Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Glazing and glazing units

Glazing and glazing units should be designed to admit light while controlling heat gain and heat loss.

Windows and doors can account for more heat gain or loss than any other element in an insulated building envelope. A well designed glazing system can improve internal daylight levels, reduce glare, and help maintain thermal comfort by reducing heat gain and loss. This contributes to energy efficiency by reducing the need for artificial heating, cooling or lighting.

By considering the transmission of heat and light through the glazing system at the design stage of the project, window performance can be significantly improved.

Controlling heat loss and gain

Heat is gained and lost through the glazing and through the frame, so it is important to consider both together.

It is also important to consider both the glazing unit’s insulating properties and its efficiency at letting solar radiation into the building (this is known as solar heat gain coefficient).

Heat loss and gain can be controlled using insulating glass units (IGUs, commonly known as double or triple glazing). In most parts of New Zealand, IGUs are necessary to meet Building Code requirements. Even where they are not required, they are advisable.

Framing and glazing materials, and glazing unit construction, also influence performance. Tinted or coated glass such as low-emissivity glass can improve thermal performance and/or reduce glare. Timber, uPVC and fibreglass frames provide better thermal insulation thank aluminium; where aluminium is used, the frame should incorporate a thermal break.

Thermally broken aluminium frames have a very strong spacer with a higher level of thermal performance between the inner and outer parts of the aluminium frame. BRANZ testing has shown that frames with this feature can be almost 60% more thermally efficient than those without it.

For best performance, consider the climatic conditions, the design and orientation of the building and its windows, and the thermal and optical properties of the glazing and glazing units.

For many window systems, performance information is available in NZS 4218.

WEERS

The Window Energy Efficiency Rating System (WEERS) is a voluntary 6-star rating programme that compares the thermal performance of windows in housing and small buildings. It was developed by BRANZ in conjunction with the Window Association of New Zealand (WANZ).

WEERS combines the thermal performance of the frame and glazing, together with the size
 of the window, to calculate an individual thermal performance rating RW for each window, and from that, its star rating. The more stars that are shown on the window, the better it will perform at restricting heat loss in winter and restricting heat entry in summer.

The weighted average RW values for all windows in a houselot are combined to give an RW(av), which is used to give a WEERS star rating for the houselot.

Houselots of windows that achieve an average RW(av) of 0.32 m2K/W (or above) will achieve ENERGY STAR® endorsement for the houselot, provided no windows in the thermal envelope are single glazed.

Code requirements

Building Code clause H1 Energy Efficiency requires that buildings are constructed to provide adequate thermal resistance. The thermal performance of glazing and glazing units will have to be considered in conjunction with other building elements to determine overall thermal performance and therefore compliance. See determining insulation requirements for details.

 

Update: 29 August 2017