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All About Glazing Options

You’ve chosen a window manufacturer, you’ve selected the frame material, and you’ve decided on casements rather than double-hungs. But how do you specify glazing?

Posted on Dec 3 2010 by Martin Holladay

UPDATED on May 5, 2016

Everybody has an opinion on windows, and there’s a lot to talk about. Which frame material do you prefer: wood or fiberglass? Do you like double-hungs, sliders, or casements? Who provides better warranty service, Marvin or Pella?

Window selection is a complicated topic, so I'll approach the issue in small bites. In this article I’ll focus on glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill..

Why windows matter

Windows are crucial to a home's thermal performance and the comfort of occupants. In a cold climate, the wrong windows will act like holes in a home's thermal envelope, leaking tremendous amounts of heat. In contrast, the best performing windows can actually collect more heat than they lose during the winter months, turning your walls' weakest link into an asset.

In a hot climate, windows with the wrong type of glazing are often the leading cause of summer overheating — they're probably the main reason that your air conditioner struggles to keep your home cool on summer afternoons. That's why the right type of glazing can transform an unlivable room into a pleasant oasis.

If you are building a new home, the cost to upgrade from run-of-the-mill windows to high-performance windows is relatively small, and the incremental cost can easily be justified by future energy savings. Upgrading to better windows will never be cheaper than during new construction.

If your existing home has bad windows, however, the cost to replace every window in your home with new high-performance windows is often prohibitive. After all, the cost to replace an existing window will always be significantly more than the incremental cost to upgrade to better windows when the house is being built.

Glazing basics

Old-fashioned single-glazed windows have been relegated to garages and barns. These days, the vast majority of new residential windows come with either double or triple glazing.

Most double- and triple-glazed windows are fitted with insulated glazing units (IGUs). The typical IGU consists of two panes of glass connected by a spacer at the perimeter. The spacer establishes the distance between the panes and seals the gap; it needs to be gas-tight so that the gas between the panes (often argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. or kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing.) doesn’t escape and so moisture can’t enter. Warm-edge spacers — for example, the Super Spacer, the Inex spacer, the Intercept spacer, or the Swiggle Seal spacer — are better at reducing heat flow through the perimeter of the window than old-fashioned aluminum spacers.

IGUs can also be produced from three (or, very rarely, four) layers of glass.

Reading the labels

There are two important numbers you need to know when choosing glazing: the U-factor and the solar heat-gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.).

The U-factor measures the rate of heat flow; it's the inverse of R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . In all climates, a low U-factor is better than a high U-factor.

The SHGC tells you how much heat enters a building through the window when the sun is shining. When it comes to SHGC, there is no simple rule to guide you. If you live in a sunny climate with high air conditioning bills, you probably want windows with a low SHGC, especially on the east and west sides of your house. (North windows don't see much sun, and south windows are relatively easy to shade, at least during the summer when the the sun is high in the sky between 10:00 a.m. and 3:00 p.m.)

If you live in a cold climate with high heating bills, you probably want windows with a high SHGC, especially on the south side of your house.

If you live in a mixed climate, you probably want to choose different types of glazing for different orientations of your house. South-facing windows should probably have high-SHGC glazing, especially if the windows are adequately shaded during the summer. West-facing windows should probably have low-SHGC glazing. Selecting the best glazing for each orientation is best done with an energy modeling program like RESFEN or the Passive House Planning Package.

[Author's note: Since this article was written, an increasing number of energy experts have begun questioning traditional passive solar design principles that call for south-facing windows to have high-solar-gain glazing. For more information on this topic, see Reassessing Passive Solar Design Principles.]

A window’s U-factor and SHGC numbers appear on the familiar NFRC label found on most new windows. Note that the NFRC reporting requirements require U-factors and SHGCs to be whole-window measurements, not glazing-only measurements. You can’t compare a whole-window U-factor with a glazing-only U-factor. If a window salesperson brags about a certain U-factor or SHGC, be sure to ask whether the advertised number measures the performance of the whole window or only the glazing.

About low-e coatings

In most of the U.S. and Canada, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows are now standard for new homes. A low-e coating is an almost invisible layer of metal that is applied to one side of the glass by the glass manufacturer. The effect of a low-e coating is to lower the U-factor of the glazing; that is, it slows heat transfer.

