An insulation contractor is installing R-19 fiberglass batt insulation, rated for a 6.25-inch cavity depth, into a 2x4 wood-framed wall with an actual cavity depth of only 3.5 inches. What is the most accurate consequence of this installation for Title 24 compliance purposes?
- AThe compressed batt delivers a reduced effective R-value, well below R-19, and does not meet the R-19 designation
- BThe batt still delivers its full rated R-19 performance regardless of compression
- CCompressing the batt increases its R-value because the fibers become denser
- DCompression has no effect on R-value but doubles the batt's fire resistance rating
Show answer & explanationAnswer & explanation
Correct answer: A. The compressed batt delivers a reduced effective R-value, well below R-19, and does not meet the R-19 designation
Fiberglass batt R-values are rated at a specific installed thickness; compressing the batt into a shallower cavity reduces the effective air pockets that provide thermal resistance, lowering the actual R-value achieved (an R-19 batt compressed to 3.5 inches typically performs closer to R-13). For Title 24 compliance, the contractor must use insulation product properly sized for the actual cavity depth, not rely on the compressed batt's nominal rating.
Why the other options are wrong
- B. R-value ratings assume installation at the batt's designed thickness; compression reduces actual performance.
- C. Compression reduces, not increases, R-value by collapsing the air spaces that provide insulating resistance.
- D. Compression does not affect fire resistance rating, which is unrelated to thermal R-value.
Batt Insulation Compression Effect
Fiberglass batt R-values are rated at their designed thickness; compressing a batt into a shallower cavity reduces its effective R-value below the nominal rating.
- R-19 batts are designed for roughly 6.25-inch cavities
- Compressing into a 3.5-inch 2x4 cavity yields an effective R-value closer to R-13
- Title 24 compliance requires using insulation matched to actual cavity depth
Memory trick: Squeeze the batt, squeeze out the R-value.