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U-factor difference between 0.2 and 0.3 on fiberglass windows

user-6990904 | Posted in General Questions on

Hi everyone,

Appreciate the feedback on my Colorado fiberglass window question posted the other day. Here is the follow up question.

How much more energy efficient and comfortable will we get in Windows that have U-factors that are 0.2 rather than 0.3? All our window choices for our new build in Colorado at 7000 feet are fiberglass. Alpen windows have U values around 0.2 and the others Migard Ultra, Kolbe Forget, and Marvin Integrity are about 0.3 on average.

The Alpen are however 30 percent more dollars.

Worth the extra money?
Will we get more comfort with the better ?

Thanks, lydia

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  1. thrifttrust | | #1

    The difference between .2U and .3U is basically the difference between twin and triple glazing. In my thinking payback is less important than comfort and convenience. A scientist will tell you that cold doesn't radiate but when you stand near a window it can feel like it. The convenience aspect has to do with condensation. I it annoys me to find the bottom edge and corners on my windows wet or frozen. This is an inconvenience with vinyl and fiberglass but can cause real damage to wood windows. The problem is greatly exacerbated if windows are set near the outside wall in thick walled buildings. Triple glazing goes a long way in preventing condensation.

    Douglas Higden

  2. Jon_R | | #2

    I like the calculator below. Note that most of the difference in comfort can be more than compensated for with a warmer indoor temperature. Or well placed radiators. It also calculates condensation (shouldn't be an issue at comfortable humidities), but you might want to use the second link.

  3. GBA Editor
    Martin Holladay | | #3

    Douglas has done a good job of explaining the comfort advantages of a U-0.2 window.

    What about energy? I'll copy and paste what Charlie Sullivan wrote elsewhere:
    "Annual heat loss through a window is:
    Q = HDD*A*U*24,
    where HDD is the number of heating degree days in your climate,
    A is the area one window (or all the windows if you want total window heat loss),
    U is the rated U-value,
    and the 24 is a conversion because U has hours in the definition but HDD is day-based."

    What follows is my math:
    If your climate has 6,000 HDD, and your house has 14 windows, and each window measures 6 square feet, then the annual heat loss through the windows is:
    6000*84*0.2*24 = 2,419,200 BTU if the windows have a U-factor of 0.2,

    or an annual heat loss of
    6000*84*0.3*24 = 3,628,800 BTU if the windows have a U-factor of 0.3.

    If you are producing this heat with an air-source heat pump with a COP of 3.0, and your electricity costs 15 cents per kWh, the cost to produce this heat is $35.44 per year if the windows have a U-factor of 0.2, or $53.16 if the windows have a U-factor of 0.3.

    So the high quality windows save you $18.72 per year.

  4. user-6990904 | | #4

    Good early Sunday morning Martin etal.

    You are up early, which is a good thing for me. Appreciate the calculation. That does answer the energy/cost question.

    Douglas’ answer is interesting as the average humidity is 50%, as it is high plains desert, so condensation should not be much of an issue, though I might be missing something.

    Now to satisfy my husband’s comfort question, when he stands by the window, be it a 0.2 or 0.3 U value, will he perceive a difference in coolness. Mind you he grew up on a farm in Nebraska, he is not soft.

    Thanks to you and every else on the site.


  5. STEPHEN SHEEHY | | #5

    Lydia:Along with comfort and energy issues, another factor is how good the hardware is. It's hard to measure, but nice solid hardware is a plus. Can you physically compare the choices?

  6. GBA Editor
    Martin Holladay | | #6

    My mother's family comes from Nebraska, where several of her cousins are still farming (or were until they retired). Your husband could probably sit near a window that is open 4 inches when it is 0 degrees out and still be comfortable. That's how he grew up.

    Ask him if there was snow on his interior windowsill in his bedroom during the winter when he grew up. (It tends to sift through the little cracks and wind up indoors.)

  7. Jon_R | | #7

    Even if he stands close to a tall window on a cold day, he probably won't notice the slight difference in draft. You might bump the temperature up 1F to compensate, costing perhaps an additional $30/year.

  8. user-6990904 | | #8

    Martin and Jon,

    My spouse says the attic where all the kids slept growing up on the farm in NE was unseated. And yes snow did appear on the window interior. He was not wild about unheated barns for early milking either.

    So now we are talk 0.2 and 0.3 U factors! As the previous poster Jon states, unlikely he will notice the difference.

    Thanks for everyone’s help


  9. Expert Member
    Michael Maines | | #9

    Lydia, for the most part I agree with everything above, except the comfort aspect--if you have small windows and/or don't sit near the windows, you may not notice the difference, but if you have larger expanses of glazing, or if you want to sit near the windows--as you might when dining or bathing--you will absolutely notice the difference, due to radiant cooling, which is the window pulling heat directly from your skin. The interior surface of a triple-glazed window is always closer to room temperature than the interior of double-glazed windows.

