# U factor assembly calculation



## Engineer22 (Aug 13, 2017)

Can someone help me figure out how to do a U-factor assembly calculation, step by step?

Let's say 100 SF of window, U (window) = .60
                500 SF of wall (net, not including window), U (wall) = .072

Is the proper procedure for calculation as follows?:

U= (.60 X 100) + (.072 x 500)

---------------------------------------- =    .16

                  600

Thank you.


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## cda (Aug 13, 2017)

Can you give code sections and editions

The codes used by people on the site are wide and varied.

It just helps answer questions easier


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## mark handler (Aug 13, 2017)

http://www.taitem.com/wp-content/uploads/2011/01/TT-NC-Calculating-U-values-Nov-2008.pdf


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## Engineer22 (Aug 13, 2017)

It's just a U-factor calculation, its not specific to a code version. However, IECC is the applicable code.

Thank you for the link Mark. However, I am looking for a U-factor weighted assembly value.


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## cda (Aug 13, 2017)

You do know that this is a self supported site and we welcome all people who would like to sign up as a sawhorse and support the site financially

And bring code and experience to the site


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## Engineer22 (Aug 13, 2017)

The U-factor assembly calculation is a required calculation under the terms of the IECC- you do know that stands for International Energy Conservation Code, right?


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## Engineer22 (Aug 13, 2017)

If anyone with "code and experience" might help me in understanding how to achieve this calculation, I would greatly appreciate it. Thank you.


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## RLGA (Aug 13, 2017)

Engineer22 said:


> The U-factor assembly calculation is a required calculation under the terms of the IECC- you do know that stands for International Energy Conservation Code, right?


The U-factor method is one of the options, but is not required. The edition of the IECC you're using is relevant to the issue, since the IECC has gone through many changes--especially between the 2012 and 2015 editions.

The 2012 has the R-value method as the prescriptive with the U-factor as an alternative; however, the weighted U-factor is not an option in the 2012 and earlier editions of the IECC, except if the ASHRAE 90.1 option is used per Section C401.2. In the IECC, the R-value prescriptive method establishes the minimum R-values of insulation materials in an assembly and does not take into consideration the thermal resistance performance of other assembly materials. The U-factor alternative establishes minimum U-factors for assemblies, but allows consideration of all materials in the assembly--including the interior and exterior air films. However, the U-factor is not weighted by area.

ASHRAE 90.1 allows the "trade-off" option, for which programs like COMcheck can be used. The 2015 IECC provides a similar method to that of ASHRAE 90.1's "trade-off" option. IECC Section C402.1.5 allows the use of the "Component Performance Alternative," where the summation of the differences between proposed U-factors x area and the tabular U-factors x area must be equal to or less than zero, but also includes summations of differences for F-factors and C-factors for slabs-on-grade and below-grade walls, respectively; and U-factors for skylights and vertical glazing using slightly different calculations.


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## Paul Sweet (Aug 14, 2017)

Your procedure is correct.


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