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Energy Solutions

‘Superwindows’ to the Rescue?

The race toward ever-more-efficient windows has hit a bit of a plateau, with climate-specific solutions taking command

This quintuple-glazed window prototype was produced by Thermotech in the 1990s as a demonstration aiming for R-20. Today, Thermotech produces very efficient windows, but focuses on climate-specific solutions, not stratospheric R-values.
Image Credit: Thermotech Fiberglass

As I’ve said before, windows are a silent but very high-tech part of our buildings. The advances in glazing in the last 30 years have been phenomenal. Will windows keep getting better and better with no end in sight?

In recent years, an increasing number of window manufacturers have been combining and refining the features that have given us today’s high-performance windows: multiple layers of glazing, multiple low-emissivity (low-e) coatings, and very-low-conductivity gases such as krypton. They’ve been creating super-high-performance windows, or “superwindows,” a term coined by Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory. In the early 1990s, Arasteh predicted that advances in technology could make all windows, even north-facing windows in northern climates, net-energy-gainers. Whether or not that day has arrived is a matter of debate, but there’s no doubt that the advances since the 1990s, when window buyers were dreaming of U-0.05 (R-20) windows (and window makers were making tantalizing demonstrations in that direction), have been astounding.

The superwindow race cools off…

Today, that race has cooled off to some extent in favor of climate-specific solutions. As Stephen Thwaites of Thermotech Fiberglass in Canada told me, “A window doesn’t have to be R-20 to be as energy-efficient as the wall around it,” due to the ability of a window to gain solar heat and provide ventilation. “A home with no windows will use more energy than a properly designed home with R-5 windows,” Thwaites said.

That’s a change from the mid-1990s when the prototype quintuple-glazed window shown above was built by Thermotech  as a demonstration of what was possible, with a goal of R-20. Today, ambitions have become more climate-specific.

… But it’s still exciting to look for what’s next

Today, R-5 windows are aspirational for many, but not really that special in the windows market. But instead of pushing past that, we are seeing more emphasis on proper design by orientation, shading, and window-to-wall ratios, and on buying the best windows for each application according to the budget.

It’s still exciting to dream of what we’ll see in the next 30 years. For example, vacuum glazing, in which most of the air is evacuated from the space between panes, reduces thermal conduction and convection to nearly zero (leaving radiation as the primary means of heat transfer), and can currently offer U-0.08 (R-12) with double-glazing and one advanced low-e coating. Vacuum glazing is still largely in research and development, however. The seal is tough to get right, since the vacuum puts a lot of pressure on it.

The biggest limit on energy performance is and may continue to be not what technology can do, but what the buyer can afford. Windows imported from Germany meeting the Passivhaus standard, for example, offer U-factors under 0.14 — at a cost of over $90 per square foot of window area. Triple-glazed Canadian windows typically cost $40–$50 per square foot, in contrast with a price range for more conventional double-glazed windows of $30–$35.

The problem with getting great windows is how they’re sold

On the subject of price, I’m going to quote a few paragraphs here from a comment I received by email after last week’s post about reading NRFC labels. This comment with some follow-up discussion is posted on last week’s column.

“Your recent columns on windows have struck a raw nerve, and it’s worth sharing why.  Construction has just begun on a small sunroom addition to my home. The problem with getting great windows is not product availability, but the awkward way they are sold.

“Building supply dealers seldom post prices of anything. Items which are ordered, like windows that have lots of options and sizes, are much worse — requiring the entry of data into a computer to get a quote.  The quote of course is for the whole window, making it really tedious to test out individual options. The retail staff, though both knowledgeable and helpful, cannot possibly keep prices for all those options in their heads, and nothing good can come from a customer mistaking a ballpark guess for a quote.

“Net result: it is ridiculously difficult to get a cost comparison of option A vs. option B, on anything. In my case of a south-facing sunroom where I knew I’d be starting a lot of veggie seedlings in spring, I depended on my own reading and research in preferring clear glass (full-spectrum, high solar gain), and triple pane (reduced heat loss). It took several days to get a price.

