Windows and Draftproofing

April 9th 2002

Is it time to do something about your windows? This is almost always a job for a qualified contractor with the proper equipment. Still the choice of what to do and what to buy remains your own, and you’ll quickly learn that there is more to windows than glass. With recent improvements in window technology, like the advent of low-emissive coatings, there is more to glass than just glass.

Always consider upgrading your windows before you consider the more expensive task of window replacement. Start by doing a thorough caulking job around the window trim and replacing worn out weather-stripping. This can cut down on uncomfortable drafts that rob your home of heat. Next you could have interior or exterior storm windows installed. The extra layer of glazing would help reduce air leakage and heat loss.

However, if your present frames are damaged or rotting, window replacement may be the only option. When shopping for windows, ask the following questions:

Do the frames take account of thermal bridging?
Will long-term maintenance be easy and cheap?
How much does glazing enhancement like “low-e” and argon gas glass improve performance?
Are the tested air leakage rates comparable?
Do the windows conform to accepted industry standards?
Are the windows rated Power Smart?


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.

Ensuring 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.

Glass Coatings

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-E”.

Gas Fill

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.

Frame Materials

Since the frame can account for as much as 25% of the total window area, it should be at least as well insulated as the glass. Several different frame types are available.

Solid Wood and Clad Wood

A solid wood frame is a good choice from an energy standpoint. The colour choices are unlimited but wood does require regular re-painting inside and outside.

Clad wood frames are protected on the exterior with a covered of prepainted aluminum or vinyl. Clad frames are more expensive than plain wood but eliminate exterior painting. Claddings must be well designed to prevent water from becoming trapped behind them.


Aluminum frames are durable and good designs are available for residential windows, aluminum conducts heat rapidly. To prevent condensation and frost from forming on the frame, the frame and sash must be equipped with well-designed thermal breaks.

It is difficult to judge whether an aluminum frame is equipped with a good thermal break. One thing you can do is ask people who have lived with the brand you are considering whether they have experienced any condensation problems.

If a metal window frame has a CCMC number,it has passed the CSA A-440 standard test for condensation resistance. Be particularly wary of inexpensive aluminum replacement windows.


Vinyl frames provide good insulation and do not require painting. However, some manufacturers reinforce the vinyl with metal, which can decrease the frames insulation value. Wood reinforcing is preferable.


Fiberglass window frames are a recent innovation available from a limited number of custom manufacturers. Fibreglass frames will hold their shape permanently and will not warp, shrink, rot, bow, dent or twist.

Most Common Q & A’S

Do you have insulated pane windows that fog up between the glass panes when the sun comes up in the morning?

Insulated glass that fogs up is an indication of glass failure. Over the years energy efficient glass has been perfected to compensate for nature’s elements such as hot and cold temperatures and direct sun and wind. Insul glass is made up of two pieces of glass sealed together with 0% relative humidity in-between giving clear vision with no fog during the cold weather. However, with aging some insulated glass will fail at the seal because of severe sun exposure. For example on a south elevation, continuous expansion and contraction takes place fracturing the seal and allowing moist air to get between the glass panes. During direct morning sunshine, this moisture shows on the outside glass panel leaving a foggy film, which is impossible to clean, therefore, replacement glass is the only answer when this happens.

If you are going to the expense and trouble of replacing windows, make energy efficiency one of your most important considerations.

A lot of energy dollars are blowing away in the wind. For savings and comfort, draft proofing is a good idea. Adequate air is required for combustions appliances and breathing, and to vent odours and moisture to the outdoors, but you don’t have to live in a sieve. Take charge by sealing off excessive air leakage, then provide adequate, minimum, controlled ventilation.

Start with the windows and doors. The right caulking material, properly applied can block drafts around trim. It may also be time to replace weather-stripping. Know which products can do the best job in each situation.

There are some major air leakage areas in the basement. After construction the sill plate can shrink away from the concrete foundation and leave you with a wind chill factor. Leakage in the header area wastes money and can make the floors upstairs chilly.

Let’s not forget the attic. Care should be taken to seal around all the openings in the attic floor, such as ceiling light fixtures, plumbing stacks, exhaust fans and chimney chases. Otherwise warm moist air can escape into the attic, where it can condense and cause moisture damage. Before leaving the attic remember to weather-strip the hatch and latch it snugly.

Extreme air sealing will make power ventilating a necessity. Care should be taken to enable all power exhaust systems to have adequate replacement air inlets.