Explaining Windows

Explaining Windows

A window can be broadly defined as an opening in the wall of a building, typically providing light and ventilation. Various window types are available with most containing certain common components.
- The pieces of glass are called sealed units.
- The sealed units are held in a sash that may move as the window is opened
- The sides of the sash are called the stiles.
- The top and bottom pieces are the rails.
- When the window within the sash is divided into several small panes, the dividing pieces are  muntins.
- The sides of the window frame are the jambs, the sill is the bottom assembly of the frame and the head is the top.
- The casing or trim (exterior and interior) covers the frame edge to finish the window edges in relation to wall finishes.

Conventional window glass is 3/32 inch to 1/8 inch with thicker glass used where increased thermal insulating value is desired.  Glass can be strengthened by tempering, tinted to reduce glare or absorb heat, coated to reflect heat, installed with wire to provide additional fire resistance or laminated (made up of two or more layers of glass and a plastic film between the layers) to improve strength, safety and/or sound insulation.


The term glazing refers to the act of furnishing and/or fitting panes or sheets of glass as in the case of windows and doors.  Prior to 1950, all windows were single glazed (one pane of glass). The insulation value of a single conventional pane of glass is approximately R-1. Presently, double glazing is either factory sealed or vented.  The factory sealed, double glazing is designed to have no air infiltration or exfiltration between the two panes.  Vented double glazing allows for outside air movement to the space between the two panes.

Optimum air space between the panes is widely debated but is usually considered to roughly to ¾ inch.
Typical double glazed windows have an approximate R-value of 2.  The triple glazed window provides two air spaces between three panes of glass with an R-3 or slightly higher insulating value.

Window Standards & Efficiency Ratings

Windows can contribute up to 25% or overall heat loss in a residential structure and consequently, any strategy to improve energy efficiency must closely address this issue. Significantly improved technologies during the past few years have resulted in appreciable gains in overall efficiency and durability.  Windows are primarily rated according to the following two standards.

CSA Standards (CSA-A440)
CSA standards provide for window rating using three primary criteria:

Air Tightness
(A Criteria)

Air tightness refers to leakage based on a 40 km/h (25 mph) wind.  A1 represents the maximum acceptable standard, with A2 being 60% of A1 and A3 being 20% or A1.

Water Tightness
(B Criteria)

Water tightness is a tes involving a constant stream of water against the window plus varying levels of wind load. B1 represents 57 km/h (35 mph) wind load with progressive levels up to B7 which represents 123 km/h (76 mph).

Wind Resistance
(C Criteria)

Wind load and blow out ratings range from C1 to C5.  In the case of wind load deflection, 104 km/h (64 mph) is C1 with highest load (C5) at 329 km/h (204 mph).

The CSA standard also addresses condensation resistance, forced entry resistance and ease of operation.  In addition, components related to the window system (i.e. screens) are tested.

Efficiency Rating (ER)

The ER represents the heating required for a window during the heating season based on the combined analysis of solar heat gain, heat loss (through frames, spacers and glass) and air leakage.  The efficiency rating measure thermal performance in which a positive number indicates that the window generates more solar heat gain than heat loss.  A negative number indicates that more heat is lost than gained.  Value ranges are from a low of -80 (very poor) to +12 (excellent).  To place the ratings in perspective, a rating of -8 to +12 would roughly correspond to an R-value of 5 and a window between -15 and +5 would represent an R-value of 4.

Efficiency with Window Features

Low-e (low emissivity) refers to the ability of a surface to reflect long wave radiation.  Low emissivity glass contains a thin metallic layer that allows sunlight into the structure during winter months.  This layer retards the outward flow of internally generated furnace heat.  |In summer, this same glazing allows sunlight in to rooms, but significantly reduces unwanted heat (long wave radiation), that is absorbed by reflective surfaces i.e. driveways, adjacent structures, decks, sidewalks and other materials near the building and radiated upwards to windows.

As a result, a significant portion of this heat fails to enter the building, which typically translates into lower cooling costs.  New Low E coatings can now limit both short wave and long wave infrared solar radiation, as well as ultraviolet rays than can damage draperies, furnishings, wood floors and carpeting.


U-value is a measure of heat flow through an object (i.e. Window glazing), often referred to as the heat transfer co efficient. The U-value is the reciprocal of the R-value commonly associated with insulation; i.e., if the U value decreases, the R-value increases. U value is a component in establishing the efficiency rating (ER) of windows.

Inert Gas Fill

Inert gases such as argon and krypton are currently used as fill between glazings given their ability to reduce window heat transfer and cold spots at the window base between the panes.  Both inert gas fills are non-toxic and colourless.  Argon is most common, given the more expensive krypton option.

Other considerations

High efficiency windows also boast other features; i.e. double and triple glazing and low conductivity spacers between window panes. However, the single most important factor to consider involves installation.  Window system efficiency hinges on how well windows are fitted and insulated.  The best ER value can be effectively nullified without prudent attention to installation details.
Homeowners should also carefully review the merits or partial replacement (retrofit of window components) versus a full tear out. While costs associated with a tear out and full replacement using high performance windows can be substantially higher, long term benefits in terms of comfort, energy efficiency and durability are usually realized.