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Windows for a passive solar sunroom

PAsun | Posted in Green Products and Materials on

I am adding a 21′ wide x 16′ deep passive solar sunroom to the back of my home here in Whitehall Pennsylvania, plant growing zone 6b. The south wall of the sunroom will face southeast (slightly more south than east). The sunroom will have a 4″ thick paver floor that along with the concrete block back wall of the house will serve as thermal mass to absorb sun energy and then radiate heat to help warm the space when temperatures drop. Right now my back door will be the only opening into the sunroom and I intend on leaving that door open to allow the sunroom to help warm up the house during sunny days during fall/winter. My plan is to use the least amount of glazing that I can to have enough sunlight to heat and light the space. The structure will have a gable frame shingle roof with cathedral ceilings and insulated concrete foundation walls that extended 2′ above grade. The roof will be vented via ridge vent and soffits. Insulation values in the roof and walls will be higher than code since my goal is to retain solar gains on sunny days during the winter when the sun’s solar angle is never more than 37 degrees and keep out the heat during summer. Roof overhangs will help block glazing to keep things cooler during warmer months. I will be starting many vegetable seedlings during late winter/early spring and propagating trees in the sunroom but this will not be a greenhouse year round. Sunlight will be beneficial for plants too but glazing will only exist along the south wall and part of the west wall and will not exceed a 20% glazing-to-floor area ratio

I am still undecided about the type of windows I will use but since casements are very efficient when it comes to air leakage, I am leaning towards using them and possibly awning windows below the casements I think this combination would help a lot with ventilation too. I have done a lot of research about passive solar, the use of thermal mass, ventilation techniques, glazing etc. I have learned about high SHGC glazing made by glass manufacturers like Cardinal and Guardian but I am having a hard time finding window manufacturers that use high SHGC glass. ClimaGuard 80/70 looks like it would be ideal as does Cardinal’s i89 4th surface coating on a double pane combined with a LoE – 180 on the 2nd surface. Unless I am not talking to the right people, high SHGC glass is not used in windows that are readily available here even in the northeast. I knew when I talked to window companies it would be uncommon that a customer ask if they offer windows with a code required U-factor and a SHGC in the 60-70 range. Sunrise does not offer this and I talked to a window company owner who installs/carries Okna and he had never been asked this kind of question before and had to get back to me after speaking with Guardian. He called back saying he can’t supply a window with that type of SHGC then proceeded to tell me that my sunroom would be getting too hot with such glazing lol. Despite my initial explanation of the sunroom build, controlling direct gains with window shading, storing heat gains to prevent overheating etc I don’t think he understood passive solar concepts.

I welcome thoughts and expertise regarding the sunroom I am going to build, tips and if anyone knows if there are some good window manufacturers that offer high SHGC windows without having to buy from Canadian manufacturers?

Thanks,

Chris

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    Pella offers a glazing option called NaturalSun. This is a high-solar-gain glazing. Here's how the glazing option is described on a Pella web page: "NaturalSun Low-E insulating glass with argon– features a single layer low-E coating that provides a low U-Factor and high Solar Heat Gain Coefficient (SHGC) that make it the ideal glass for passive solar applications."

  2. Expert Member
    Peter Yost | | #2

    Chris -

    Be careful of the moisture load you could introduce to the rest of your home if you use the sunroom for plants.

    Be a good idea to have a hygrometer in the sunroom and the main space to which it connects (for recommended hygrometers, see this GBA blog: https://www.greenbuildingadvisor.com/article/measuring-and-understanding-humidity.

    Peter

  3. Aedi | | #3

    I would not rule out double hung windows for this application. Air sealing isn't very important for a room like this. If there is sun, the room will likely have more than enough energy to deal with air leakage through the windows. If there is not sun, then it will be closed off anyway, and so it won't matter.

    In addition to being cheaper, double hung windows are also easy to remove, so you can convert the sunroom into a porch during the summer (if you design it right).

  4. PAsun | | #4

    Martin thanks for the Pella NaturalSun information. I was not taking into consideration that some of the really good U-factor and SHGC numbers I was reading about were for the glazing alone. Once in a frame those numbers are not as good (especially SHGC) since averaging in the frame can only decrease the solar heat gain average.

    Peter thanks for the heads up about moisture being introduced to the home via the sunroom. Really like your idea of having a hygrometer in both spaces.

    Aedi I was thinking double hung windows initially. Then I thought windows with better air sealing might help especially when the sun goes down during the winter or nights on those days that there is no sun to help keep the sunroom itself warmer. How much of a difference do you think having casement windows would help to keep the sunroom warmer during those nights without sun during the day or for several days?

    Aedi when you say "double hung windows are also easy to remove" do you mean just completely sliding and leaving open either the top or bottom section of the window?

    Chris

    1. Aedi | | #5

      When the sun goes down at night, the room will cool down pretty quickly -- I'd be surprised if you could keep it warm more than a couple hours. Likewise, when it's cloudy the room will not be much warmer than the outside, no matter the windows. The thermal mass of the concrete will help store some energy, but it takes much more than that to store heat overnight. Storing heat like that for several days is impossible. That's why I think the tighter seal you get with casement windows is not worthwhile: the difference in room heat retention time when using casement instead of double hung windows would be measured in minutes.

      It is important to have realistic expectations of a sunroom like this. It will only help on sunny days. When the sun goes down, or it is cloudy, you should close it off, otherwise you'll be paying to heat the sunroom. It is not a reliable or cost effective way to heat a home: if that is your primary goal you are much better off putting the money towards solar panels and a heat pump instead. A sunroom is only a worthwhile investment if you enjoy spending time in a sunroom, any occasional heating benefits should be considered an pleasant side effect.

      By remove, I mean taking out the sashes completely. Modern windows make this very easy, no tools required: https://www.youtube.com/watch?v=Roi2b3tMizA

  5. PAsun | | #6

    Aedi,

    Thanks for the insight! I do want to retain as much of the solar gain as I can and I will do that with all the concrete serving as thermal mass, really good insulation in the foundation, walls and ceiling and by limiting the square foot area of windows. The extra expense of casement windows looks like the minutes of added heat retention you mentioned is not worth it. Similar to me previously wondering if a 4th surface coating like Cardinal's i89 is worth the extra money. Yes, it will retain more heat in the sunroom but at what extra expense? Also, any heat retention gain may be completely countered by the i89 coating decreasing the SHGC compared to the higher SHGC of a LoE 180 coating alone. I can see why Martin suggested the Pella NaturalSun which is just a 3rd surface LoE 180.

    Ok, I see what you mean by "remove". I've done that to have glazing replaced that was cracked. That's actually a simple andcreally good ideađź‘Ť.

    The main purpose of the sunroom is definitely not to heat the home but to enjoy the sunroom. My existing back door will be the only opening from the sunroom into the house. Therefore, the sunroom will be completely closed off when that door is not open.

    I can now focus on finding good quality, well insulated double hung windows that offer LoE 180 or similar glazing. As Martin suggested Pella NaturalSun is one brand I am looking into, any other window manufacturer recommendations? I hear Sunrise has a good reputation.

    Thanks,

    Chris

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