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Estimating U-Factor

severaltypesofnerd | Posted in Energy Efficiency and Durability on

How can I guesstimate U-Factors for older unmarked windows?

I do work legalizing unpermitted dwellings, of which my area has tens of thousands.   These were all built at a time when such units were not allowed, so often zero permits.  Now, these can become ADU’s (Accessory Dwelling Units) but must generally be brought up to Title-24.

I have a unit with vinyl double hung double pane windows, as inserts into a early 1900’s redwood frames.  They’re perfectly good, but I can’t exactly date them, and there’s no real maker mark, and no U-Factor.

Could anyone point to a resource by sale date, of the required minimum U-Factor and SHGC for windows in California USA?

Without this we’ll have to tear the windows out, and frankly there seems to be lower hanging fruit for energy efficiency.  I’m guessing it’s carbon negative to rip these out.

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  1. oberon476 | | #1

    Assume that a single sheet of glass has a U factor of 1. Add a second lite and the U factor improves to U.5, add a third sheet and figure U.33. These numbers are general and are dependent on both the spacing between the lites as well as glass thickness, but for guesstimating pure generic performance they are about as close as you can get.

    For SHGC assume that a single lite of 1/8" glass has an SHGC of about .9, two lites .8, and three lites .7 (notice the pattern?).

    Keeping in mind that these are glass-only numbers that do not include LowE coatings or any sort of gas fill other than air. Also frame material is going to make a difference of course. Vinyl and wood not so much, aluminum a lot more (in a bad way).

    LowE coatings have become much more common in the past 20 years or so (not sure how much that translates to SoCal though), so if the windows are more recent it's entirely possible that the glass may be coated which will improve both U factor and SHGC. If the glass isn't coated then applying an appropriate aftermarket film to the uncoated clear glass can help improve both U factor and SHGC as well.

    There could also be also be concerns about installation and air leakage (a lot more so if you were in a less forgiving environment), or if the windows meet (or not) basic safety glazing requirements depending on where and how they were installed.

    No matter the code requirements in place at the time of construction, materials that didn't meet current codes would have probably been readily and legally available to anyone who wanted them, especially if they were building an unpermitted structure in the first place, so not sure how helpful knowing the codes in place at the time might be?

    1. severaltypesofnerd | | #4

      What's sold as "Shed" and "Barn" windows are single pane. These are clearly meant for residential.

      I suspect the cheapest window at Home Depot at the time was purchased, and that it met the then current minimum U-Factor and SHGC.

      Note: This particular area, due to fog, is largely a heating climate. So designs that reject incoming sunlight are undesirable, but by energy code frequently used. There are far more heating days than cooling days.

  2. Expert Member
    NICK KEENAN | | #2

    If you're required to certify the work I think you're out of luck. Even if there were minimum standards in place at the time these windows were installed, who's to say they complied then? Maybe they were sold for agricultural use or brought in from another state.

    I'd work harder on identifying them. You say "there’s no real maker mark." What is there? Replacement windows like that are often made in small shops, but the glass units are purchased pre-assembled and my understanding is there's only a handful of glass manufacturers in the country.

    1. severaltypesofnerd | | #3

      The same glass units are used in multiple makers of window product. So while I found some numbers and codes on the glass unit, I'm not sure what that gets me.
      And the glass unit says only limited things about the performance of the entire assembly.

      What about regulations. As of X date, a certain U factor was required.
      Where could I look up that regulatory history for new windows sold?

      1. Expert Member
        NICK KEENAN | | #6

        The problem is there's no way to show that these windows met code when they were brand new. You can buy windows labeled as "shed" windows or "barn" windows that don't meet code. If someone is doing an illegal renovation why would they concern themselves with whether the windows are code-compliant?

  3. walta100 | | #5

    Short of identifying and finding the original certificate seems like you must assume the worst and enter something like the numbers Greg posted.

    The question becomes will the regulators accept your made-up numbers.

    Thought the regulators required the building to meet whatever code is current as of the day of the application no grandfathering allowed for unpermitted building.


    1. severaltypesofnerd | | #9

      The AJH's here will sometimes apply the "code applicable as of date of construction", and they have that authority. California Senate Bill 1226 (passed in 2018) reaffirms that existing authority.

      Usually, they want today's code applied to yesterday's work. The California Existing Building Code (CEBC, based on IEBC) applies, but it's pretty modest in what it's helpful for vs. new construction. Adaptive reuse of existing buildings is dramatically more complicated than greenfield building.

  4. Expert Member
    NICK KEENAN | | #7

    At the end of the day what matters is what the guy who's going to be signing off on this thinks.

    Can you ask him? He might say, "Mid 90's? Let's just say U-0.5 and call it a day." Or he might say, "No certificate, no good." We're just guessing.

    1. severaltypesofnerd | | #11

      Well, if I could track down the data to say when this was built.
      Then find a table of what regulations say was at Home Despot.
      I've at least got a chance of an argument.

      Is there a "history" of U factor regulations, by date, that someone has compiled?

      1. Expert Member
        NICK KEENAN | | #12

        It seems like an awfully big gamble to do all your work first and then ask the AHJ if he agrees with you, but whatever.

        California is governed by the California Energy Code, which is not based on the model codes that other states use. So what you're looking for is California-specific. The California Department of Energy actually has a website dedicated to past codes at , but they just say to "Contact Title 24 Hotline at the email and phone numbers to the right for any 1978 to 2008 documents."

  5. user-723121 | | #8

    It might be time to challenge the powers that be and do a cost benefit analysis for your climate. Where is money best spent on thermal improvements. Thermopane glass in a mild climate should suffice for heating. Solar gain, Low E coatings, shading and overhangs are factors to consider from a cooling standpoint.

    How does this fit in with the idea of affordable housing?

    1. severaltypesofnerd | | #10

      Our local AJH's are far better checklist checkers, than systems thinkers. Case in point the California Title 24 regulations until this year did not recognize top of slab foam below flooring. Zero credit for that. Our 1 hour fire separation requirements are similarly unthinking, resulting in fire assemblies that meet code but won't work, but at the same time are expensive and harmful to historic finishes. This all differentially raises the cost of retrofit housing, compared to greenfield.

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