CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
A contractor installs R-19 fiberglass batt insulation (designed for a 6-1/4 inch cavity) into a 2×4 wall cavity that is only 3-1/2 inches deep. What is the most accurate outcome of this installation?
- AThe batt will still perform at its full rated R-19 value since the material itself hasn't changed
- BThe compressed batt's effective R-value will be lower than R-19 because compression reduces trapped air volume
- CThe R-value will increase because compression makes the fibers denser and more effective
- DCompression has no effect on thermal performance, only on vapor permeability
Show answer & explanationAnswer & explanation
Correct answer: B. The compressed batt's effective R-value will be lower than R-19 because compression reduces trapped air volume
Fiberglass batt insulation relies on trapped air pockets between fibers for its R-value rating. Compressing a batt designed for 6-1/4 inches into a shallower 3-1/2 inch cavity reduces the air space, resulting in a lower effective R-value (an R-19 batt compressed this way typically performs closer to R-13).
Why the other options are wrong
- A. Incorrect — R-value ratings assume full, uncompressed loft thickness.
- C. Density increase does not improve R-value; trapped air is what matters.
- D. Compression significantly affects thermal performance, not just vapor permeability.
Insulation Compression Effect
Fiberglass batt R-values are based on installation at full designed thickness; compressing batts into shallower cavities reduces trapped air and lowers effective R-value.
- R-value depends on trapped air volume
- Compressed batts underperform their rating
- R-19 compressed to 3.5" often performs near R-13
Memory trick: Squeeze the fluff, lose the stuff (R-value drops).