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Heat loss Calc, correct wi ndow measurments

Robert_HRobert_H Member Posts: 135
The many heat loss calculation instructions I have found tell you to measure the windows or enter the window dimensions, but what should I measure? glass edge to glass edge, sash to sash, use the actual frame size of the window etc? What about when looking at a window catalog which provides a rough in size?

this seemingly critical piece of information is missing from everything I have read so far.




  • STEVEusaPASTEVEusaPA Member Posts: 1,506

    The easiest way I can think of to explain, just measure it like you were fitting it for mini blinds, to cover the glass.  It's more important to get the right type of window (r value).  A few sq inches wont matter
  • Robert_HRobert_H Member Posts: 135

    Thanks Steve
  • SpenceSpence Member Posts: 316
    Window Measuring

    When you are working with load calculations with respect to windows, you are working with "fenestration." It is a term used to describe all types of glass (windows, doors, skylights, etc.). The term applies to the entire assembly of glazing, sash, and framework that fits into a rough opening.
  • Robert_HRobert_H Member Posts: 135
    Thanks Spence

    So the overall dimensions or minimum rough opening if I'm looking at a catalog.
  • SpenceSpence Member Posts: 316

    Correct! Then, once you have your type and size, take into account blinds, shading, awnings, orientation, and whether or not the glazing is flat against the wall (as opposed to a bay, bow, skylight, or garden window) and you're in pretty good shape. If you're wondering about the different grades of windows, glazing insulation value is shown in a "u" factor. "R" and "u" are a reciprocal; 1/R = u, and 1/u = R. For example: if your window has a u-factor of .36, then insulating value is equal to R-2.8.
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