Many houses built before 1978 Building Code regulations took effect have no insulation. Even post-1978 houses may lack sufficient insulation. We look at how to insulate the roof space, walls, floors and windows of existing homes.

Insulation is most effective when achieves unbroken coverage around the building. If there are any breaks in the insulating material, heat can escape.

If you are putting in insulation in an existing house, for example when you are renovating, be conscious that any gaps or interruptions in the insulating material will significantly reduce its insulating qualities.

Roof space

For older homes with a space under the roof, concentrate on ceiling insulation first. It's usually easiest, and that is where you lose most heat. Skillion type roofing will need a different approach.

If you're doing the installation yourself, your choices are blanket segments or a blanket roll of insulation (see Home insulation materials). Loose or blown products require specialist equipment. Any of the glass fibre, wool or polyester products will do the job.

If you use your roof space for storage, be careful of damaging or disturbing the insulation. Glass fibre insulation is more easily damaged than wool or polyester. Loose fill is easily disturbed.

  • Check the access into your roof space. If it's too small to let you get a bulky roll of insulation through the access hole, you may need insulation that is cut into segments. Or you may need to install a larger access door. Access hatches should be insulated too.

  • Ensure the insulation fits snugly between joists as even small air gaps will significantly reduce effectiveness. With bulk materials fit 1 layer between the joists and another over the top. If you use a blanket-type insulation to cover the ceiling joists and prevent thermal bridging you need an insulation thickness of at least 150mm, preferably 200mm (about R2.4 to R3.2 (see Insulation basics)). However, covering the ceiling joists could make moving safely around the ceiling space difficult if you cannot easily see the joists. Using a plank to span several joists may be a safer option.

  • It's important to insulate right out to the edges of the ceiling, but not over the top plate. Gaps will either allow heat to escape or cold air to blow under the insulation.

  • In areas prone to frost insulate cold water pipes separately with special pipe insulation. They will no longer be warmed by heat from the house if they are above the insulation. Always insulate hot water pipes in this way to avoid wasting heat.

  • Try to get the insulation under electrical wiring. Wires may overheat if they are covered by thick insulation and a large current is flowing in them.

  • Recessed light fittings can compromise the effectiveness of ceiling insulation. If you can afford it we recommend that any recessed lighting that is not rated CA (closed abutted) be removed. Such downlights need air holes to be cut in the surrounding insulation to avoid overheating and risking fire. These holes reduce the effectiveness of your insulation. CA-rated recessed light fittings are now available that allow insulation to touch them.

  • Some kitchen and bathroom vents may also allow warm air to escape.

  • Leave a clearance around any heating appliance flue that penetrates the insulation. The manufacturers of the appliance will have specified the size of the clearance.

  • Loose fill insulation products are worth considering if cost is a concern, if you have a low-pitched roof that makes it difficult to access the ceiling space, and/or if you don't need to get into your roof space very often. The insulation can be built up to give total coverage over the joists, reducing heat loss through the timber. It can also be blown into the inaccessible corners and edges of low-pitched roofs. Loose fill insulation is prone to moving around if draughts blow through your roof space. It will also settle over time and become less effective.

Walls

It is possible to fit insulation into existing walls – especially if outer claddings or inner linings are at least partly removed. But there’s a set of problems ...

  • You need a Building Consent.
  • The work must comply with the building code.
  • There’s no acceptable standard way for an installer to demonstrate compliance. Each installation becomes a specific design that requires judgements to be made by building professionals.

All the above probably puts wall insulation into the “too hard” basket for many homeowners.

We suggest using a professional insulation installer if a retrofit is planned. Ask them to get the Building Consent, and provide proof of compliance to the Inspectors.

Floors

They may look nice, but bare tongue-and-groove floorboards can let a lot of cold air through the joints, especially when it's windy. Traditionally foil was stapled under the floor joist to create a still-air gap. Foil insulation is still allowed under the revised Building Code. However, it is difficult to install to achieve the R-value required by the Code.

Note: installing underfloor foil insulation can be risky. Be careful not to pierce electrical cabling with staples. Existing foil insulation, if improperly installed, can be live. If you are unsure about anything, hire a professional installer or get an electrician to check things out.

These days bulk insulation products made from polyester, polystyrene, wool or glass-fibre are more commonly used. Non-foil insulation is fitted against the underside of the floor between the joists. There is no air gap required.

You'll need reasonable access under the floor and in windy sites the foil will need a rigid lining to hold it in place rather than just staples and strapping. Any gaps in foil must be sealed to provide a still air space.

Other types of products will have their own specific installation requirements. Check with the supplier before starting the installation.

If the ground under the floor is constantly damp a layer of polythene sheet laid on the ground will be useful in reducing underfloor damp air. A well ventilated under floor space also helps keep the area dry.

In areas prone to frost insulate cold water pipes separately with special pipe insulation. Always insulate hot water pipes in this way to avoid wasting heat.

Try to get the insulation under electrical wiring. Wires may overheat if they are covered by thick insulation and a large current is flowing in them.

Windows

Houses lose heat through window glass and metal window frames. Cold air flowing down windows can cause draughts. Heavy drapes or thermal drapes that fit closely around the window frames help prevent heat loss and reduce draughts. Pelmets also help.

If you are replacing windows consider double glazing and insulated frames. These can dramatically reduce heat loss and condensation compared to aluminium-framed single glazed windows. Double glazing also reduces sound transfer.

Double glazing can, however, be an expensive option and may not necessarily be suitable or easy to install in your design of window. The choice of a double glazing product and installation needs to be by an experienced professional.