“Going through that process for the dozens of decisions in a given project is simply not going to happen.  Even a highly motivated customer has little choice but to just give up and trust the contractor to do something reasonable. And still miss here and there. You recently noted that using argon gas was a no-brainer; that column came too late, and I didn’t expend the effort to price argon vs. air.”

It may not help to say this, but if you’ve faced this window-pricing issue, you’re not the only one. Martin Holladay had some great comments on this two weeks ago which I’ll paraphrase here. There is little transparency on pricing in the window industry, and someone who wants to really shop around and price out different options easily faces dozens of hours of research on a new home. Hopefully they can get good cooperation from dealers, but that’s not always a given. It would be great to see just one window manufacturer post accurate and complete prices online. I suspect it would make their business stronger, not weaker.

Tristan Roberts is Editorial Director at BuildingGreen, Inc., in Brattleboro, Vermont.


  1. user-939142 | | #1

    it really isn't that complicated
    there are more details in ordering a burger king whopper and choosing amongst all the condiments
    burger king has invested in the order management and advertising to make it easy, the window company and lumber yard hasn't

    and for good reason. it is easier to sell high priced items when price is not part of the comparison.
    you create a window, quote a price, claim its on sale, and the customer really has no choice but to take it or try their luck elsewhere. making the process time consuming encourages them not to go through it again elsewhere.

    if the shipping ever became economical and practical, the chinese factories would fix this problem for us

  2. user-659915 | | #2

    On low bid shopping
    I have to disagree. The complexity of window specs is a reality, an inevitable result of the significant competing developments in window technology as documented above. A shopper, even an experienced one, can easily make expensive mistakes without good guidance from the supplier. That means the supplier is running a service business, not a commodity business, and you can't provide decent service on the tightest of competitive margins. How do the builders amongst you like bidding against the local low-bid truck jockey who you know will screw up the job and end up costing the client much more in the long run?

    "It's unwise to pay too much, but it's unwise to pay too little. When you pay too much you lose a little money, that is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing you bought it to do.

    The common law of business balance prohibits paying a little and getting a lot. It can't be done. If you deal with the lowest bidder, it's as well to add something for the risk you run.

    And if you do that, you will have enough to pay for something better"

    John Ruskin

  3. oberon476 | | #3

    Chinese windows to the rescue?
    Currently North America has 37 active float glass plants, with the newest float plant (Cardinal's Winlock Washington facility) opening about 6 years ago. China currently has over 110 floats either operating or under construction.

    The last I heard, something like 11 of the Chinese float lines fabricate glass that would meet North American quality specifications. Much of the glass from the higher quality plants is intended primarily for export.

    It takes about twice as long and costs about double to build a float line in North America than it does in China primarily due to evironmental considerations. The Cardinal plant in Winlock is currently the "cleanest" (lowest emissions) float plant in the world.

    Chinese windows are readily available in North America for people who are looking for a bargain. Sam's Club, for example, does a thriving business in them, and they are becoming much more common on the west coast - particulary in the southern Califormia. It is curious how a window made in China and shipped to North America can be less expensive than a window manufactured locally, but that is a fact for people looking to save a few bucks.

    There is also a lot of glass being imported from China. Again it is amazing that glass (and glass is very heavy and very bulky and requires a lot of care in shipping to see that it arrives unbroken), imported from China into the Pacific northwest can be less expensive than glass coming off a line from a plant within 100 miles.

    I have had the opportunity to work with something like 40 different window and/or door companies (all North American except two German and one Chinese) in product development and testing over the last 15 years or so.

    One thing I have learned over the years is that while every North American (and German, but in their case they just assumed that their product was superior already) company that I have personally dealt with truly wanted to manufacture the best possible window or door that they could within the scope of their product line or business (without exception the folks that I dealt with really believed in what they were doing). The Chinese folks that I dealt with had a slightly different philosophy.

