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Community and Q&A

Decision time: 3-glazed or 2-glazed windows in zone 4A?

David McNeely | Posted in GBA Pro Help on

New Construction: walls = R-26; unvented roof = R-65.8; slab = R-10. Hip roof with 4’ eaves all around. Aiming for airtight. Energy rater estimates load for both floors will be less than two tons (“1 ½ could get the job done”).

Main floor = 2155 sq.ft; doors & windows = 534 sq.ft = 24.7% of floor space
Main floor WALLS = 1992 sq.ft.
Lower floor (shop/future use/resale value) = 1549 sq.ft; door & windows = 265 sq.ft. = 17%
Lower floor walls = 2058 sq.ft.

Intus windows Arcade series, which provide Ug only:
2-glazed windows = 0.27 Btu/sq.ft.
3-glazed windows = 0.1264 Btu/sq.ft.

Note that Lstiburek et.al. recommends <.3 U for high R-value enclosures in zone 4 (http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view) Cost increase from 2-glazed to 3-glazed = $6,800 = 13% increase Electricity costs $0.088/kWh Q1: Does it make sense economically? Q2: Will it make a difference in comfort? Plan is to use 1-9K & 1-12K minisplit on Main floor; 1-12K on Lower, all with short ducts. Since the air movement will be minimal, will the lower Ug make a difference is temperature stratification or variability across the space of a room? Q3: Recommendations for alternative choices for the windows?

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Replies

  1. Charlie Sullivan | | #1

    Q1. Really rough numbers: The range of heating and cooling degree days in 4A is pretty big but suppose it's 5000 HDD and 4000 CDD. 5000 HDD for U 0.27 is 32 kBTU/y vs. 15k for the triple pane, so that's a 17k savings. For cooling, it's just 4/5 of that, so 13.6k savings. If the heating is delivered at COP = 3 and the cooling at COP = 4, that's a total of 9 kBTU of electric input saved per year per square foot, or 2.66 kWh, or 23 cents at current electric rates. That's at a cost of $8.5/sq ft, which makes it a 36 year simple payback time or a 2.75% return on investment. What rate are you getting on your mortgage? And, more of a wild card, how much do you expect electric rates will rise over the next 30 years?

    Another way to look at it is the cost of PV to provide that energy. At at modest 1.2 kWh harvested per watt installed per year, you'd need 2.2 W of PV per square foot of window to make up the difference. Depending on what you pay for a PV installation (what is it now, $3.50 to $4 for residential installations?), that would cost about the same as the window upgrade.

    Based on that analysis, overall it's not a financial benefit to get the triple pane, but it's not unreasonable either.

    Caveats: Assumptions about HDD and CDD may not match your actual climate. I also assumed the the same solar heat gain for either option, and assumed the heat loss through the frame is the same for each. I also assumed the COP is not changed by the window choice, which may not be true.

    Q2: Yes and yes, it will make a differences in comfort and stratification. And as noted in Martin's recent blog, stratification may affect minisplit COP. Also, your overall heat load will be changed significantly by the choice, which could make the minisplit COP better or worse, depending on whether it starts out closer to being undersized or closer to being oversized. I don't think temperature variability is as much of a comfort issue as some people do, but there's also the very real effect that being near a cold window makes you feel cold because of radiant heat loss from your body to the window surface.

    Q3. The runner-up when we were looking for triple-pane windows was Fiberframe Comfortline, for offering triple pane at a good price. I have no direct experience but if I were you I'd get a quote.

  2. Reid Baldwin | | #2

    Did you do an economic analysis when you decided to go with R-26 walls or R-66 roof? I suspect that if you apply the same criteria to your windows, you will decide that R-4 is not high enough. There are quite a few products in between the R-4 double pane and R-8 triple pane options you mention. I have selected U=0.18 windows from Inline fiberglass for my house with R-24 walls. I don't have them yet, so I cannot endorse them beyond what you could figure out from web searching.

  3. KEVIN ZORSKI | | #3

    David - Charlie has presented a good breakdown of things. I live in zone 6A, and even here it's not clear that it makes sense to go with triple pane in every situation, though if you end up sitting right next to a window, the triple will always be more comfortable.I'm no energy modeler, but do like to get "rules of thumb" as the design process starts. As I read your description/question, some things stood out: 1.) Going with triple pane and R-26 walls made me think " go with double pane" or thicken the walls first. Thickened walls are a lot cheaper than super-windows. 2.) The Intus windows are reasonably popular in my area (Maine). One issue I have with them is what happens when you break a window? Things like that do happen with kids, birds, hailstones, manufacturers small defects in sealing,etc.How long will you wait for a replacement, and will there be readily available help to install it? I like the idea of an American ( or possibly Canadian ) window in terms of the future. 3.) You have a lot of window square footage, especially on the main floor. Could those windows be downsized some?Reducing the square footage saves money no matter what you go with. Fixed windows also save money no matter which way you go. Again, thick walls are a lot cheaper than good performing windows. What percentage of them are south facing?Double pane will probably win out on the south side, if there is good sun exposure.( Though with 4' overhangs,I'm not sure you'll have a lot of solar gain,especially on the upper, main floor.)It's all too complicated, isn't it?Some things to think about. Best of luck to you with your project.

  4. Nate G | | #4

    FWIW, I have a nice rocking chair right next to my U-0.26 double-pane 4x8 living room window and it is in no way uncomfortable to sit there in the winter (Zone 5B). I think comfort concerns with high-quality double-pane windows are overrated unless you're in like zone 7.

    At 2.75% rate of return, you're getting into the territory where it makes more sense to simply bank the money, put it into T-bonds, and use the interest payments to defray the slightly higher energy costs. But such a cole economic analysis leaves out many factors. Seems like a real judgement call.

  5. Stephen Sheehy | | #5

    When comparing triple pane with double pane, remember that installation time is identical, framing is identical, taping/air sealing is identical, so when you add up the total cost of an installed window, the difference remains $6800, but the 13% will be less. Depending on how experienced your builder is with Intus windows, installation can be costly.

    We put Intus windows in our new house and are very pleased with them. We used triple pane for all conditioned space, but went with double pane for a room outside the conditioned space.

  6. GBA Editor
    Martin Holladay | | #6

    David,
    Using Charlie's analysis, I figure that with 799 square feet of windows, the $6,800 upgrade saves you $184 per year.

    But borrowing that money (30-year mortgage at 3.8%) costs you $380 per year. So your increased mortgage costs are a lot higher than your savings.

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