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Understanding the Thermal Side of Windows

Thermal features for windows are a fairly important aspect of their quality. One wants to ensure that they are getting the correct windows for their given situation and locale. Local climate plays a big part in what features will benefit you most. Different windows, materials, and features all respond differently to environmental stimuli. To simplify this process, the National Fenestration Ratings Council (NFRC) created a series of ratings and guidelines that allow manufacturers and representatives to communicate the benefits of a product easier.

U-Factor Rating

The U-Factor Rating of a window is normally the first rating a potential buyer looks for. Most individuals want to purchase thermal replacement windows because their older models are too drafty or do not insulate very well. This rating gives you an idea of how much heat loss occurs through the window. The typical window will feature a rating between 1.3 and 0.2. The lower that this number is, the better it is at insulating and keeping the respective climate on its appropriate side.

The following values are by no means exhaustive; and only given as a rule of thumb for comparison purposes. These are some average values one would expect to find with a given window.

Metal Casement Window 1.3

High Quality Single-Pane Window 1.0

Quality Single-Pane and Storm Window 0.6

Low-E Glass, Double-Paned Window 0.4

Low-E Glass, Triple-Paned Window 0.25

As we can see, the triple-paned window with Low-E glass rates near the lowest of the scale. This is what one would expect to see in a side-by-side comparison.

General Ratings

A homeowner really does not need to have a college-equivalent knowledge of windows and their ratings to effectively shop for them. One can easily get lost in the many numbers that are available. The following section will introduce you to and explain thermal ratings that could be of importance when shopping for windows. To note, these deal with the thermal aspects of the window only.

Solar Heat Gain Coefficient (SHGC)

The SHGC is the opposite of the U-Factor. This rating will tell you how effective the window is at keeping heat out of the house from a warm exterior. This number will normally be rated between 0 and 1; with lower being better.

Air Leakage

Air leaking around the window and window frame can cause any number of problems if the window is not sealed correctly. This value tends to deal more with the seams and how the window was constructed. The numerical range of this value tends between 0.1 and 0.3; with a lower rating being better.

Condensation Resistance

The press for quality windows and high efficiency has created a minor problem with condensation collection in many types of windows. These value will give you an idea of how well the windows resist the collection of moisture and condensation formation on the windows. A higher value represents better efficiency with Condensation Resistance.

Visible Transmittance

This value is not a thermal value; but it will be included here for clarification purposes. This rating expresses how much sunlight the window allows through it. This should not be taken to mean solar energy or heat. It specifically means how much light the window allows to pass through it; completely separate from heat or other energy.

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