    The original batch of glass/patio doors that came from China failed to meet North American standards for a variety of reasons, but the primary one I was concerned with at the time was safety.

    When I brought it to the attention of the company representitives their first comment was "do we meet minimum standards?" When I told them no, in my opinion they did not meet even minimum standards, their reply was "there are ways to get around that if you know the right people."

    That was my first and last professional association with Chinese manufactured windows or doors.

    It is absolutely true that the Chinese will supply us with whatever products that we want, be it pet foods, children's toys, drywall, or doors and windows....good luck with that


  4. albertrooks | | #4

    I got lost on this one...

    I'm happy to see more about "super windows" earning their place in the market and I hope that you continue to post on the subject.

    Can I point out that I see considerable movement to windows with high solar heat gain to take advantage off all that "free solar energy" in Passivhaus or ZEB projects. As we are learning (and as Martin had commented on earlier), it wouldn't be too hard to set up a "summer overheating" scenario, or other mis-applications. This new breed of "super window" really needs to be designed into the application while overhangs and operable exterior shading is taken into account.

    That's where i lost the thread of where your going. On the one hand, you (and I) are excited to see more progress in highly efficient products, but on the other hand your kind of asking that the selection process gets "dumbed down" and easier.

    I'm in the process of setting up my small company as the Regional dealer for the German window Optiwin for the Pacific Northwest. I just got back from visiting the plant in Germany. We are all working hard to bring both the current and planned new innovative windows and doors to our the area.

    While these things are not rocket science, they probably should not be "self serve or off the shelf" products.

    I'd like to suggest a that there should be a distinction between how "easy" we expect it to be when choosing a "super window" Vs. a "frame with a glazing unit in it... would you like low e with that?".

    I think it's ok to expect that you have to work a little to get the good stuff in the right spot. That's the point of the work.

  5. Tristan Roberts | | #5

    dumbing down?
    Albert, I'm not sure where you see me suggesting that anything should get "dumbed down"?

    I am strongly in favor of clear labels, clear recommendations, and accessible, reliable window performance information. The super-engaged consumer who takes the time to learn this stuff, understand the implications on their existing home or design, and get quotes from four manufacturers can get high-performance windows at a reasonable cost—and they deserve the superior result that they are getting.

    If anything I am asking for more complication in some ways. Part of the point I tried to make here is that "superwindows" with ever-greater U-factor are not the future. The future is climate- and building-specific applications that have carefully selected but not necessarily off-the-charts performance numbers. That may be harder to do than simply training everyone to look for better and better NFRC numbers.

    I submit that there are a lot of consumers and even builders out there who for one reason or another need things to be a bit more accessible, and those people equally need a quality home, and for the sake of our planet we want them to have a quality home.

    Shouldn't there be a middle ground between boutique German-imported windows, and McDonald's style imported-from-China windows?

  6. user-659915 | | #6

    Different strokes
    Tristan makes a very good point: we don't all have the same needs from a window supplier. Custom builders - and custom designers like myself - operate in a very service-oriented world where every project is different and we expect to pay a little more for specialized expertise. High-volume production builders can afford to do the extensive research on price and quality for themselves and write off the overhead on a streetful of homes. Sounds like it's the folks in the middle that are being under-served in the present market not in available quality of product but in good information about that product.

  7. Tristan Roberts | | #7

    width of krypton
    Greg, I was aware that windows with krypton fill are thinner but understood the reason was that the molecules are more "slippery" and so convection gets going faster. While this might also be true, I hadn't thought about the basic geometry and cost, and the fact that a thinner profile means less space to fill with a pricey gas!

    Thanks for this tidbit and your other astute thoughts.

    For anyone who's an EBN subscriber and wants a look at the vacuum glass with the pillars, there's a link for you.

