Beautify your home and save on energy costs.

Terms and Definitions


The rate at which a window, door or skylight conducts non-solar heat flow. This is expressed in units of BTU/hr/sq.ft. The lower the number, the better relative thermal performance of the window. A typical aluminum framed insulated sliding window would have a U-value of .65. A typical vinyl framed insulated sliding window with low-e glass would have a U-value of .35.

Solar heat gain co-efficient (SHGC)

The amount of solar heat that passes through a window divided by that amount of solar heat that is contacting the exterior of the window is its SHGC. Again, the smaller the number the better the performance. A typical aluminum framed insulated sliding window with clear glass would have an SHGC of .60. A typical vinyl framed insulated sliding window with low-e glass would have an SHGC of .29. This rating is particularly important in climates where our energy dollars are spent running air conditioners in the summer months.

Visible light transmittance (VT)

As the name implies VT is the measurement of light that is blocked from entering through the window. This is important because while we want to limit the SHGC to reduce unwanted heat via ultra violet energy, we don’t necessarily want to diminish one of the primary functions of a window …to let in light. Keep in mind, the best window will never be as energy efficient as the wall surface next to it but the wall does a poor job of letting light in and letting us look at the world around us. Thank God for windows!

How do windows work to conserve energy?

Frame materials

Window and door products are comprised of a variety of glass types and frame materials. The predominant frame materials are wood, fiberglass, composites, vinyl, aluminum and steel. Three factors determine frame material selection. Cost to manufacture, durability and thermal performance. To the consumer, you can add the element of perceived value as well for a fourth factor. Manufacturers frequently align themselves with one of the basic frame materials due to the types of processing equipment involved. Vinyl framed windows and doors have experienced the greatest increase in U.S. market share in recent years in all parts of the country and in both new construction and replacement applications. Initially chosen for their improved thermal characteristics, it soon became apparent that Polyvinyl chloride window frames are very durable and require little maintenance other than periodic cleaning. Add to that scratches don’t show, the corners are welded to eliminate potential water leaks and the price point is very affordable, it is easy to see why they are popular.

Glass types

Insulated glass for residential applications has been around for more than forty years now and contributed to incredible amounts of energy conserved in that period. It is virtually universal in application throughout the U.S. and has led to ongoing research into better ways to reduce heat loss / heat gain through windows such as triple and quad pane and suspended films within the unit. There is even research into making the glass surface work like a solar panel and actually produce electricity.

For all the good insulated glass has produced, there was less than adequate information given to consumers about its lifespan. Sealants that bond the two pieces of glass to form an insulated unit will deteriorate over time. This can lead to a cloudy appearance between the panes commonly known as a “moisture failure”. Today’s insulated glass continues to benefit from new and better materials and techniques providing consumers with a greater expectation of trouble free performance. It should be noted that such “moisture failure” problems are confined to the environment between the two panes of glass and cannot introduce water into the home. In fact, the condition has little or no impact on the thermal performance of the window and is really just unsightly and can be repaired by most full-service glass shops.

The biggest improvement to window glass has been the introduction of coated glass about fifteen years ago. This technology works by breaking down molecules of silver and other particles and applying them to one surface of the glass destined to become the inside of an insulated unit. This low emissivity or “Low-e” coating brings spectrally selective properties to the insulated glass, filtering out harmful ultra violet rays from the sun in the summer and re-reflecting desirable infra-red rays back into the home during cold winter months. A truly tangible improvement which significantly improves the thermal performance of windows and doors.

What’s inside?

One of the most common mis-understandings about insulated glass is that there is either a vacuum between the glass or that the unit is pressurized. The truth is that most insulated glass has the ambient air that was present in the factory on the day it was assembled and that “dead air space” serves to insulate us from temperature extremes. There are two inert gases that can be injected to displace the air in the manufacturing process, argon and krypton. The problem is that they add little to the overall thermal properties of the unit and it can be difficult to verify their presence. Consequently, gas-filling is more of a feel-good feature with minimal impact on a homeowner’s energy consumption.