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Musings of an Energy Nerd

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?

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 glazing.

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…

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  1. Rob | | #1

    Climate definitions
    Fantastic post, and one I hope to utilize if/when we replace our nine windows in the future. I've always wondered where builders' definitions of cold climate and hot climate start/stop... up here in Chicago I've always assumed we're cold climate, but we get plenty of 90 degree days in the summer!

    Maybe cold and hot climates should be defined by USDA hardiness zone... USDA zones 1-4 are cold, 5-6 are marginal, 7+ are hot?

  2. GBA Editor
    Martin Holladay | | #2

    Response to Rob
    When it comes to broad climate categories in North America, many builders have adopted the hygrothermal regions promoted by the Building Science Corporation. Here's a map of the BSC Hygrothermal Regions:

    According to this map, in Chicago you are Cold. (Take heart, however -- at least you are not Very Cold).

    To determine whether a specific orientation of a house in Chicago will be better served by a low-SHGC window or a high-SHGC window, you have two choices:
    1. Common sense (tempered by experience), or
    2. Computer modeling using RESFEN or PHPP.

  3. Armando Cobo | | #3

    Gas filled windows
    Gas filled windows have different issues worth mentioning. Windows installed within 3,500 ft altitude difference from where they were made need to have a “breather tube” for pressure, and the gas will disappear in a short time. Windows in the western states are seldom sold with gas in it (I do know some retailers disregard this fact and use the gas as an “advantage”)
    In the flat lands, depending on the manufacturer you talk to, gas can disappear about 2%-10% per year. However, a lot of the windows are coming with foam spacers who apparently doing a better job on energy efficiency and keeping the gas in. Interesting enough, are there any window manufacturers that warranty keeping the gas in?

  4. GBA Editor
    Martin Holladay | | #4

    Response to Armando Cobo
    If you are building west of the Rocky Mountains, you need to seek out window manufacturers who assemble their windows west of the Rocky Mountains. (That way, the windows don't need to make the perilous journey over the Rocky Mountains, with the attendant changes in altitude that wreak havoc with glazing.)

    I know they exist. Pozzi Windows manufactures windows in Oregon, and there are several window manufacturers in British Columbia. Any readers want to chime in with recommendations for Armando?

  5. J Chesnut | | #5

    worth my money
    Excellent post. I got some work now so I'll sign up for a membership again to support the excellent information this website offers. This post has great content to forward to the local window reps. (You should get AIA education credits for this stuff.)
    Why though do you not include information about the advanced European glazing options?

  6. GBA Editor
    Martin Holladay | | #6

    Response to J Chesnut
    J Chesnut,
    I have already written a blog about European windows and European glazing. If you missed it, here's the link:
    Passivhaus Windows.

  7. Armando Cobo | | #7

    My post was to mention the
    My post was to mention the gas issue... here in the SW we use most manufacturers, we just order windows without gas. Sierra Pacific, Serious and Pozzi, to name a few, build their windows out this way.

  8. GBA Editor
    Martin Holladay | | #8

    Second response to Armando
    There are two related issues about breather tubes and elevation.

    The first concerns windows installed at a low altitude that have to be delivered over the Rocky Mountains. These windows are sometimes shipped with breather tubes that can be sealed shut after they make the crossing. Some, but not all, gas is lost during transit.

    Some manufacturers who deliver windows over the Rocky Mountains just omit the gas. That works, but the performance of the glazing suffers.

    The second type of customer is one who is installing the window at a high elevation. In that case, the ideal solution is to find a manufacturer assembling IGUs and windows at your altitude. Failing that, you may have to settle for windows without argon.

  9. user-716970 | | #9

    Testing for argon
    Is there any way to know for certain whether or not argon is present in a window? Any sort of test for varification purposes? If not, this seems like something that could easily be "gamed" by the industry...

  10. wjrobinson | | #10

    great blog once again Martin
    Keep up the great work Martin, I appreciate all that you post here and always learn something from you that is useful knowledge for this aspiring green builder.

  11. GBA Editor
    Martin Holladay | | #11

    Response to Garth Sproule
    The glass industry has developed standards for argon leakage. If the IGUs are certified to the EN1279 standard, the argon leakage rate should be between 0.5% to 1% per year. Assuming the unit is filled with 90% argon when assembled, then even after 20 years you can expect the unit to retain 73% of its argon.

    Equipment has been developed to detect argon in IGUs. One device is the GasGlass Handheld V2 unit manufactured by Sparklike. Here is more information on the GasGlass Handheld V2:

  12. M. Johnson | | #12

    Do you know this from a book? Which one?
    Martin you said something no HVAC engineer would agree is true:
    "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..."

