Access hatches and doors. Access doors from conditioned spaces to unconditioned spaces (e.g., attics and crawl spaces) shall be weatherstripped and insulated to a level equivalent to the insulation on the surrounding surfaces.
A batt must be undersize, else too much drag, a problem especially in closure. Even where staples hold, a tight-fitting batt will be pulled out of contact with drywall in pulling down, then having negligible value. A batt with reasonable clearances, in contact with drywall, does not come close to the needless hope of matching surrounding insulation value. In fact, know surrounding loose-fill insulation is progressively destroyed as the clumsy plunker makes assorted landings.
This frame is 22.8” x 31.4”. The R38 unfaced batt is nicely square-cut 22” x 23”.
The fraction of drywall area covered by the batt is 22*23/22.8*31.4 = 0.707.
1/(Reff + 3) = 0.293/3 + 0.707/41
Reff = 5.7
Reff + 3 is the total heat transfer resistance number, and the inverse, U = 1/(Reff + 3), is
U = 0.10
I can do better with a factory-made hatch that is safer to use, and with lower installed cost if mass-produced. This hatch will not lose its R-value by handling, and is not subject to other heat loss where the plunker rarely has an effective gasket.
Apply Insulation Math:
Plug is 18.25" x 20", 2.53 sf
R7.5 foam is 16.75" x 18.5", 2.15 sf
Wood frame at R1.4 is 2.53 - 2.15 = 0.38 sf
2.53/(Reff +3) = 2.15/(7.5 + 3) + 0.38/(1.4 + 3)
Reff = 5.7
It is so much better to have a lid that retracts to space below, if at-risk attic insulation about the hatch is fragile loose fill.
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