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Climate Minute #3 Estimating Roof Insulation Appendix

 

Estimating the Amount of Insulation Above Your Roof Deck

 

For Deck Houses

*From p. 2 in AcornDeckHouse's "Helpful Hints for Deck House Owners". 

The best way to tell the makeup of the roof is to remove the fascia or lift up a shingle, but if you are just looking for what the approximate thickness of the insulation is (if original): 

  • 0.75” fascia overhang + 2.25” to 2.625” for roof decking (+/- depending on year) + 0.5” plywood (if after 1982) + 0.375” for shingles = 3-7/8” 

  • If 1” of insulation, then the roof should measure 4-3/8” from bottom of fascia to top of shingles (if one layer and no Plywood and no venting) 

  • If 2” of insulation, then the roof should measure 5-7/8” from bottom of fascia to top of shingles (if one layer and no venting. The extra ½” is due to the addition of ½” plywood over the insulation.) 

  • If 4” of insulation, then the roof should measure 7-7/8” from bottom of fascia to top of shingles (if one layer and no venting)

  • Etc.

 

For Techbuilt Houses or other Mid-Century Modern Homes

For an accurate assessment of what’s in place on their roof, you (or your builder) can pry one end of a rake board or fascia board loose to expose the roof planking and insulation ends/edges. The board can be easily nailed back in place.

 

To get just an approximate thickness of the insulation above your roof deck, follow these steps:

  • Measure the thickness of your roof assembly (from the top of the shingles or roof membrane to the bottom of the rake or fascia board.)  

  • Subtract 7/8” for shingles and plywood. (If membrane roof, just subtract ½” for plywood)

  • Subtract thickness of roof planks/deck (1-1/2” if your roof beams are 4’ apart, or 2-1/2” if beams are 8’ apart)

  • The result is the approximate thickness of your roof insulation.

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