    As a self declared "energy nerd" (I consider myself one too) it would have been correct to say this is a leading cause of excessive ENERGY USE. Comfort on the other hand, relies on proper AC sizing and very much depends on good ductwork which delivers air in proportion to the BTU heat gain for each room. Comfortable temperatures can be delivered even for an energy hog with yards of single pane windows. Good windows (and treatment) just make this more energy efficient.

    Seeing as you live off-grid in Vermont, all this talk of air conditioning must be academic not experiencial to you.

    Regards -- MJ

  13. David Meiland | | #13

    That would be
    That would be "experiential"...

  14. Bob Ellenberg | | #14

    Finding Cardinal Glass
    Their website doesn't list any maufacturers that use their glass. A search of one of the window manufacturers you list doesn't either. Appears we will have to really hunt to find this. I have emailed them.

  15. GBA Editor
    Martin Holladay | | #15

    Response to M. Johnson
    M. Johnson,
    It's fairly common for a home or office to have a room with west windows that overheats on summer afternoons. I know from experience, for example, that this is a big problem with the entryway of the Union Bank building (built, I think, in about 2005) in Lyndonville, Vermont.

    This is a design problem. You're right that the HVAC engineer who designed the ductwork and AC controls made several errors.

    But there are two ways to fix this problem. One solution (the one you suggest) is to install larger ductwork and perhaps a zoned system so that the AC can always keep the room at 74°F. That'll work, but energy bills will rise.

    The other solution would have been to specify low-SHGC glazing. That would improve the comfort of the room for the life of the building, without increasing the owner's energy bills.

    You may quibble with the wording in my blog, but I'm trying to address the mindset that says, "All you need to do is install a big enough air conditioner, big enough ductwork, and a big enough air-handler fan, and you can install any windows you want."

    Go to Houston or Miami and ask people if there is one room in their house that overheats on summer afternoons. Many homeowners will show you such a room. What is the best way to solve this problem?

  16. GBA Editor
    Martin Holladay | | #16

    Response to Bob Ellenberg
    Bob Ellenberg,
    You wrote that I didn't mention any window manufacturers that use Cardinal glass. In fact I did: in one of the paragraphs under the "Buying high-SHGC windows is hard" subhead, I mentioned that Pella sells windows with Cardinal glass.

    Although many window manufacturers don't mention the source of their glazing, they all report U-factor and SHGC numbers. You can shop for windows by comparing these numbers.

  17. Bob Ellenberg | | #17

    Who uses Cardinal
    I didn't say YOU didn't mention. I saw that you did and went to those websites. The problem is they list many glass options and describe them but don't specify the Cardinal model # you gave. I am assuming they offer other glass as well and was simply saying it appears it will take some work to determine which model window the manufacturer uses the specific Cardinal glass in. Getting exact pricing and specifications will be harder than usual but without your information I would not have known what to go looking for. Thanks!

  18. qHNVm6qn8J | | #18

    Two Questions for you
    Thanks for a great post, and timely for me as I’m trying to select windows for our new construction home in south central PA. The house is oriented to that it has a wall facing solar south. On the south wall I’ll try to get the highest SHGC I can afford and will shade windows in summer. My initial conversations with local window suppliers indicated that this might require a “special order” and even a window in a “high end product line” that I hadn’t necessarily been considering. Not the best news, but I guess that’s the way it is.

    On the west, I will have three windows under an 8’ porch roof, and three smaller windows above the porch (unfortunately not shaded). Unfortunately trees to shade the 2nd floor are many years away.
    - I think I should go with lowest SHGC I can get on the west wall windows to reduce summer cooling costs. This would be different than the south wall windows. Would you agree?
    - If I can see some south face and west face windows from an interior room (open plan), should I expect that the “view” through the window will be significantly different due to the different coatings?


  19. GBA Editor
    Martin Holladay | | #19

    Response to Mary Shultz
    I'm glad you are considering orientation-specific glazing. That's the right thing to do.

    Q. "I think I should go with lowest SHGC I can get on the west wall windows to reduce summer cooling costs. This would be different than the south wall windows. Would you agree?"

    A. Yes -- especially for the unshaded windows. For the west windows protected by the porch roof, the SHGC may not matter very much, unless the porch is narrow and the windows will receive sun.

    Q. "If I can see some south-facing and west-facing windows from an interior room (open plan), should I expect that the “view” through the window will be significantly different due to the different coatings?"

    A. That's a hard question to answer. Some people notice a slightly different tint to windows with different coatings, while others never notice the difference. It depends on the coatings you are comparing as well as the sensitivity (or perhaps artistic sensibility?) of the viewer. I would hazard a guess that most residents in homes with orientation-specific glazing never notice the difference between their west-facing windows and their south-facing windows.

