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Are high performance windows worth it?

user-957077 | Posted in Energy Efficiency and Durability on

We’re in the final stages of planning a new house that we hope to build this summer. We have just met with our contractor who has prepared an estimate. Our builiding criteria includes R45 walls, R65 roof, R20 slab, lower than 1 ACH at 50 pascals, and windows with a u value of at most .2. Our contractor is ready to meet our specs, but he told us that he’s not convinced that our window spec. is worth the expense. All along, I’ve felt that one of the high-performance Canadian windows would be critical to the performance of our house. But since today’s meeting, I’ve been playing with a heat load calculator mostly to make sure that we’ll be able to heat the house with a minimal system. When plugging the above specs into the system, we need 20k btu’s for our heating system requirements. I was amazed to find that if I reduce the R value of our windows (we have planned for about 375 sq. ft. of window total) from 5 to 3, the heating system requirements go from 20k btu to 22,250– a little over our goal, but not by much… and I’m assuming that there is a big savings up front by doing this. The annual heating costs (assuming propane at $2.50 a gal., although we are actually likely use a mini-split or a pellet stove) go up by $150 a year (from $650 to $800) with the lower grade windows. I’m assuming that the cost difference between these windows would be several thousand. This is making me question the wisdom of the good windows. Can anyone tell me what I’m missing in all of this, orr is there a little too much hype surrounding the “good” windows?


Randy George
Moretown, VT

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  1. jklingel | | #1

    3 things to consider. (1) What is the solar heat gain coefficient of your glass? Cheapos will not likely have what you need. My guess is a value of about 0.4 or higher, at least on the south side. I'll be using 0.58. I suspect that proper SHGC windows will do a lot toward paying for themselves. Search here. Martin, and others, have posted some good info on all this. (2) How much of your glass will be south facing? (3) What is the air leakage rate of the spendy vs cheap windows? Cheap, leaky windows will be expensive and you may get ice problems inside. I just discovered tilt/turn windows a few weeks ago. Incredible windows. Very tight, yet very simple. And expensive. But worth it, to me.

  2. GBA Editor
    Martin Holladay | | #2

    Your energy use calculations sound about right. A few comments:

    1. At current energy prices, many energy-efficiency measures are hard to justify on payback alone. Fossil fuel energy is still cheap -- which is why so many people still use double-glazed windows. However, propane may not be $2.50 a gallon for long. (I've been paying an average of about $3 a gallon for my propane for the last 4 or 5 years.)

    2. Triple-glazed windows provide comfort advantages that can't be quantified. When it's below zero outdoors, you can sit in a chair beside a triple-glazed window and not feel cold. That's worth something.

    3. A home with triple-glazed windows will ride out an electrical outage (an ice storm, perhaps) better than a house with double-glazed windows, because the house won't cool off as fast. That's worth something.

    4. It's your money -- spend it how you want. Personally, I'd rather have a smaller house with triple-glazed windows than a bigger house with double-glazed windows.

  3. gusfhb | | #3

    What Delta T are you using? For that little of a difference it looks to me like that only works at about 45 degrees. Shouldn't your area be more like 70? At that Delta the difference is more like 3500btu, with a total window loss at 8750, or about 43 percent of the buildings loss, which makes more sense to me, given your stated insulation, unless you have a larger house with smallish windows.

    Not to unnecessarily trash a profession, but contractors make money by using slightly cheaper materials than asked for. It is just business. It is your job to make sure that you get what you want.

  4. Natur Haus | | #4

    I have been researching this a LOT lately, and I am not sure what to believe. The only thing that triple pain is gaining you is R-value, which is worth something, but what is it worth? In my current pricing, triple glazing with my preferred window manufacturer would cost 30% (roughly $6,000)more than double glazing, and is not available with certain Cardinal glass. Also, it lowers the SHGC along with lowering the U factor. My best option with this window would be to spec Cardinal 179 glass and get a U factor of .35(barely meeting code), and a SHGC of .52. I am not sure if it is the hard coat suspended Low-E in the Serious windows or what that is making the difference. Most standard Low-E glass is going to lower the SHGC along with U factor when triple glazed, and I don't think I can honestly sell someone on clear glass and get fading call backs, so the dilemma continues. Let's tickle two birds with one feather and get to some specifics on window glass and glazing on this thread to help answer both Randy and my dilemmas.

  5. GBA Editor
    Martin Holladay | | #5

    What's your climate?

