The use of various pigments in the glass and oxide coatings on glass surfaces is the most effective way to alter the thermal performance of the glazing system. Tints and reflective coatings on glass are often used for esthetic (for either external appearance or glare control) and architectural reasons in commercial applications, but their primary purpose is to control solar gains, especially in large or high-density commercial buildings where cooling is a concern.
Tinted glass absorbs part of the solar energy that strikes its outer surface (the part that is absorbed gives the glass its colour). This energy is then re-radiated, about half of it to the outside, to reduce the cooling load. Reflective glass uses a metallic coating as a mirror to reflect solar gains away from the building. The trick is finding glass that reduces solar gains without noticeably reducing visible light transmission: otherwise, the view appears grey to the occupants.
Glass with high visible transmission allows the designer to use “daylighting” strategies. Daylighting reduces the use of artificial lighting, which leads to reduced heat gains from the lights, which in turn means a lower cooling load. Thus, proper glass selection can reduce operating costs in commercial buildings, both for lighting and cooling energy consumption.
Any glass that absorbs or reflects part of the solar spectrum (that is, the part that causes solar heat gains) and allows another part (visible light) to pass through is referred to as “spectrally selective glass.” Products of this type are used for solar control in commercial buildings. Other spectrally selective products are used as privacy glass, to block visible light yet allow solar gains to pass through. This is useful in residential applications where solar gains are desirable, but privacy is required, such as in bathrooms or bedrooms.
A common application of glass coating technology uses metallic oxide films to reduce radiant heat transfer across the glazing cavity, thereby reducing radiative heat loss at night and solar heat gains during the day. Layers of different metal oxides can be used to determine which parts of the spectrum will be transmitted through the glass, and which will be reflected or absorbed. These films are referred to as low-emissivity (“low-e”) coatings.
Low-e coatings are the most common method used to control IGU thermal performance in residential and small commercial applications. There are several different types of low-e coatings. “Hard-coats” are durable coatings that are baked into the glass, with emissivities in the range of 0.2–0.6. As hard-coats are quite robust, they can be used on removable storm windows.
“Soft-coats” are sputtered onto the glass by a vapour deposition method. They are actually metal films, not metallic oxides. This means that they will oxidize in the presence of air and are quite fragile (and therefore can only be used inside an IGU).
An oxidized low-e coating will produce a smoky or yellowish tinge to the glass. Softcoats have emissivities in the range of 0.04–0.2 and produce a lower U-value than hard-coats, but also allow less solar gain. Recent advances in coating technology have produced hard-coats with low emissivities (in the 0.17–0.2 range), which combine high durability with low U-factors and reasonably high solar gains.
The location of a low-e coating is also important to the performance of the glazing system. The standard convention for referencing glazing layers and surfaces is to number them from the outside in. Thus, the outermost glass surface that faces the outdoors is always Surface #1. In a doubleglazed window, the surface facing the room is Surface #4 (in a triple-glazed window, it would be Surface #6). Table 4 illustrates the effect of putting a low-e coating on the various surfaces of a double-glazed system.
All glazing systems in the table were evaluated at an interior temperature of 21°C and an outdoor temperature of -18°C, the standard conditions in CSA A440.2. This discussion is specific to glass effects, so the values listed in the table are centre-glass only. A coating on Surface #1 provides little benefit in reducing the U-factor, and is not recommended. A coating on Surface #2 helps to reduce solar gains, and so is useful for non-residential applications, or to address overheating in residences that have large glazed areas.
Putting the low-e coating on Surface #3 is the best option for northern climates, or where the design objective is to minimize the heating load. A low-e coating on Surface #3 of a double-glazed window provides the highest SHGC (most solar gain), and the lowest U-factor (least conductance losses) of all options. Some manufacturers have reported an increase in glass breakage (which they attribute to higher thermal stresses) with a low-e coating on Surface #3, but this is rare.
A low-e coating on Surface #4 reduces the radiative heat transfer between the room and the window, so the glass surface is more like the outdoor temperature. In the case shown in Table 4, this results in low glass temperatures, which lead to condensation and occupant discomfort. Therefore, low-e coatings on the interior glass surface of a window are not recommended.
Some manufacturers offer films intended to be applied to existing windows. Such products are useful in certain retrofit applications (to reduce glare, solar gains, or heat loss), it is much better to choose the correct glazing system in the first place and avoid the additional cost of applied films.
These films can be quite expensive, and must be applied correctly to be of any benefit: their cost is only justified in retrofit situations (and even then, it is often better to replace the IGU with a properly selected glazing system).