    As for me, I would never hesitate to choose the best-performing window, even at the slight risk that the glass would have a slightly different tint.

  20. Donald Lintner | | #20

    Can one just buy the glass?
    I'm building my own entrance and garage doors. Local glass suppliers can provide a thermopane but I haven't found one who can do the warm edge spacers and gas. Do the glass manufacturers sell the high SHGC glass products you list above for someone like me or as replacement glass for existing windows or only in new windows?

  21. GBA Editor
    Martin Holladay | | #21

    Response to Donald Lintner
    I'm not sure. If I were you, I would call as many glazing suppliers as possible. If that doesn't work, you might want to call Cardinal, PPG, Guardian, or Pilkington.

  22. M. Johnson | | #22

    Windows are part of some solutions, but not the whole solution
    HVAC engineers understand clearly that when you have an uncomfortable room, remedies include adjusting the building envelope (i.e. windows) as well as correcting errors in design and implementation of the HVAC system. You are absolutely right that replacing windows can be a solution to some too-hot room problems, and that new construction should always use these advanced windows of the right type. I wish people would respect more the issue of old construction, where there is a limited number of cost effective solutions. For example a too-hot room can use solar shades instead of new windows, the plus trade-off is effectiveness at low cost, the minus trade-off is these are less esthetic than whole new windows. I did this in my own house for $100 vs. several thousand would be for new windows.

    Even though it does not accommodate the energy nerd, the HVAC system is responsible for maintaining even temperatures throughout the house, otherwise it is not a good system. All too often it fails at this because the system was merely installed without being designed competently. I don't believe wanting the system to function as it should have been originally designed, is incompatible with being an energy nerd... keeping in mind the absolute most energy savings is to turn off heating and cooling.

    Thanks for listening.

  23. Patrick McCombe | | #23

    All about Glazing
    Great post Martin.

  24. Allen Schloss | | #24

    low-e on inner or outer pane?
    So much good information, thanks Martin.

    We live near the coast in California, never need A/C but use heating a lot during the winter. We have two south facing rooms, both apparently with "standard" low-e glass windows. When we moved here I was disappointed to discover that if I stood at one of these windows with the sun on me, I felt very little heat from the sun. When we decided to replace one of the windows with a large patio door,
    I was determined to get a door that provided better passive solar heating.

    I went to three different sellers before I found one who was sympathetic to the idea of passive solar. He called several window manufacturers before finding one, (Milgard) that offered a relatively high SHGC glass (.50) as standard with their double glazed vinyl patio doors. Milgard refers to it as "hard coat" low-e, and when the door arrived it was referred to as "low-e 2" on the invoice but I've been told it's Pilkington energy advantage glass. The door now lists the SHGC as .43, which I assume is the "whole door" specification, not just the glass. Thankfully I've read a lot of posts on this site so I could make sense of all this.

    So far, so good. I requested that the low-e pane be installed on the inward side of the door as I had read that that will radiate more heat into the room rather than to the outside. Both the salesman and the factory rep said that was a bad idea, that in fact the door could "blow up" from heat build-up.

    The door arrived yesterday with the glass uninstalled. I'm inclined to install the low-e glass on the inner side, as this type of low-e glass (high SHGC) passes most solar heat anyway and shouldn't result in much heat build-up. Any advice?

  25. GBA Editor
    Martin Holladay | | #25

    Response to Allen Schloss
    I'll answer your question by quoting a knowledgeable visitor to Breaktime, the Fine Homebuilding Web forum; he posts comments under the name "Oberon."

    According to Oberon, "A high solar heat gain (HSHG) LowE coating will allow direct solar energy to pass thru the coating and will prevent heat from passing back thru the coating to the outdoors when the sun isn’t shining. This coating offers the advantages of passive solar energy gain with the advantages of the heat-blocking ability of the LowE.  Placing this coating on the #3 surface of the IG (the outer side of the interior lite) affords a bit more solar gain advantage as well. But, placing the coating on that surface will make the coating less effective in the summer months in keeping outside heat outside, so there is a trade-off. LowE coatings are generally placed on surface #2 – interior of the exterior lite – except in extreme northern climates."

  26. ZEQfbYvNAN | | #26

    Still trying to make sense in my mind
    VERY informative reading—thanks Martin—yet puzzling to my situation.

    I'm in a cold climate zone (zip code 99362) and about to purchase double pane sliding doors/windows with low E366+argon. The U-factor is 0.29 and SHGC 0.21.

    Big south-facing openings with overhangs designed to keep the sun out in the summer. No windows facing west, some good-sized windows facing the north. Winter is not very sunny, lots of fog and cloudy days.