    In southern Canada and northern New England, the best glazing for south walls is the one with the best ER rating. Stephen Thwaites has a very useful chart here:

    Thermotech's designation for the best triple glazing is "322 Gain." In a casement window, the glazing gives you a whole-window U-factor of 0.19 and a whole-window SHGC of 0.42. (Glazing-only SHGC is 0.61).

  6. user-957077 | | #6

    Mea Culpa... how embarrassing... I must have had a number wrong before because now I'm rechecking it and the difference is actually more like 5500 btu's. With the propane reset to $3.50 (agree, we'll be there before we know it), the heating cost difference becomes $200/yr.. With wood at $250 a cord, it's $50/yr. So even though I was off by more than 100%, these aren't huge numbers if your only concern is payback (and I have other concerns, but I want to know what the payback is nonetheless). I have been fixated on the btu requirements because that saves money up front in terms of reducing the system requirements. But 25k btu's still seems like we're in the realm of a low-cost, single source heater rather than a boiler and a distribution system. I should say that our plan is to use a wood stove most of the time. The other source (as yet to be determined) is there for back-up and to satisfy the bank. The most compelling argument yet for me was Martin's point about how it feels to be near a double-glazed window. That's important to me. It can't be justified mathematically, but livability at the top our list of priorities. The SHGC issue is important, but we're talking about VT-- we get a lot of cloudy days and during the coldest months of the year the sun isn't going to be shining into those window for more than a few hours most days anyway. That's a bonus when it happens, but the rest of the time I've got what amounts to 400 or so sq. ft. of very poorly insulated wall (even with high end, R5 windows). That said, we're putting about 75% of the glazing on the south side, so it should have a noticable effect when we're lucky enough to have sunshine. Thanks for all the advice... I think that in the end we're going to pony up... windows aren't something that you can easily upgrade later.

  7. Natur Haus | | #7

    @Martin , I'm in Southern Oregon, which is at a zone change, so nothing is cut in stone here. This project is in a 4,700 HDD area, where as I live 20 miles away in the Cascades and am closer to 6,000 HDD. Thanks for that link as well Martin, I will look at it shortly. As far as the all the manufacturers I have been looking at , which is either Cardinal or PPG glass, the U factors and the SHGC are piss poor compared to the Serious and others. The .52 SHGC I mentioned was the window, not just the glass. Again, when I add triple glazing, that number goes down, along with the U factor. @ Randy, as far as performance, VT doesn't mean a thing, SHGC is CRUCIAL to passive solar.

  8. Natur Haus | | #8

    I should have mentioned that the my preferred window(aluminum clad wood) in triple glazed option only lowered the U factor to .28, from .35 and the SHGC lowered as well. It seems to get the ideal south glazing for passive solar design, we may be forced to use the Serious, Thermotech, Accurate Dorwin windows to get the U factor and SHGC spreads we need.

  9. gusfhb | | #9

    A lot of people get hung up on opening windows, and a lot of architects just design from the Anderson catalog[witness six opening windows above the garage door at my last house, never opened once]
    It 'may' be cheaper to investigate fixed glazing. Good ventilation and good 'breezing' require few operable windows, although you may need an egress window in bedrooms for code. My current house has 350 sq ft of fixed windows vs 120 of opening [plus 4 sliders, silly damn house]
    I hate sliders and am converting 3 of the 4 to swinging [read airtight] doors. They are also 'wicked' expensive compared to a fixed glass panel and a swinging door. Just a thought.....

  10. SvgjEajrmP | | #10

    You should also keep in mind, if you're financing the house construction, payback might be less important than net cash flow. By that I mean you can calculate that at 5% for a 30-year mortgage, borrowing $1000 costs roughly $65/yr. (Insert figures appropriate to your financing situation here)

    So, if you spend $3000 to upgrade the windows, that will add about $195/yr to the mortgage. But, if that upgrade saves $200 or more a year in energy costs, it will be cash flow positive from day one. You could also say that it has a 15-year payback, but that really only applies if you're paying out-of-pocket.

  11. bdrfab | | #11

    "Not to unnecessarily trash a profession, but contractors make money by using slightly cheaper materials than asked for. It is just business. It is your job to make sure that you get what you want."

    I'm sorry you feel this way. Perhaps you need a different contractor. At a minimum, you need a contract. Actually, we make more money, due to discounts, by using higher cost materials. 10% off $1000 in poplar trim is more than 10% of $500 in MDF. And if you're getting billed for poplar, but getting MDF, then that's fraud. Windows are even easier to see if you're getting what you want by looking at the sticker right on the window. If your contractor hurries up and takes those off immediatly, be afraid.Get a reputable contractor. one you can TRUST. By the way, we probably won't be the cheapest.

