GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Q&A Spotlight

In Search of the Most Energy-Efficient Windows

How do offerings from Serious Materials, Thermotech, Inline, and others match up out there in the real world?

Windows are an expensive but vital part of the building envelope. A number of manufacturers now sell super high performance windows at hefty prices, making the right choice all the more difficult.

It seems like a very long time ago, doesn’t it, that windows were considered simple building components? As long as they opened and closed and let in sunlight most of us were content. We know now that windows are anything but simple. They’re an essential part of an energy efficient building envelope; they must simultaneously admit sunlight (and a certain amount of solar energy — but not too much), minimize heat loss or gain, prevent drafts, and last a generation or two.

This week’s Q&A Spotlight from Green Building Advisor starts with a simple question from someone looking for just such a window. Claire Anderson wondered about the experiences that GBA readers have had with windows from Serious Windows, Thermotech Fiberglass, Fibertec, Inline and Accurate Dorwin. “I’m having a difficult time,” she wrote, “finding high SHGC windows (with a U-value less than or equal to 0.30) for my passive solar home that are affordable.”

As simple as it sounds, the question proved complex: Who makes a window with a high solar heat gain coefficient (SHGC), for solar gain during the winter, that’s also an effective insulator (low U-value equals high R-value), and doesn’t cost an arm and a leg?

So who’s really tops? It didn’t take long for GBA readers to dive in with their opinions.

Experience counts, but your mileage may vary

Among the most often mentioned high performance windows are SeriousWindows, manufactured by Serious Materials Inc. of Sunnyvale, California. The windows are made with foam-filled fiberglass frames, suspended films inside the glass, inert gas fill, and high R-values, common attributes in this category of uber efficiency.

Scott Heeschen selected Serious units for a south-facing clerestory and Marvin Integrity units for the rest of the house after getting quotes from a number of suppliers. He found SeriousWindows were the most expensive: about $60 a square foot for operable units and $40 a square foot for fixed units in the 725 series.

Heeschen also considered Fibertec windows (made by Fibertec Window & Door Mfg. of Concord, Ontario), which were about $10 cheaper per square foot in both operable and fixed versions. While the price was attractive, Fibertec had no local dealers. Serious Materials was only about 10 miles from where he lived, presumably making it easier to get service if he needed it.

Fibertec’s lower price was attractive to poster Boulder, CO, who “tried to save a few bucks” by choosing its windows over Serious Windows. But Boulder found poor customer service, fingerprints between panes of glass, and gaps in the mitered corners. Ken Huck also found Fibertec windows “not the highest quality,” although they were much less expensive than another oft-mentioned brand, Thermotech, made by Thermotech Fiberglass Fenestration Ltd (not to be confused with Thermotech Windows Ltd).

Andrew Henry found Thermotechs were a pleasure to install and came with high quality Truth hardware that drew the sash tightly to the frame in the awnings and casements he selected. The triple-glazed units did a much better job of keeping out a cold winter chill than the older Marvin double-hungs he had in the original part of the house. Heeschen, however, found Marvin’s Integrity windows a step up.

Brent, however, was totally unimpressed with his Thermotech experience, citing unresponsive customer service and lackluster tech support. He found more to like with Inline and Heinzmann European windows.

Accurate Dorwin was another brand that wins rave reviews. “These were by far the best windows we ever had…tremendous strength and quality,” writes Stan, adding that the service and installation experience was the best he’d had in 25 years of building.

In short, there are no absolutes for defining “quality” and no universal experience even with the same manufacturers. Anecdotal evidence from builders and homeowners varies. In addition to the window itself, how local distributors deal with potential customers is certainly a factor. Poor customer service can make a great window seem like a bad buy, while responsive tech support can cure a lot of ills.

A detailed explanation of performance features and how they relate to the German Passivhaus standards was the subject of an earlier Holladay blog at GBA. He also offers some details on Serious Materials products, which use multiple layers of Heat Mirror film instead of triple glass glazing, an approach that has some drawbacks.

How to sort it all out? Site-specific requirements are a good starting point. Then, in addition to manufacturers’ web sites, visit the National Fenestration Rating Council. Its data should weigh heavily in making a selection.

Yes, but are they really ‘green?’

The very advances that make high performance windows possible are in themselves off-putting because they suggest an endless cycle of buying and replacing as technology improves, one of several anonymous posters laments.

Anonymous was in the midst of replacing 100-year-old sash. His observation? Windows are not so much rebuilt and repaired these days, simply replaced.

“To my knowledge, no one manufactures a sash where when the glass seal fails, or the glass gets broken all you need do is go to your local hardware store and buy new glass. As for being green, the direction window manufactures have taken us appears to be contradictory. What are the energy costs of manufacture and how much oil is being consumed to manufacture all this plastic.

“In my opinion, green needs to take the whole picture into account, including manufacture, maintenance, and product life.”

Although Anonymous worries he’d strayed off topic, Robert Riversong assures him he’s right on target. “In fact, your complaint is equally applicable to almost every product we buy and use today,” Riversong writes. “There is very little in the ‘green’ movement that is truly sustainable or earth-friendly or even sensible.”

Riversong would have us ponder the thoughts of writer Wendell Berry, who suggests several criteria for the introduction of new technology, including requirements that new tools should be cheaper, smaller, more efficient than what they replace.

Tax credits and third-party testing

Homeowners are now eligible for a federal tax credit of 30% of the cost of energy efficient windows under the American Recovery and Reinvestment Act, but there’s a catch. In addition to having a U-value of no more than 0.30, windows also must have a solar heat-gain coefficient of 0.30 or less, and that’s a problem, as described by Holladay in an earlier post.

In cold climates, a SHGC of between 0.39 and 0.65 actually saves more energy. Low SHGC windows are a good choice in Florida to lower air conditioning costs, but elsewhere a high SHGC means more solar gain and lower heating bills in the winter.

It’s difficult and in some cases impossible to find high windows with high solar gain potential, as our original poster Claire had found. Coupled with the federal government’s bungled attempt to promote energy efficiency with the AARA initiative, homeowners are likely to install windows that will deliver second-best energy performance.

Finally, there is the issue of third-party performance testing, which backs up manufacturers’ marketing claims and ensures homeowners are getting what they pay for.

While Thermotech Fiberglass Fenestration is roundly praised, Steven asks how come the company doesn’t have its windows listed with the NFRC?

We did, says the company’s technical director Stephen Thwaites, back in 1999 when we were part of Thermotech Windows. The company didn’t renew in 2003 because “building officials weren’t (and in our experience still aren’t) looking for NFRC stickers. More importantly it didn’t seem to matter to our customers either.”

However, because of a Canadian rebate program, Thermotech Fiberglass will be rejoining fold, Thwaites writes.

A GBA expert speaks

We have invited senior editor Martin Holladay to provide an expert opinion on high-performance windows.

Martin Holladay’s advice:

Cold-climate builders need windows with a very low U-factor, a very high SHGC, and a very high VT. That magic window doesn’t exist, so we have to spec our windows based on a series of compromises.

Although most of the Heat Mirror glazing (that is, glazing that includes interior plastic films) used in Serious Windows has a very low U-factor, these windows come with a couple of drawbacks: the windows have a lower SHGC than comparable all-glass triple-glazed windows, as well as a low VT rating. Some homeowners with Serious Windows have been bothered by the appearance of these low-VT windows; they can make everything look gray.

Most of the Canadian manufacturers of windows with pultruded fiberglass frames are knowledgeable about glazing options. As long as you specify high-solar-gain triple glazing, you’ll probably be satisfied. (Aim for a window with a whole-window SHGC of at least 0.39.)


  1. mike | | #1

    broken windows
    "To my knowledge, no one manufactures a sash where when the glass seal fails, or the glass gets broken all you need do is go to your local hardware store and buy new glass."

    since most windows these days are custom jobs, with double or triple pane glass - it isn't feasible for hardware stores to stock the stuff...

    however, most of the euro wood window mfrs can ship new glass relatively quickly and the glass can be replaced by removing glazing stops... so it is serviceable...

  2. Brian O' Hanlon | | #2

    European glass
    Getting any information about climate zones in Europe like you have in north America, seems to be quite difficult. This is one of the few things I have managed to come across, in relation to window technology.

    I'll be honest, the big European manufacturers ship their product all over the EU region. But it never occured to me, until I learned something about your north American difficulties with glazing - from reading Martin Holladay's blog entries - that the glass in southern europe, may need to be different from that installed up north.

    I know a lot of folk in the glazing industry and have handled some fairly large orders. But it took GBA website to educate me in relation to this. This website offers a world of value for research, by the average European.

  3. Valerie | | #3

    Very interesting comparisons.
    Very interesting comparisons. I would like to add some important points for clarification:

    - Reiterating what was said (but we can’t emphasize this enough): the “most” energy efficient window depends on climate zone, placement and orientation, and % of window to wall area. No one size fits all.

    - Based on Martin’s criteria for the best window – very low u-factor (that means high full frame R-value), high SHGC, high VT – we can’t find another manufacturer that has a standard product that comes close to a 0.14 u-value (R 7.1), 0.42 SHGC, and 0.57 VT. That’s our standard fiberglass 925 series picture window. For Passive House projects in Northern U.S. areas, this may be the best performing window available anywhere at this time. You can browse through our standard fiberglass series and glazing options based on performance here:

    - Those are our standard offerings. Last week on our blog, we highlighted a net zero project in Oregon – where “tuning” was critical to performance – and allowed for max glass use in the design. Now, the state of Oregon has its own climate challenges, certainly. Our solution was 4 different glazing packages for all 4 orientations of the house. The South facade (43% window coverage), required windows with U-value of .22 (R 4.5) and SHGC .54 – for maximum winter heat gain. Read more about the tuned approach here:

    - There are some drawbacks of triple pane windows, including the limited performance range that can be achieved (some SeriousWindows have up to 4 chambers to improve R-value at center of glass).

    - We make both fiberglass and vinyl high R-value windows with selectable high or low solar heat gain. Vinyl windows may be 70% less expensive than fiberglass windows, for those who want very high energy performance on a limited budget.

  4. Dan Kolbert | | #4

    We built a house last year (I wrote about it in JLC - a cold climate and used Thermotechs. The client has raved about how warm they are on blistering winter nights. We've outperformed our energy model by about 40%, and I think the windows had a lot to do with it.

    We've had some technical problems, and recently the owner himself came to our jobsite. I'd use them again.

  5. John Brooks | | #5

    Dan Kolbert Article & Project
    Your Article is one of my favorites.
    The article, project and illustrations are very good.
    I would recommend the JLC Online subscription to Brian and others.
    The Thorsten Chlupp article and a couple of recent ones by David Joyce ....very good.

    The JLC Building Science Forum can be very good at times also.

  6. Dan Kolbert | | #6

    Thanks, John. Our next project will, it looks like, use Serious, and I'm looking forward to seeing how those perform as well. Heaven forbid we have several choices for excellent windows!

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Valerie
    You're right that Serious Windows can offer decent specs in a fixed window -- but so can the Canadian manufacturers.

    Too bad the same specs aren't available in a Serious operable window.

  8. Tomas Kocis | | #8

    PH Windows Ug-10, SHGC of 0.6, VT of 0.74
    I would like to contribute with our specs for windows that we import from Europe.
    We started to import European PVC Schuco windows for passive house projects.
    We offer PH Certified windows with the following glazing specs:
    1. Ug-0.105, SHGC of 0.5, VT of 0.71
    2. Ug-0.105, SHGC of 0.6, VT of 0.74

    I am happy to answer any question here or you can contact me directly at [email protected]


  9. Russ Hellem | | #9

    We have been asked for years how the various windows compare, so last year we decided to put all of the information we could find in one place. The spreadsheet is comprised of all the high performance (we deemed .30 or better good enough to make the spreadsheet) windows we could find and got performance info and pricing for a 2040 Casement window. Granted we do not have every window available, but we do have a good start. If anyone has the applicable information available please email the information to us. You can find the spreadsheet on our website at

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Russ
    As far as I can tell, you forgot to answer an important question: are these whole-window specifications (including the frame) or glazing-only specifications? As you probably know, European manufacturers usually provide glazing-only specifications (especially for SHGC), while US manufacturers usually provide whole-window specifications (as required by the NFRC).

  11. Garth Sproule | | #11

    Question for Martin
    Would it be possible for GBA to post and maintain a window comparison spreadsheet similar to the one @"energetechs" ? Perhaps adding separate comparisons of fixed and operable windows?

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Garth
    I published my own shreadsheet back in March, but you may have missed it. Here's the link:
    Choosing Triple-Glazed Windows.

  13. Garth Sproule | | #13

    Thanks Martin
    Yes, I remember that now... all info is there except prices. But I am sure you have your reasons for this...

  14. User avater GBA Editor
    Martin Holladay | | #14

    Price information
    I love price information! I always invite people to share it. It's all over the map, however, because pricing is set by local dealers, not manufacturers.

    Although many people give prices in $/sq. ft., this is tricky, since a large window sometimes costs about the same as a small window.

    I urge everyone to share their pricing information.

    In 2008 I paid $58 per square foot for two large triple-glazed Thermotech windows.

    This year I paid $65 per square foot for 4 triple-glazed Inline windows (one is a fixed window).

    I was quoted a price of $51 per square foot for the same sized windows by a Paradigm dealer (vinyl windows, triple-glazed, with krypton).

  15. Garth Sproule | | #15

    Maybe a column with MSRP's for a certain size of window would be a useful comparison ?

  16. Joe - ecoENERGY advisor | | #16

    Fibertec Windows
    I have Fibertec triple glazed, Krypton filled, fiberglass frame windows in my home.
    The low e coatings are in a "glazed by house orientation" approach.
    Installed them about 3 years ago and am very happy with the performance and quality.
    Paid $54 per sq ft, mostly casements.

  17. Jim | | #17

    As pricing is very subjective, an apples to apples comparison should include whether you went directly to manufacturer or through a dealer. This way this would be a true and fair comparison. I agree with Garth, someone should direct prices from the manufacturer.