Most low-e windows have double-pane IGUs with a low-e coating on one side of one pane. In almost all cases, the low-e coating faces the air space between the panes, so that the coating is protected from abuse. (It should be noted, however, that hard-coat or pyrolytic low-e coatings are tough enough to be used on single-pane storm windows.)

Low-e coatings don't just reflect heat in one direction; they work in both directions. Since the addition of a low-emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. coating adjacent to an air space has the effect of lowering the U-factor (that is, raising the R-value) of the air space, a low-e coating helps slow heat transfer from the interior to the exterior during the winter, and also helps slow heat transfer from the exterior to the interior during the summer.

Low-e triple glazing can have either one or two low-e coatings. (If a triple-glazed IGU has two low-e coatings, it's sometimes referred to as “double-low-e” glass.) It's also possible to order triple glazing without any low-e coatings; such windows may allow more solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. on sunny days, but they will lose more heat at night than a triple-glazed window with a low-e coating.

Knowing that a window has low-e glazing tells you almost nothing about its solar heat-gain coefficient (SHGC). A low-e window might have a low SHGC, making it a good choice for a house in Florida, or a high SHGC, making it a good choice for a house in Minnesota. Just because a window is low-e, doesn’t mean it’s good at reducing solar gain.

Argon and krypton gas

Filling the space between the panes of an IGU with argon gas instead of air improves the performance of the glazing by lowering the unit's U-factor. Most low-e glazing units are filled with argon gas.

The optimal thickness of the space between panes of argon-filled glazing units is 1/2 inch. Increasing or decreasing the thickness of this space degrades performance. For krypton, the optimal space is thinner — only 3/8 inch — so krypton, a much more expensive gas than argon, is usually reserved for applications where the total glazing unit thickness must be minimized.

Don't be tempted to buy glazing without argon or krypton gas. Like a low-e coating, argon gas is a cost-effective upgrade. In almost any location in the U.S., argon gas and low-e coatings will quickly yield energy savings exceeding their cost.

Buying low-SHGC windows is easy

The vast majority of low-e windows sold in the U.S. have low-SHGC glazing. These windows perform well in Texas and Florida, but they make little sense up north, where homes would benefit from high-SHGC glazing. Every time a cold-climate builder installs a low-SHGC window instead of a high-SHGC window, the homeowner is condemned to higher energy bills for the life of the window.

There are two main reasons that low-SHGC windows dominate the market:

  • Window manufacturers prefer to stock a single type of glazing from Florida to the Canadian border, to simplify the management of their inventory.
  • Because of lobbying by the window industry, the federal tax credit penalizes cold-climate builders who choose energy-saving windows instead of energy-wasting windows. By relieving window manufacturers of the need to stock many types of glazing, the tax credit streamlines the window distribution business — at the expense of cold-climate homeowners, who end up paying higher energy bills.

If you live in a sunny climate, low-solar-gain double glazing makes sense. Popular options include:

  • LoĒ²-270 from Cardinal;
  • ClimaGuard 55/27 from Guardian; and
  • Solarban R100 from PPG.

Buying high-SHGC windows is hard

If you want to buy a high-SHGC window, you'll probably need to do some research before placing your order. Many window reps are ignorant about glazing options, so be prepared to educate the sales staff (or go over their heads) to get what you want.

Several glass manufacturers make high-SHGC double glazing. Good bets include:

  • LOF Energy Advantage from Pilkington;
  • LoE-179 from Cardinal;
  • Sungate 500 from PPG; and
  • ClimaGuard 75/68 from Guardian.

When ordering your windows, you can mention these products by name. At this time, Pilkington's Energy Advantage is probably the best-performing high-solar-gain double glazing available.

Until recently, few U.S. window manufacturers were able to supply high-solar-gain windows. Even those who were able to supply them often erected hurdles that made special orders difficult. (There hurdles included technical ignorance, high prices, and in some cases an outright refusal to supply special-order glazing.) That's why U.S. builders looking for high-SHGC windows often seek out Canadian window manufacturers, many of whom have an excellent understanding of the technical advantages of high-solar-gain windows for cold climates.

During the past few years, however, some U.S. manufacturers have begun to market high-solar-gain glass. For example, Pella now offers a glazing option called “NaturalSun”; it's a high-solar-gain double glazing from Cardinal. The center-of-glass specs for NaturalSun glazing are pretty good: a SHGC of 0.70 and a U-factor of 0.27. This glazing results in a whole-window SHGC of 0.51 and a whole-window U-factor of 0.32.

