R value question
Hi guys,
I am a little confused by R value question. So if the question tell you for example: 2” base coating, R value is 0.78, 1 1/2”brick veneer R value is 5.2, 1/2” finish coating is 0.56. What’s the total value? ( sorry I made this question up, I know the wall section doesn’t make sense, lol)
Should I use the R value multiply the thickness? I don’t think I should, but some of the practice question, the right answer is to multiply with the thickness. Isn’t the R value itself is the materials R value not the R value per inch. Right?
Thank you in advance.
Sha

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.

Hi Sha,
This type of problem usually comes with two pieces  a wall assembly and rvalue table
If the Rvalue needs to be multiplied by the thickness it will be listed in the table as: Rvalue/in
For example, A wall assembly calls out 3.5" batt insulation and the Rvalue table lists batt insulation = 3.3/in you would calculate the Rvalue accordingly.
Rvalue = 3.5x3.3 = 11.55
If the Rvalue in the table is shown as just a number and does not have /in you just have to use the Rvalue that is given for that material.
For example, A wall assembly calls out 1/2 in exterior sheathing and the Rvalue table lists 1/2 in sheathing = 0.60 the Rvalue for the material would simply be 0.60 and no multiplication is needed.
Once you have found the Rvalue for each material you add them all up to find the Rvalue for the total wall assembly.

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 Rvalues, Uvalues, 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://energymodels.com/heattransfer
I hope this helps!
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