Types of insulation

Bulk insulation traps air in still layers.

Bulk insulation comes in a variety of materials (glass fibre, polyester, polystyrene, wool, paper) and formats (blankets, segments, rigid sheets or loose fill).

Insulation materials

There are many insulation materials available including wool, polystyrene, glass fibre, mineral wool, paper-based insulation and polyester.

Insulation materials has a more detailed explanation of your options.

Straw bale construction, structural insulation panels (SIP) or insulated concrete form work also provide high levels of insulation.

When should you install insulation?

Anytime is a good time to insulate.

The most economical time to install insulation is during construction of new buildings and during extensive renovations before walls, floors and ceiling spaces are closed in. This is especially true for wall insulation.

However, insulation is also important for existing homes. Houses built before 1978 are unlikely to have any insulation, unless it has been subsequently added, and houses built before 2007 have lower than currently required levels of insulation and single glazing. Retrofitting insulation will prove cost-effective because it will keep your home warmer and reduce your heating costs over the long term.

Where should you install insulation?

When thinking about insulation, it’s important that you consider all of the following areas:

• ceiling
• under the floor
• walls
• windows including the frames.

These four areas form the ‘thermal envelope’. The thermal envelope is the barrier between heated spaces and the exterior climate. Imagine it like a complete blanket protecting the inside of the home from the outside climate. The better insulated your thermal envelope is, the easier it is to keep your home warm.

The design and construction of your home will affect the specific types of insulation you can use, and where the insulation can be placed.

In houses with a suspended floor it can be helpful to install a polythene blanket on the ground underneath the house. This stops cold moist air evaporating from the ground and entering the house, which will make it drier and easier to keep warm.

With some construction systems – such as, polystyrene concrete form work, SIP and straw bales – little or no additional insulation may be required.

Construction systems has more information.

Ceiling insulation

About 35 per cent of heat loss from an average uninsulated home occurs through this area.

The most effective way to insulate the ceiling is to install bulk insulation between joists, combined with a blanket rolled over the top – this eliminates thermal bridging, which is the heat escaping through the wooden joists.

Some types of house – for example, art deco homes – have small roof spaces that can make retrofitting ceiling insulation difficult.

In the ceiling, you will need to leave gaps around downlights – unless they are new CA, Insulation Contact (IC) and Insulation Contact – Fire Resistant (IC-F) rated and other fire hazards. That will create insulation gaps through which a lot of heat can escape. See Downlights and recessed lights.

Ceiling insulation on the Gen Less website has more information.

Floor insulation

Fourteen per cent more heat loss can occur through the floor of uninsulated houses.

Insulation should be used underneath suspended timber and suspended concrete floors, around the edge and underside of concrete floor slabs in new homes, and around the edge of concrete slabs in existing homes if possible.

While you are installing your under-floor insulation, don’t forget to lay a vapour barrier over the ground. BRANZ research has shown that 0.4 litres of water can evaporate from 1m² of ground in 24 hours – that’s 60 litres per day on average for a 150m² house – it can damage your insulation, and cause dampness problems in your house.

Wall insulation

Up to 25 per cent of heat from an average uninsulated home is lost through the walls. Once you have insulated your ceilings and under your floor, think about wall insulation. You should especially consider adding wall insulation when you are removing cladding or linings for any reason – it is the easiest and most cost-effective time.

Under some circumstances, retrofitting wall insulation to external walls may require a building consent.

Guidance on Building Code compliance for retrofitting insulation in external walls on the Building performance website has more information.

If you're using a framed construction system, insulation is usually placed within the wall framing. Insulation can also be installed outside the framing (but the insulation must be weatherproof to be effective). If there is a complete insulation layer installed on the outside of the framing to meet insulation requirements, it is important that there are no vented cavities between the insulation and the framing, because the air flow would by-pass much of the insulation.

Pay particular attention if your house has steel framing. As metal is a good conductor of heat, it can lose a lot of heat. It is usually recommended to add strips of special insulation over the framing members before any cladding is put on the wall.

If you're using a solid construction system such as concrete, insulation should be placed on the outside of the solid wall. If the house has good exposure to sunlight the exposed concrete functions as thermal mass, storing surplus heat during the day (less cooling required) and releasing it during the evening (less heating needed). If there are no significant solar gains the inside of the wall can be insulated instead or in addition to the outside.

Wall insulation can be fitted to existing homes by:

• removing wall claddings and installing blanket or segment insulation, as well as building paper to prevent moisture coming into the wall – this is the best option for timber frames
• injecting or 'blowing’ bulk loose-fill or foam insulation into the top of the wall cavity, or through holes in the top of an external wall. This can be done for timber frames, but is labour-intensive and difficult to get a consistent fill. Houses with brick cladding should not use this type of insulation product, as the cavity is a key part of keeping the building weathertight. In all types of houses, a building consent is needed to install this type of insulation
• fixing rigid sheet insulation to the outside of solid walls, such as concrete block – this 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 by-pass the insulation layer. Any drainage cavity should therefore be on the outside of the insulation layer.

Note that the amount of insulation you can install may depend on the thickness of the walls and the size of the framing. If you're building a new home or renovating, consider increasing the framing size to fit in more insulation.