  8. GBA Editor
    Martin Holladay | | #8

    Response to James Morgan
    You wrote, "A shopper, even an experienced one, can easily make expensive mistakes without good guidance from the supplier. That means the supplier is running a service business, not a commodity business."

    I wish! I have yet to meet an American window rep who was capable of providing good window advice for a builder in a northern climate interested in high-solar-gain windows on the south orientation and windows with different specs on the other orientations. Such reps don't exist. [Later edit: As I noted in a subsequent comment, "I owe conscientious window dealers an apology for painting them with such a broad brush; I am guilty of hyperbole."]

    I wouldn't mind paying higher prices for high quality service if I were being provided high quality service, but that's not what I'm getting (except from the Canadians -- Thermotech excepted, of course).

    So in the U.S. we have we have the worst of all possible worlds: the window buyer has to do his or her own research, and then educate the window rep or dealer, and then we cross our fingers and hope that our order wasn't screwed up -- and we pay extra, because "this isn't a commodity business."

  9. user-659915 | | #9

    "I have yet to meet

    "I have yet to meet an American window rep who was capable of providing good window advice for a builder in a northern climate interested in high-solar-gain windows on the south orientation and windows with different specs on the other orientations. Such reps don't exist."

    Sorry to hear your experience has been so unsatisfying but I think your extrapolation is unmerited. I have certainly met my share of window reps who barely know their product even in general terms let alone the particular applications, but also a few who were very knowledgeable. My experience though is that for consistent and reliable advice a local dealer beats a manufacturer's rep every time, especially if they carry several product lines. I suppose I am fortunate that the staff at my preferred local window supplier stay well-informed on SHGC orientation issues and have been working with us on specs like these for nearly two decades, but I find it hard to believe they are unique in the North American market. They certainly quote their share of commodity projects off stock plans which have front, left, right and rear to describe the various elevations rather than cardinal points, but when a customer appears with an interest in correctly-oriented glazing specs they are perfectly capable of giving proper advice. I also like that they also check that our schedules are fully coordinated and code-compliant. Boy, can that be a money-saver. Altogether a relationship worth cultivating, IMHO.

  10. GBA Editor
    Martin Holladay | | #10

    Another response to James
    I owe conscientious window dealers an apology for painting them with such a broad brush; I am guilty of hyperbole. But I am not the only person who has been frustrated by the problems I mentioned.

  11. user-939142 | | #11

    windows are not that complicated

    if manufacturers and distributors WANTED to simplify the process for those ordering the standard fair middle class models, they could

    the current system of hide and seek and salesmen working on commission means that won't happen. it also is true that most window installers on large jobs are the ones selling them (or collaborating) so the buildes have no incentive on making it easy and having homeowners take over the job or argue price with facts.

    that is why i say a chinese like system would fix this problem. not quality, but more a modern ordering system

    as an example, roof types and styles are just as complicated as windows. yet homeowners can easily comparison shop, even if they do it poorly. there is no need to sit down with a shingle rep and try to get a quote. of course you still can if you choose.

  12. user-659915 | | #12

    Re: again
    "if manufacturers and distributors WANTED to simplify the process for those ordering the standard fair middle class models, they could"
    But why would they? What would be the motivation? Let me remind you how this thing works. All manufacturers need to protect their margins in order to stay in business and pay for the R & D to improve their product. To do this they strive to differentiate their higher end product with specialized offerings: extruded versus folded aluminum or vinyl cladding, extended color ranges, and yes, steadily improved thermal performance. Yes it makes it harder to comparison shop: this is by intent. It also protects the manufacturer and their vulnerable employees from being at the desperate end of a cut-throat commodity market and produces a consistent improvement across the whole range of the product line over time. Tell me again why this is bad for the consumer or the industry?

