Passive Design

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 insulated glazing units (IGUs, commonly known as double 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 and PVC frames provide better thermal insulation than aluminium; where aluminium is used, the frame should incorporate a thermal break.

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 or through the Window Energy Rating System (WERS). Computer modelling can also provide an accurate assessment of thermal and optical performance, but can be expensive.

As well as glazing and glazing units, thermal performance of windows and glazed doors will of course be influenced by other features such as size, orientation, shading, and use of curtains and shutters to prevent heat loss at night.

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.