    So, if winter sun is not always coming through the south glass, does it really matter if the SHGC is not high?

    What would be, in general terms, the optimal values for an orientation-specific glazing for conditions like mine?

    The statement above: "In a cold climate, the wrong windows will act like holes in a home's thermal envelope, leaking tremendous amounts of heat." makes me wonder if there's anything I should consider on my current glazing specs.


  27. Bill Burke | | #27

    Special Ordering High Solar Gain Low-e
    You are dead on about special ordering high solar gain low-e. I did that when I replaced the windows in my flat in San Francisco. Made a big point of it. The company selling me the windows kept telling me solar control glass would prevent fading. I told them I didn't care about fading and wanted the heat gain. The crew arrived early and tore out the old windows on the front of my flat. Then the truck arrived with the new windows. When I looked at the NFRC labels on the windows, they all showed low SHGC solar-control glass. With the old windows already out, I was screwed! So I have solar control glass even though I special ordered high solar gain glass. One lesson I learned is that with window replacement, make sure the new windows are on-site and are what you ordered before the crew takes out your old windows.

  28. Jim B | | #28

    Argon in high altitudes
    Insulated units with gas are now available in the Rockies. The breather tube problem was solved (by Cardinal glass, I think) by adding a balloon to the end of the tube. That way the glass cavity could expand and contract without losing the gas. I found that quite a number of manufacturors will supply this option, so go back and hound your suppliers.

  29. Don Otto | | #29

    high or low SHGC?
    I build in Iowa City, roughly 6500 HDD, and have been using Andersen's "Smart Sun" low-e, with a SHGC of about 0.19 and Tviz of about 0.5. My rationale for using it is that the SHGC is created by the low-e coating, which reflects infrared. In winter we have many more hours of dark in Iowa than light, and the HVAC contractor has to keep my clients warm at night and on cold cloudy days. We both feel that he has to size a larger heat pump or furnace if the glass loses more heat through a higher infrared-transmitting glass. As well, the radiant heat loss is reduced with low SHGC windows, leading to greater client comfort.

    Please let me know what you think.

  30. GBA Editor
    Martin Holladay | | #30

    Response to Robert Car
    After playing detective with your Zip code, I discovered that you live near Walla Walla, Washington -- kind of in the Western part of the state, towards Idaho, in Climate Zone 5. What's your elevation? How cold does it get in winter?

    You describe your climate as cold, and you inform us that you have "big south-facing openings with overhangs designed to keep the sun out in the summer." All of these factors point to the same conclusion: you will be better off with high-solar-gain glazing. This can be confirmed by consulting a good energy-modeling program.

    However, you have chosen to buy low-solar-gain glazing with a SHGC of 0.21. (That's extremely low -- are you sure you have that right?) As a result, you will be spending more to heat your house during the winter than you would if you bought the right kind of glazing.

    You ask, "if winter sun is not always coming through the south glass, does it really matter if the SHGC is not high?" The answer is, "Yes, it matters," for two reasons:
    1. Even on hazy days without full sun, high-solar-gain glazing on the south side of the house will admit more heat than low-solar-gain glazing.
    2. You WILL get at least a little sun during the heating season, and energy researchers have studied homes like yours and concluded that you will save energy with high-solar-gain glass, even considering heat loss at night and during cloudy weather.

    Finally, you ask "What would be, in general terms, the optimal values for an orientation-specific glazing for conditions like mine?" As I wrote in the article, your south-facing glass should have a whole-window SHGC of 0.42 to 0.55 (or a center-of-glass SHGC of 0.59 to 0.76). Higher is better than lower.

  31. GBA Editor
    Martin Holladay | | #31

    Response to Bill Burke
    Bill Burke,
    I'm sorry to hear about your experience with your replacement windows. Sadly, stories like yours are all too common.

    It is rather infuriating that window companies care so little about glazing options. Few window reps bother to educate themselves on these issues, in spite of the fact that they have chosen to become window dealers. The fact that window companies care more about managing their inventory problems than helping customers choose windows that will save them energy -- it's maddening.

  32. GBA Editor
    Martin Holladay | | #32

    Response to Don Otto
    Don Otto,
    Like Robert Car, you have chosen the wrong glazing for your climate -- at least for your south-facing windows.

    Why choose glazing with a very low SHGC for south-facing windows? You are throwing money out the window.

    Your statement, "the SHGC is created by the low-e coating, which reflects infrared," is true -- but you reach the wrong conclusion. There are a variety of low-e coatings out there. Pyrolytic coatings -- also called hard-coat low-e coatings -- tend to create glazing with a high SHGC. That's what you want. Other types of low-e coatings (sputtered coatings or soft-coat low-e coatings) generally tend to create glazing with a low SHGC (although there are exceptions to this rule).

