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Manual J and Floor Over Crawlspace

seeking_comfort | Posted in General Questions on

Hi ,

I am doing a Manual J spreadsheet and need some suggestions on how to classify situations that dont have an exact match in the book: Manual J, Abridged edition , Residential Load Calculation

Specifically , I am unsure how to classify the floor over the crawlspace.

The building is a: 1000 sf, 28’x36′,  single story, 2×4 framed  on a block and poured concrete foundation.

Crawlspace:  36″ high from dirt floor to subfloor,   dirt floor has no insulation ,  block walls have 1″ blue foam board and joist end bays have 3.5″ fiberglass + 1 foam board.  There is no insulation on the ceiling of the crawlspace.

Since the forced air furnace is in the crawlspace and the temperature is almost the same as the living space above ,  how should the floor over the crawlspace  be classified on the Manual J sheet:

The book lists U-values for the floor above the crawlspace on Table 4A, “Construction Number 19 , 20 ; Floor over unconditioned Crawl Space or Basement”,  but the description doesn’t really match my situation:

–  “Unconditioned crawl space” (but it is heated by the radiant heat from the furnace)
– “No insulation on Exposed Walls” …  but crawl space walls are insulated with 1″ blue foam

So,  do I just wing it and guesstimate a U-value for the floor over the crawlspace?


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