Designing your home – what you need to know before you start:

The New Zealand Building Code requires new residential construction to achieve a minimum energy performance.

 Thermal Calculations

There are three ways of doing this:

 

1.      By using the ready made “schedule” method of building elements with known energy performance values.

2.      By calculating the building performance using a manual “calculation” method.

3.      By “modelling” the building using approved computer software.

The details of these methods are described in New Zealand Standard NZS 4218.

Mix and Match

The benefits of doing the extra work of calculating and modelling your building (whether a new building or an existing building that you are thinking of changing) is that you can mix and match the thermal benefit of different building elements and trade off the cost and benefits of each option. It is the overall BPI (Building Performance Index) that matters, and there are lots of different ways of achieving that.

Simple Solutions

WANZ recommends that a great deal of attention is given to energy efficiency at the building design stage. There are many simple things that can be incorporated into a building design that can significantly reduce construction costs and provide a building that is cheaper to run during its lifetime.

The simplest of these is orientating your building so that it benefits from the free heating that the sun provides. You can demonstrate the benefit of good orientation by using the modelling method described above. The schedule method and the calculation method do not reward good orientation.

Consider designing the roof eaves so that they allow the low angle of the winter sun to enter the building and warm it free of charge, and wide enough to provide some shade from the high angle of the summer sun. These calculations are easy to do from published sun angle data for your location.

Roof, wall and floor insulation is cheap compared with the cost of most other building materials that could be used for their thermal insulation qualities. Use insulation products liberally to keep the indoor temperature stable. Insulation does not heat or cool your house -  it simply slows down the rate of energy loss or gain. Insulation is less important in mild climates.

Glass provides a wide range of very clever answers for insulating windows. Because the glass portion of a window is usually by far the biggest surface area, it therefore has the most effect on saving energy. (see point 4 below)

Quick Summary

As a general rule, you can minimise your construction costs, and your ongoing utility costs for years to come, by:

1.      Orientating your building to the sun for cheaper winter warmth.

2.      Using the cheapest means of summer cooling such as large indoor/outdoor flow type of windows, doors and louvres to provide free cooling from ventilation.

3.      Use high levels of ceiling, wall and floor insulation. Put the maximum amount in the walls because it is unlikely that you can add more insulation at a later time.

4.      Spend your window dollars knowledgeably:

You get the most thermal benefit from your window dollar spend by using higher performance glass – e.g. Low e, double glazing, spectrally selective, tinted etc. (Glass can do a lot more for you than just thermal – for example you can combine thermal with reduced  fading, noise reduction and enhanced security).

Thermally broken frames do improve insulation, reduce condensation and enable large windows and doors to be used without any reduction in the structural strength of the window.