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Estimate of u factor on old sliding door?

canada_deck | Posted in General Questions on

I’m replacing an old sliding door. It’s probably about 20 years old. Aluminum frame with no thermal break. Double pane window but likely no low-e coating and the seal is in the glass unit has broken (window is fogged up inside.) The weather seals are also not in great shape.

As I debate the pros and cons of a u-factor of 1.2 vs 1.4, I’m wondering if anyone would have an estimate for what the u-factor of the current sliding door would be?

Also interested in knowing if I will see much practical difference between a 272/CL and a 366/i89.

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  1. Trevor_Lambert | | #1

    Centre of glass u-factor on a broken seal double pane is probably around 1. Don't really know how to estimate the frame contribution other than to say it's higher. Where are the 1.2 vs 1.4 numbers coming from? Surely that's not the u-factor of a new sliding door.

    1. canada_deck | | #3

      See thread below - Looks like we are using different units off by a factor of 5.678.

  2. Expert Member
    Michael Maines | | #2

    I'd guess that the existing glass is probably closer to U-0.5, similar to old single-glazed windows with a separate storm window; even a broken seal will still slow air flow. It's hard to find accurate values for the aluminum frame, because different alloys have different thermal properties and the frame details (wall thickness, frame depth, air spaces, etc.) will affect the final value. But I agree that it's higher than glass, resulting in a door that might be somewhere around U-0.5 to U-1.5.

    I also agree that U-1.2 to 1.4 does not make sense for a new door.

    What is your climate zone? Do you have indoor humidity control? The i89 coating has good performance characteristics for colder climate zones but it keeps the glass surface cool, potentially leading to condensation.

    1. canada_deck | | #4

      Sliding glass door with a vinyl frame. About 80" tall and 72" wide. I have looked at a number of manufacturers and they are all coming in at around the same value.

      Vendor 1: The U-Value is 1.42
      Vendor 2: U-Factor: 1.55
      Vendor 3: U-Value range from 1.08 for a triple pane 366/180/i89 to 1.59 for a double pane 272/CL

      What U value would you aim for?

      1. Expert Member
        Michael Maines | | #5

        Ah, I just noticed by your name that you're probably in Canada, so you're using SI units, not the IP units we use in the US.

        Typical PVC windows and doors for colder climate zones in the US are around U-0.27 to U-0.30; that would be U-1.53 to U-1.70 in SI units.

        Windows and doors usually perform much worse than the rest of the walls they are in so it makes sense to get the best-performing ones you can; I aim for around U-0.2 to U-0.25; in SI, that's U-1.14 to U-1.42.

  3. canada_deck | | #6

    Ah got it. Didn't even think to check the units since they were not provided by the vendors. Hopefully they are talking about the SI units! When you buy insulation in the stores here, everyone quotes the imperial units I believe. E.g. you might aim for R22 of insulation in a wall. Confusing enough that we talk about U instead of R for windows and doors - now they are switching units as well....

    So your rough guess would translate to 2.839 to 8.517 in my local units which means any of these doors should be a noticeable improvement.

    I think I will eliminate the 1.55 from consideration.

    One option that looked pretty good to me was a double pane 366/i89 at U 1.31.

    Maybe I am being paranoid but it feels like it is just an extra set of seals to break.

    1. Expert Member
      Michael Maines | | #7

      Sounds good, just note that i89 can result in condensation if you don't keep your indoor humidity levels pretty low.

      Maybe Oberon will chime in. He's a glazing expert and we were recently on a webinar together; he said that Cardinal Glass, where he used to work, has something like a 0.2% failure rate on their glazing seals, and that's after 20 or 30 years of service, so I wouldn't worry about seal failure. Triple glazing does cost more than double glazing and for just one door it won't make a huge difference in your energy costs, but it will make a difference in how comfortable it is to be near the door in cold weather.

      1. canada_deck | | #8

        Ah interesting. I'm near the coast in British Columbia and we have all sorts of moisture issues so I will keep that in mind. I keep hearing that seals don't fail that often but I am also having to replace a lot of my windows due to broken seals. One of windows is going to set my back $2000 - just to replace the sealed glass unit - no frame replacement. To be fair, they are fairly old now.

  4. [email protected] | | #9

    As Michael Maines already pointed out, the glass package in your existing door is going to be right around U factor of .5 based on two glass lites with no argon, whether sealed or unsealed, because it doesn't matter if there is a seal failure or not. Simply, single clear glass lite - U1/R1. Two clear glass lites - U.5/R2. Three clear glass lites - U.33/R3. Definite pattern.

    Can you share who are the manufacturers that you are considering?
    Vendor 1, U 1.42 metric - U.25 or R4 imperial.
    Vendor 2, U 1.55 metric - U.27 or R3.7
    Vendor 3t, U 1.08 metric - U.19 or R5.2
    Vendor 3d, U 1.59 metric - U.28 or R3.6

    Clearly the triple pane option stands out when compared with U factor performance of the other three choices . And since you are in Canada I would expect that the doors were tested to NFRC with imperial results that were then converted to metric. Also I converted your options specifically to show the R value numbers because I think it really is easier to compare performance using whole numbers when the higher the number equates to the better the performance (the reason why R value was "invented" in the first place).

    I would also be looking at SHGC and VT, I think those are especially important where you live, and while a higher or lower SHGC might be debatable, I think that you need to look into getting the highest VT that you can, while still keeping an acceptable U factor. That said, depending on which direction your door is facing, I might seriously ask about other coating options besides 366. I would give serious consideration to using 180/180 in a triple configuration as long as the door isn't facing west.

    Per your concern about seal failure, once again I agree with Michael Maines and depending on the system used to construct the IGU's, seal failure shouldn't be a concern, but another reason I asked what manufacturers you are talking with. Knowing who is building the door, might make a difference.

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