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Community and Q&A

Window mix-up

Ryan Knight | Posted in General Questions on

We are in the process of dramatically upgrading the insulation of our 120 year old farm house and adding on a mudroom/laundry room, new bathroom and replacing the sunporch. We are in northern Michigan (zone 6a). Our original plans had Andersen 400 windows with a U-factor of 0.26 but our builder inadvertently ordered, purchased and installed an older version of the same window that is 0.30 for 12 windows. Everything else about the window is the same — the difference is a coating on the glass.

Our builder is telling us that there isn’t much difference between 0.26 and 0.30 and the work and cost involved would not be worth the gains in U-factor. I don’t know anything about windows and wanted a non-biased opinion. Should I push the issue and ask him to replace the sash with what was originally planned? Or is he correct in saying there really won’t be much of a difference between 0.30 and 0.26?

Thanks for your opinion,


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  1. GBA Editor
    Martin Holladay | | #1

    The U-0.30 windows have an R-value of R-3.33.
    The U-0.26 windows have an R-value of R-3.84.

    The U-0.30 windows will leak heat at a rate that is 15% faster than the U-0.26 windows.
    The U-0.26 windows will leak heat at a rate that is only 87% as fast as the rate of the U-0.30 windows.

  2. Alan B | | #2

    I don't know what your energy loss increase over the next 25 years will be (since windows are a percent of wall area) but i would at the least ask for a discount for the energy loss and not getting what you ordered. Someone got more money by paying for the lower cost product, the builder or the manufacturer, and as the end user paying the retail cost you should get some reimbursement for your loses.

  3. Mel Tillyard | | #3

    "Our original plans had Andersen 400 windows with a U-factor of 0.26 but our builder inadvertently ordered, purchased and installed an older version of the same window that is 0.30 for 12 windows."

    Not to be a doubting thomas but I would doubt that it was inadvertent. I found on my building project that things got ordered because it was cheaper or at a discount. Did you catch this or did the builder catch it and bring it to your attention?

  4. Charlie Sullivan | | #4

    If you tell us the total square footage of the windows, what fuel you heat with, and what you pay for that fuel, we could estimate the energy cost over 30 years. We could do it even better with the efficiency of the heating system.

  5. Expert Member
    Dana Dorsett | | #5

    An IRC code-min window in zone 6 would be U0.32, so it's slightly better than code min, but not the 23% lower loss than code min windows you specified.

    Some dumb napkin math:

    U0.30 - U-0.36= U0.04

    A middle-of-the zone 6 location sees about 8000 heating degree days per year which is (x24=) 192,000 heating degree hours oer year. The difference of U0.04 (BTUs per hour per degree F) results in losing an excess of U0.04 x 192,000 = 7680 BTU per year for every square foot.

    If you have 200 square feet of windows (12 windows at ~17 square feet per? Your real size is probably smaller) that's an annual difference of (7680 x 200=) 1,536,000 BTU per year.

    Over a presumptive 25 year lifecycle that adds up to (x 25= ) 38,400,000 BTU.

    At a buck a therm net cost for heating energy that would be (/100,000 =) $384 over 25 years, assuming 200 square feet of window.

    Clearly replacing that much sash would cost considerably more than $384. But run the 25 year cumulative BTU numbers on the real square footage and your real energy costs at current fuel costs. If you then multiplied the dollar amount by two (or even three) you'd be covered against potential energy price inflation over the coming 25 years, and the contractor would be thrilled to not have to replace them.

  6. Ryan Knight | | #6

    Thanks for the very thoughtful answers. I was the one who caught the mistake but I truly do think it was an oversight on the part of the person ordering the windows. Our builder seemed genuinely surprised when I pointed it out to him. We live in a poorer, rural area where few people care much about minor differences in efficiency.

    We were given an estimate today that it would cost an additional $3869 to replace the sash in all 12 windows. I am not sure if the builder will be willing to eat the difference if I push the issue or if it will just be passed on to us. Dana, from your calculations, it looks like it will take about 250 years before the upgraded window will pay for themselves.

    The total square feet of the 12 windows is 160 sq ft. We heat our home with radiant floors and a 5 ton GSHP. Power from the grid is about 13 cents per kWh (cheap). We also have a 17.1kw PV that goes live tomorrow. Initial calculations showed that the solar should supply 90% of our energy (home and one electric car) but there are a lot of new things this year including the addition and lots of added insulation. This is part of the reason I did not want to stray far from the plans.

  7. Expert Member
    Michael Maines | | #7

    Ryan, the "how many years to pay back" argument is not the best way to think of it. Better is to say "what would that money do if invested in something else." If you invested the $15/year extra you will be paying in additional energy costs at 8% interest, and assume that energy costs grow at 1.5% each year, the real cost of the window mistake is more like $1800. Still a long way from the $3869 charge to correct the mistake, but if you each paid half you would break even.