    The comparison with the roof shingle market is telling. All manufacturers produce shingles of the same dimension and pretty much the same range of colors, with minor variations only in thickness and quality denoted by expected life, making for the easy comparison shopping you desire. Could this be why there are just a few manufacturers serving this huge market and the technology has been essentially stagnant for fifty years? And why alternative higher-performance products from smaller manufacturers have such a hard time gaining a foothold in the market?

  13. albertrooks | | #13

    Back on track.

    Thanks for your comments. I get it now. I had read Martin's points earlier and then probably created a personal "mish mash" of yours and Martins thinking that the goal was for "things to get easy".

    TR: "Part of the point I tried to make here is that "superwindows" with ever-greater U-factor are not the future. The future is climate- and building-specific applications that have carefully selected but not necessarily off-the-charts performance numbers. That may be harder to do than simply training everyone to look for better and better NFRC numbers."

    Your re-statement is music to my ears... or... it's the "picturesque view" seen through the "super window" (sorry, I know that was lame).

    My point that I was trying to make is along the same lines: We don't have the "magic bullets" of ever increasing U factors in Euro or domestic windows. What we've got is getting better. The addition of "boutique German-imported windows" on top of the Canadian offerings will drive the middle market up. That's always the case: When the top end stratus of the market capabilities raise, the middle market is subsequently forced to raise and fill a "void near the top". Failure to do so invites new competition into the market, that un-checked, will displace the existing middle players.

    What's also new, and in fact more important to me, is your statement: "The future is climate- and building-specific applications". My primary interest in "the game" is that building envelope quality quickly take a large leap forward and consume less energy by a factor of 10.

    One significant component of that goal is how the window is placed into the "whole envelop", not just a wall. Good and improving products won't reach their potential without intelligent use. The intelligence has to come from good building designers first, and good windows suppliers second. We can't rely on the consumer for this level of knowledge.

    Yes. I agree that "boutique German-imported windows" are a sliver of the market and that the vast middle ground is where significant improvements in product and it's supply chain needs to happen. I'm hoping that we can affect that change by continuing with top end product improvements that require a good discussion for every application. A kind of a "higher tide requires all boats to rise" effect.

  14. albertrooks | | #14

    Comment to Martin Holladay and to a degree Bob Coleman
    To Martin and Bobs complaints about window vendors: "I wouldn't mind paying higher prices for high quality service if I were being provided high quality service, but that's not what I'm getting (except from the Canadians -- Thermotech excepted, of course).
    So in the U.S. we have we have the worst of all possible worlds: the window buyer has to do his or her own research, and then educate the window rep or dealer, and then we cross our fingers and hope that our order wasn't screwed up -- and we pay extra, because "this isn't a commodity business.""

    How sad for you both!

    Have you tried breath mints?

    Okay, that's probably not it...

    Being on the "selling side of the table" for many years, and still sticking my face out there for more, those comments strike a nerve.

    Guy's, I have to say that the American buyer is often just getting what he's paying for. It's always been the case, and now with a down economy, it's worse.

    What do you guy's want?? [Sorry to paint you with the broad brush but MH picked it up first.] Do you want intelligent reps and sales people... Or the race to the bottom on your price??? Your words say intelligence and availability for help. your actions on the other hand say the lowest price in the market.

    This Education that Martin and Bob want costs a lot of money. I'm quite sure that both of you underestimate how much and how long it takes to get it. If you don't want your suppliers to treat the products your interested in as a commodity, then don't treat your suppliers as a commodity. They are not, and are just as vulnerable to failure as anyone else.

    This attitude to products and suppliers has never been worse as it is in this downturn. For the most part buyers are wanting the help and then placing the order with the lowest price vendor.

    Come on! Where is there going to be margin in that scenario to pay for training and experience retention.

    There are plenty of vendors in the market that still care about what they do and what they sell, And would like to grow in product quality and range. Frankly, if you toss them aside for being over the price of another, then I'm happy if they toss you over too. What value do you bring? A sustainable partnership? Not.

