally answer a lot of public enquiries, too, so I can relate well to your observations. My take on this is that people are free to make their own decisions. I see our role as informing people as best as we can so that they can make well informed decisions. In the example of retrofitting wall insulation I think it is useful for people to know about the risks and what can be done to mitigate them. If wall insulation is retrofitted incorrectly you could end up with a leaky home and you are worse off than before. I talk to people from all walks of life, some are able to follow our advice by the book, others have no means to do it. I can’t do much about that, what I can do is give advice on what could be done and how to do it safely and effectively.

  2. I agree, this is an excellent post. And thankfully not a mention of UFFI. Retrofit wall insulation will have to be huge in this country if we are ever going to really get anywhere. We all know that ceiling and underfloor is a good start , but that’s all it is, a start!

  3. Had a chat with my colleague in building policy here at Auckland Council, and he has been involved in the on-again off-again romance with UFFI which I understand led to the requirement for consents for wall insulation in the first place. We are now consenting that product for all except brick construction (so watch this space to see if the marriage will last, same sex or otherwise…) Would be great to know what testing has been done by BRANZ or other agencies – as Christian says, there is not much research into this area, but plenty of anecdotal evidence. Which was used for the creation of the guidelines?

  4. Thanks Christian, but this is only testing on brick veneer and doesn’t cover direct fixed cladding. Nor does it answer why we have now a higher standard of insulation requirement for retrofitting bulk insulation into wall cavities than for UFFI (which is allowed to penetrate the wall wrap, assuming there is one). It seems like we are making it harder to retrofit walls by taking the lining off and adding bulk insulation. So my question remains, what evidence do we have that led to the change in recommendation from inserting squares of building wrap under 4246 to the latest guidelines that end up limiting the amount of insulation you can insert in order to provide that cavity? By doing so, we are unlikely to end up with higher R-values than with UFFI.

  5. I’ve been interested in this issue for a while, but have heard very little about UFFI over the last year or so (I’m in Wellington). I notice in a BRANZ article Dated August/September 2010, they were looking to do some more research on whether there seemed to be moisture issues in houses which have had UFFI installed for some years. Does anyone know if there is any more to report on this?

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