    If you assume that energy costs rise at 3% each year and the stock market averages 11%, at the end of 25 years your net present value is $4,100.

  8. Bill Dietze | | #8

    Another consideration is comfort. The more insulating windows would be more comfortable to sit next to on a cold cold day. For me, that might be half the value of the better windows.

  9. Stephen Sheehy | | #9

    Reply to Bill: I doubt anyone sitting next to the window would notice the difference between U.30 and U.26.
    The better question is why specify only U.26 in the first place in zone 6?

  10. Ryan Knight | | #10

    Thanks for all of the great advice and discussion. I am relatively naive about building and have certainly learned a lot with this project. After a family discussion, we've decided to just leave the windows. It sounds like it is a relatively minimal difference in efficiency and we feel that throwing 12 new windows in the dumpster is probably more of an environmental sin than loosing a small amount of heat yearly.

  11. Walter Ahlgrim | | #11

    Did the contract you and the builder signed state the U value of the windows to be installed?

    What you need to decide is, did the contractor bid the job with the intent of installing U-30 window to win the job with a low bid and finesse you should you happen to notice?

    The fact that he is telling you how much it will cost you to fix his mistake instead of offering to refund the cost deferential between the products makes me think this is his normal business plan and it is likely this is not the only mistake/ profit center on this job.


  12. D Dorsett | | #12

    The napkin math assumed some number which may or may not correlate with the actual window sizes, local climate HDD, local energy costs, etc. and was for example only. You have to run the math using the real data to come up with a closer estimate.

    That said, there is (almost) never "payback" in finincial terms on replacing windows even when moving from U0.50 clear glass double panes to U0.26 (cutting the losses nearly in half) over a 25 year presumed lifecycle, and a ridiculously miniscule return for moving from U0.30 to U0.26. (about a 13% cut from an even lower loss number.)

    In a zone 6 climate it's "worth it" to replace U0.50 windows with something better on both comfort and window condensation terms. But it's never "worth it" to replace a brand new U0.30 window with a U0.26 window on those terms.

    The only question is whether and how much the contractor is going to compensate you in some way for the error. I suggested a starting point for that negotiation, but it would be a waste of time, money, and goodwill to force the contractor to swap them out on their own dime.

  13. Bill Dietze | | #13

    Ryan, As for the comfort and payback issues of highly insulating windows, you might read the following:

    Are the windows in question right next to where you sit? The reason I bring up comfort is that, if it's an issue, you won't get that back wth a refund on the difference in window costs.

  14. Ryan Knight | | #14

    Our builder updates us monthly with a detailed invoice, as well as, a spreadsheet on how we are faring (plus or minus) on our overall budget. He left me November's on Friday and I noticed the U-30's were invoiced and the difference in cost was credited. I will call that good. I trust our builder and he has fixed more than a couple unanticipated problems at minimal cost.

    I don't think the difference will affect our comfort level. Eight of the windows (120 sq ft) are for a sunroom that is separated from the rest of the house by a french door. The other four windows (40 sq ft) went into an addition that added a much needed laundry room and bathroom onto the house. Our washer and dryer is currently next to our refrigerator.

    For the rest of the house, we replaced replacement windows about 10 years ago with double paned, low-e windows. This Q&A has certainly spurred a lot of discussion about windows in our household over the past week including a discussion about the 10 year old windows. These are windows that are still in perfect condition but somewhat uncomfortable to sit beside when watching a movie. You need a sweater and cup of something hot when it's 15F outside. I've read a little on interior storm windows like "indows" being a cost effective way to add almost a full r to single pane windows. How about double paned windows like ours? Logically, they would add 4 inches of dead air space between the interior of the home and the window. However, it seems like there might also be unanticipated problems with moisture buildup, decreased solar heat gain and maybe a pocket of air that gets too hot on a sunny day and damages the seals/frame.

    Thanks for all of the great advice. I am not in the building industry but have learned a lot from this site. Local contractors are continually asking about some of our improvements and I hope that they will incorporate some of these energy improvements into their future projects.

  15. Ryan Knight | | #15

    My wife was at work and the kids were at a sleepover yesterday so I decided to do a backyard experiment. There are two side by side double hung windows in our family room that feel cold (not drafty) when you are sitting. I purchased an inexpensive single pane of glass and built an interior window with some scrap wood, caulk and left over weatherstripping. This created an air gap of approximately 5.5" between the actual window and this new interior window. I did not alter the other window and used it as my control.

    I left the new window in overnight to eliminate any greenhouse warming effect and to allow the temperature to equalize. I have an inexpensive FLIR thermal camera that I've used to track down cold areas in my house. The measured temperature of a piece of masking tape on the new window was 3.1F warmer (64.6F versus 61.5F) than the control. Inside temperature and wall temperature was measured at 70F. Outside temperature was 33F.

    This was such a simple experiment that I don't want to read into it BUT the couch definitely felt more comfortable this morning. Total cost was less than $50.

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