  12. davidmeiland | | #12

    Thanks for picking on that Aaron, I found it to be a tasteless generalization.

    One thing that I haven't heard discussed is the replacement cost of triple glazing when the seals fail. I do at least a couple of glass replacement jobs every year and the cost and difficulty varies quite a bit. Are the triple glazed units that are typically mentioned in these threads designed to be reglazed in the field, or do the sash have to be replaced entirely? It would really hurt my feelings to invest in expensive windows and then find later that they couldn't be serviced. I know that's ~20 years out, and almost no one thinks that far ahead, but it's something to consider.

    Also, what about the possibility of using double glazing plus storm windows instead of triple glazing?

  13. gusfhb | | #13


    First, I apologize

    Second: Fer cryin out loud, how polite must one be.

    I quite specifically did not indict all contractors. It must be mentioned, that for one reason or another, if a contractor suggests either a more expensive or less expensive product, their motives are fair game.

    i have several times done projects as contracted labor, at a mutually agreed supplier that I have an account with, just for this reason. It was not to save money, but to ensure that when I spec CVG red cedar I am not getting mixed grain because the project is running over time. Have you seen the sticky tar paper that passes as ice and water barrier?
    My last house was a better house because the builder I hired built it, no question. I also had to push him a bunch on detail issues. Did he love me at the end? I don't know, this is business.

    If you guys don't think that a lot of contractors out there would push an inferior product because the margin is 2 percent better, well, you have lived a charmed life.

    Note,I did not say 'all' or 'most' or ' every' contractor.
    I hope that every one of your customers is your friend for life, that would be awesome.
    Life is not always that perfect, and some people are not as worried about it as you are.

    Now this has taken waaaaaay more text than I intended

  14. DrDanger | | #14

    I am living my first winter inside our new home which has Serious' 925 windows, which have two glass and 2 mylar panes. I live in a 8600 HDD climate, and would echo Martin's remarks, about comfort and livabilty. We have lots of windows on our south side of the house, and had these been conventional 2 paned windows we would have had a very unconfortable house thanks to the cold drafts that these would have created. We went to a friend's house for Thanksgiving with a big picture window next to their dining table, and even with the thick drapes pulled it was miserable to be sitted next to the feet were freezing!
    There is a goverment sponsored program to entice window manfacturers to produce more economical triple windows which are avaiable from a variety of manufactures. It basically specifies that that have a U of 0.2 and cost $4/sg ft of glazing. The down side is most are vinly frames and have SGHC of around 0.25, which would be OK if not building for passive solar gains, or alternatively they could be used on all but the south facing windows.
    This still leaves you spending more for your south facing windows ifyu wanted a high SHGC for passive solar purposes but it would lower your expense. It seems that your building envelope is being built to rather high standards, so if if compared to a chain, your weak link is definitely your windows. It doesn't make much sense to me if the rest of your thermal envelope is built so well only to have your windows be such a weak link.

  15. albertrooks | | #15

    Hi Randy,

    No science, no guessing motives. My two cents... The specs on the project look extremely good. Well done to take the envelop this far!

    I'd like to suggest that you buy the best performing window that you like (for whatever reason) and can afford.

    Then sit by them with your favorite beverage at every thanksgiving. With the right window you'll be as warm next to the window as anywhere else. It'll give you a few minutes every year at the holidays to reflect on how smart you were.

    Best of luck!

  16. bennywinks | | #16

    I was in the same situation about a year ago when choosing windows/glass for our 500sqft glass curtainwall. The S, S/E facing wall is one of the walls of the main living room and it was crutial to have the best performing glass available. I didn't want to install 10 heating registers along the wall just to make up for the shortcomings of todays window offerings. After researching/costing and comparing double vs. tripple performance data, I felt that the additional cost of tripple pane windows was not worth the marginal performance gains. The glass industry has a lot of catching up to do, but with only three or four major glass suppliers, it might be a while before we see affordable, high performance windows. IMO, the current market offerings are falling short of the type of performance we should be installing in our high performance homes.
    Heat Mirror technology was the only option for us, and Randy, I recommend you look into it for your 400sqft glass wall. Feeling is believing... ask a local dealer for the " heat lamp" test, the proof is in the

  17. bennywinks | | #17

    Kevin wrote: "It doesn't make much sense to me if the rest of your thermal envelope is built so well only to have your windows be such a weak link."

    See it too much around here, SC Ontario, Canada, with ICF, strawbale homes, people spend the extra effort and resources to super this and eco that, just to install insane amounts of Pella/Andersen/Marvin windows. Tripple or double, still not enough for what should be the standard for todays high performance homes IMO

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