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Best choice for windows in passive solar house?

Lynn McDonald | Posted in Energy Efficiency and Durability on

We have a passive solar house in northern New Mexico and we need to replace our 19 4’x6′ clerestory windows. We seem to have the choice between clear glass double pane windows, or Low E windows with an SHGC of .29 and a U factor of .31. I know that a higher SHGC window would be ideal, but that doesn’t appear to be an option in our price range.
So, is it better to sacrifice heat gain for better insulation, or sacrifice insulation for better heat gain? Summer overheating is not an issue — I’m worried about winter heating. Thanks for any advice!

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Replies

  1. Greg Smith | | #1

    You do not want clear glass windows, you want a LowE coating. However for a passive solar home you don't want a low solar heat gain coating, which is what you are getting with a .29 SHGC, you want a high solar heat gain coating.

    What window company are you dealing with? And did they tell you what coating is in the windows that you are looking at?

    You do have other options besides the two that you mention.

  2. Lynn McDonald | | #2

    Hi Greg, thanks for your response. The windows are Simonton brand, soft coat, argon filled, sold and guaranteed through one of our local big box stores, and the only option they offer is the one described above (and clear glass).
    This is the second time in 8 years that we've had to replace all of these windows because the install was done incorrectly the last time and the window company wouldn't honor the warranty. It's important to us to have the windows and installation guaranteed by the same company this time to avoid the finger pointing and another expensive replacement cycle! I'm having trouble finding a high SHGC lowE coating in our area, guaranteed by a company that is likely to be around awhile.

  3. Lynn McDonald | | #3

    Hi Kevin. It's definitely true that the window dealers don't seem to understand the concept of passive solar heating, so it gets frustrating. It looks like Pella may have a glazing that is pretty good, but I'm not sure we can afford them. I have an appointment for an estimate, so we'll see. However, I guess my question is really more general -- given a choice between low SHGC but well insulated windows, and a clear glass high SHGC window, which would be better for a passive solar house? I know that neither is ideal, but which is the best choice between the two? Thanks!

  4. Greg Smith | | #4

    Good morning Lynn,

    I am not knocking the big box stores, but when you go there you are dealing with a clerk, however helpful, who has very limited exposure to different options that are available - and not just in windows.

    When talking about window options, the clerk has been taught that the lower the U value and the more solar heat blocked, the better the window. "Why would anyone want a window that lets the sun heat in? That just makes no sense at all." He or she is responding as they have been trained to respond.

    I would suggest that you consider moving away from the big box store and finding a Simonton (or other window) dealer near you and talking with them. If that isn't an option then I would recommend that you return to the big box and tell the clerk that you want LoE-180 coating on IGU surface 3 of your windows or else you will not be able to buy your windows thru them.

    LoE-180 is a designed high solar heat gain coating that is readily available to Simonton thru their glass supplier. It starts with follow up on the part of the clerk, and his or her rep with the window supplier.

    With clerestory windows, especially the size and number that you posted, and your only options being clear glass or low solar heat gain coating, I would be inclined to go with the coated glass because of both comfort and energy performance.

    Summer not being a problem, unless you have some way to insulate those windows when the sun isn't shining in winter I would be concerned that when using clear glass they are losing more energy than they are gaining.

  5. Lynn McDonald | | #5

    Hi Greg,
    Thanks so much for your most helpful advice. I am convinced that we need to try to find a better window option, so will do some more research with dealers in our area. Pella looks promising with their Natural Sun product, so I suppose I need to balance the higher initial cost against the long term savings (and increased comfort).

  6. Kevin Dickson, MSME | | #6

    Lynn,

    I've considered going with clear glass on the south elevation of a passive solar house. The performance could be pretty good combined with nighttime window insulation, but it's a distant second choice to using the right glass to begin with. Greg's concern is valid, especially if the occupants aren't diligent with the window insulation.

