Calculating U Value of a Wall

Mechanical & Electrical Plan Vignette and Multiple Choice

Calculating U Value of a Wall

Postby mdrnarch » Sun Jul 23, 2017 11:51 am

Hi All...

If you have a wall that is 10 ft x 12 ft and there are (2) window openings that are 12 sq.ft total....

Total R-value for Assembly: 25.99
U Value of Window: 0.32

What is total u value of the wall?
Last edited by mdrnarch on Sun Jul 30, 2017 7:31 am, edited 1 time in total.
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Re: Calculating U Value of a Wall

Postby Anthony4 » Sun Jul 23, 2017 11:01 pm

I think we need to know the R-value of the windows, otherwise we have 2 unknowns...
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Re: Calculating U Value of a Wall

Postby mdrnarch » Sun Jul 30, 2017 7:32 am

Anthony4 wrote:I think we need to know the R-value of the windows, otherwise we have 2 unknowns...


Sorry totally left that out.
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Re: Calculating U Value of a Wall

Postby AJD » Mon Sep 25, 2017 10:26 pm

I am assuming that the "Total R-Value for Assembly" means just that, that it is the R-value for the entire assembly including the windows and not just the opaque wall element.

If the total R-value of the whole assembly is already given then can you not just solve for U by using the formula U=1/R (R being the sum of all resistances)? That would mean that U = 1/25.99. Therefore U = .038. Can anyone weight in on this?
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Re: Calculating U Value of a Wall

Postby ccolley12 » Thu Sep 28, 2017 1:00 pm

mdrnarch wrote:Hi All...

If you have a wall that is 10 ft x 12 ft and there are (2) window openings that are 12 sq.ft total....

Total R-value for Assembly: 25.99
U Value of Window: 0.32

What is total u value of the wall?


This is a strange question, but I think they are asking you to understand 2 main concepts... one is the relationship of R-vaules and U-values... They are recipricals of each other... U=1/R and R=1/U. Secondly, they are asking you to understand how a window/opening (that has a poor thermal performance compared to the opaque wall) impacts the thermal performance of an overall elevation... So I would think about this in the following way:

The opaque wall is 90% of the overall area and the glazing is then 10%. The assembly is then the average of the R-values given their % area of the wall.
90% of the wall has an R-Value of 25.99
10% of the wall has an R-Value of 3.125

I would answer something around R 23.5 +/-
(25.99 * .9) + (3.125*.1)
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