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Insulation for existing homes

Many old homes have too little or no insulation. Here is how you can add some.

Give that old house a thermal boost

Many old houses, especially if built before 1978, have little or no insulation. It makes them uncomfortable, expensive to heat and probably also less healthy.

There are some things that can be done relatively easily to improve this situation. For some of the measures there are even government grants that make them very affordable.

If you retrofit insulation in your home no building consent is required for the ceiling or floors. However, you are required to have a building consent for retrofitting insulation in external walls, or alternatively, specific approval from your council’s building inspector that it is not required. Make sure you are safe doing it, so turn off the power supply to the house before starting any work.


Ceilings with roof spaces

Ceilings are often the easiest and most effective areas to insulate in an existing house. If the roof space is accessible bulk insulation can simply be placed over your ceiling lining and framing.

Even if there already is insulation it may be less than optimal or may have settled over the years and has lost some of its performance. A very rough rule of thumb is to see whether the insulation is still more than 120mm thick, ie thicker than the framing. If it is less it is probably worth adding some more. Also look out for any gaps in the coverage. Even small uninsulated areas will contribute to heat loss. But remember that some old downlights and other electrical equipment may require a gap for fire safety reasons.

If the existing insulation is in reasonable condition (dry and no rodents in it) it can often remain in place and is simply covered with a new additional layer.

Ceiling insulation on the Gen Less website has more tips and advice.

Ceilings without roofspaces

It is quite difficult to retrofit insulation in ceilings if there is no or insufficient roof space. A good opportunity to insulate those is when the lining is replaced.

Keep in mind moisture problems could arise if insulation is poorly installed. Talk to your designer or professional installer about whether you need a gap between your insulation and roofing underlay.


Concrete slabs on ground

There is little that can be done about existing concrete slab floors that directly sit on the ground. The good thing is that the ground underneath the slab actually improves the insulation performance to a reasonable level.

Most heat will escape through the edges of the slab. It would therefore be beneficial to add a strip of vertical insulation around the slab edges, if that is practical. The insulation material (typically polystyrene) has to be protected from ground moisture and have some drainage.

You also have to take care that the insulation is installed in a way so water at the top of the insulation panels is not directed behind the cladding or onto the bottom plate (the horizontal piece of timber at the bottom of the wall sitting on the concrete).

Suspended floors

Uninsulated suspended floors can be a major source of unnecessary heat loss. In many cases it is easy and cost-effective to add under-floor insulation. If the space under the floor is accessible and high enough insulation can be installed between the joists. Most commonly bulk fibre insulation or polystyrene is used for underfloor insulation.

Many old houses had previously been insulated with aluminium foil. That was cost-effective and easy to do, but the foil does not provide sufficient insulation and when in contact with electric wiring can become a serious health hazard. If the foil insulation has rips or holes, it should be replaced.

When checking or removing existing foil insulation, always turn off the power supply to the house and if unsure of anything, get an electrician to assist you. Note that retrofitting or repairing foil insulation in houses is now banned, so your only option is to replace with bulk insulation.

In most cases it also pays to install a layer of polythene ground sheet on the ground under the house. This is especially critical if your section is quite moist. The sheet will reduce moisture entry into the house, help the insulation work better and make your home healthier and easier to heat.

If you are making changes to the floor such as re-piling or replacing floor boards it is a great time to add some insulation.

Underfloor insulation on the Gen Less website has more details.


In most cases it is not easy to retrofit insulation in existing walls and a building consent may be required.

The most effective way is to do it when the cladding or the lining is being replaced. If there is no building paper present this should also be installed at the time to prevent moisture wicking into the insulation and the lining.

Fixing rigid sheet insulation to the outside of solid walls, such as concrete block will include external cladding as part of the system. This may require window flashings and sills to be modified. Remember that a vented air gap between the insulation and the blockwall would allow airflow between them and therefore much heat would bypass the insulation layer. Any drainage cavity should therefore be on the outside of the insulation layer.

There are some products that can be blown into the wall cavity through holes drilled through the cladding or the lining. Some blown-in and injected foam insulation suffers from shrinkage or settling over time, which can affect its performance. Also, sometimes determining whether the cavity has been effectively filled with the product is difficult, resulting in reduced performance. These products can only be used if there is building paper present in the wall. If you plan to have one of these products installed, make sure that you visit the BRANZ website and talk to your local council first.

Wall insulation on the Gen Less website has further information.

BRANZ Find can help you find more information on foam insulation.


Replacing old single glazed windows with new double glazed insulated glazing units (IGU) is usually easy to do, however, it is not cheap. The key thing to look for is its thermal effectiveness, or R value – the higher the value the better.

Living areas such as lounges and living rooms tend to have large glazing areas. Single glazing has a very low insulation value, ie only about 1/20th of an insulated wall. That means that as much heat escapes through 1m² of single glazing as through 20m² of insulated wall. Because it is not cheap to replace old windows it can be smart to replace them only in rooms that are heated and occupied a lot.

Other options are secondary glazing systems (acrylic or glass). These will achieve insulation performance about as good as new double glazing.

There are also cheap temporary films available that can be attached to the window frames and create a gap between the glass and the window. They can be found at most hardware stores and last for a winter season before having to be replaced.

Some window films can be placed directly onto the glass. They often use a special 'low-E' filter to reduce radiant heat loss. If you are planning to install one of these make sure you check that the product has been tested by a reputable independent testing facility.

Window frames

When choosing your new windows also think of the window frames. Simple aluminium frames are great heat conductors and it may pay to choose some thermally broken frames or polyvinyl chloride (PVC) or timber frames that have a much better insulation performance. These will also reduce the likelihood of condensation and mould forming on the frames.

If you have old wooden frames that don’t close properly a lot of warm air escapes through these gaps. Install some draft stopping strips, which you can buy from most hardware stores.

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Note that this document is published by the Ministry of Business, Innovation and Employment Chief Executive as Guidance under Section 175 of the Building Act 2004. This is a 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.