    It consistently appears to me that America see's it's value in the price and not much more than the price. If that's our behavior, then the market has no incentive to add value where there is no return to pay for the added cost.

    Vendors, Designers and Builders are all stuck in the same ailing industry. Develop a "culture of mutuality" with your vendors or quit bitchin'!

  15. GBA Editor
    Martin Holladay | | #15

    Response to Albert Rooks
    I've already apologized for my earlier hyperbole. But I'm happy to respond to your questions.

    You asked, "What do you guys want?" Speaking only for myself, and not for any other guys, nor for the 50% of the population that is female: I want anyone who chooses to be a window dealer or a window rep to understand glazing and SHGC. I think that isn't too much to ask. While the situation may be improving, it's been dismal in the U.S. for years.

    I wrote an article on the problem for the November 2006 issue of Energy Design Update. To give you more background on what I found, I'll quote extensively from that article, "Choosing High-Solar-Gain Windows":

    "Window manufacturers have little incentive to educate their sales representatives on glazing issues. When EDU asked representatives of major US window manufacturers about high-solar-gain glazing options, many misunderstood the question. EDU contacted a representative for Hurd Windows, who responded to the question by listing the virtues of Hurd’s low-solar-gain glazing. Several manufacturers, including Pella and Weather Shield, seemed unaware of the existence of low-e glazing with a high SHGC; their representatives could only recommend clear insulated glass. When EDU called Jeld-Wen’s Bend Window Division, the company’s manager for codes and certification, Steve Strawn, responded, ‘We don’t have much in the way of high-solar-gain glass. It’s kind of a dead thing....’

    "Representatives at Great Lakes Windows were stymied by a request for high-solar-gain glass. Eventually a Great Lakes Windows representative named Molly Kersten responded by e-mail. ‘We at Great Lakes Window appreciate that you thought of us,’ Kersten wrote. ‘However, after learning a bit more about what kind of information you’re looking for, we’ve realized that we are not a good fit. We really don’t get many requests for passive solar applications.’

    "John Kurowski, a well-known green builder in Littleton, Colorado, has had similar experiences. ‘Most window manufacturers do not understand the glazing options that are out there, or the benefits and the challenges of different glazing types,’ Kurowski told EDU. Another Coloradoan, energy consultant Paul Kriescher of Denver, picked up the same theme. ‘Window manufacturers don’t understand glazing,’ said Kriescher. ‘When you talk to the window manufacturers’ sales representatives, they have no idea why you would want a high-solar-gain window on the south side of a house. It can be very frustrating. Sometimes they’ll say, ‘Well, we’ll have to research that.’ Then I have to tell them where they can get the glazing. It seems that the sales representatives have been given the mantra, ‘Go as low as possible with the solar heat gain coefficient. Keep it simple.’ Windows represent one of the most profitable areas of building component manufacturing, but window manufacturers are not spending any time to educate their sales people on this issue. It’s a very big problem.’

    "John F. Robbins, a designer of energy-efficient houses in Morningview, Kentucky, remembers the days when window manufacturers offered more glazing options. “Marvin used to have both a northern low-e glass and a southern low-e as standard options,” said Robbins. “Weather Shield used to also. But they have eliminated all those options as standards. ..."

    "Another frustrated solar home designer is Steven Strong, president of Solar Design Associates in Harvard, Massachusetts. ‘We used to use Hurd windows because they had a suite of Heat Mirror glazing options,’ Strong told EDU. ‘Then Hurd decided to get out of the Heat Mirror business. The American market just didn’t care, and they stopped making it. Now it’s hard to get a window with decent solar transmission, and I’m yearning for the good old days. I’d encourage the manufacturers by saying the leading edge of the industry is asking for this stuff. With energy costs having tripled, it’s time to get serious on this issue.’

    "... In much of the US, obtaining high-solar-gain windows is an uphill battle. In an e-mail to EDU, Dariush Arasteh, a staff scientist at Lawrence Berkeley National Laboratory, noted, ‘I know about the difficulty of finding high-solar-gain glass first-hand.’

