Air Leakage measures the rate at which air passes through joints in the window, in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Better constructed windows exhibit lower AL values. Most industry standards and building codes require an AL of 0.3 cf/m/ft².
Two other properties characterize the performance of windows. These properties do not affect the energy usage or thermal comfort of the home very much but have other implications.
Visual Transmissivity (VT) that measures the proportion of light that passes through the glass. The choice depends on the amount of light that is desired in a room, based on the design and purpose of the room, number of windows and the direction that the windows are facing.
Condensation Resistance (CR), that measures how easily condensation can occur. Frequent condensation build-up can wet the walls and the window frames themselves if they are made of wood, leading to mold growth. CR is rated from 0 to 100, higher means less likely for condensation to take place. CR depends on a combination of the frame material property and the assembly of the window.
Frames
Many difference types of frames are available these days, including wood, vinyl, aluminum and fiberglass. They vary in looks, durability, thermal conductivity and price. These are summarized in the table below.
Some window frames are made of a combination of the above materials. For instance, there are aluminum clad wood windows, which requires no maintenance on the exterior and provides the equivalent of thermal breaking between the aluminum and the interior. Some manufacturers also offer fiber glass exterior and wood interior, providing the maintenance-free exterior and the classic wood look in the interior. These products give the consumer more choices to fit their preference and life style.
