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I’m overthinking windows again… SHGC question

Griffin728 | Posted in PassivHaus on

So I think we’re close to choosing a local window producer to furnish our windows. Richlin 500 casements offer U values as low as 0.13, very good air seal numbers, are built down the road from us with a short lead time and an economy price. The only downfall that I’m aware of is that they insist on LoE272 glazing for their triple panes. This gives them a dismal 0.22 SHGC. Since I have 36SF of summer shaded, winter exposed, south facing window, I was thinking of going with a different window on the south side. Richlin does offer a clear glass (no LoE) in double pane, which gives a SHGC of 0.51, but it has a poor 0.39 U-value.

Is there any way that it would make sense to downgrade to double pane for the south facing windows?

I could also look for another manufacturer, but it’s got to be a vinyl crank out casement that looks similar and is in the same price braket. Kolbe Windquest would get me slightly better 0.38 SHGC while still offering a 0.19 U-value.

Is there a quick and dirty way to gauge the tradeoff between R-value and SHGC without doing a full energy model? Thanks!

Ryan
Minneapolis
Climate zone 6

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Replies

  1. iLikeDirt | | #1

    Run the numbers for yourself with a DIY calculation for just the windows. Here's a quick, dirty, and possibly questionable example (take it with a grain of salt and do your own calculation):

    U-0.13 / SHGC 0.22
    Heat loss: delta T 82 degrees * 0.13 = 10.66 BTUs/hour lost/square foot
    Solar heat gain: Average of 60 BTUs/hour *0.22 = 13.2 BTUs/hour gained/square foot
    Total = 2.54 BTUs/hour winter heat gain/square foot * 36 square feet = 91.4 BTUs gained/hour

    U-0.19 / SHGC 0.38
    Heat loss: delta T 82 degrees * 0.13 = 15.58 BTUs/hour lost/square foot
    Solar heat gain: Average of 60 BTUs/hour *0.38 = 22.8 BTUs/hour gained/square foot
    Total =7.22 BTUs/hour winter heat gain/square foot * 36 square feet = 260 BTUs gained/hour

    So on paper your higher SHGC windows win ever so slightly in winter performance. But let's face it, 170 BTUs is not exactly a big difference. If the higher SHGC windows cost more, it will take approximately a zillion years to pay back vs the other ones. And furthermore, they will perform slightly worse in the summer because they will let in slightly more solar heat and do a slightly worse job of keeping air-conditioned air cool. Honestly, I would recommend choosing the windows that are cheaper, look better, are made from better materials (e.g. fiberglass over vinyl) offer you the best functionality, etc.

    I just wrestled with this very issue in my own house and wound up concluding that it made no real difference. I ran the numbers and found the heat gain and loss figures to be nearly identical between my choices. I wound up going with the slightly higher SHGC windows on the East side of the house with better views because their VT values were higher on paper. In practice, I can't tell the difference looking through them vs the lower VT windows. Good windows are good windows. You can fret over these tiny details forever if you let yourself!

  2. GBA Editor
    Martin Holladay | | #2

    Ryan,
    Nathaniel has given a pretty good answer.

    Good for you for asking these questions. I can't for the life of me understand why window manufacturers offer low-solar-gain triple glazing. The only people who are buying triple glazing live in cold climates, and they all want high-solar-gain glazing. No one in Texas and Florida is buying triple glazing -- so what's with this strange product, low-solar-gain triple glazing? Why can't window manufacturers figure this out?

  3. rocket190 | | #3

    Martin, isn't the answer as simple as common sense being trumped by politics? I think the window manufacturers have determined that it's in their best interest to product windows that are eligible for the federal energy rebate, regardless of if it's in the best interest of the buyer (location dependent).

    That being said, there are small regional companies that have figured it out. I ended up buying my windows from Wasco as they had a glass package tailored to my climate.

  4. jackofalltrades777 | | #4

    People do buy triple pane windows in climates like Texas and Arizona. I personally know of a few people who installed low < 0.25 SHGC triple pane windows in hot and sunny climates. Intus Windows offers uPVC triple pane windows with SHGC numbers from 0.25 - 0.63, all depending what climate location and what elevation (south, west, etc) you want to install them in.

    I'm installing uPVC triple pane Intus Windows with a higher SHGC on the south elevation and the lower SHGC of 0.25 in the west and east elevations. This is for a Zone4 climate.

  5. STEPHEN SHEEHY | | #5

    Have you looked at Kohltech casements? They make a vinyl high SHGC casement window.

  6. jackofalltrades777 | | #6

    Stephen,

    I am not a fan of casement windows because of the hardware and inevitable wear and tear that happens with the levers and cranks. On my old house that had casements I've had a few wind gusts damage the hinges and caused frame damage to the window. Casements are a wind sail and a nice gust of wind will cause damage to an open casement.

    I prefer the tilt & turn design of windows like Intus. No hand cranks to deal with and since they open in they are safe from wind gusts. The hardware on an Intus Window is like a bank vault. Super heavy duty and designed to last a lifetime. Intus gets DP70 ratings on most of its windows and doors and AI ratings of <0.03.

  7. STEPHEN SHEEHY | | #7

    Peter: the original poster wants casements. I agree with you about Intus tilt turn windows. I have a houseful waiting to get installed. I also have several Kohltech casements which will be installed in an unconditional porch. We wanted windows that open out.

  8. jackofalltrades777 | | #8

    Martin,

    You stated:
    "I can't for the life of me understand why window manufacturers offer low-solar-gain triple glazing. The only people who are buying triple glazing live in cold climates, and they all want high-solar-gain glazing. No one in Texas and Florida is buying triple glazing -- so what's with this strange product, low-solar-gain triple glazing?"

    People in warm climates do buy low SHGC triple pane windows. It's not a strange product. If you lived in Texas or Arizona and had the option of buying a low SHGC dual pane window with an R-Value of R-2/R-3 or buying a low SHGC triple pane window with an R-Value of R-7/R-8, would you choose the window that is 1/3 as efficient?

  9. Griffin728 | | #9

    Peter, of course I agree that tilt turn is a superior product. They just do not work well with the design of our current house (OK, my wife refuses to have a window that opens in).

    She does have a point though, it is a very small house and the sills are very low in every room, so they are blocked by furniture in many cases and would be inoperable. In other cases they would block flow in rooms or hallways when open. I know... next question... what's the SHGC of my couch... just kidding.

  10. jinmtvt | | #10

    Martin: i can't recall the exact numbers,
    but when i purchased the windows for my house i specified different glazing for
    2 sides of the house ( the house is ~35deg from south ) than the 2 others, using both tripple pane glazings.

    the solar side had something in the vincinity of 0.5shg
    and the "north side" in the ~ 0.3 but with much higher U value

    i never calculated the energy diff. because did not have the proper knowledge/interest back then,
    but it seemed basic logic back then to use them on the "gain" side.

    1 manufacturer back then, refused to give me a quote if i did not use the same glazings all around...
    ahah

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