    70° air at 50% RH will condense on surfaces a little over 50°, which is common on double glazed windows in cold weather, especially where the glass meets the frame. You will often see a bit of mold growth there in the form of small black spots. It's nothing to worry about for most people, but for those sensitive to mold it can be an issue.

    If the triple glazing is a lot more expensive it might not be worth it, but if prices are close it's worth considering. In my experience, high-quality, triple-glazed UPVC windows are very close in price to Marvin Integrity double glazed windows. The calculator Jon linked to is a useful one.

  10. GBA Editor
    Martin Holladay | | #10

    I agree with Michael. My earlier comment concerned the hardiness of Nebraska farmers and was not intended to refer to glazing characteristics.

    For most humans, Michael is right. Once the temperature gets close to zero, the triple-glazed windows are noticeably more comfortable for anyone sitting near a window.

  11. Jon_R | | #11

    According to the Payette calculator, the difference in comfort due to radiant effect at -20F changes from 7.3% to 7.7% dissatisfied (.2U to .3U change, 2' from the glass, etc). I suggest that this is not detectable by most humans.

    The draft effect is more significant and neither are likely to be comfortable without a room temperature increase. In this fairly extreme case, the .3U would need +2F more that .2U.

    ASHRE lists as low as 30% relative humidity as being comfortable and you definitely won't see any window condensation at this level.

  12. walta100 | | #12

    If you mix them you will be able to tell the difference.

    I have a wall of windows with a glass door at the end. The windows are .27 and the door is a .30
    We like to keep the humidly at 45%, when it gets below 18° the door is covered in condensation and the windows will have a few drops in the corners. When you walk past the windows and the door you do can tell the difference while moving. If it is below 40° and you touch the glass the difference is noticeable.


  13. marianelson | | #13

    U-value indicates the rate of heat flow due to conduction, convection, and radiation through a window as a result of a temperature difference between the inside and outside. The higher the U-factor the more heat is transferred (lost) through the window in winter.

    The units of U-value are: Btus per hour per square foot per °F (Btu/hr · ft² · °F)

    U-factors usually range from a high of 1.3 (for a typical aluminum frame single glazed window) to a low of around 0.2 (for a multi-paned, high-performance window with low-emissivity coatings and insulated frames).

    A window with a U-factor of 0.6 will lose twice as much heat under the same conditions as one with a U-factor of 0.3.

    Total (or net) window U-factors can be considerably higher than the center-of-glass U-factors...

  14. billingsdave | | #14

    I don't want to hijack this thread but it got me thinking about the windows I had selected for my cabin project (elev. 4140' Zone 6b). The price difference between the Marvin Integrity windows I had planned to use and the comparable Kolbe Forgent windows is considerable. The Kolbe windows are over 1/3 less expensive. Am I missing something or are the Kolbe windows kind of a no-brainer?

  15. user-6908581 | | #15

    David, I'm finding myself reconsidering my window choice also. Martin's math was an eye-opener. Do I want to pay $4000 more to save $20/year? ROI is in 200 years (for all practical purposes, never). I'd be better off putting those dollars elsewhere, perhaps more insulation below the basement floor or go for the UltimateAir vs the Renewaire. Yes there are other factors (comfort being the main one mentioned), but ROI is always a big one for me.

  16. GBA Editor
    Martin Holladay | | #16

    A GBA blogger named David Posluszny came to a similar conclusion. (Posluszny built the controversial Massachusetts house with a wrong-side vapor barrier. Here is the link: An Affordable Zero-Energy House in Massachusetts.)

    Posluszny did the math, and concluded that the upcharge for triple-glazed windows (compared to double-glazed windows) was a worse investment than PV.

    Posluszny wrote, "The windows are wood-framed vinyl-clad Andersens. The two south-facing windows are fixed, the four north windows are casements, and the two gable windows are awning. I decided not to install sliders or double-hungs because of the air leakage associated with them.

    "The windows have a U-factor of 0.29. I would have preferred windows with a lower U-factor, but I bought the windows inexpensively as odd-lots. Prior to purchasing them I ran some heat load calculations. My home is heated with electricity produced by a solar array. I had to purchase all the solar panels to make the electricity for the heat. After my heat-loss calculations, I found that the extra solar panels were less expensive than windows with lower U-factors.

    "I applied this thinking to all my decisions, and found that the least expensive way to be net-zero is not always the most energy-efficient way."

  17. user-6990904 | | #17

    Update from the original poster,
    In the end, we were hesitant to use the Kolbe Forgent even though the price was sweet as they are a new product. We tend to like tried and try in our windows and marriage. Then it came down to Miguard Ultra and Alpen 525. The u value was better on the Alpen for the same price, though we will be missing the support from the local wonderful vendor, Mariposa windows and doors. Tough choice, but we went with the Alpen.

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