When it comes to triple glazing, a surprising number of U.S. window manufacturers are still confused. For inexplicable reasons, the triple glazing usually offered to U.S. buyers is almost always low-solar-gain triple glazing. These window manufacturers fail to understand that no one in Florida or Texas is buying triple glazing. The buyers are all up north, in Montana and Minnesota and Maine, where only high-solar-gain glazing makes sense. So wake up, window manufacturers! If you're going to offer triple glazing, make sure it is high-solar-gain triple glazing.

Visible transmittance

Although I’ve focused my attention on glazing U-factor and SHGC, there’s another number that matters: visible transmittance, or VT.

A high VT is better than a low VT, unless there is reason to believe that glare will be a problem. Windows with a low VT are less likely to cause glare than windows with a high VT.

Every time you add another layer of glazing to a window, the VT goes down. When the VT drops below 0.40, everything seen through the window begins to look a little gray.

What about triple glazing?

In general, neither glass manufacturers nor window manufacturers have “branded” any particular type of triple glazing. As a result, those interested in triple-glazed windows are forced to compare U-factors and SHGC numbers.

For more guidance on buying triple-glazed windows, see “Choosing Triple-Glazed Windows.”

What numbers should I aim for?

In you are in a hot climate, you probably want a double-glazed low-solar-gain window. Look for these specs:

  • A whole-window SHGC of 0.25 or lower. Lower is better than higher.
  • A center-of-glass SHGC of 0.20 to 0.27. Lower is better than higher.
  • A whole-window U-factor of 0.30 or less. Lower is better than higher.
  • A center-of-glass U-factor of 0.24 or 0.25. Lower is better than higher.

If you are in a cold climate, you probably want a double- or triple-glazed high-solar-gain window. If you settle for a double-glazed window, look for these specs:

  • A whole-window SHGC of 0.42 to 0.55. Higher is better than lower.
  • A center-of-glass SHGC of 0.59 to 0.76. Higher is better than lower.
  • A whole-window U-factor of 0.30 to 0.39. Lower is better than higher.
  • A center-of-glass U-factor of 0.27 to 0.29. Lower is better than higher.

If you can spring for a triple-glazed window, look for these specs:

  • A whole-window SHGC of 0.33 to 0.47. Higher is better than lower.
  • A center-of-glass SHGC of 0.50 to 0.63. Higher is better than lower.
  • A whole-window U-factor of 0.19 to 0.26. Lower is better than higher.
  • A center-of-glass U-factor of 0.16 to 0.22. Lower is better than higher.

Last week’s blog: “Fastening Furring Strips to a Foam-Sheathed Wall.”

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Sep 6, 2012 7:31 AM ET

Response to K.E. Jenkins
by Martin Holladay

K.E. Jenkins,
There are three possible explanations for the phenomenon you describe as a "blue haze":

1. This is a slight tint that occurs with some types of low-e glazing. In other words, it's in the normal range, and you are one of the rare homeowners who is driven bonkers by the slight blue tint. Most homeowners who look through low-e glazing have no complaints.

2. You are observing condensation between the two window panes, a phenomenon which occurs when the edge seals fail. This is usually described as a whitish haze, however.

3. The manufacturer of the glazing supplied windows with some defect in the coating resulting in an unacceptable tint.

It is impossible to determine which of these three possibilities is true without a site visit.

In the U.S., a window installer or glazing supplier would certainly honor a warranty if the defect proved to be failed seals leading to condensation.

Good luck with your negotiations.

Aug 15, 2013 5:37 AM ET

Why is 1/2" (12.7mm) the optimal space for Argon Gas fill?
by Jerry Chwang

I understand it might have something to do with convection currents developing vs. total amount of gas between glass panes, but I have had 2 European window manufacturers come back to me with IGUs that use larger spacing.

One IGU from PressGlas uses 18mm in both gas cavities of a triple-glazed unit
The other from Guardian uses 24/20, 20/18, and 14/14 depending on the size of the window.

Any ideas as to the reason for these differences?

Aug 15, 2013 5:57 AM ET

Edited Aug 15, 2013 5:59 AM ET.

Response to Jerry Chwang
by Martin Holladay

The answer to your question is provided by Stephen Thwaites, in a comment (Comment #10) posted on this GBA page: Passivhaus Windows.

