Most window manufacturers offer several types of glazing. The types vary considerably in insulation value of the window and the likelihood of condensation forming on the glass. Glazings can be sealed (thermal pane) or unsealed. Sealed glazing units eliminate condensation between glazings.
All windows should be at least double-glazed. If you have difficulty determining the number of glazings in a particular window, hold a light next to the glass and count the reflections, which will correspond to the number of glazings.
Air Space Size & Insulation Value
Glass by itself is not a good insulator. However motionless air between panes of glass improves thermal resistance (resistance to heat flow).
The width of air space between two panes of glass influences the insulation value of the glazing.
Insulation values are measured in RSI (metric) and R (imperial): the higher the value, the better the material insulates.
Typical double-glazing with air space width of 12mm (1/2′) gives an insulation value for RSI.35 (R-2). In comparison, double-glazing with a 6mm (1/4′) air space provides only RSU.26 (R-1.5). Avoid glazings with an airspace of less than 10 mm (1/2″), unless the space is filled with argon gas as this enhances the glass to be more energy efficient.
Standard Sealed Units
A standard sealed unit consists of two sheets of glass spaced about 12 mm (1/2″) apart. A desiccant (moisture-removing substance) is included in the edge spacer to absorb moisture and the unit is sealed around the outside edge to prevent air or moisture from entering the air space.
If moist air finds its way into the sealed air space, condensation may form between the glazing. This is usually caused by a faulty sealant and can be corrected only through replacement of the sealed unit, it does not require the whole window being replaced, just the sealed unit.
Before buying, you can determine whether a standard sealed unit is likely to perform well by:
Ensuring that the edge spacer at the perimeter of the glass, or in some cases, the glass itself is etched with IGMAC (Insulated Glass Manufacturers Association of Canada) label. New technology in edge spacers such as Super Edge performs much better and companies using it normally give a better warranty.
Ensure that the air space between glazings is no less than 12mm (1/2″)
Checking the warranty period; Most manufacturers guarantee sealed units for five years, some offer an extended warranty.
Improved Sealed Units
Not long ago the only way to make standard sealed glass more thermal resistant was to upgrade to triple glazing. Now several new technologies offer improved performance. Prices vary, so shop around for the high efficiency products outlined below.
Standard Triple Glazing
Triple glazing increases the insulation value of a sealed unit by adding extra air space. Both air spaces in a standard triple glazed should be 12mm (1/2″) wide.
Low-emissivity (Low-E) coatings increase the insulation value of glazing by reducing radiant heat loss through the glass. They reflect radiant heat emitted by room temperature objects and people. These coatings are invisible (or almost invisible) and a coated double-glazed unit has about the same insulation value as an uncoated triple-glazed unit. The lower the emissivity of a particular unit, the more effective it is at reflecting radiant heat. Glass with Low-E coating should be labeled Low-E2.
The insulation value of a sealed unit is greater if an inert gas (argon or krypton) is used to fill the space between the two panes of glass. Argon is less expensive than krypton but the latter is effective in air spaces as narrow as 1/4″. If you order gas filled units make sure the manufacturers label indicates this.
High Performance Triple Glazing
Standard triple glazing has been largely replaced by Low-E double-glazing. However, extra high performance glazing is possible using Low-E triple glazing and gas fill. Some manufacturers use plastic film with Low-E coating as the middle glazing to reduce the weight of the unit and increase the amount of solar energy that can pass through glazing.
Improved Edge Seals
Aluminum edge spacers conduct heat rapidly and can substantially decrease a window’s insulation value.
Non-conductive spacers such as butyl-metal or insulated spacers reduce heat loss and the potential for perimeter condensation (I.e.: warm edge spacer).
Choosing The Right Glazing
It may not be easy to decide which type of glazing is right for your windows. Improved sealed units will increase comfort and energy efficiency, but it may be many years before their extra cost is repaid through energy savings.
If you are in doubt follow these rules:
Large north facing windows or very exposed windows should have improved sealed units.
If your favorite chair sits in front of a window, consider installing a high performance unit for increased comfort.
Standard double-glazing may provide adequate comfort and energy efficiency in mild climates especially in south facing windows.