    As I wrote in the article, "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 window in Minnesota. Just because a window is low-e, doesn’t mean it’s good at reducing solar gain."

    There is no getting around the fact that you have to read the labels on your windows! If you want a window with a high SHGC -- and I think you do -- seek out low-e glazings with high SHGC. They are out there. You just have to educate your window rep and insist you want to buy them.

    If you buy low-e windows with a high SHGC, they will still slow down heat loss at night. Over the course of the winter, they will perform much better than windows with a low SHGC -- because they bring in free heat during the day.

  33. Mike T | | #33

    low-E coating location and climate
    Should the low-E coating be located on a different side of the glass in a heating climate versus a cooling climate? I.e. does the low-E coating reflect heat equally well in both directions?

  34. Steve McCarthy | | #34

    high SHGC
    It's nice to know that Pella offers a window with a high SHGC but there must be other window builders that offer that option. Who are they?
    How difficult is it to buy windows from Canada? What Canadian manufacturers offer high SHGC?
    Are there any builders of replacement windows that offer high SHGC?

    Thanks for any info you can share.

  35. GBA Editor
    Martin Holladay | | #35

    Response to Mike T
    Mike T,
    A low-e coating lowers the U-factor of the window glazing. It works to slow down heat flow in both directions. During the winter, it slows down heat flow from the interior to the exterior; during the summer, it slows down heat flow from the exterior to the interior.

    That's a general statement about low-e coatings, and it's true. Nevertheless, glazing manufacturers still have to decide which glazing surface should receive the low-e coating; depending on where the low-e coating is placed, slight performance differences are possible. Here's how "Oberon" -- the Web name of a knowledgeable visitor to the Breaktime forum -- explained it:

    "A high solar heat gain (HSHG) LowE coating will allow direct solar energy to pass thru the coating and will prevent heat from passing back thru the coating to the outdoors when the sun isn’t shining. This coating offers the advantages of passive solar energy gain with the advantages of the heat-blocking ability of the LowE. Placing this coating on the #3 surface of the IG (the outer side of the interior lite) affords a bit more solar gain advantage as well. But, placing the coating on that surface will make the coating less effective in the summer months in keeping outside heat outside, so there is a trade-off. LowE coatings are generally placed on surface #2 – interior of the exterior lite – except in extreme northern climates."

  36. GBA Editor
    Martin Holladay | | #36

    Response to Steve McCarthy
    Some builders have had success ordering high-SHGC windows from Marvin; others have been frustrated by unhelpful window reps. If you are persistent, however, Marvin should be able to fill your order.

    The best-known Canadian window manufacturers -- all of which should be familiar with high-SHGC glazing -- are Thermotech Fiberglass, Inline, Accurate Dorwin, Duxton, and Fibertec. Here in Vermont, I've had good luck ordering windows from Thermotech Fiberglass and Inline, and I have friends who have purchased many Accurate Dorwin windows.

    I'd love to hear from GBA readers who have had positive experiences ordering high-SHGC glazing from window manufacturers.

  37. jeff_williams | | #37

    It is maddening
    As you said Martin, it is maddening that window companies and reps don't care about this stuff. I just replaced all the windows in my house in MN with windows from a very large MN window manufacturer and not a peep about SHGC. It turns out that I have the ones with low values (.27 is common on my lables) and those were from the window line that were eligible for the government rebate. My house is apparently well suited for Florida. Ugh.

  38. Robert Car | | #38

    Good guess Martin, as I am in
    Good guess Martin, as I am in Walla Walla. Thanks to the information provided here, I'm able to change all south facing windows to Low-E 179 right before placing the order!

    Would you say for all non-south facing glass, the Low-E366 will be a good choice? It's the one with the best U-value from Cardinal.

    On a side note, Cardinal shows what it seems to be a new product, Low-E i81, that can be added to any other Low-E flavor. I'm having the window rep investigate this. Does anybody have any experience or info?

  39. GBA Editor
    Martin Holladay | | #39

    Response to Robert Car's latest post
    You ask whether it makes sense to use Cardinal Low-E366 on your east and west windows. It depends on many factors: the glazing area, your climate, and the results of your energy modeling program.

    Cardinal Low-E366 is a low-SHGC glass. That means when the sun shines through your east windows on a cool morning, the sun won't do much to help heat up your house.

    Is that what you want? It depends. Do mornings start out hot in your climate during the summer? Or do mornings start out cool? Do you want the morning sun to help heat up your house in the morning? Or is the sun your dreaded foe, a harbinger of a dreadful day in the 90s?

    Of course there is no single answer. I know that where I live in Vermont, I want the morning sun to help heat my house. Where you live, you might be scared of the morning sun.

