Walls and floors
The difficulty with insulating existing walls is that the internal lining or external cladding must be removed first, which can make it an expensive exercise. If renovating, and having to spend a lot of effort to get a good finish for new plaster and painting, consider replacing the old linings and while it’s off install insulation (and building paper if missing – see detail in NZS4246). Target south-facing walls, which do not get sun and may also be hit by cold winds. (Please note that a Building Consent is required for work involving wall insulation due to bracing and weathertightness issues that may result.)
Floors, unlike walls, are relatively easy to insulate, provided there is reasonable access. Between 12 per cent and 14 per cent of heat loss is through the floor. Gaps in floors leak heat. Even small gaps can have a big effect. (The same, in fact, applies to the installation of insulation: a gap of 5 per cent can result in a 50 per cent loss of the potential insulation effect in the immediate area of the gap.) Installing your own underfloor insulation is possible, though not always advisable, because electrical cabling is sometimes laid between floor joists. This is especially hazardous if insulating with foil.
Costs and subsidies
Insulation pays. In 2008, the New Zealand Building Code set new minimum standards for insulation. According to the Building and Housing Group, Ministry of Business, Innovation and Employment, the higher standards have increased the cost of building an average house by about 2 per cent, but produce an annual energy bill saving for occupants of nearly 30 per cent. The group has calculated that the cost of installing better insulation pays for itself in six years.*
Ceiling insulation, according to an Energy Efficiency and Conservation Authority estimate, costs about $15 a square metre to install, and underfloor insulation about $18 (accessed 2012 – see www.energywise.govt.nz) The authority offers subsidies to promote the insulation of existing homes and the installation of clean-burning fires and heat pumps in a polluted airshed to replace an open fire or non-compliant wood burner. Owners of houses built before 2000 can get a grant of up to a third of the cost of installation (nearly two-thirds for Community Services card holders or landlords with tenants who have the cards).
* This calculation was based on an average construction cost of $253,000 for a medium-sized house and additional costs from tougher insulation rules of between $3000 and $5000. Annual energy savings were estimated at $940. This assumed the whole house is heated all day to 16º, and the living areas to 20º in the morning and evening.
Make your house as airtight as possible. 6-9 per cent or more of heat loss is through draughts. You must have some ventilation, of course. Ventilation is controlled air change; draughts are uncontrolled, constant air change.
- There are some simple ways to keep warm air in and cold air out.
- Seal up openings such as unused cat doors and open fireplaces.
- Get some draught “sausages” to lie against the bottom of doors.
Fit self-adhesive weather strips around windows and strips and draught excluders around and under external doors.
Selecting the best product
Your choice of insulation products includes fibreglass, polyester, polystyrene, cellulose or mineral fibre and wool. Before deciding on an insulation product, get answers from suppliers to the following questions:
- What is the R-value?**
- Does it lose effectiveness if it gets damp?
- Will it eventually “settle”, that is, slowly collapse, and become less effective?
- Is exposure to it hazardous to your health?
- Has an independent body such as BRANZ appraised or endorsed it?
- Has Enviro-choice or another eco-labelling system certified it?
- Is it recyclable or biodegradable in a landfill?
- Is there a guarantee?
- What is the expected lifespan?
** R-values are a way of rating the insulation effectiveness of walls, windows, floors and roofs. The higher the R-value is, the greater the insulation a product offers. The minimum R-value for roof insulation is R 2.9, though a rating of R 5.0 may be necessary in colder areas of the country.