Tuesday, October 25, 2016

The True R-Value Of A Best-Example Attic Drywall Plunker With Twined R38 Batt

This "insulated" drywall plunker is the common result of building code statement such as this of 2012 International Energy Conservation Code, Section R402.2.4:

 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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