    Use your gut instinct, or use an energy modeling program -- your choice.

  40. imt | | #40

    Trying to decide on the right glass
    I an in NJ and live about 20 miles west of NYC. Thus a mixed environment. I also looked at the BSC hygrothermal regions map but its hard to discern where exactly I fall on that map. I am probably close to the line between the humid and cold. I am ordering an Andersen 4 panel slider that will be installed facing east. Its on the north side of the house. I have woods behind the house (mature tree's) and the tree line is approx 50' from the back of the house. There are no tree's covering the house so when the sun clears the tree's the house is fully exposed.

    From what I understand from the article, am I correct that I would want the slider with a low SGHC for my climate? The window with the ecoexcel package (Low E4 sun) has a SGHC of .18 vs. the .27 for the standard low E4 Glass.

  41. GBA Editor
    Martin Holladay | | #41

    Response to Todd
    My response to you would be similar to the advice I just posted to Robert Car.

    Use your gut instinct, or use an energy modeling program -- your choice.

  42. imt | | #42

    Response to Martin
    Not sure if I understand how to use resfen but I attempted. I basically just entered in one user defined window for the east side of the house and put the sq footage and properties in based on that one slider. I then zeroed out the other variables. I choose my state etc. None of the envelope packages really fit with my older home but I played around with a few. actually it really didn't make much difference the deviation is about the same since I am just playing with one window (assuming I did it right).

    the difference between both option with trying all types of solar gain reduction as well, is a total annual cost of approx $2 - $3. This would then come down to which utility cost will escalate more (gas or electric). If gas escalates then higher SHGC could be better but if electric costs go up more then a lower SHGC.

  43. GBA Editor
    Martin Holladay | | #43

    Response to Todd Sherman
    I'm glad you used RESFEN. I have no reason to doubt your conclusion that the difference between two glazing options in a single east-facing window aren't too significant -- especially if both options have low U-factors. So, take your pick.

  44. qHNVm6qn8J | | #44

    Reviewing the Quotes After Applying Your Advice
    Hi Martin,

    I’m returning to this blog post after getting several window quotes (see my original Question, Two Questions for You above dated 12/05/10). I used the info here to go out and get some window quotes for our new house. I discussed what I was trying to do (passive solar) and what I was looking for (high SHGC, low/reasonable U) with each vendor and provided names of some glass that would work (the LOF Energy Advantage from Pilkington and LoE-179 from Cardinal).

    The resulting vendors’ quotes for south-side window glass options included:
    - “High Performance Low-E4 Glass” (from Pella)
    - “High Performance Low-E4 Glass” (from another Anderson supplier)
    - “High Performance Low-E4 Glass” as part of the window description with than an additional comment, “ADD ON FOR 179 CARDINAL GLASS” (from an Anderson supplier)

    So, a range of responses. Interestingly, the one supplier to list the Cardinal glass was also the lowest cost, and the most prompt in his reply. I believe he will win my business.

    After reviewing the info and comments on this blog again, I have a few questions:

    In your blog, under “What numbers should I aim for?” – are you referring to a buyer looking at generic numbers provided in the product literature … or should the buyer try to get the numbers for the actual window they’re going to order/purchase. I notice that none of the quotes provide this info. And with all the variables in window size and grills, maybe that’s not a reasonable request. (?)

    Some people have posted questions here about where the special coatings should go (which pane, which side). When I place and order for a window that shows a “add on for 179 Cardinal Glass”, can I assume that the window manufacturing is putting it in the window assembly to make a high SHGC window? Or does a buyer need to be more specific on this? FYI, the windows I’m considering are double pane A-Series Anderson casements.

    I’ve learned a lot by reading your blog and all of the comments/responses. It is a tremendous resource for an owner-builder like me!

  45. GBA Editor
    Martin Holladay | | #45

    Response to Mary Schultz
    I would insist that the window supplier put the SHGC of the glass in writing, in the quote.

    Most window suppliers are totally ignorant when it comes to SHGC. The words “High Performance Low-E4 Glass” tell you nothing. They refer to a range of glazing options -- some of which are high SHGC and some of which are low SHGC.

    The type of glass you want (assuming you go for “High Performance Low-E4 Glass”) is called “High Performance Low-E4 Glass Sun” when offered by Andersen. It has to have the word "Sun" in it.

    If you are ordering from Pella, you want "Natural Sun," not “High Performance Low-E4 Glass.”

    If you get Cardinal 179, you should be OK -- as far as I understand it, that's a high SHGC product.

    Ask for the SHGC numbers!

