CSLB General Building (B)Finishes and Building EnvelopeHard
A wall assembly for a Title 24 energy calculation includes R-13 fiberglass batt insulation between studs, R-5 continuous rigid foam sheathing over the exterior, and a combined R-2 contribution from interior/exterior air films and finish materials. What is the approximate total R-value of the wall assembly?
- AR-25
- BR-13
- CR-20
- DR-18
Show answer & explanationAnswer & explanation
Correct answer: C. R-20
Total assembly R-value is found by summing the R-values of each layer in series: 13 (batt) + 5 (continuous insulation) + 2 (air films/finishes) = 20, giving an approximate total assembly R-value of R-20.
Why the other options are wrong
- A. Overstates the total; does not match the given component values.
- B. This only counts the batt insulation, ignoring the sheathing and finishes.
- D. Undercounts one of the layers in the sum.
Assembly R-Value Calculation
Total thermal resistance of a wall assembly is calculated by adding the R-values of each material layer in series, including insulation, sheathing, and air films.
- R-values in series are additive
- Continuous insulation adds to cavity insulation value
- Air films contribute small but real R-value
Memory trick: 'Stack the R's like bricks to build the wall's true value.'