Thursday, April 10, 2008

Cleaning Up A Blown-Cellulose Attic

This attic floor of 2x6 joists on 16" centers had found insulation of a relatively pleasant form of cellulose, up to the tops of joists, R18. The cellulose is compacted to give space for kraft-faced R11 batts. Unfaced R19 batts cross joists to complete an upgrade to about R45. A vapor barrier is acceptable anywhere it will remain securely at temperature above the dew point. It is nice to not place unfaced fiberglass against the cellulose, where picked-up batts would again raise cellulose to the surface.

(Edited at 11/13/2009) The claim of R45 is high. I will call the compressed cellulose R13. Applying my Insulation Math, consider parallel paths for the thickness of 2x6 floor joists.

1/Reff = (1.5/16)/ Rjoists + (14.5/16)/ Rinsulation
where Rjoists is taken as 0.94 per inch (douglas fir), 2.4 for 2x3, 3.3 for 2x4, 5.2 for 2x6, 6.8 for 2x8.
1/Reff = (1.5/16)/ 5.2 + (14.5/16)/ 24. Reff = 18.
Adding R19 batts, total insulation value above the ceiling is R37.

This added exercise tests my installation against a two-thirds rule commonly applied, where less than one third of the total insulation blanket is allowed to the warm side of a vapor retarder. Credit R29 above the kraft facing, R8 below. 29/37 = 0.78. The installation passes the test, but that is not an appropriate end of the thinking.
From a Google search, subject: "vapor barrier two thirds rule"
I have done right for the climate of Portland, Oregon. From study of dew point data for Portland, I have justified a one-half rule. In fact I have never departed from a two thirds rule.

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