  46. user-954346 | | #46

    high heat gain glazing and software
    Martin, A window designer in SD told me Cardinal has replaced the 179 with 180. There is also a i81 for energy star N locations. I also looked into the Pilkington hard coat- Energy Advantage.

    Do you know of any window manufacturers that offers Energy Advantage with their window?

    I called the Pilkington rep. He could only offer where the glazing has been used in Pella (with their tri-pane) and North Star (vinyl in Canada). Lowes as I kinda expected, didn't have a clue. I had them call Pella in Iowa. The Pella rep. did not know of Engergy Advantage in their products nor would they acknowlege who their glass supplier was. And only offered that the Pilkington rep must have referenced a third party installation-modification.

    Is it correct that the reference you have to Energy Advange as to it being a better performing product is due to the pyrolitic- hard coat in the glass?

    I have a 1945 sf Ranch house with basement in zone 5A or southeast SD and plan to super-insulate it. In regards to energy modeling for comparing glazing I have used -- window selection tool. The results for the lowest annual cost are with high SHGC compared to low SHGC for all walls. Resfen 5 is more selective, and it too gives the lowest cost and energy for using high SHGC in all walls.
    I have read your advise about using high gain on south walls and low on north walls-- this seems the best.
    Would over heating in summer be a valid concern if one chose all the walls to have high gain glazing? Should I trust the sofware or go with a gut feeling-common sense approach?

  47. GBA Editor
    Martin Holladay | | #47

    Response to Mark
    An earlier comment on this page mentioned that Milgard Wildows offers Energy Advantage glazing; you might check with Milgard to see if this is true.

    You are correct that Energy Advantage is a hard-coat (pyrolytic) low-e coating that offers a high solar heat gain coefficient.

    Concerning your question about summer overheating: yes, your house design needs to take the possibility of summer overheating into consideration. This is a complicated topic, impossible to answer in a brief paragraph -- but the standard design solutions include sizing your south roof overhang to shade your south windows during the hottest days of the summer (while still allowing the low-angle winter sun to hit your south windows) as well as including interior thermal mass.

  48. [email protected] | | #48

    Response to Mark

    The window designer was correct that Cardinal is phasing out LoE-179 (and LoE-178) in favor of LoE-180.

    Like 178 and 179, LoE-180 is a HSHG coating and performance numbers are very similar.

    The "1" in all three coating designations refers to the number of layers of silver in the coating and the second two numbers refers to the VT or visible transimittance of the coating. So, 180 has a slightly improved VT compared with the earlier coatings.

    LoE-180 has a U-value of .26 versus .28 for 179 and 180 has a SHGC of .69 versus .70 for 179. The LoE 178 performance numbers are slightly worse than either 179 or 180 in all three catagories.

    I81 is a different critter altogether. I81 can be used as a stand-alone coating, but it was designed to be used as a surface 4 coating in conjunction to a surface 2 coating. Currently it isn't possible to get I81 with a surface 3 coating - technology isn't there yet.

    I81 is a sputter or softcoat with the durability of a hardcoat - and with very similar energy performance numbers as a typical hardcoat when used in either a dual pane or stand-alone application. I81 energy performance numbers, if you are looking for higher SHGC and lower U-values, are not as good as the LoE-180 in the dual pane application.

    The reason Pella currently offers the Energy Advantage in their tri-pane is because it's not a sealed three-lite unit. While the two outer lites are sealed (a dual pane IGU), the inner lite can be removed which works with Energy Advantage since it is a hardcoat and it would not be damaged in that set up whereas a softcoat would corrode in a non-sealed application.

    Martin quoted me in two of his earlier replies as Oberon, which is the name that I post under in Breaktime and JLC, while I am not sure why but for some reason GBA lists me as Greg Smith....but in the interests of disclosure...anyway,

    To (hopefully) clarify the quote that Martin used earlier, both HSHG or LSHG LowE coatings, when installed on the number 3 surface of a dual-glazed IGU, will result in increased passive solar heat gain and a warmer interior glass surface versus a surface 2 application, while both HSHG or LSHG LowE coatings will, when installed on the number 2 surface of a dual pane IGU, result in less passive solar heat gain and a cooler interior glass surface versus a surface 3 application.

    So in the interest of complicating things even more, the NFRC rates coatings as not only HSHG and LSHG, but also as moderate solar heat gain. In the case of Cardinal coatings (and I use Cardinal primarily because about 65-70% of residential coatings used are from Cardinal), LoE-180 would be considered a HSHG coating at SHGC of .69 when installed on surface 3, LoE²-272 would be considered a moderate solar heat gain coating with a SHGC of .41 when installed on surface 2 (but, that number would INCREASE if that coating were installed on surface 3), and LoE³-366 would be considered a LSHG coating with a SHGC of .27 when installed on surface 2.

