SAB HOMES 2 | Windows in a nutshell
A primer to help choose what to buy
by Hugh Perry and Jim Taggart
In our very seasonal Canadian climate, home designers, builders and homeowners have to balance the desire for large windows that give style, views and natural light with the need to conserve energy. This article reviews the essential things to consider in choosing and sizing windows for a new home, or replacing old windows.
GLAZING
Double glazing has been a familiar feature of North American homes for more than 50 years, but recent rapid advances in glazing technology now offer builders and homeowners a wide choice of windows that vary greatly in quality, price and performance.
Thermal Performance
The most common reason for choosing double, triple or even quadruple pane windows is for enhanced thermal performance, which saves energy and ultimately money. In Canada, the most widespread system for measuring the thermal performance of windows is the Energy Rating [ER] value used by the federal government’s Energy Star program.
The ER value measures the thermal resistance of the whole window assembly [not simply the glazing unit], and therefore takes into consideration many variables including frame design, air leakage and edge spacer type. The most basic Energy Star-rated window is a double pane unit with a heat reflecting coating on one pane that has an ER of 27. This is equivalent to an R-value of 2.85.
The best windows currently on the market have an ER value in excess of 100 – equivalent to an R-value of about 10.5. While these high performance windows may be twice the price of the basic ones, they save about four times as much energy.
Energy Star prescribes minimum levels of performance for each of Canada’s climate zones. The climate zones for which a window, door or skylight is ENERGY STAR qualified will be shown on the product label, or ask the manufacturer.
It is worth noting that the Argon gas used in most multi-pane windows, works best with air spaces between 1/2” and 3/4”. Smaller spaces – most common in triple and quadruple-pane windows – should be filled with Krypton which is more expensive, but a better insulator.
Also, decorative window bars or shading devices that divide large windows into smaller panes lower the overall thermal performance of the unit. Operable windows should only be used where needed, as they are less efficient than fixed windows of equivalent specification. Hinged windows are more energy efficient and more durable than sliding ones.
REFLECTIVE COATINGS
Factory-applied to the glass surface, reflective coatings are designed to do one of two things: either reflect exterior solar heat in summer to keep the building interior cool; or reflect internal heat back into the building to keep it warm in winter. If only one coating is used, its placement will depend on whether summer or winter performance is more critical. The recommended coatings for shaded and unshaded windows with different orientations are shown in the Table [Guideline for window exposure] in the thumbnails images.
Low Emissivity [low-E]
Low-E coatings are applied to the interior surface of one or more panes of glass and usually take the form of microscopically thin metal foil. The efficiency of these reflective coatings, referred to as the Solar Heat Gain Coefficient [SHGC] will vary according to latitude and window orientation.
Heat Mirror
A European adaptation of triple-paned windows that use only two panes with a free standing film between them, thus creating two air spaces similar to triple-pane windows. According to the orientation of the film, it will either reflect exterior heat or reduce heat loss from the interior.
UV Blocking
Ultra-violet light [UV] blocking protects valuables like furnishings. A standard low-E coating will block about 80% of UV rays, while Heat Mirror blocks 99.5%.
Manufacturers promote their products under various names, such as High Performance, Insulated Low-E, Eco-Insulated, Sun Guard, etc.
FRAME TYPES
There is a variety of frame materials on the market, and for residential applications vinyl, wood and fiberglass predominate. Aluminum is both durable and relatively inexpensive, but should only be considered in moderate climates, and then only if the frame is thermally broken and its thermal performance can be verified.
Vinyl
Low initial cost, is durable and virtually maintenance free – but has serious drawbacks when the life cycle of the material is taken into consideration. Many environmental agencies have identified vinyl as a ‘red list’ material – one that is to be avoided because of the toxic by-products of the manufacturing process.
Fiberglass
Can be filled with laser-cut rigid insulation and can be painted on site. The Consumer’s Guide published by the Office of Energy Efficiency, Natural Resources Canada states that foam-filled fiberglass frames have a higher insulation value than any other frame type.
Wood
A 4” thick softwood frame will have an R-value of about 5.6 – similar to that of mid-range Energy Star window assemblies. With its other environmental advantages, this makes wood a suitable material for frames if issues of durability can be addressed. Traditionally wood frames are thoroughly primed and paint-ed prior to installation, and require regular maintenance to ensure long life.
Nowadays, some manufacturers offer wood frames clad in aluminum. This aluminum typically has about 30 to 35% recycled content. Emerging technologies include frames that combine plastic with recycled wood or wood fibre.
Glass
Construction glass that is collected at depots is generally reprocessed into products other than windows. However, some manufacturers do offer window glass with recycled content – usually around 10%. The low-E and other coatings used on windows contaminate the glass preventing it from being reused for clear glass applications.
DURABILITY
Energy Star focuses on energy performance, which of course depends to some extent on quality materials and construction. However, builders and homeowners will need to verify the reputation of any given product, one measure being the manufacturer’s warranty which may vary from 5 to 10 years.
The most common cause of failure is the breakdown of the seal between the panes of glass. This enables humidity to enter the unit and condensation to occur. While it somewhat reduces the thermal efficiency of the unit some companies remedy this problem in-situ by drilling small holes in the inner pane to permit ventilation of the cavity. This can end up being a green solution by extending the life of the unit.
Product development is ongoing and is now driven by the industry itself, as manufacturers strive to produce more energy-efficient lines, and to reduce the environmental impact of manufacturing processes for both their employees and end users. We’ll re-visit this topic in future issues of SAB Homes.
Hugh Perry and Jim Taggart are associate editor and editor, respectively, of SAB Homes
