Zero Energy House

Investment in high performance glazing and joinery increases comfort and helps the ZEH avoid the need for winter heating

Glazing and Joinery

Glazing in a house provides access to:

1. Daylight
2. Solar gain
3. External views
4. Connection with the outside
5. Views of the inside which helps with security

It is important to optimise the window to wall ratio of a building to allow sufficient daylight without compromising the thermal performance of the building.  Glazing will allow good solar gain on sunny days the Winter but will also have the highest thermal conductivity, so is a weak point for heat loss.  It will also result in solar gain in the Summer which will lead to overheating if shading features are not used to control this and ventilation is not used to reduce any heat build-up inside. We investigated various window to wall ratios using thermal modelling and came to the conclusion that an overall window to wall ratio of around 30% was optimal for thermal performance.  

The amount of glazing varies on each elevation:

• NORTH - We have maximised glazing on the North elevation to allow good solar gain during the Winter.  Northern (midday) sun is also easy to control with the use of overhangs (see Shading).  
• EAST - The Eastern elevation has the second highest percentage of glazing to encourage heat build up over the day in the Winter.  We are considering the use of vegetative shading to control the solar gain on this elevation during the Summer months. 
• SOUTH - The South and West elevations have the least glazing. The South elevation will be the coldest and therefore higher levels of glazing on this elevation lead to greater heat loss. However we have optimised the level of glazing on this elevation to allow sufficient opening windows for good cross ventilation, natural ventilation of bathrooms, daylight access and interaction with the street.  
• WEST - The West elevation will be exposed to strong afternoon Summer sun, so minimising the glazing on this elevation helps to reduce solar gain in the warmest months.

Glazing has a significant impact on the heat loss from a building. The thermal performance of a building element is usually defined by its thermal resistance (R Value). The building code requires for example that walls of residential homes have a R value of 2. The bigger the value the harder it is for heat to escape throught the wall. While the walls of the ZEH have an R value of R3, windows have the following properties:

Source = NZS4218:2004

The R value of windows are determined by the glazing but also the window joinery. The table shows that timber  increases the R value of the window over standard aluminium joinery (thermally broken² aluminium will also improve the R value over standard aluminium). Double glazing adds an air gap between 2 panes of glass, as air is a poor ocnductor of heat this increase the thermal performance of the window. Low E coatings¹ reduce the amount of heat that is lost via radiation.  

We concluded that the additional thermal performance of double glazing with a low E coatings1 (to reduce radiant heat loss) was worth the investment. In combination with passive solar design and good insulation levels we are expecting not to need heating in the winter.

Up until late 2009 it was easy to get single glazed domestic buildings through consent.  Changes in the building code have made it much harder for single glazed buildings to pass and generally most domestic buildings now require double glazing.  This is a positive step, however shading should also be included to avoid overheating and increased use of heat pumps which have become commonplace.

1. Low E, is an abreviation for low emissivity. By coating the surface of glass with a low E coating the amount of infrared radition that is lost out through the window can be reduced. 
2. Aluminium is a very good conductor of heat meaning that you can lose quite a lot of heat through the windown frame. The window frame will also be colder because of this and you will often see condensation forming on the inside of aluminium joinery when its cold outside. To reduce this problem a plastic separator (plastic being a poor conductor of heat) is placed between the inside and outside of the aluminium frame, reducing the amount of heat lost through the window frame.