A balancing act
Windows have so many jobs to perform that their importance cannot be understated. They contribute to the house external aesthetic, bring daylight, view and passive solar gains to the interior, provide ventilation and passage (in the case of glazed doors), all the while doing their best at keeping the noise and the weather out (read cold, heat and drafts) when they are shut. Windows are well loved and if it wasn’t for this last reservation about excluding the weather, they could be used without worrying much about their size, location and type – apart from privacy and budget considerations that is. But even the best windows are a thermally weak building component, and one needs to exercise caution when it comes to their location and design to maximize their great benefits and minimize their thermal disadvantages.
How thermally weak are windows?
A well insulated wall of standard construction boasts an R value of 2 or 2.5 and you would expect windows and glazed doors, which are a component of the wall, to not unduly reduce its energy efficiency. Not so. The weakest window of all is the one all other (better) windows are compared to. That basic window is of the most ubiquitous metal-framed single-glazed type and its R value is somewhere between 0.20 and 0.25. A tenth of a well insulated wall. A hole in the wall fitted with a good flywire screen is not much worse and is much cheaper to boot! Now consider the best window on the market: insulating frame, double glazing with the right air gap and Argon gas filling. This window’s R value is 0.7. Indeed, it performs three times better than the base window but it is still three times weaker than the wall it’s installed in. Because of this, you can be sure that homes that feature large areas of glass in all directions pay dearly for this in terms of comfort and energy use.
Windows like north
The smartest way to redeem windows’ thermal weakness is to locate them to the north of the building, as they will allow winter heat gains which more than make up for the heat loss they themselves create. North is indeed the only orientation where they will be actively contributing to the thermal performance of the building year round, rather than just detracting from it. The low wintertime sun only has warmth when it faces north (roughly between the hours of 11am to 3pm), which north-facing windows will harvest very obligingly. The summertime sun on the other hand will be vicious to east and west facing windows – which are otherwise not hit by the sun in winter. Only locate windows to face those directions when there is no alternative and keep them to a reasonable size.
A word of caution: whichever state Australians live in, they seem to believe they are in the tropics – there certainly is some truth to that if you compare our climate with that of England… The facts don’t stack up with passive solar design however! So please do not stick a verandah in front of your north-facing windows unless you truly live north of the Tropic of Capricorn. Everyone else lives in a so-called heating climate and needs to use temporary shading for those three to four months of the year where you’d rather keep the sun out.
Locating windows facing East, West or South for ventilation.
While these windows should be kept small to medium size for the reasons explained above, they can play a useful role particularly in summer where passive ventilation is desirable. South-facing windows don’t need to be large to bring cooling breezes in and where loosely aligned with north-facing windows to let them out, will create a most pleasant cross-ventilation in summer. Windows facing east or west can also contribute to passive ventilation if they are of the casement type which is hinged on the side and open out like a door. Locate the hinges on the north side and the cool wind will be funnelled by the open pane of glass into the room.
Casements, awnings, double-hung, sliding, frameless, fixed? Effective ventilation and draft proofing are the key factors to consider here. Awnings are most common in Australia – they are hinged at the top – and while they can be effectively weather-stripped, they are poor ventilators. Only consider when the window is very wide and squat (ribbon type) and you use hardware such as Truth which allows you to swing the sash right out. Anything sliding such as sliding, double-hung and frameless windows offers decent ventilation but will be difficult to draft proof, as the sash cannot be tightly held against the frame when the window is shut. These have their place in situations where an outdoor passage is needed in the vicinity of a window and you don’t want an open sash in the way. Double-hung windows may also be the preferred ones for heritage homes to preserve the period feel. Fixed windows are usually draft proof enough, but obviously are used for light and view rather than ventilation. Casements are hinged on the side and are a winner in many cases. Team them with rubber strips, sash locks and brackets winders from companies such as Truth hardware and you have a window that ventilates well and is well sealed too.
Metal windows are cheap and low-maintenance but conduct heat and unless they are so-called “thermally-broken”, will not be energy-efficient. Thermally-broken windows are rare and far apart on the market and will only be available in limited types and dimensions. While common in Europe, PVC windows are relatively new on the Australian market. They are maintenance-free and attractively priced while offering good seals and effective insulation. The material itself is controversial however and a quick online search will let you know that both its manufacture and disposal by incineration release significant quantities of dioxin, a human carcinogenic, while its use at home exposes you to toxic phtalates. Timber windows are great insulators but require some maintenance, and you also need to be aware of conservation issues. Stay away from south-east Asian timbers (Meranti, Merbau) and stick to Australian hardwoods, selecting timbers with the Forest Stewardship Council (FSC) label where available. Miglas and Paarhammer Windows are great Victorian manufacturers which supply Australia-wide. If you find these options too onerous, employ a local joiner who uses 40mm thick timber sections of kiln-dried hardwood (KDHW), and is willing to use double-glazing and effective weather stripping. The performance of all branded windows can be checked online on the Window Energy Rating Scheme (WERS) website.
Double-glazing is the best glazing by far for all climatic zones, as it is the best at keeping the indoor climate separate from the indoor one. Be sure that the gap between the two sheets of glass is 12-16mm, as anything outside of this range spells reduced performance. Filling the gap with Argon gas increases the glass performance by another 1-15% which is well worth it. Some manufacturers such as Paarhammer Windows offer triple-glazing, a sound idea in the cooler states, especially for clerestory-type windows located at ceiling level – more heat is lost near ceilings.
If you live in the tropics, you can specify a low-e coating for all your windows. It is a transparent metallic finish which reflects solar radiation and reduces the Solar Heat Gain Coefficient (SHGC). In a heating climate however, do not use this treatment for north-facing windows or you might unduly reduce solar heat gains in winter. Note that south-facing windows don’t generally receive enough radiation in summer to warrant a low-e coating, unless they face south-west rather than true south.
Short of changing your windows, you can consider getting some rubber strips to draft proof them and having some secondary acrylic glazing installed by companies such as Magnetite, ecoMaster or MagicSeal. A less durable but inexpensive and effective system is Clearview, a DIY product consisting of a clear plastic sheet heat-shrunk to fit on the window frame. Coloured films much like that of car windows can also be applied to reduce glare and excessive heat gains where present in warmer climates.