I'll quote the relevant information:

"U-factor [in Europe is] calculated for a smaller Delta T. As I understand it, in Europe U is based a 0°C (32°F) outdoor temperature and a 20°C (68°F) indoor temperature. In North America U is based on -18°C (0°F) outdoor temperature and a 21°C (70°F) indoor temperature. While most materials don't change their insulating properties over this temperature range, gases can. As a result in Europe the optimal air/gas space is 16 mm, while in North America the optimal air/gas space is 12.5 mm. North American windows with optimal spacing between panes, when evaluated to European norms are sub-optimal. (The reverse is not true).

"(As an aside, there are strong arguments for following Europe on this temperature issue. In North America the -18°C ( 0°F) outdoor temperature to calculate Uwindow was adopted to size heating equipment. Given that;
- heating systems are routinely obscenely oversized
- most homes in North America only rarely experience -18°C (0°F) wintertime temperatures
- Uwindow's role in predicting seasonal energy use is more important, than is role in sizing heating systems,
there is a strong argument that the delta T used to calculate Uwindow should be decreased to something that represents typical rather than extreme conditions - more like what is used in Europe.)"

Nov 15, 2014 6:14 PM ET

Warm edge spacers
by John Burk

"Warm-edge spacers — for example, the Super Spacer, the Inex spacer, the Intercept spacer, or the Swiggle Seal spacer — are better at reducing heat flow through the perimeter of the window than old-fashioned aluminum spacers."

Martin if the Swiggle Seal spacer is made by Tremco they had a high failure rate. WhenI owned a local glass and glazing company I refused to use any product with swiggle which I felt would only be problems down the road. It allowed windows to be produced in substandard conditions and with little quality control. Personally although I've been out of the business for years I thought it had faded into obscurity. Generally Tremco made some great products and stood behind them.

Not much mention of Heat Mirror which I think is a great product due to the convection reduction especially in sloped glazing and solariums. Marvin carries it I think.

I am in the process of designing a Coach House with 3 bay garage under on my cottage lot North of Parry Sound, Ontario, full Southern exposure on the water. Keep up the good work I really enjoy the discussions and articles.


Sep 25, 2017 5:50 AM ET

Buying the right glazing
by David Martin

35 years ago I built fixed windows on the south side of my Vermont house using two panes of glass with an inch of air in the middle. Now dirt has found its way between the panes and it is time to replace them with real insulated glass. I see that I should aim for:
A center-of-glass SHGC of 0.59 to 0.76
A center-of-glass U-factor of 0.27 to 0.29 But I have not been able to find glass that meets both of these goals. Can someone recommend glass that I should consider?
-- David Martin

Sep 25, 2017 5:52 AM ET

Response to David Martin
by Martin Holladay

You're right that it can be hard to purchase the glazing you want -- especially if you are buying glazing from a local shop that supplies replacement glazing.

You'll probably have to compromise on the SHGC. I advise you to buy low-U-factor glazing, even if the SHGC isn't as high as you'd like.

Dec 27, 2017 5:20 PM ET

Which surface gets the Low-e?
by Craig Hassell

In a cold climate, which surface should get the low-e? Some building science experts tell me surface three, but manufacturers are telling me they always put it on surface 2 - otherwise the heat build-up inside the insulated unit can cause premature seal failure. Thoughts?

Dec 27, 2017 5:31 PM ET

Response to Craig Hassell
by Martin Holladay

Here is how GBA reader Tex McLeod answered the question in a previous thread:

"The industry numbers the surfaces from the outside to the inside to help in these kinds of discussions. Using this numbering system makes the inside surface of the outermost pane, surface 2 and the outside of the innermost pane, surface 3.

"In very general terms, you would look for surface 2 in an air conditioning climate and surface 3 in a heating climate. The U value [U factor or U-factor] of the window remains constant in both placements but the SHGC varies. According to lab work conducted by LOF, solar heat gain through Energy Advantage glass is reduced by about 10% by installing the sealed unit so that the coating is on surface 2 rather than surface 3.

"In theory, you can advise your window manufacturer which way you want the sealed unit to be installed in the frame by virtue of whether you want the additional solar heat gain or not. In other words, what is more important to your client - passive solar heat or reduced air conditioning loads.

