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Thread: Question about windows and manual J

  1. #1
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    Question about windows and manual J

    Hi,

    For anyone who is familiar with Manual J as it relates to windows, which has a larger heat gain for the same size window, 1. double-pane, low E, clear glass, vinyl window or 2. double-pane, tinted, aluminum framed (no thermal break) window. I assume it is #2 but wanted to have someone to confirm it. Thanks.

  2. #2
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    Should be #2 just based on the metal frame, but why are you asking? Are you purchasing new windows? They should be able to give you heat gain ratings from the manufacturer which are always more accurate than the numbers from Manual J. Generally I’ll look up the U-Value and SHGC then use a manual J window type that has matching numbers.


    Sent from my iPhone using Tapatalk

  3. #3
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    Based on the default values given in the 2017 Florida Building Code - Energy Conservation (which is based on the International Building Code):

    • Non-metal, double pane, clear windows: U=0.55/SHGC=0.7
    • Metal, double pane, tinted: U=0.8/SHGC=0.6



    You can look up the combinations on this calculator: https://www.adicotengineering.com/window-u-shgc-values

    The U-factor is based on the frame and the # of panes, the SHGC is based on the number of panes and clear or tinted.
    Adrienne Gould-Choquette, P.E.
    www.adicotengineering.com

  4. #4
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    Thread Starter
    AdrienneGC, thanks for the window U/SHGC calculator link. The Manual J software package I use is showing more Total Heat Gain for the vinyl frame windows than the aluminum frame ones for the same square feet of windows. The house currently has the aluminum frame windows (at least 15 years old) so there is no energy label is available for them. I've been told that the metal frames gets very hot in the summer when the sun is shining on them (south exposure) (naturally). I guess I will reach out to the company that makes the software to ask about it.

  5. #5
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    Hi @drose,

    ASHRAE's equation for Fenestrataion Cooling Load,q, is: q/A= U(Δt – 0.46DR) + PXI SHGC × IAC × FFs

    I suppose if your Δt (change in temperature) is relatively high compared to your DR (daily temperature range), the U-factor will be more predominant than the SHGC. So it seems that your question is dependent on location. In some cases, a better U-value will be the deciding factor, and in other cases, the SHGC will be the deciding factor.

    I am definitely interested to know if your software provider gives feedback. Thanks for the distraction from work
    Adrienne Gould-Choquette, P.E.
    www.adicotengineering.com

  6. #6
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    Quote Originally Posted by AdrienneGC View Post
    Hi @drose,

    ASHRAE's equation for Fenestrataion Cooling Load,q, is: q/A= U(Δt – 0.46DR) + PXI SHGC × IAC × FFs

    I suppose if your Δt (change in temperature) is relatively high compared to your DR (daily temperature range), the U-factor will be more predominant than the SHGC. So it seems that your question is dependent on location. In some cases, a better U-value will be the deciding factor, and in other cases, the SHGC will be the deciding factor.

    I am definitely interested to know if your software provider gives feedback. Thanks for the distraction from work
    Also, the term PXI (peak exterior irradiance) is dependent on latitude, so this further points to the location as being a factor in determining which window is more efficient.
    Adrienne Gould-Choquette, P.E.
    www.adicotengineering.com

  7. #7
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    Thread Starter
    Hi AdrienneGC, with regard to your wondering if I would hear back from the software provider. The software provider did reply and told me that the support offered with my package was only related to how to use the program such as "how to add a door". The package is very straightforward so I don't have any trouble with how to use the package. I just thought there might be some error in the software algorithm. If there is, I guess I will have to find it on my own through other resources. I'm not saying there is a problem.

    I did set up another dummy job/house with one room. The room had 3 windows with the same parameters (same size, double pane, clear glass) except for the frame (vinyl, wood, and metal (no thermal break)). There was no difference in heat gain but the metal had more heat loss. That seems to corresponds for the most part to the calculator link you posted.

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