  49. user-942656 | | #49

    Pella offers windows with a
    Pella offers windows with a high SHGC
    Pella® (ProLine) Clad
    Vented double hung: 11/16” clear IG with 2.5 mm glass 0.50 0.64 66 41
    Fixed: 11/16” clear IG with 2.5 mm glass 0.47 0.65 (SHGC) 68 54
    Clear glass is what we are specifying for our Passive Solar homes in Jaffrey NH for the True South Side. All you have to do is tell them what you want and make sure they give you what you want.

  50. Kejenkins | | #50

    HELP....BLUE HAZE on our newly installed double glazing windows
    PLEASE HELP.......Three weeks ago we had newly installed double glazed windows - which looked great. I came home from shopping walked into the kitchen only to discover to our horror an opaque Blue haze which wasn't there before that day.
    The installer told me the manufacturer he gets his supplies from claims its the coating sprayed on the inside of the inside pane of glass and is normal for energy saving windows that the government (UK) requires.

    My argument is neighbours have newly installed energy saving windows one week prior to ours being installed and they do not have the blue haze

    I have since told the installer, whom we really feel for as well as ourselves, that we do not accept the excuse given and that if we were told we would be left with an opaque blue haze when the sun shines on them we would never have had them installed as We cannot live with this and that if this was the case why is it that not everyone who has had newly installed dbl glzg windows have the same opaque blue haze, because they don't. Told installer rather have them all ripped out and boarded up than live with this

    I tried some research and have learn't that a blue haze appears when the window has been breached, and can happen with age too..there is no moisture though, just a blue opaque haze and so vivid I find depressing to look out of.

    Any advice and help will be much appreciated.

    [Editor's note: The answer to this question is posted on Page 2; click the link below to reach this page.]

  51. GBA Editor
    Martin Holladay | | #51

    Response to K.E. Jenkins
    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.

  52. jchwang | | #52

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

  53. GBA Editor
    Martin Holladay | | #53

    Response to Jerry Chwang
    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.)"

  54. Ikillyou | | #54

    Warm edge spacers
    "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.


  55. davidsmartin | | #55

    Buying the right glazing
    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

  56. GBA Editor
    Martin Holladay | | #56

    Response to David Martin
    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.

  57. user-2374770 | | #57

    Which surface gets the Low-e?
    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?

  58. GBA Editor
    Martin Holladay | | #58

    Response to Craig Hassell
    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."

  59. Expert Member
    Dana Dorsett | | #59

    There's more- try #4! @ Craig Hassell
    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.)

  60. maine_tyler | | #60

    comparing glass vs IGU's vs windows
    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."

  61. GBA Editor
    Martin Holladay | | #61

    Response to Tyler Keniston
    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.

  62. maine_tyler | | #62

    so does size matter if glazing only
    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...

  63. GBA Editor
    Martin Holladay | | #63

    Response to Tyler Keniston
    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.

  64. Expert Member
    Michael Maines | | #64

    Tyler, NFRC whole-window
    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.

  65. lesse | | #65

    I bought IG from ECO Insulating glass Inc, 1416 BonhillRoad Mississauga, Ont. L5T 1L3, Canada.
    In total 193 units QUAD SC75 with Heat Mirror film and filled with krypton in April 2009.
    The first unit imploded in 2015 we had no idea what happend. Now all 193 units are concave and 8 units have imploded.
    ECO Insulating Glass used a inferior sealant as a result the Krypton gas is leaking through the sealant air molecules are bigger and do not go through fill up the IG glass with a result that it puls vacuem between the panes, panes go concave and implode. for more info about this problem see This is a warning for anybody that want there panes filled with gas make sure your supplier know what he is doing.

  66. SRI_Energy | | #66

    At SRI Energy, we made a commitment to replacement windows that provide the best insulation value at an affordable price. We understand that the construction and installation of the windows are as important as the components. Learn more about Energy Star Replacement Windows

  67. evergreenhomes | | #67

    I’m in climate 5, lake front house, and my client wants windows that protect hardwood flooring from changing color/fading.
    With common considerations, we’d have low U factor and high SHGC. However, if we want to reduce the shgc of the south facing windows, maybe we put a low SHGC, low E3, on the south side and our more common, low E2 window with higher SHGC on the north.
    Do you think a low shgc window will provide the “tint” to minimize wood fading?
    Looking forward to nerd camp 2019!

    1. GBA Editor
      Martin Holladay | | #68

      Low-solar-gain low-e windows do a better job of preventing the fading of wood and fabric than high-solar-gain low-e windows.

      For more information on this issue, see this article from the Efficient Windows Collaborative: "Reduced Fading."

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