R value comparison

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Yi Li

March 01, 2020 23:25

Which Of the following steel stud wall assemblies gives a higher overall R-value? In both assemblies, the total amount of insulation is R-15. A. Exterior sheathing of R -5 and within stud insulation of R-10 B. Exterior sheathing of R-3 and within stud insulation of R-12. Can someone please explain why the answer is A??? I thought they would be the same ??? Many thanks !!!

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  David Kaplan March 02, 2020 02:00

  Yi
  The reasoning is that the continuous insulation gives you the better insulation because it is "continuous." When you put batt insulation in between studs, you only get insulation value in between the studs, but the actual stud itself becomes a thermal break. Thus, every 16" on center, you technically have a cold spot. That's where continuous insulation comes into play. It covers the full exterior wall and is not interrupted (except for at windows, say). So, the higher R-value co continuous insulation gives you a better assembly.

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  aagangchen March 02, 2020 03:15 (Edited March 02, 2020 03:24)

  Agree with David. Here is another way to look at this:

  The studs, especially steel studs, will transfer more heat than the insulation, and will compromise the insulation effect.  

  For A, you have a solid R-5 plus a compromised R-10; For B, you have a Solid R-3, plus a compromised R-12. So, A will give a higher overall R rating, and transfer less heat.

  Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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  Yi Li March 02, 2020 16:49

  Thanks you David and Gang! 

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