Double glazing in Northland


Double glazing will be required if designers choose to show compliance with H1 using the Schedule Method, included in Acceptable Solution H1/AS1.

However, if designers choose to use the Building Performance Index to show compliance with H1, in most cases, double glazing will not be required for housing in Northland.

Because of the warmer climate in Northland, most designs can demonstrate compliance with H1 (using the Building Performance Index) with single glazing.

Using single glazing in Northland

Calculation of the BPI for a wide range of typical house designs in Northland have shown that most houses in Northland will comply with H1 using single glazing provided:

  • the glazing area is 50% or less than the total wall area
  • the R-values for walls, floors and ceilings in the table below are used
  • all the notes in the table below are observed.
Table of minimum R-values (m2 °C / W) for Northland
Building componentNon-solid wallsHigh thermal mass walls
Roof R 2.9 R 3.5
Wall R 1.9 R 0.8
Floor R 1.3 R 1.3
Glazing (vertical) R 0.15 R 0.15
Glazing (skylights) R 0.26 R 0.26
  • Using double glazing Carpets or floor coverings are not included in the floor R-value.

The R-values for glazing refer to whole window R-values (glass and frame). The values in this table are for standard WERS windows (see Appendix G, of NZS 4218).

There are no R-value requirements for the opaque parts of a door or a door set.
Total area of skylights must be no more than 3 m2.

While the above table may be of assistance, it does not form part of an acceptable solution. Accordingly, building consent applications must be assessed on a case by case basis and it is up to applicants to show compliance with the clause H1 of the Building Code.

Designs outside of the parameters

Designs that do not comply with the above limits may still comply with clause H1, but compliance will need to be shown by ensuring that the BPI calculation for the design does not exceed 1.55.

If the BPI exceeds 1.55, there are a range of modifications that can change a non-compliant design to a compliant one, such as:

  • using higher performance windows
  • moving windows (to increase solar gain)
  • reducing the window area (particularly on the south side of the house)
  • improving the floor, wall or roof insulation
  • using additional thermal mass.

Good design for energy efficiency

Designers and homeowners who want to exceed the above limits should seek expert advice on thermal performance.

Large areas of glazing may cause over-heating in summer and excessive heat loss in winter. Effective shading and other passive thermal design features are beneficial.

The ideal area of glazing, for good thermal performance, is not easy to determine. Design factors that should be taken into account include:

  • climate
  • insulation levels
  • amount of thermal mass
  • orientation of the glazing
  • size and geometry of the house.

Information on the principles of passive solar design is available in a booklet titled “Designing comfortable homes: Guidelines on the use of glass, mass and insulation for energy efficiency”, published by the Cement and Concrete Association of New Zealand and the Energy Efficiency and Conservation Authority.

Advantages of using double glazing include:

  • reduced on-going energy bills
  • improved comfort
  • a wider range of solar control options (by the use of selective coatings)
  • reduced condensation
  • reduced cold spots and draughts near windows
  • reduced noise
  • increased re-sale value.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general 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. Expert advice may be required in specific circumstances. Where this information relates to assisting people: