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For southern exposure, what do you think is the better glazing option: 1. U=.29, SHG=.49 or 2. U=.19, SHG=.37 ?

rocket190 | Posted in Energy Efficiency and Durability on

Both windows would be installed near Green Bay, WI, climate zone 6. The lower u-value window is vinyl and the other is fiberglass, but aesthetically, both are acceptable and equal for my tastes. The vinyl are manufactured by Wasco Windows (based near Milwaukee, WI) and the fiberglass alternative are Marvin Integritys.

The square footage of glazing along this wall is approximately 5.5% of the floor space. The finish floor will be a 5″ concrete slab that will have no additional floor covering. The slab will be insulated to R-10 and the perimeter frost walls are insulated to R-20 except at the slab edge, where I only had “room” for R-10.

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  1. rocket190 | | #1

    Also, the Marvin's are dual pane. The Wasco spec is for triple pane. If I chose to go with a Wasco dual pane, they would have similar specs to the Marvin, although they would probably have a slightly better u-value.

    Based on prior reading, it seems as if some people feel the additional solar heat gain of the dual pane is more attractive for the southern exposure, at the expense of r-value. Others seem to argue that triple panes are necessary if you're serious about glazing.

    Since I carefully sited my building and placed the majority of the windows on the southern exposure to take advantage of the winter sun, I want to make sure I'm making the right call.

    Thanks for any insight.

  2. mackstann | | #2

    I'm a bit naive here, but you're cutting 24% of the heat gain and cutting 34% of the U-value. Not only is the U-value improvement proportionally greater, but it is useful year-round, unlike the solar heat gain.

  3. STEPHEN SHEEHY | | #3

    Is there some reason why you are limited to those two? You should be able to get better u value and better solar gain in the same window.

  4. jackofalltrades777 | | #4

    Why are you limiting yourself to only 2 options? There are better options out there and you can have your cake and eat it to by getting triple pane windows (high R-Value of R-7) while also having the high SHGC you need/want (0.49+). Intus Windows makes awesome performing triple-pane windows that have uPVC steel-reinforced frames and get DP ratings of up to DP70 (225 mph winds). One can get a U-Value of 0.10 (window) - U-Value 0.15 (window + frame) and still get a SHGC of 0.37 or all the way up to SHGC 0.62.

    I would expand my search and get a bid from Intus Windows.
    A lot of forum members have utilized those windows with great results. You are in a Zone 6 climate so you would be throwing money out the window by not utilizing a high SHGC on the south side of the home. I'm in Zone 4 and I am utilizing passive solar on my home.

  5. GBA Editor
    Martin Holladay | | #5

    First of all, I agree with the other responders that you may wish to consider other window manufacturers who offer other glazing options. For more on this topic, see:

    All About Glazing Options

    Choosing Triple-Glazed Windows

    If you really want an answer to your question, you'll need to use a software program like RESFEN or PHPP to model the performance of the two options for your house and your proposed window sizes.

    One final point: it doesn't make economic sense to install large south-facing windows in hopes that the solar gain will lower your energy bills. So keep your windows reasonably sized -- you don't want oversized windows. For more on this topic, see Study Shows That Expensive Windows Yield Meager Energy Returns.

  6. rocket190 | | #6

    Thanks for the help and for the links. I've read and re-read all those articles. I'm not limiting myself to those two windows, but they approximate the energy range of many "pretty good" windows available. I would like to buy from a local supplier for various reasons, which is also a factor. I'm very intrigued with the Intus windows, but there is no dealer in Wisconsin. The nearest dealer is Michigan. Has anyone ordered factory direct from Intus with good success?

    I'm really trying to figure out the economic gain (or loss) of shooting for high solar gain. I'm by no means trying to do a passive solar building, but since my finish floor is concrete I thought it might make sense. To Nick Welch, in my climate, useful solar gain will be useful for approx 75% of the year. I typically have a heating bill from late September-Early May,

    I haven't used RESFEN before. Will that help quantify the economic impact of glazing?

    In my discussion with the owner of WASCO (and the chief engineer) I can order a window with a u-value down to the U.13 range, but the price is approx double of their standard triple glazed model with the U-value of .19

    It's an internal struggle. Do you buy the best now and pay through the nose? Or do you pocket the savings and upgrade in approx 20 years to a hopefully far superior product? Regarding comfort, I have used some of the higher quality Simonton vinyl windows before, and even in a double pane, I had no condensation or comfort issues.

  7. STEPHEN SHEEHY | | #7

    Rick- I am about to order a house full of upvc Intus windows. As an example, the price quoted for a 40 x 48 window, U .10, SHGC .49, VT .71 is $575. That is a triple pane, tilt turn window.

  8. rocket190 | | #8

    Wow, that is a pretty darn good price. Do you know what their lead time is from order date to arrival? Also, did you have a local dealer to work through or did you go factory direct?