    (BTW the best window insulation is probably magnetic-edge window quilts, which make an R6 assembly with mylar. There is no problem with window condensation like you get with cellular shades - http://www.1windowquilts.com/products/full_broch.htm)

    The extra effort needed to get the correct south facing high SGHC low E glass is worth it for two reasons:
    1. When the sun is coming through the window during the day, the window loses significantly less heat than clear glass.
    2. The window loses significantly less heat when the occupants aren't around or forget to close the window insulation at night.

    The big box stores typically use local independent subcontractors for installation. So you could still wind up with finger pointing.

    Since your house has 19 windows, that's a lot of time to move the window insulation every day. So assuming no nighttime window insulation, the answer to your original question, "is it (1)better to sacrifice heat gain for better insulation, or (2)sacrifice insulation for better heat gain?" is (1) "low SHGC but well insulated windows".

    RESFEN window simulation software can be used to understand the answer in finer detail:
    http://windows.lbl.gov/software/resfen/resfen.html

    Note - I also made the assumption that you prefer the view and the light provided by the 19 windows to a solid wall. A solid R19 wall would actually perform better and be a lot cheaper than low SHGC windows.

    Have you figured out why the windows keep failing?

  7. Kevin Dickson, MSME | | #7

    Nine out of ten window companies you won't get the answers you are looking for. Solar glass to them means low SHGC and no amount of explanation will help.

    Sometimes they understand when you mention "hard coat" Lo E.

    Alpen and Hurd are the only US manufacturers I know of that understand what you want.

    Here are some threads to read: https://www.greenbuildingadvisor.com/blogs/dept/musings/high-solar-gain-glazing
    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/14850/serious-windows-vs-thermotech-fibertec-inline

    https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-glazing-options

  8. Lynn McDonald | | #8

    Hi Kevin,
    Thanks for the comprehensive advice! You're right -- dealing with window insulation every night would not be fun, especially since these 19 windows are all clerestory, so difficult to reach. We do rely on the solar gain from these windows, since they are our primary heat source, so a solid wall isn't an option either. It's pretty clear now that the high SHGC low E windows are the way to go, so I'm working on getting quotes for those. At this point it looks like Pella is going to be our best bet, as I haven't found any other dealers locally who carry anything suitable.
    The windows have been failing because of they way they were initially installed 40 years ago -- mostly related to problems where the clerestory meets the roof. We tried to fix that the last time we replaced all the windows, but the contractor we hired (on the advice of the local window dealer), didn't do it properly and then left town. The seal on all of our windows failed within a year, which I guess could also be due to the altitude (we're at 6800 feet). On closer inspection we discovered that the contractor had installed several of the windows upside down, along with a host of other issues. It's a mess up there now, so we're going to have our roofer and the window installer work together to agree on a plan to fix the structural problems once and for all. Hopefully when it's all done we'll have efficient windows and a non-leaking roof for the first time since we bought the house!
    Thanks again for all the helpful information!

  9. Nelisa Heddin | | #9

    Lynn - I'm curious what you ended up doing and if you are happy with your choice. We are building a house in Angel Fire NM - last week it got to -29 at night (summer high doesn't get over 80). We are working on a passive solar design with large windows facing south. I'm curious what you went with and if you're happy with your selection. We are considering the Pella Natural Sun product also; the rep thought we lost our mind when we asked for a bid with this glass - he was concerned we'd be too hot in the summer. We intend to use automated blinds, and will also have large overhangs on that side of the house.

    That said, if you wouldn't mind letting me know what you went with and whether you're happy with your decision, i'd greatly appreciate it.

  10. GBA Editor
    Martin Holladay | | #10

    Lynn,
    To answer your question: If I had to choose between the two options you listed, I would choose low-e glazing with a relatively low SHGC over clear double glazing, because the low-e glazing has a lower U-factor.

    Frankly, it's hard to justify large areas of south-facing glazing using the passive solar principles of the 1970s. Research has shown that you will never gather enough heat from the 19 south-facing windows to justify the cost of the windows (and to make up for the heat loss at night and during cloudy weather).

    Here is a link to an article with more information on this topic: Reassessing Passive Solar Design Principles.

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