understanding windows and energy efficiency

Understanding Windows And Energy Efficiency


PART IVerbalizing TheQuestion


Part I Verbalizing The Question

Studies show that windows account for anywhere between 15 – 20% of a home’s heat loss to over 30%, depending on the source and the data modeling being used. Despite the discrepancy in the figures, it is clear that windows are a significant source of potential loss of energy efficiency in our homes. Before an actual discussion of how windows affect your energy efficiency, however, it is important to discuss the terms that will be used to discuss the features that make a window energy efficient, or not, as the case may be.

An item’s U-value is the measure of how much heat the item allows to flow through it; specifically, it is the rate of heat transmission through a square foot of the material in question when there is a degree of difference in the temperature of the air on either side. The lower the U-factor, the less heat it allows to escape, and the more energy efficient the item is.

Directly related is an item’s R-value; where the U-value measures the amount of heat transmission through an object, the R-value measures an object’s resistance to heat flow. The higher the R-value, the better an insulator the item is.


Part I Verbalizing The Question

A window’s Solar Heat Gain Coefficient, or SHGC, refers to the amount of radiant heat the window admits in the form of solar radiation. Given as a value between zero and one, the lower the SHGC, the less heat a window structure allows through.

Except for the air leakage value, all of the above values are typically listed on a window; if either U-value or R-value is not included, they can easily be calculated, as one is the inverse of the other.

Air leakageis mostly self-explanatory; it is the measure, expressed in cubic feet per square foot, of the air that passes through a window’s structure. The lower the value, the less air leaks through, and thus the lower heat loss or gain via heated or cooled air through the window.

Visible Transmittance, or VT, measures how much sunlight passes through a window. Again given as a value between 0 and 1, the higher the value, the more sunlight the window allows to shine through.


PART 2Framing theDiscussion


Part 2 Framing the Discussion

Much of a window’s energy efficiency is determined by the material that a window’s frame is made of. Except for the pane itself, all of a window’s other parts are generally made from the same material, from jambs to sill to rails to stiles. Because of this, the material’s particular energy efficient aspects dictate much of the same for a window.

Aluminum, being a metal is largely, not very energy efficient. It gains and loses heat readily (low R-value), and thus contributes a lot to heat loss (during the cold) and heat gain (during the summer). Aluminum is, however, generally the least expensive window material option, and in some cases is more practical than others, especially in places where the weather poses severe impact stress on windows.

As the other less expensive option, much is touted about vinyl’s cost-effectiveness in addition to its good R-value. Generally, good, “tight” construction also means that vinyl generally has a low amount of air leakage.

Wood is by far the most expensive of the different window materials. It is, however, a better insulator than even vinyl. The spacing between the panes in a wooden frame tend to be set farther apart than vinyl or aluminum, which provides a better buffer against solar heat gain.



One of the most energy efficient materials is Fibrex. It blocks thermal transfer almost 700 times better than aluminum helping to reduce heating and cooling bills. Made with wood fiber and thermoplastic polymer it is twice as strong as vinyl, allowing windows to be made with less bulky frames and is virtually maintenance free. Fibrex is available in a wide range colors and contours, creating unique styles including real wood interior options.



Aside from considerations of material, the style or design that a window is made in also affects its energy efficiency.

Double-hung windows are, by far, the most common window style in the United States. While generally efficient, there is a lot of potential air leakage in the design, especially if the window has aged and is no longer properly operable.

Casement windows are generally more energy-efficient than double-hung, and are actually even more efficient when the wind is blowing hard, sealing the window tighter.

Their construction requires regular maintenance to maintain the level of energy efficiency, however.

Picture windows are generally energy-efficient, especially since they don’t open at all. The relative energy efficiency of picture windows, however, is affected in large part by the structure of the windowpanes, though this applies to other window styles as well.


PART 3Focusing OnThe Glass


Part 3 Focusing On The Glass

The last, and potentially most important, item that determines a window’s relative energy efficiency is that of the windowpane itself. That is not to say that the panes alone determine all of a window’s energy efficiency. As shown previously, it is how all parts interact as a whole that dictates just how energy efficient a window setup actually is.

Low Emissivity Glass – commonly referred to as “Low-E,” this is glass that has been treated with an invisible metal oxide coating that reflects both infrared and ultraviolet light, lowering the window’s solar heat gain coefficient while generally not affecting visible light transmittance. This is important because if a window limits the amount of visible light coming in, any savings due to reduced heating or cooling is offset, at least in part, by the increase in energy expenditure for lighting.


Part 3 Focusing On The Glass

While older houses generally feature windows with only a single pane of glass, modern windows usually come with double- and even triple-pane options. The idea behind

this is that the air between the multiple panes of glass forms an insulating layer in the window, reducing heat loss through the window, and thus your home. Naturally, triple pane windows will have a lower U-value than double-pane windows, but are proportionally more expensive.

Not content to rely solely on “normal” air to act as a heat loss buffer between windowpanes, glass manufacturers have found that inserting certain clear, inert gases in between the glass panes has a positive effect on a window’s relative energy efficiency. The two most common gases used for this purpose are krypton and argon. Argon is, by far, the more common of the two, and the less expensive. Krypton, for all its expense, is significantly more effective than argon in reducing the amount of heat lost through the windowpane.

No single feature, ultimately, determines a window’s particular energy efficiency rating. Rather, it is the different combinations of features that make up the entire window structure that determines that. It’s the different interactions between these that make all the difference between a good window, energy-efficiency wise, and a truly great, energy-efficient window.

DIAL ONE WINDOWS23230 Del Lago Drive Laguna Hills, CA 92653

(631) 247-9597www.DialOneWindows.com

http://www.DialOneWindows.com

understanding windows and energy efficiency

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