  9. STEPHEN SHEEHY | | #9

    Rick-12 week lead time. My contractor is going through Maine Green Building in Portland. I was also leery about contracting directly with a European company, in case there is a problem of any sort. Having a local dealer with a good reputation eases my worry. Obviously, extreme care with the ordering is essential, since getting the wrong window would cause a major delay.

  10. Stephen Thwaites | | #10


    The numbers you provided - are they Uglass and SHGCglass?
    If so, can you provide Uwindow and SHGCwindow?
    (The latter better characterizes the effect the window will have on your energy bills - and makes it easier to answer your question)

    Thermotech Fiberglass Fenestration

  11. GBA Editor
    Martin Holladay | | #11

    Translation of Stephen's question:
    Are we talking about whole-window U-factors and whole-window SHGC numbers (as reported on NFRC window labels) or are we talking glazing-only U-factors and glazing-only SHGC numbers (as some European manufacturers like to tout)?

  12. oberon476 | | #12

    I would venture to guess that the performance numbers from Rick's original post were whole window while the numbers that Stephen Sheehy received from Intus are glass-only "translated" from the original european specs.

  13. rocket190 | | #13

    The numbers I used in the question heading are whole window NFRC (for the Wasco and Marvin). Based on the incredible specs that Stephen Sheehy shared, I would presume those are center glass. I've initiated an inquiry with Intus and will post with additional information as soon as it becomes available.

    One thing that is very disappointing is the long lead time from order to delivery. Intus is saying 12-14 weeks. Wasco is quoting similar long lead times for their European style windows built in the Milwaukee, WI factory. One would think that a quicker turn around would be possible, but it seems that WASCO is experiencing massive sales growth of their higher end models. Intus Windows are manufactured in Lithuania, which partially explains their long lead time.

    Either way, it's a far cry from the 3-4 weeks I was accustomed to from the more popular domestic manufacturers.

  14. jackofalltrades777 | | #14


    Whatever you do, please don't use RESFEN. It is very problematic and gives incorrect results. A few GBA members have gotten some bizzare results that were contradictory. The program is a "free" government program so take that for what it's worth. I would go the route of PHPP if you want an accurate result.

    As far as Intus lead time goes. It is about 8-12 weeks. If you call them directly they can put you in contact with your local reps. Bang-for-buck they cannot be beat. Many forum users have used them and some are even Passive House certified homes.

  15. jackofalltrades777 | | #15


    Most domestic manufacturers are mega-volume standard size window manufacturers. That means at any given time they got hundreds, if not thousands of "standard" window sizes available on the warehouse floor. Just go to Home Depot and you will see what I am talking about. Any custom home with custom sized windows will require more lead time.

    In the end, it's about quality and performance. If you don't mind waiting an extra 8 weeks for quality windows, then it's a simple decision on the high-performance windows. When you compare an Intus Window to a mega-volume domestic manufacturer, there is no comparison. It's like comparing a Maybach to a Yugo.

  16. STEPHEN SHEEHY | | #16

    Here's what I got from the INTUS supplier:

    Fittings: Multi-Matic
    Frame: INOUTIC passive house window profile
    Color: White/White
    1: Prem4x20Arx4x18ArxPrem4, warm edge spacer, Ug=0.088 Btu/hr sq.ft F, SHGC=0.494, VT=0.709 (4)
    Sash: 1: Tilt & turn

    This is from the INTUS website:

    Values: SHGC 0.25 – 0.62, Designed Pressure (DP) 70, Air Seal 0.01, dB – 47 (noise insulation)
    U-Value Ranges: Ug 0.053 – 0.106 (glass), Uf 0.1287 – 0.16 (frame), Uw 0.102 – 0.16 (window)

  17. Stephen Thwaites | | #17

    Short Answer:

    Martin is correct even simple software like RESFEN will give a better answer than conjecture. My experience on this subject is that in heating climates conjecture, is often wrong.
    In heating climates, despite a lesser insulating ability, Solar Gain low-e almost always yields lower net energy bills compared to Solar Shade low-e.
    But Rick's question is not about comparing doubles to doubles or triples to triples. His question is about comparing a double to a triple, both with what appears to be a Solar Gain low-e for a south elevation in Green Bay.
    In this case I think within the error limits of simple software, both options will produce similar heating bills.
    I would go with the triples based on the extra comfort and condensation resistance.

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  18. Stephen Thwaites | | #18

    Long Answer:

    Martin is correct even simple software like RESFEN will give a better answer than conjecture. My experience on this subject is that in heating climates conjecture, is often wrong. While PHPP, Energy 10, HOT2000 or ESP-r/HOT3000 are all great tools, few homeowners will pay a simulator for a better answer than RESFEN.

    In Canada we are 'lucky' to have only heating climates. This allowed the development of a single number window rating that combines the effect of window gains & losses (similar approaches exist in the UK and Denmark).

