What Do U Value? A Glass Efficiency Primer

Posted On Wednesday, Aug. 24th, 2016


Seaway Manufacturing of Erie, PA, manufacturer of residential replacement windows, patio doors and sunrooms, presents “Look Into Your Windows,” an educational series designed to answer the most frequent questions about choosing replacement windows for your home.  In this installment, we offer an explanation of window glass efficiency standards.

As if shopping for replacement windows wasn’t confusing enough, now you’ve seen the term “U-Value” for the first time.  What does it mean?  Is it like the R-Value rating you’ve seen on those bundles of pink insulation? Yes.  And no.

In case you’re getting ready to take the SAT, here’s the official definition: A U-Value – also called a U-Factor – is the overall heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in watts) through one square meter of a structure divided by the difference in temperature across the structure.

Glad we cleared that up.  Now, in English, the U-Value of a window or other building component is a measure of heat loss.  R-Value is a measure of resistance (the “R” in R-Value) to heat loss.  Here’s why that’s important: with an R-Value a high number is better.  This is easy to see in the insulation aisle at your local home center; the higher the R-Value, the thicker the insulation and vice versa.

But because U-Values measure thermal loss, a lower number means a more efficient window glass package, and that number will appear in manufacturers’ literature as well as the NFRC label affixed to every new window.  All other things being equal, a window with a lower U-Value will insulate better.

Bear in mind, however, that all other things are not necessarily equal.  While glass makes up the largest area of a window, the construction of the window also has a big impact on how well it will resist the elements.  The most efficient glass, if installed in a poorly-constructed window, will not be very effective.  Be sure to evaluate the entire window and not just the glass.

One more note on the numbers: on that same NFRC label you’ll see a rating for Solar Heat Gain Coefficient (SHGC).  This is similar to the U-Value except that it measures only how well the product blocks heat caused by the sun.  The number will be between 0 and 1, and again, lower is better unless you’re in a region where heating days outnumber cooling days quite substantially.

One final note: the right answer for your home will depend in part on where it’s located. The EPA’s Energy Star program has divided the country into climate zones based on the number of heating and cooling days for each. A window or door that’s appropriate for the Northern Zone might not be right for the South-Central or Southern Zones. Here’s a link to a map showing the Energy Star climate zones, or use this tool to enter your state and county.

Questions about replacement windows? Contact Seaway Manufacturing, or click here and enter your ZIP code to find a Seaway dealer in your area.

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Window Glass, Part One: Look Into Your Windows

Posted On Wednesday, Aug. 3rd, 2016


Seaway Manufacturing of Erie, PA, manufacturer of residential replacement windows, patio doors and sunrooms, presents “Look Into Your Windows,” an educational series designed to answer the most frequent questions about choosing replacement windows for your home.  In this installment, we offer an overview of the options in window glass.

In the earliest homes, windows were very small – or nonexistent.  Glass was extremely expensive, and a home with windows almost certainly meant that the occupants were wealthy.  Over the course of a few centuries, manufacturing processes improved and costs came down dramatically, so now it’s hard to imagine a home without large vistas in nearly every direction.

But today’s larger window openings bring their own set of problems.  By itself, the glass that makes up most of a window’s area is not much of an insulator; that is, the single pane of glass found in most older windows is not very good at keeping outside temperatures outside and your expensive heating and cooling inside.

Modern replacement windows answer this need by using an insulated glass package, two or more panes of glass with a sealed airspace in between.  This design provides energy efficiency that’s far superior to single-pane windows, but can also bring confusion with a dizzying array of options: clear or low-E glass, double- or triple-glazing, Argon or Krypton gas, spacers, U-values … what are the best choices?

We’ll address each of those items in this series, but the answer depends on your individual situation, starting with which region of the country you live in.  Recognizing that a home in Arizona is likely to have very different insulating needs than, say, a home in Vermont, the EPA has divided the United States into several Energy Star Climate Zones.  These can be a good first point of reference in determining what to look for in a window.

Next, consider your home specifically.  Does one side receive much more sunlight than the others?  If so, you may want to consider a higher-efficiency glass package for the windows on that side of the house.  Similarly, certain combinations are better at reducing outside noise, so if your home faces a busy street you might want an upgraded glass package in the front windows only.

With those general considerations as a starting point, we’ll move through the elements of a window glass package one by one beginning in the next post.

Questions about replacement windows? Contact Seaway Manufacturing, or click here and enter your ZIP code to find a Seaway dealer in your area.

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