    "According to Kriescher, ‘At Milgard Windows, there are signs of diminishing interest -- we’ve been running into roadblocks lately. First, it’s hard to find the person who could order the glazing. Secondly, we’re just getting general reluctance -- they seem to be saying, "Why don’t you talk to a competitor? We don’t really want to do this.” ’

    "Another energy expert who ran into difficulties obtaining high-solar-gain glazing is Scott Pigg, a senior project manager at the Energy Center of Wisconsin. Pigg recently finished overseeing remodeling work on his own house. ‘I had done a RESFEN analysis,’ Pigg told EDU. ‘I ran the simulations with hard-coat low-e and soft-coat low-e, and it made a big difference. We had decided to go with Marvin windows. When you call Marvin, they just ask, "Do you want a low-e coating?" That’s it for glazing options. I liked everything else about the Marvin windows, but I wanted a different low-e coating -- I wanted a hard-coat low-e for better solar gain. It turned into a long, involved nightmare. I asked my contractor, and my contractor had no idea what I wanted. When he talked to Marvin, the message was garbled. Then he came back to me, and said it would take three extra weeks. Then he said it couldn’t be done. I didn’t believe it, so I called Marvin directly. They said it was possible. Then my contractor said it would cost an extra $200 per window. I asked Marvin about that, and they wouldn’t give me any pricing information.’ Pigg ended up settling for low-solar-gain glazing, noting, ‘I will just have to suffer with windows that are somewhat better suited for southern cooling climates, as this seems to be where the entire industry is headed.’ ” [End of quote]

    So, Albert, what about pricing? The retail world has changed a lot because of all the technological revolutions that the Internet has made possible. I really don't think that greater transparency in window pricing is as complicated as you imply.

    The first window manufacturer to develop a Web site with an online calculator that displays window prices in response to shoppers who enter window sizes, window styles, and window glazing specs will garner lots of business. It will happen. Manufacturers' reps who complain about the pace of change can continue to put their heads in the sand; on the other hand, those who see changing technology as an opportunity to provide better service will find customers flocking to their doors.

  16. oberon476 | | #16

    A rather rambling response to Superwindows to the Rescue
    Since my earlier post was a good bit off topic (my apologies for that), I will attempt to stay on topic this time, but it's still likely to ramble....

    Direct solar heat gain aside for the moment, approximately 60% (give or take a bit) of heat gain or heat loss thru a window is radiant and the remainder is conductive. If a window manufacturer wants to limit radiant heat gain or loss thru the window (including direct solar heat gain this time) then they will be needing to use an appropriate LowE coating for that purpose.

    However, LowE coatings are not intended to limit conductive loss or gain thru the window system. In fact, LowE coatings actually increase conductive gains and losses thru the glass in a window system. How much they increase the gains or losses depends on both the coating and coated surface in the IG configuration.

    Given that the gain/loss increase is small and that it is many times offset by the improvement in radiant performance that the coating brings to the glass/window system, it's still a surprise to most folks when they find out that little bit of trivia.

    Modern LowE coatings really are to the point where radiant heat gains and losses thru the glass are becoming almost inconsequential when compared with conductive gains and losses; the push within the industry is to control conductive performance in the window.

    Tristan mentioned vacuum glazing which, as he said, is still somewhat in R&D mode at the moment, at least in our part of the world, but the seal is not the only issue being addressed; currently a vacuum IGU requires tiny stainless steel "pillars" placed approximately 1" on center throughout the entire IGU to keep the glass from collapsing inward from the vacuum between the lites. The pillars work, but they are visible in the glass and not everyone would find them aesthetically pleasing when looking out their window.

    As an aside, some folks may have heard that the older welded-glass dual panes had a vacuum between the lites...they didn't. They had air between the lites.

