Windows

Why pay attention to something you just look right through? Windows can represent a significant problem with insulation, as most are thin and drafty. Green building incorporates high insulation, double paned, low-e, glazed windows.

Double paned means the window has two sets of glass, which creates an insulating space in between. This space is often filled with an inert dense gas, like argon or krypton, which increases insulation value. Think of it like covering up with a thick quilt instead of a thin sheet. Low- emission (low-e) coatings are fine metal oxides applied to a window’s outer surface, this coating allows visible light to enter and exit, but blocks the vast majority of infrared light (heat) from entering or exiting. This means that a low-e coated window will block heat from entering in the summer and will trap heat inside during the winter.

What to Look For in Energy Efficient Windows

Whole House System: Windows play an important role in the function of a home as a system. To take advantage of passive solar design, large windows are placed on the south side of a home while smaller windows are on the north side. This allows the heat from the low laying winter sun to enter the home from the south, and prevents heat from easily escaping from the shadier north side. Also, highly insulated windows help to keep heat from escaping and reduce a home’s energy requirements.

Glass:The glass in an efficient window is covered with a low-e coating. These millions of tiny metal slivers implanted on the surface of the glass are invisible but make the window much more thermally effective. The coating allows visible light to enter without any impediment, but it blocks 90% infrared light, infrared light is better known as heat. By blocking heat the window keeps the home cool in the summer and warm in the winter. It does this by blocking the sun’s infrared light from entering home during summer. In the winter, the heat you are producing in your home is reflected back inside; reducing the amount of time you need to run your furnace and saving money.

Construction: Efficient windows have two panes of glass. The space between the two panes of glass is filled with a high density gas. This creates a barrier between the inside temperature and the outside. Some windows also have a thermal break in the frame, an air space in the middle of the frame similar to the space between the glass panes.

Window Facts Guide

The National Fenestration Rating Council (NFRC) provides objective data to help consumers make wise choices. How do you read this information so that you can make the best choice for your home? The U-Value measures the amount of heat that passes through a window in an hour for every degree of difference in outside temperature; the lower the U-Value the better its insulation value. The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat caused by sunlight; the lower the SHGC the better it is at blocking heat from sunlight. Visible transmittance measures the amount of light that can pass through the window; a higher number means more light goes through. Air leakage measure the amount of air that can leak through the window, the lower the number the better its insulation value.

More info at: The National Fenestration Rating Council website