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Existing window glass efficiency

CARL SEVILLE | Posted in General Questions on

Does anyone know how to determine the U and SHGC of existing window glass? I have heard of tools that can identify low e coatings on existing windows. Need to determine efficiency of existing glazing on a multifamily high rise for energy modeling. Thanks

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  1. user-626934 | | #1

    Carl - here's the low-e detector that I use. It's been very helpful for both existing buildings and new construction. $200

    Inert gas (argon, etc.) detection is much more difficult. Here's the product I'm familiar with, but haven't used - It looks expensive enough that I'm afraid to price it out.

    If you wanted to be precise, you'd need to know the glass thickness and spacer thickness, along with the gas fill (if other than air) and whether or not there were low-e coating/s. With this information, you could generate U-values and SHGC using LBNL's WINDOW software. If you did have a low-e coating, then you'd have several different coating types to choose from in the software that would effect both the U and the SHGC. Note - I'm not saying that you SHOULD do this...just that you COULD. ;-)

  2. davidmeiland | | #2

    I think it's going to be hard to get exact numbers from a field inspection, but you can get fairly close. Check out the glass inspection tools from EDTM (which include what John mentioned above).

  3. charlie_sullivan | | #3

    There are two general types of approaches. One is to identify the coatings and perhaps the fill gas, and then look up or calculate the performance. The other is to directly measure the parameters of interest.

    Coatings can be detected by detectors like what John and David pointed to. They generally work by measuring reflectance with an IR laser. Some such units can figure out more details about where the coating is located and what type of coating it is, while others give more of a yes/no result. It does not measure the performance of the coating--it just recognized its signature.

    For fill gas I would be inclined to assume Ar if it's low-e and air if low-e is not detected, unless it's really important to get it right or you have reasons to doubt it would follow that pattern.

    Then you can use what you have found out to look at similar configurations in glass or window catalogs to find out the performance parameters, or use the software John suggests.

    For SHGC, direct measurement is possible, but problematic. One possibility is the meters made by ETDM that reach around the glass and shine different wavelengths through and give you a direct reading of SHGC. Another is to measure the total irradiance when the sun is shining through the glass, and compare readings inside and outside the glass. But you need a meter than has flat spectral response (not a light meter that matches the eye's response) over the full solar spectrum. And even then, the measurement doesn't include any of the heat absorbed by the glass (not reflected or transmitted), whereas in practice and in official SHGC numbers, a fraction of that heat is included based on how much of it moves from where it is absorbed into the inside our outside air.

    For U-value, you can make very rough estimates by comparing the temperature of the interior surface to the room temperature. With that difference, a known indoor-outdoor temperature difference, and an approximation for the R-value corresponding to the heat transfer between the room and the glass surface, you can calculate the U-value. The accuracy of that approach is limited. If verification of the estimate from this or from the other approach was important, a possibility to consider would be temporarily installing a known sample that has the estimated U-value next to the mystery window and comparing the surface temperatures.

  4. Expert Member
    CARL SEVILLE | | #4

    Thanks for all the feedback.

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