    Absolutely stagnant air has an R-value of 5, but as soon as that air is moving, then that R5 plummets. In the airspace of an IGU (Insulating Glass Unit) there are going to be convection currents that greatly affect the conductive performance of the window - the wider the space, the more the currents affect performance. The optimum airspace for a sealed IGU is going to be about 7/16" to 5/8".

    As Tristan pointed out, using an inert gas such as argon or krypton is going to improve performance by helping to limit those convection currents as well as affecting conductive heat movement across the airspace width as well. Krypton does it better than argon, but krypton also costs about 600 times as much as argon; which is one reason why you will rarely see an IGU with a wider airspace filled with krypton - it's darned expensive. And for the same reason, windows that have krypton fill are often actually filled using an argon/krypton blend rather than pure krypton.


  17. oberon476 | | #17

    Reply to Tristan

    First, thank you and you are welcome


    As you know, conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole, while convection is heat transfer by mass motion of a fluid (air for example) when the heated fluid is caused to move away from the source of heat.

    Both conduction and convection play a role in heat transfer inside an IG airspace. Conduction occurs molecule by molecule across the space. Convection occurs when warmer air (next to the warmer lite) moves up and cooler air (next to the cooler lite) moves down. Argon and krypton affect both convection and conduction by "gumming up the works". This may be the worst analogy that I have ever used (and I may regret it), but think of air as "water" and think of argon as "honey" and think of krypton as "molasses". Now imagine swimming from one side of a pool to the other thru the different liquids...water is easy, honey is harder, and molasses is harder yet....

    While this doesn't precisely (and that's an understatement!) approximate convection and conduction heat transfer, maybe it gives a very simple visceral image of the process? Or not????

    As noted, krypton is a good bit superior to argon in airspaces thinner than ~3/8" (the narrower, the more the improvment), while argon in that thin airspace isn't a huge improvement over air. Even at 1/2", krypton will slightly outperform argon as an insulating gas, but a cost versus added benefit graph would show that the slight insulating advantage of krypton doesn't come close to matching the increased cost of krypton.

    Also, LowE coatings will affect the formation of convection currents, good or bad, because of the effect the coating has on glass temperature.

    Side note - after posting this thing, I began thinking about the relationship between convection and conduction in the airspace and really comes down much more to convection to conduction at all widths. I called a friend of mine who knows more about that area than I do and he agreed that while conduction across the airspace plays a roll, convection is still the dominant factor.


  18. albertrooks | | #18

    Response to Martin Holladay
    Okay… Uncle… I give!

    Wow. Long and sad list. I should know better than to engage in hyperbola with an experience researcher and writer.

    I guess that's why us "boutique guy's" might have a place in the market. We might be those "one eyed guy's" in a blind industry. We can certainly do our part in teaching the benefits of glazing and placement with passive heating and transmisivity as the goals to both consumers and competitors.

    Pardon my defense of the sales reps. They get blamed for a lot. If the manufacturers Product Managers, don't understand and offer glazing and thermally broken frames for passive solar , then I think it's a little strong to lay the blame at the reps feet. These folks only are only going to know about the windows that they can sell, not the ones that they can't. You edited your self... I admit that they are not well versed in the industry needs outside of their own product range... End of story.

    Your online pricing idea is great and does not conflict with my "price shopping rant" at all. It could all be based on the list price. The dealers would then discount from list on a project by project basis based on project size. That's pretty much how it's done in the auto industry now. you can build a car from variables on the manufacturers web site and know the MSRP. From there, purchasing the car is an individual negotiation. I could see that the next step is that actual project quotes are created on the site as the site gets perfected with "dealer interfaces".

    My beef has only been with those who learn what they need to from a rep, and then shop it down in an unforgiving market. It always happens, and just like a builder who designs a house at his expense that another builder builds for less, I dislike it.

    Tristan's blog started out as education, sorry to jump into the "defensive rant".

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