U-value measures how well a product prevents heat from escaping. It is a measure of the rate of non solar heat loss or gain through a material or assembly. U-value ratings generally fall between 2.0-10.0 W/m2.K for Australian products. The rate of heat is indicated in the terms of the U-value of a window assembly which includes the effect of the frame, glass, seals and any spacers. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value. The U-value for a window takes account of the various U-values for the components making up the window, so you may see these in technical literature; UW is the value for whole window and because of its importance is usually abbreviated to U, UC is the value at the centre of glass, UF is the value for the frame.
R-value and U-value are essentially two sides of the same coin. R-value is usually cited when discussing things such as wall and ceiling insulation values. The term does not translate well to windows and other fenestration products. That industry prefers U-values. The two are actually inversely related. The higher the R-value, the better insulated are the walls and ceilings. The lower the U-value, the better job a window does in keeping out the heat and cold.
SHGC measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits.
Visible Light Transmission of light (VLT) measures how much light comes in through a window. This is important as the amount of natural light in a room during the day will determine whether or not lights need to be turned on (not turning on the lights is an easy way to reduce energy consumption).
VLT is an optical property that is expressed as a number between 0 and 1. The higher the number, the more light is transmitted (a VLT of 1.00 allows all of the available light through, a VLT of 0.75 allows 75% of the light through, and a VLT of 0.00 allows no light through). When there is a lot of sun, a lower VLT means that the SHGC is reduced.
The ANAC ratings have been available for many years and are based on "Australian National Average Conditions". This rating system is being replaced by a more advanced and globally acknowledged system developed by the NFRC (National Fenestration Rating Council) in the USA. Unfortunately, the building code called for its use before the "whole of house" rating software (called Accurate - for use in First Rate, BASIX, BERS, etc) was updated. The result is that there are two options available until the transition is complete. Companies can continue with ANAC ratings, and for those with NFRC ratings, as an interim measure, these have been adjusted so that they may run in the "whole of house" software packages. These will be re-rated again when the Accurate software upgrade has been completed, and the ANAC ratings will be discontinued.
Air infiltration (or leakage) is indicated on a test report and expressed as L/s m2. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the air infiltration figure, the less air will pass through the cracks in the window assembly, improving the energy rating.
CR measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the rating, the better the window is at resisting condensation formation. The CR is expressed as a number between 0 and 100. Whilst this rating cannot predict condensation, it provides a method of comparing the potential of various window types.
Assume that aluminium frames are standard unless the manufacturer provides a Window Energy Rating (WERS) which demonstrates that better energy performance will be achieved by using the particular frame.
An aluminium improved frame is defined as one which is broken but has a gap of less than 5.30mm between the pieces of aluminium. This may be achieved by a variety of design improvements, some (or all) of which may be present. To be classified as broken, a window must conform to the following definition by the U.S. National Fenestration Rating Council (ref: NFRC 100-2001).
To be classified as thermally broken, a window must conform to the following definition by the U.S. National Fenestration Rating Council (ref: NFRC 100-2001).
"Thermal break: a material of low thermal conductivity that is inserted between members of high conductivity in order to reduce heat transfer. Thermal barrier material conductivity shall be no more than 0.5 W/m.K.
Thermally broken (TB) members: system members with a minimum of 5.30mm separation provided by a low-conductance material (where thermal conductivity is less than or equal to 0.5 W/m.K) or open air space between the interior and exterior surfaces. Such systems include members with exposed interior or exterior trim attached with clips and all skip/debridged systems".
A coated or tinted glazing with optical properties that is transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.
For operating windows, the operating type has little direct effect on the U-value or SHGC of the unit, but it can have a significant effect on the air leakage and ventilation characteristics. Hinged windows, such as awnings and casements, generally have lower air leakage rates than sliding windows, either horizontal or double hung. A compressive seal and latch increases the effectiveness of the weather stripping in preventing air leakage through hinged windows.
It is better to install efficient windows than to rely on heating, ventilation and air conditioning (HVAC) systems to solve thermal comfort problems. Not only do HVAC systems create non uniform interior conditions, only partly relieving thermal discomfort, but power outages on peak days can lead to extremely uncomfortable conditions.
In very cold climates, look for windows with very low U-values in order to minimise discomfort. If summer discomfort is expected look for very low SHGC values, but bear in mind that lower SHGCs lead to increased comfort in the summer at the expense of less winter solar warmth.
Cooling Climate Zones
The priority in a cooling climate is to keep unwanted heat out of the home. In order of decreasing importance, unwanted heat gain takes three forms; radiant heat gain, conducted heat gain and infiltration of hot outside air.
In a cooling climate (tropical, subtropical or hot arid), coloured red on the map, the best results are obtained from windows that limit solar heat gain on all orientations (low solar heat gain coefficient). Although the first two climates are frequently humid while the last one is not, they can all subject the home to the risk of overheating at any time of the year. Good insulation (a low U-value) is also beneficial, especially if the home is air-conditioned.
When selecting a window product from WERS, look for products with a high cooling Star Rating.
Radiant Heat Gain
Radiant heat gain occurs when solar energy is transmitted through the glass. In addition, some solar energy enters indirectly. This occurs when the glass (and frame) are exposed to sunlight and heat up. Some of this heat is transferred inside by conduction and convection (air currents).
The best way to stop radiant solar heat gain is to shade the glass, but this is not always possible. In addition, shading systems sometimes interfere with the view, reduce natural light, inhibit ventilation or add cost to the window system. Every situation is unique. An alternative is to select a window with a built-in, low solar heat gain coefficient. Under WERS, windows with more than three cooling stars have a low solar heat gain coefficient. A poor cooling star rating tells us that the window, on its own, 'needs help' to reduce solar heat gain. A good cooling rating implies the window reduces solar heat gain very substantially and may need no further shading treatment.
Note that even in a heating climate, it may be desirable to specify a good 'cooling' window if the orientation is to the west and the window is large. However, for other elevations, the guidelines for that climate type should be followed.
Conducted Heat Gain
To reduce unwanted conducted heat gain in a cooling climate, a window with a low U-value is desirable, especially if the house is air-conditioned. The best 'cooling' windows have low U-values as well as low solar heat gain coefficients. If in doubt, look for a low U-value on the WERS certificate. Insulating glass units (IGUs) with some sort of low-e coating and an argon gas fill between the panes give the lowest U-values: as low; as 1.8, compared with at least 5 in the case of a single-glazed clear window. A frame with a low U-value assists in reducing the whole-window U-value. Frames that use thermal breaks or other improvements, or timber or uPVC frames, outperform standard aluminium windows. This performance difference shows up in the WERS stars.
Infiltration Heat Gain
The final way that unwanted heat enters the home is through gaps and cracks around sashes and frames. When a window is shut it should be shut. WERS-rated windows must satisfy Australian Standard AS 2047 for air infiltration performance. Most easily exceed it and achieve air leakage figures below 1 litre per second per square metre of window area. Look for windows with good weather seals. Traditionally, those with 'compression' seals, as fitted to awning and casement windows, tended to have superior long-term infitration performance. However, recent advances in some sliding window seals have reduced the gap.