"As mentioned earlier, it is not that easy because of the range of glass products (hard coat, soft coat, suspended films etc.), their properties and the way in which manufacturers use the products. For example, some manufacturers have a low E coating on both surface 2 and surface 3 to virtually eliminate solar heat gain. Most builders haven't done a very good job of managing solar heat gain and as a result, many manufacturers have way more problems with overheating than general comfort. Therefore the bulk of glass offerings are adjusted accordingly to design out solar gains, much like you would find in commercial installations.

"The bottom line - there isn't a right or a wrong - it comes down to knowing the products available to ensure you are getting what you want from the windows you are buying. Keep in mind, this is a specialized field and we are all a little dangerous, which is why we need to push our suppliers to provide better technical information. I hope this helps some, the best of luck."

Dec 27, 2017 7:25 PM ET

There's more- try #4! @ Craig Hassell
by Dana Dorsett

There is fairly high-gain/low-U double pane glass out there with low-E coatings on surfaces #2 & #4 that work well in cold climates, as long as it's not TOO cold. The coating on surface #4 makes the interior surface colder, and if it's cold enough to form condensation the high emissivity of the film of water renders the coating on #4 useless (until it dries.) When it's dry the coating on #4 reflects heat back into the room, and body-heat back to the mammals standing next to the window.

The condensation issue is primarily a problem in US climate zone 7 or colder (or zone 6 if the interior is maintained at higher than average humidity in winter.)

Jan 5, 2018 10:40 PM ET

comparing glass vs IGU's vs windows
by Tyler Keniston

I am a bit confused about how to compare numbers. I understand that NFRC is a third party and rates the entire window. So it would seem that 'how good' a number is depends not just on the glazing, but the size of the window, the operation type, and the frame.

If that is true, why do people throw around U-factor, SHGC, and VT numbers without a word about the window size, style, etc. If a builder or consumer wanted to shop for the best GLAZING, do the NFRC numbers help compare? Is there a standard square footage that the NFRC uses to test for these numbers?

I asked a local glass shop if they had U-factor, SHGC, and VT numbers for some IGU's and their response was, "what size? The numbers depend on the size."

Jan 6, 2018 7:46 AM ET

Response to Tyler Keniston
by Martin Holladay

Glass manufacturers like Cardinal provide information on the U-factor, SHGC, and VT for various IGUs -- so-called "glazing-only" specs. The same specs are provided by European (and some Canadian) window manufacturers.

It's perfectly OK to compare glazing-only specs, as long as you do so with open eyes. You should realize that high-SHGC glazing won't provide as much winter heat when installed in a fat frame as it would in a skinny frame. You should also realize that low-U-factor glazing won't perform as well when installed in a window frame with a high U-factor as it would when installed in a window frame with a low U-factor.

In other words, frames matter, because they affect performance. But if you want to get into the nitty-gritty of window design -- as all window manufacturers must -- of course you care about glazing-only specs.

Jan 6, 2018 1:11 PM ET

so does size matter if glazing only
by Tyler Keniston

Thanks Martin.

So if one were looking at glazing only specs (I assume this would be center of glass numbers? or maybe that doesn't account for the spacers?) would size matter? In other words, If I want to compare IGU's to IGU's (no window frame) should I need to be providing a size, as my local glass shop has requested?

My worry is that if I give them a size and they get back to me with some numbers, it won't be an apples to apples comparison if other manufactures base their numbers on some standardized test size for their IGU's. Perhaps I should just provide the size to everyone...

Jan 6, 2018 1:41 PM ET

Response to Tyler Keniston
by Martin Holladay

Yes, size matters. Heat loss (which affects U-factor) is greatest at the edges of the IGU and lowest at the center of glass. Since the width of the spacers is the same regardless of the size of the IGU, the edge penalty is greater for small IGUs than for large UGUs.

Jan 6, 2018 1:44 PM ET

Edited Jan 6, 2018 1:44 PM ET.

Tyler, NFRC whole-window
by Michael Maines

Tyler, NFRC whole-window U-factors, sometimes noted as U-w, relate to a standard size window, but if you ask for U-g, or center-of-glass U-factor, it's just that--the value in the middle of a pane, regardless of size or spacers.

For precise calculations, in Passive House design we look at center-of-glass U-factor, frame U-factor, the insulating value of the spacer, and the insulating value of the installation (is the gap filled with insulation, is there insulation covering the window frame, etc.), as each of these elements and their sizes affect window performance. But it's pretty common to get a general sense of window performance by looking at both U-w and U-g.

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