    The prices above don't add up.sorry.

  18. Jerry | | #18

    The bigger (window) picture
    Concern for high R-value and low air infiltration decreases significantly when a home's orientation is switched from super-insulating to passive solar design. A strong passive solar design has such a massive thermal flywheel that the largest percentage of glazing, the south wall in cold regions, can be double pane clear glass in low-embodied-energy wood frames. This saves a tremendous amount of money and environmental impact. Night-time insulation can be added, especially to reduce the radiation effect, but most important is that strong passive solar design: one story homes, wing-insulated foundations, and plenty of mass inside the insulation envelope. KISS rules.

  19. User avater
    Brian Knight | | #19

    I disagree with Jerry, Weathershield Prices
    The most important thing for a Passive Solar Design is an Air-tight, Continuously insulated envelope. Free heat does you little good if you cant control where it goes and this depends on the building envelope FAR more than internal thermal mass.

    These new windows are very exciting. I hope that consumers begin to recognize their benefits and prices come down. Weathershield fiberglass line was a good option for us last year. On the passive solar side we went with a double pane Cardinal 179 glazing: the 6x6 and 6x4 fixed were around 23$ a sqft while the triple pane 3x3 casement was 35$ a sqft through a dealer before taxes. Interestingly, the 2x3 triple pane jumps to 55$ a sqft. The 2x3 double pane with HSGC glass was only 6 bucks less at 48$ per sqft. This is a good window sizing lesson to those designing Passive Solar on a budget. The performance was a little lower with their product but the price difference made for a very long payback in our climate with the other brands mentioned.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Maurice
    In a comment posted on this GBA page, builder Dan Kolbert wrote, "while we think the ThermoTech windows have clearly added to the house's performance, we have had ongoing warranty issues over some relatively minor problems that have soured our relationship with the company. As I've told them directly, they desperately need a service and warranty division or they will just add to their list of former fans."

    I responded with my own comment: "Thermotech Fiberglass makes great windows, but reports of slow customer service from Stephen Thwaites, the technical director of Thermotech Fiberglass, are legion.

    "Such reports are so common, in fact, that builders of superinsulated houses have a shorthand expression they use to refer to the customer service situation one experiences after contacting the company about a problem -- it's called "Thwaiting for Thermotech." (I can't take credit for coining this jewel, unfortunately.)

    "After I requested a price quote for some windows for my own house, I had to thwait for Thermotech for almost a year before I got a response.

    "Another customer who got tired of thwaiting for Thermotech was David Pill, who posted a report of his unhappy experiences on this GBA page."

  21. Maurice Bouchard | | #21

    Don't Buy ThermoTech
    I agree with Brent (and shame on me for not reading the above article sooner)

    My wife and I are building a passive solar home in upstate NY and have selected Thermotech Fiberglass as the window vendor... a huge mistake as it turns out. They may have a good window but they are extremely difficult to deal with. We are now in week 17 of an alleged 8-10 week lead time (we wired $15K on 6 Jul 11 as a deposit) with no end in site. When we haven't been ignored, we've been misled, misdirected, and lied to. The order processing and quoting is a joke. Customer service is non-existent and their shop floor systems and procedures are farcical. Stephen Thwaites, putative managing director, has no control over the company, has no idea what is happening on his shop floor and may have the worst customer service "skills" of anyone I have had the misfortune to deal with. When asked a specific question about why my order wasn't getting the attention it deserved, Thwaites said, "You're assuming there is a logical rationale in our [shop floor] procedures." He blames his employees for his company's problems, won't return phone calls, and won't stand behind what he says. The company culture is excuse-making and equivocating. Everyone I've spoken to at Thermotech makes excuses first, before any commitment (which then aren't kept). It sickens me to think that, eventually, I will have put ~$52K in the coffers of this poor excuse for a company. Don't buy from ThermoTech. You'll regret it. (Mr. Thwaites, I challenge you to point out where I'm wrong or where I've exaggerated).

    I will be blogging about the entire ordeal at Comments welcome.

  22. Ben Raterman | | #22

    Suspended Film windows
    I am in the market for quality windows for a house I will be building. I am considering Serious Windows among others but have heard that over time the suspended film used in these windows can break down and "pit". I can find no documented evidence of this tho. Does anyone have any corroborating evidence (or evidence to the contrary) of such a phenomenon?

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Ben Raterman
    Serious Windows use Heat Mirror glazing with suspended plastic films. The glazing has been around for a long time -- since the late 1980s, I think.

    There were, indeed, problems with the original glazing. Southwall Technologies (the manufacturer of Heat Mirror glazing) claims to have improved the product and overcome the problems.

    My opinion: the jury is still out. Time will tell.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |