Building Envelope  »  Fenestration

The thermal performance of windows has improved enormously through the use of multiple glazing layers, low-conductivity gases (argon in particular) between glazing layers, low-emissivity coatings on one or more glazing surfaces and the use of framing materials with much lower conductivity. Operable windows are available with heat flows that have only 25-35% of the heat loss of standard non-coated double-glazed (15-20% of single-glazed) windows (Smith, 2005). In recent years the performance of glazing design has been improved from single glazing with U-value as high as 5.6 W/m²K (undesirable), to triple or even quadruple glazing with special treatment, achieving U-values as low as 0.4 W/m²K (desirable). However, the overall window U-value depends on the window frame and thermal bridging and thus reduces the glazing U-value by a marginal factor in highly insulated frames and by considerable factor in poorly insulated metal or wooden frames.

Typical U-Values of glazing

Windows frames most often have the lowest U-Value of a window and can have a large impact on the insulating properties of the window. Care must be taken to use good insulating materials and to reduce thermal bridging especially in metal frames and in advanced window systems. Thermal breaks can be used in windows frames to reduce the thermal bridging through the frame. In addition to this window frames should be constructed so as minimise infiltration. This should be done both between the frames themselves and between the frame and the wall. The optimum window to wall ratio varies according to location climate the purpose of the window whether for ventilation lighting or both and the use of the room. Sealed windows with at least double-glazing also help to reduce sound transmission from outside. Windows should be installed in the centre of the insulation.

The glazing type of the windows is also important in terms of solar heat gains through the glass. Important characteristic of glazing that determines its effectiveness is a factor called “shading coefficient (SC)”. Shading coefficient is the value of the amount of direct solar radiation passing through a particular glazing divided by the direct solar radiation passing through a clear float glass. SC for a glazing is between 0 to 1. However, SC is giving way to a new factor which is referred to as ‘Solar Heat Gain Coefficient (SHGC)’ (also referred to as g-value in Europe). SHGC, unlike SC, takes in to account the amount of solar radiation passing though an entire window system including its frame. SHGC is roughly given as 0.87 multiplied by SC. Lower SHGC value represents a better performing glazing system. Higher values of SHGC could potentially contain tint to cut down on the transmitted radiation, which could potentially reduce the visual light transmittance.

Visual light transmittance is a measure of the amount of daylight that passes through the glazing of a window. A window with a higher VLT transmits more visible light and leads to a reduction of the need for artificial lighting. In all cases good day lighting or Visual light transmittance (VLT) should be guaranteed. Care must be taken, though, not to produce glare inside the internal spaces. Optimising the balance between window U-value, solar heat gain coefficient and visual light transmittance is therefore, important in order to find the right solution for the prevailing climatic conditions in each case.

Tinted glazing, formerly used to reduce the solar heat gain in buildings, also reduces the amount of visible light that enters the building. The energy saved in this way by the air-conditioning units is replaced by the additional need for artificial lighting. Tinted glazing is therefore not highly recommended.

The SHGC can be changed by toning the glazing or using reflective deposits on the glazing surface. Certain coatings can also allow high levels of visible light into a building and at the same time reduce the long-wave radiation losses through the window.

Light to solar gain (LSG) ratio is an important determinant of the window quality. It is usually expressed at the ratio of the Solar Heat Gain Coefficient (SHGC) to the Visual Light Transmittance (VLT) both of which can take values between 0 and 1. Higher values of the LSG (SHGC/VLT) ratio indicate higher visible light transmittance at lower heat gains.

Windows frames most often have the lowest U-Value of a window and can have a large impact on the insulating properties of the window. Care must be taken to use good insulating materials and to reduce thermal bridging especially in metal frames and in advanced window systems. Thermal breaks can be used in windows frames to reduce the thermal bridging through the frame. In addition to this window frames should be constructed so as minimise infiltration. This should be done both between the frames themselves and between the frame and the wall.

In hot climates, glazing should be specified to reduce the heat gain, both by direct solar transmission and by conduction. High performance glazing with a low U-value, low shading coefficient and high visual light transmittance should be selected. By keeping the daylight transmission high and the solar heat gain low, adequate daylight will be admitted while still blocking significant quantities of solar radiant heat.

In order to achieve a reduction in heat transfer by conduction, a double glazed unit with a reflective outer layer can be combined with a low emissivity (Low E) coated inner layer to reflect the heat that is transmitted, outwards. Glazing that reflects and absorbs a large fraction of the incidental solar radiation reduces solar heat gain by up to 75%. This leads to a significant reduction in cooling load and presents a cost-effective way to improve the building’s energy performance.

In temperate and cool climates, solar heat gain is welcome during the cooler seasons as well as low U-values and high levels of daylight. Because low U-values slightly decrease the scope for solar heat gain, a balance needs to be struck between solar heat gain potential and heat losses at night.

Generally speaking, a low U-value with a lower SHGC is recommended for glazing on the side of a building facing away from the sun (south in the southern hemisphere and north in the northern hemisphere). This is due to a much lower solar gain potential on that side.

Windows should be at least double glazed for temperate climates and triple glazed for cold climates. Windows should be kept to the minimum and facing south. Windows should have a positive net solar gain. On all other facades, the solar gain that is achievable with a lower U-value needs to be checked against the higher heat losses at night. A balance needs to be found between the two extremes. A higher solar heat gain coefficient is useful when higher solar gains are desirable and sufficient thermal mass is available to store it.

For Passive Houses in cool climates a U-value of 0.7 W/m²K is required (Stockholm and Helsinki). For a Passive House in Rome, the requirement is very much lower at 1.4 W/m²K (Harvey, 2006). In a built example in Brampton, Canada, the added cost for advanced triple glazed, low-e, argon filled windows was about 30%, with a simple payback period of just 3-6 years (Harvey, 2006).