Passive Design
Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.
Glazing for Temperature Control
The overall thermal performance of an external wall envelope is mainly determined by:
- the level of insulation in the solid portions of the wall and
- the amount of that envelope that is glazed.
The R-value of glazing – even high performance glazing – is significantly less than that of an insulated framed wall. The schedule method of NZS 4218 is based on a maximum glazed areas of 30% of the wall area.
The current insulation requirements (NZS 4218 and Building Code Clause H1 3rd edition) set a minimum R-value for glazing in new homes at R 0.26 or R 0.31. This can be achieved through use of Insulated Glazing Units (IGUs, or double glazing) of various kinds, as follows.
Insulated glazing units
IGUs are now the default option for all new houses, and are an option for retro-fitting to existing buildings.
IGUs can have a significant effect on heat loss and condensation. They can also reduce:
- solar gain (if the unit incorporates tinted or reflective glass) and
- transfer of noise.
The most common type of IGU is made up of two sheets of glass held apart by a hollow bar, which is sealed to both sheets. The space between the sheets normally contains dry air, but some units are filled with a gas (argon) to improve performance.
IGUs must be correctly installed to ensure maximum durability – units must be installed so that the edge seals are protected from UV and continual dampness.
- Typical insulated glazing unit construction
Double glazing, more correctly called insulated glazing units (IGUs), can have a significant effect on heat loss and condensation. IGUs can also reduce solar gain (especially if the unit incorporates tinted or reflective glass), and the transfer of noise.
The most common type of IGU is made up of two sheets of glass held apart by an aluminium hollow bar which is sealed to both sheets. The space between the sheets normally contains dry air, but some units are filled with a gas (argon) to improve performance. The heat transfer through IGUs can be reduced further by using low-E glass. This is glass with a microscopically thin, virtually (but not quite) invisible metallic coating that reduces radiant heat transfer through glass.
Calculating R-values for IGUs
Because window size and construction varies so widely, a ‘standard window’ has been defined, to allow standardised calculations of R-values for glazed areas. The standard window size is 1800 x 1600mm, and IGUs are assumed to have a 12mm air gap. For more detail see the Glass Association of New Zealand (GANZ) website.
A standard IGU with timber or PVC framing and two panes of clear glass has an R-value of 0.37, compared with an R-value of 0.15 for an aluminium-framed window with a single pane of clear glass.
Low E glass
Heat transfer through windows can be reduced by using low-E glass. This is glass with a microscopically thin, almost invisible metallic coating that reduces radiant heat transfer. A standard IGU with an inner pane of low E-glass will have an R-value of 0.31 in an aluminium frame, or 0.47 in a PVC or timber frame.
There are different types of coatings to suit various purposes – while a high solar gain coating may be suitable for winter conditions, a low solar gain coating performs better in summer. It is important to consult the manufacturer to ensure that the correct product is used for specific conditions. See also the Window Efficiency Rating System (WERS) section.
Minimising heat loss through glazing design
Heat loss through windows can be minimised through design by:
- minimising the number of south-facing windows
- restricting the size of east- and west-facing windows – these windows admit heat in the mornings and afternoons but over a 24 hour period heat loss exceeds heat gain (Australia’s house energy rating scheme recommends keeping east-facing glazing to less than 5% of the home’s total floor area and windows between 20° west of true north and 40° west of south to less than 3% of the home’s total floor area.
Window joinery
Heat loss through timber, composite timber/aluminium and PVC window frames is significantly less than for an aluminium frame unless the aluminium frame incorporates a thermal break.
There is composite window framing which is usually referred to as thermally improved, and then there is thermally-broken window systems, which are also sometimes referred to as thermally improved, but both have different performance.
If a single glazed aluminium frame is used as a benchmark and assessed as having 100% heat loss, the comparative heat loss for other options is:
- 88% for single-glazed thermally improved aluminium
- 79% for single-glazed timber or PVC
- 58% for double-glazed industry typical aluminium
- 56% for double-glazed thermally improved aluminium
- 42% for double-glazed timber or PVC.
Controlling heat gain
The amount of heat entering a room through the windows can be reduced by:
- solar control glass which is fitted into the window frames
- reflective glass
- solar films or coatings
- IGUs.
Solar glass, reflective glass, and glass with applied coatings all reduce light, so designers will need to strike a balance between reducing heat gain and achieving required levels of daylight. IGUs reduce heat transfer and have a small effect on heat gain (which is increased if solar control glass or film is incorporated into the unit).
Solar control glass is similar to clear glass. Its appearance and thermal performance are altered by tinting or by adding a reflective coating. It restricts heat build-up from the sun by:
- absorbing energy and radiating most of it to the outside but some to the inside, or
- reflecting most of the energy.
Solar films or coatings filter or reflect the sun’s energy. There is a range of films available with varying properties.
When specifying a product, remember that glass and films are made for a wide range of purposes. It is important to decide what your requirements are and to establish clearly with the manufacturer which product will give the specified performance. Glass and applied finishes must be installed and maintained as instructed by the manufacturer. The length of warranty for films should be checked.