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Community and Q&A

Another triple-pane thread…they just are not making a lot of sense

user-1012653 | Posted in Energy Efficiency and Durability on

First off, I have read about every post, blog, and entry made on here about triple pane windows, have talked with the companies, received bids, etc. So with that being said…

On paper, they do not seem to make any sense now matter how I look at it. I have ran numbers in multiple programs from as simple as RESFEN to high end stuff in my Revit MEP and other modeling programs. I am happy to say all of my loads and energy costs are very similar from the free programs to the high end however.

Looking closely at full fiberglass options, I have compared Pella Impervia, Marvin Ultrex, Milgard Ultra, Inline, Fibertec, Thermotech and Serious. The Marvins have the worse numbers (in terms of infiltration, U-factor, and SHGC) out of the group, so I tossed them. Serious, while good numbers, I am hestiant, like many, of the films so I will make it easy and toss them too. Thermotech has the best numbers of the group, but the price came in pushing double any of the rest, so they are out.
Inline and Fibertec are basically the same number and cost wise, so to be easy, I will look at Inline vs Pella Impervia.

Let’s start with the facts. THese are also the numbers I used in the programs:
Pella: Dual pane lowE, U=.28, SHGC .30, air infil= .05, VLT 57 for all north glazing
Dual pane LowE w/ NaturalSun, U=.30, SHGC .57, VLT 64 for all south glazing

Inline: Dual pane lowE U= .31, SHGC= .50, air infill= .03 VLT .51 for south glazing
Triple pane, south glazing, U=.22, SHGC= .39, VLT .43
Triple pane, north glazing, U=.20, SHGC= .25, VLT .39

GIven those numbers, Pella is not looking to shabby at all. Lets add in that Pella is about $2500-3500 less. They are about 2500 less if you use Inline dual on the south and triple on the north, and 3500 less if you use triple on both.

Energy savings, on average from my programs, are putting out a super low $70 a year. I only have north and south glazing, so it makes calcs easy as well. So assuming they are a 30+ year payback, are my findings typical for some? I know based on responses from Robert Riversong he seems to really favor the Impervia line. I would agree as they perform better on southern elevations, and not far off on the north.

I understand the human thermal comfort argument….but what else am I missing?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you didn't tell us your location or climate.

    Second of all, congratulations on your thorough analysis. I really admire the way you are doing your research. You have reached out to a variety of manufacturers; you have tracked down the relevant specifications; and you have run the numbers through several energy modeling programs. Bravo! That's exactly what you should be doing.

    I've said it before, and I'll say it again: energy prices are very low in the U.S. right now. Many experts predict that natural gas will continue to be cheap for at least another decade. We don't have any political leaders in this country who are willing to discuss or implement carbon taxes.

    Because our energy is cheap, it is hard to justify many superinsulation techniques on payback alone. So if, after running the numbers, you decide to go with double-glazed windows, your decision is logical and understandable.

    Some energy nerds and environmentalists are willing to invest a little more in triple-glazed windows because we worry about future energy price spikes, because we are willing to invest more to lower our carbon footprint, or because we want the increased comfort that triple-glazed windows provide.

  2. gusfhb | | #2

    If energy were free, would you buy single pane glass?

    I just bought a 150 square feet of pretty much state of the art triple pane IGU's at around $23 a square foot delivered and I do not know what the payback is. I do know that the inner pane of glass will be a lot warmer than if it were double pane.

  3. user-716970 | | #3

    Take a look at Cardinal website...some good info
    Their dual glazed IG units have some impressive "center of glass" numbers. For example
    LoE-180 (#2)/ i81 (#4)...U value (argon) .21 SHGC .59

    I really like that they make their own IG units in a quality controlled factory setting and have some of the best long term seal performance. This may be the glass the Pella and Inline uses...might be worth asking for.

    You might also consider using the same high SHGC glazing on both north and south. There will likely be enough incident solar gleaned on the north windows to help make up for the slightly lower U values of the low SHGC glazing.

  4. user-1012653 | | #4

    assumed you would know by now from all of my annoying questions! I am in NE Iowa, zone 6.
    I plan on doing a 10-12" double stud with ZIP, so I may as well take the windows to the top as well. However throwing payback out the window, it almost appears the Pella or Inline double pane will give me a higher performance then the triple pane option on the south. Running the numbers in RESFEN and looking at feedback from individual windows, the higher SHG is offsetting the higher u of the double pane by quite a bit. With that being said, if the triple and the double were the same price, knowing the double perform better for heat loading, would you still select the triple for the "human comfort"? (I will add we really do not have any windows you would "sit by" with exception of a large window in the dining room that could be triple for a comfort standpoint, but this is also one of my main solar collectors too...)

    I will eat my own words and stop being as concerned about cost, since we are not hestitating to use soapstone or something in the kitchen instead of laminate....for example. I want to install the best performing and best quality we can safely afford. If this means dual on south and triple on north, triple everywhere, or dual everywhere. I am, of the opinion however that triple on the north and dual on the south is what is looking like the best performance combination. I would like to use Inline/Fibertech triple on the north, and Pella on the south since they are lower cost and higher gain ratings....but I do not think I want to mix and match window brands. Although both being simple white fiberglass frames may not make any difference.

    My heat loading is right around 18k BTU for 2900 sqft. Going from all Pella double to all triple only reduces the total BTU to 17,500.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for providing your climate zone.

    I'm trying to encourage people to tell us their climate zone every time they post a question. After all, I'm not the only reader. We get new readers every day, and there are many GBA readers who many not remember who you are, and who will want to know where you live so they can provide the advice you seek.

  6. user-869687 | | #6


    If you do go with Inline, you get a better quality frame. Download the cad drawings and zoom way in. The Pella extrusions have thinner walls, and at least with casement windows they appear to have more parts than the Inline frames, which have a more logical design. I like the interior glazing stop on Inline, vs. the exterior PVC stop on Pella.

    This image shows Pella on the left, and Cascadia at right (same as Inline).

  7. gusfhb | | #7

    They get better:

    I believe that the 1 1/8 triples are their thickest standard, as they did not want to go higher.As it is the ones that I selected end up with a U of .13, [R 7.7] COG.

    Since much of the money in a manufactured window is in the opening mechanism and sealing, consider fixed glass. It would be pretty simple to design a frame that buts up to an inch of foam so no thermal bridge. Any half sober carpenter could make a houseful in a few days.

  8. user-626934 | | #8


    In case you missed it - you might want to check out the newer PVC and wood imports (such as Intus Windows) that Matrin reviewed a few blogs ago (

  9. user-1012653 | | #9

    thanks for the detail Thomas. You are correct. And it also appears that the glass is wider too.

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