    The Energy Rating (ER) is actually a good window screening tool for PHPP, as higher Energy Ratings virtually always produce lower heating bills in heating climates.
    Unlike PH window rating schemes, the ER accounts for the light blocking effect of the frame, so bulkier framed windows get penalized from 'the get go' rather than later, in the running of PHPP.

    To address the 'averaginess' of ER (it's based on an average number of degree days and an average orientation in an average CDN city), there is a more specific variant; the ERS (Energy Rating Specific). It is for a specific City and Orientation.

    Here in Ottawa, ON we have about (pronouced a-boot in these parts) 3% more Heating Degree Days than Green Bay. Not sure about the differences in Solar Gain – probably somewhat sunnier there than here. So while the weather data is not perfect, it is a first step in helping Rick with an answer to his question.

    For his choices, my calculated ERS's for the south facing windows were both Net Gains. The double glazed windows gained an about 1 kWh/ft^2/yr. more than the triples. I wouldn't ever 'bank on' these numbers. However, it was comforting to see RESFEN5 (using Green Bay weather files) produce a similar result. I guessed like crazy on the characterisitics of the house and got 2% lower space conditioning load with the double glazed windows facing south compared to triples everywhere. Different guesses will produce different results.

    So one last thought on Rick's question -- Based on Greg Smith's comments and some more digging I will guess that Rick's numbers are for operable windows, casement perhaps? Where possible, particularly facing south, I would suggest using low profile fixed windows rather than that matching higher profile “fixed casment” to increase glass area and increase the whole window SHGC.

    So if it was me i'd take the comfort and condensation resistance of the triples and pick up some extra solar gain with low profile fixed windows on the south. Could be a good compromise.

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  19. oberon476 | | #19

    An interesting follow up to the information in the thread comes from Window&Door magazine this morning:

    Wasco Window System Earns Passive House Certification

    Window & Door
    July 3, 2014

    Wasco Windows of Milwaukee has achieved PHIUS (Passive House Institute U.S.) certification for its Geneo window. Developed by German company Rehau, Wasco's Geneo window is the first American-made window system that meets European Passivhaus standards, it reports. Wasco has been manufacturing Geneo windows since 2012.

    “The certification will give an architect designing a passive house all of the inputs needed for the detailed software inputs required. And, with the PHIUS certification, it is easy to keep the proper paper trail for certifying a passive house," says David Paulus, part owner of Wasco.

    “In the past few years, people who wanted these top-of-the-line products in their new construction homes had to have them shipped from Germany, Austria or Poland,” Paulus adds. “We knew it would be much more economical and sustainable to move the profiles and hardware here to the U.S.”

  20. oberon476 | | #20

    Good morning Rick,

    Both Marvin and Wasco use Cardinal glass in their windows.

    In North American both SHGC and VT window performance numbers are determined from the full window and not just the glass performance.

    I agree with Stephen Thwaites that the numbers you posted are based on the use of high solar heat gain products, but I am surprised that the Wasco SHGC is only 37%; that just seems to be lower than I would expect unless the window that you are looking at has interior grills or else the sash/frame covers a higher percentage of the overall window size than most other windows (North American).

    Also like Stephen Thwaites mentioned, I personally happen to like triples more for increased comfort and condensation resistance than for the amount of energy saved, which is not to say that I don't like the improvement in energy savings as well, but a big part of energy savings can be based on comfort simply because comfortable people don't play with the thermostat.

    Which also happens to echo in some part why Passivhaus is so adamant about window performance.

  21. rocket190 | | #21

    Wow, thanks for the tremendous help from all the experts out there.

    In response to Stephen Thwaites, the windows on my southern exposure are all to be fixed picture windows. However, I had some of my specs incorrect, which I will correct below

    Wasco Triple "Sustainability Glass" HSHG
    Price $390 for my size window.....approx 16 sf of glass per window
    Of note, these are not the Geneo series windows from Wasco, which would have gotten me to approx Uw=.14.

    I also have specs from Intus:
    SHG=.62 HOWEVER.......They don't provide a whole window SHG, so their number is center glass. It probably will be higher than Wasco after adjusting for frame losses.

    The Intus represents a 35% increase in R for a 37% price premium. As they say, there is no free lunch.

    I'm leaning toward Wasco since they are made approx 90 miles from me and have a lifetime warranty that can be transferred once.

  22. rocket190 | | #22

    I forgot to add the Intus window price of $536 for the same size.

    Both manufacturers have 12 week or longer lead times.

  23. Stephen Thwaites | | #23

    For what its worth;

    Comparing the south facing Ottawa ERS for the double glazed option and the revised triple glazed option;
    By my calculations, the revised triple glazed option will gain about 5 kWh/ft^2/yr more than the double glazed option (this is a 6 kWh/ft^2/yr improvement over the original triple glazed option)
    This result shows the effect of a higher whole window SHGC.

    Thermotech Fiberglass Fenestration

    PS in case it wasn't clear from my previous post, the ER and ERS are not intended to replace simulations. They are intended to rank window option's effects on heating bills - so my use of them is comparitive rather than predictive.

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