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 The R-value measures resistance to heat flow, or heat loss, and is sometimes described as thermal resistance. The R-value is a measure of conductivity. If you're shopping for cooking pans, you want high conductivity – but in windows it's not so desirable, because it means that the heat you're paying for (and sometimes very dearly!) is escaping. Low conductivity, or high resistance, is what the R-value measures.

The R-value was originally created as a way to sell insulation. Generally speaking, a high R-value means the window is better, more insulated, and more energy efficient, because it means it is more resistant to losing heat.

More windows today are rated using the U-Factor, however, which is considered a more accurate measurement of energy efficiency. The R-value is the reverse of the U-factor, or R=1/U.

What is the U-Factor?

While the R-value was developed for insulation, and can be used to rate windows, it's not the best metric to use when shopping for replacement windows. Insulation and windows interact with their environments very differently. A window lets light in and reacts to indoor and outdoor temperatures, wind, and so on. There are just more variables to consider.

Windows react to solar radiation (or emissivity) and air flow, which the R-value doesn't measure. The U-factor assesses the energy efficiency of windows in terms of its thermal transmission.

More specifically, the U-factor measures the rate of heat transfer, or heat loss, through one square foot. In this type of measurement, the lower the number, the greater the insular quality and energy efficiency.

U-factor ratings are typically between .20 – 1.20, and anything at or below .40 is considered energy efficient. Some states mandate a minimum U-factor, so be sure to check.

Emissivity and Low-E

Emissivity factors into a window's U-factor as well, so it helps to understand what emissivity is. Emissivity is a window's ability to absorb and transfer energy (especially infrared). A window with high emissivity, such as the old single-pane windows, can transfer about 84% of the infrared energy in a warm room to the cold air outside. A window with low conductivity and emissivity translates to a lower rate of heat loss, or a lower U-factor.

What can contribute to low emissivity? Recent advances in window technology have resulted in various microscopically thin metal or metallic oxide coatings known as Low-E. Low-E windows will significantly impede radiative heat flow – that means radiant heat from a warm room to the cold air outside, or the heat from a summer day to a cool room.

To be most effective, in a hot climate or an east- or west-facing window, the Low-E should be on the outside pane. If you live in a cold climate and want to keep heat inside in the winter, it should be applied to the inside pane. The US Department of Energy notes that Low-E coatings can cause a window be 10-15% more expensive than a non-coated window, but can "reduce energy loss by as much as 30-50%."

There are many hard and soft Low-E options for different climates and to allow for high, moderate, and low solar gain. Low-E coatings can have a minimal impact on visible light, or may appear slightly tinted, unless you look for a spectrally selective Low-E window. The Department of Energy estimates that spectrally selective coatings can "reduce the electric space cooling requirements of new homes in hot climates by more than 40%."

Gas Fills

Another characteristic of windows that can impact the U-factor is the sealed space between the panes. Not all windows have gas fills (plain air is just fine), but some inert gases, which are denser than air, can improve a window's performance.

That's because this space normally allows for convection, with warm currents rising to the top and cold currents floating to the bottom. Using inert gases instead provides less movement and greater resistance to heat flow, and reduces conduction and transfer. This improves energy efficiency and lowers a window's U-factor.

Argon gas, which can improve thermal performance, is the standard for Cambridge Doors & Windows Inc. and an upgrade for others. It is nontoxic, inexpensive, clear, and odorless, and requires about a 1/2-inch space.

Krypton gas is a high-end upgrade, and offers better insular values. It is also nontoxic, odorless, and clear. It also requires less space between glazing’s (3/8-inch is optimal), so if space is an issue, it is a better choice. Krypton gas performs better than argon, but is also significantly more expensive. A mixture of krypton and argon is a nice compromise that spits the difference on price and performance.

Gas fills are best used in combination with Low-E coatings and required for the Tax credit rebate.

Other Important Ratings

Other factors that are used to rate windows are Visible Transmittance, Solar Heat Gain Coefficient, and Air Leakage.

Visible Transmittance (VT) measures the amount of visible light transmitted. The VT range is 0 to 1, but most values are .3 - .8. The higher the VT value, the more daylight will enter the room. If energy savings are your goal, consider that reducing visible light may require increased lighting inside the house, possibly offsetting your efforts. However, spectrally selective glass will block or re-radiate solar heat without mitigating visible light rays, providing just as much daylight as clear glass.

Solar Heat Gain Coefficient (SHGC) measures how well a window blocks the heat of the sun. It is a fraction of the heat from the sun that enters through a window, door, or skylight. Like VT, the SHGC value range is 0 to 1, but in this case you want the lowest possible number appropriate for your climate and goals. The lower the number, the less solar heat the window transmits and the more it shades. A rating of .3 means it will allow 30% of the sun's heat to pass through.

Air Leakage (AL) is just what it sounds like – air that seeps in or out through the mechanics of the frame itself. It is the rate of air infiltration expressed in units of cubic feet per minute per square foot. A lower air leakage rate indicates a tighter seal, and is obviously more desirable. The best choice is a window with an AL of .3 or less.

Light-to-Solar Gain (LSG) is a combination measurement that doesn't appear on all windows. It is the ratio of Visible Transmittance to Solar Heat Gain Coefficient. A higher number means that there is more light transmitted without adding heat.

Condensation Resistance (CR) measures just what you think it would – condensation. Specifically, how well a window resists forming it. Like LSG, the CR is an optional rating that not all manufacturers embrace. Its range is 1 to 100, with higher numbers being more favorable.

Saving energy is a great idea for your personal budget, but it is also beneficial for the environment. Insulating your home properly and guarding against unwanted heat gain or loss reduces your drain on the energy supply, and that bodes well for the planet. When choosing new windows, there are a variety of factors to take into consideration. The most important among them is the U-factor, but you also want to weigh the Visible Transmittance, Solar Heat Gain Coefficient, and Air Leakage as well. The appropriate combination of these traits will depend on where you live, and specific situations such as east- or west-facing windows, and so on.

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