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BPI Written Exam  Section 1 Building Science Fundamentals5. Understand area weighted RValue
The area weighted Rvalue is for finding the Rvalue of a series of materials together as one. It is most often used when calculating a walls Rvalue because it considers multiple materials such as drywall, vapor barriers, siding and wall studs, not only the insulation in the walls. This may be a problem you see on the BPI test, so here is what you need to know.
Terms:
Relationship between Uvalue and Rvalues Uvalues are the fraternal twins of Rvalues, meaning that to find a Uvalue for a material, simply put 1 over the Rvalue = Uvalue. The opposite is true also, to find the Rvalue of a material given the Uvalue, take 1 over the Uvalue = Rvalue.
The difference between Rvalue and Uvalue
Practice Problem:
Find the Rvalue of this madeup, allinsulation and no stud wall, given the following: Rvalues –Brick = R0.10 per inch –Sheet rock = 0.90 per inch –Batt insulation = 2.6 per inch –Outdoor air film = R0.17 –Indoor air film = R0.68 First create a table of 4 columns of material, Rvalue per inch, Thickness and Total Rvalue.
Then you can multiply all the Rvalues by their thickness to get the total Rvalue for each material. Before going further, try it on your own an see if your number matches mine. In homes and buildings, walls are not all just insulation. We need to account for the studs and other materials as well.
On the test:
What BPI will give you:
Approach:
Here is another sample problem you may see on the BPI test to find the Rvalue of the wall composition. Follow the approach above with your 4 columns. Once you've added the total Rvalues for all the wall components, remember to multiply the cavity and frame by 75% and 25% respectively.
Next Section1a. Basic terms and definitions
1b. Principals of energy, air & moisture
1c. Combustion science
