R value question

Comments

5 comments

  • Avatar
    Anton Gross

    R values are added up for all components in an assembly. You multiply per inch for some substances like insulation. Think about it. If you had 2" of rigid insulation in a wall or if you had 6" rigid insulation in a wall, wouldn't your resistance be totally different? So, for some materials, you will multiply an R rating per inch, etc. for that material, and then add that number to your total. Always remember that the U is 1/R as well. One is for transmission and one is for resistance.

    Hope this helps some.

    0
    Comment actions Permalink
  • Avatar
    Sha Liu

    Hi Anton,

    I understand the insulation thickness matters but my confusion is that if the question tell you 2” brick R value is 6.5, is that 6.5 already have the thickness in count.? if that the case we shouldn’t multiply the thickness.

    0
    Comment actions Permalink
  • Avatar
    Emily Greenstreet

    Hi Sha,

    This type of problem usually comes with two pieces - a wall assembly and r-value table

    If the R-value needs to be multiplied by the thickness it will be listed in the table as: R-value/in

    For example, A wall assembly calls out 3.5" batt insulation and the R-value table lists batt insulation = 3.3/in you would calculate the R-value accordingly.

    R-value = 3.5x3.3 = 11.55

    If the R-value in the table is shown as just a number and does not have /in you just have to use the R-value that is given for that material.

    For example, A wall assembly calls out 1/2 in exterior sheathing and the R-value table lists 1/2 in sheathing = 0.60 the R-value for the material would simply be 0.60 and no multiplication is needed.

    Once you have found the R-value for each material you add them all up to find the R-value for the total wall assembly.

    3
    Comment actions Permalink
  • Avatar
    Theresa Bort

    Hi Sha,

    The other thing to look out for is whether the question says anything about using the "k" or "c" value. The k value ("conductivity") is measured per inch, whereas the c value ("conductance") is a value for the given thickness of the material. 

    I found a great article online that goes through a variety of ways to calculate R-values, U-values, and the temperature at a given point in the wall. It may look daunting at first, but as you read through it, you may start to realize that every equation is saying the same thing in a different way. Also, note that the "Overall Heat Transfer Coefficient" equation is an equation given in the test's reference material.

    http://energy-models.com/heat-transfer

    I hope this helps!

    1
    Comment actions Permalink
  • Avatar
    Sha Liu

    Hi Theresa and Emily,

    Thank you for your reply. They are helpful. I guess next time, when a question tells  me a 4" brick R value is 0.44. I assume 0.44 already take the thickness into account. Unless I see a chart or K value that is when I should multiply the thickness. Thanks.

    0
    Comment actions Permalink

Please sign in to leave a comment.

Powered by Zendesk