The ASHRAE Handbook of Fundamentals contains a reasonably good table of values for typical glazing options in a variety of window frames. It would be appropriate to start with the Handbook to determine the effect of choosing different glass options on the total-window U-factor and SHGC and then refine the choices by talking to specific window manufacturers about the choices they offer. Once the preferred glazing options have been chosen, it is easier to specify those glass choices (and gas fills, glazing spacers, etc.) and verify that those components show up on site in the finished product.
Window installation
According to several CMHC residential field investigations, at least 25 per cent of the problems and damage to moisturesensitive components in exterior wall assemblies are directly attributable to water penetration through windows or windowto- wall interfaces. Although interior moisture sources can account for and contribute to some of the damage in wall assemblies, rain penetration through exterior gaps, inappropriate detailing and poor water management design are the most significant causes of moisture problems in exterior wall assemblies.
A recent CMHC study, Water Penetration Resistance of Windows (2003), showed that leakage paths with the highest risk for damage to moisture-sensitive areas are either through the window (L4) or at the window-wall interface (L5).
Water reaching these hidden areas can go undetected for very long periods with the consequence of extensive damage and costly repairs. Although not considered as high a risk to subsequent damage, leakage paths L1, L2 and L3 (water penetration through the fixed window unit, the operable window unit and the window-wall interface, respectively) that result in water reaching the interior space should not be dismissed either.
They could result in significant damage to interior finishes such as gypsum board, carpets, hardwood flooring, wood sills and trims. They could also affect occupant comfort through poor IAQ, high indoor relative humidities and mold growth.
Fortunately, water that penetrates to the interior space notifies the occupant of a wall problem that requires attention. To manage rainwater entry properly, some very basic principles should be followed in designing the window-to-wall interface.
Each of the following is discussed in further detail:
1. Protect the window rough opening (which will get wet).
2. Provide a water-impermeable sub-sill protection for all window openings.
3. Keep the air-barrier system components at window-wall interfaces tight and dry.
4. Ensure all elements in the wall assembly and window-to-wall interface are shiplapped to shed water to the exterior.
5. Incorporate flashings at head and sill and at other design features to shed water and protect window openings from water ingress.
Window installation must be considered
an integral part of the wall design. The continuity of critical barriers (air, vapour, moisture barrier and water-shedding surface) in the wall design must be carried through to the window and the windowto- wall interface.
Protect the window rough opening Recent laboratory experiments undertaken by CMHC and NRC/IRC, together with previous field observations, have clearly demonstrated that deficiencies in the envelope allow water to reach the underside of windowsill; in some cases, even when there is no wind pushing rain across the assembly. In other words, gravity alone is the driving force.
Codes and standards, including the 2005 NBC and the CSA A440.4 Window Installation Guideline, are moving towards requiring moisture protection (i.e., sub-sill flashings) under the windowsills to catch “incidental” water and to drain this water to the exterior. The upturned membranes extend 150 mm (5.9 in.) high at each jamb.
If wall assemblies incorporate capillary breaks (adequate air gaps) behind the cladding, draining the water from the sill to the wall drainage cavity is acceptable. However, allowing the windowsill to drain incidental water into face-sealed walls or walls with direct-applied claddings is not appropriate. In that case, draining the windowsill directly to the exterior is required. Additional moisture protection will include a sloped sub-sill and a back-dam.
Any incidental water that reaches the windowsill must be directed into a clear flowing drainage exit path or directly to the outside. However, flanged windows (with integral fins) may prevent the free drainage of water from the sill. Drainage can be promoted by furring out the window assembly or removal of the sill flange, if the window warranties are not affected. (Even a small space between the flange and the sheathing membrane/impermeable membrane will allow for drainage.)
Air barrier at window-to-wall Interface Current practices often attempt to manage rainwater entry by airtight and watertight sealing of the interface between the front of the window and the rough opening. This is especially true for flanged windows in lowrise residential construction. In those situations, installers typically apply caulking to the back of the flange around the entire perimeter of the window, place the window into the rough opening, press and secure the flange on the sheathing membrane or sheathing board. A peel-andstick membrane or tape is then installed over the flange to attach it to the sheathing membrane and so provide additional protection against water entry.
CMHC-NRC/IRC laboratory testing has shown that creating the airtightness plane as described above, at a location where it is exposed to water, is not entirely successful in managing rainwater ingress. Similar to face-sealed wall performance, any breaches in the plane of greatest pressure drop will result in water being driven into the assembly.
The testing showed that significantly less water will enter by keeping the airtightness plane continuous, dry and warm (for example,) and by ensuring that the pressure drop across the front interface between the window and the wall is low. For example, the main air barrier is at the back of the window-to-wall interface and there are no seals between the window flange and the wall assembly. It is important that the air barrier at the window-to-wall interface be continuous — the leakier the air barrier, the greater the potential of more water getting in.
Reviewed by Marty Lapedus.
