I believe loose-fill fiberglass insulation intended R19, is commonly R10 on average. Do the math.
R2.2 per inch fiberglass. Target depth 9”.
R19 peak over 20%. R15 average except at voids. R2 over voids that cover some unknown fraction, F.
1/(R avg + 3) = F/5 + .2/22 + (.8-F)/18
I believe loose-fill fiberglass insulation intended R38, is commonly R15 on average. Do the math.
R2.2 per inch fiberglass. Target depth 18”.
R38 peak over 20%. R30 average except at voids. R2 over voids that cover some unknown fraction, F.
1/(R avg + 3) = F/5 + .2/41 + (.8-F)/33
The graphics of this post are drawn from a Picasa web album,
Insulation Voids and Much More, Repaired .
Here is one photo of the found "R19" insulation that inspires this post.
Here is a review of some of my actions that challenge policy in Portland, Oregon, whereby this attic is judged just fine. Leave it as is. In truth, few attics contain greater opportunities for saving with weatherization measures. Even including the cost of a wonderful attic ladder and surrounding decks, all cost of weatherizing this 1988 attic to higher standards of 2014, will be repaid in about ten years.
Details of the storage provision are in a Picasa web album
Truss Attic, February, 2014 .
All details, including construction of R30 insulation on attic walls, are in a pdf album at Google Docs: Truss Attic February 2014 . Please know always to view pdf albums as downloads.
There is real value in 280 sf of added storage space, brightly lit, safely accessible. The inviting access is important, too, in doing the profitable weatherization.
I do all cutting in the attic, catching debris in a sheet which I shake frequently into a 55 gallon drum liner.
Here observe all stages of insulating attic walls to R30, with crossed R15 unfaced batts. The outer covering is 3/8" OSB.
A "weatherization" contractor only interested in more blow and go void-ruined insulation, throws barriers that may be insurmountable, against best savings possibilities and quality of life improvement.
A weatherization sponsor who teaches and rewards dumb work even that including fraud, devalues good work. Here I criticize the weatherization funds-squanderor I know, Energy Trust Of Oregon . Energy Trust, at article 2.1 in its Specifications , is oblivious of voids concerns. Energy Trust knows better, but further and more disastrously, does not require any "attic floor sealing" before adding more insulation. They go further, declaring that sealing costs are not repaid with energy savings. Not worth the trouble! How stupid! How evil! It doesn't pay, they say, yet they still reward only the contractors who lie about value of blower door testing, Clean Energy Works Oregon . Insulation voids and attic floor pits are never found with a blower door. Lee voids are created, always, by the blow and go practitioners of Clean Energy Works Oregon.
At 12/31/2014, I am disappointed to find this post buried in a Google search, at Page 27, with search terms "voids under attic insulation ." That seems unfair. I think this and related posts at this blog , are more meaningful than any found more-easily in the Google search. No other person has offered the involved math. With this edit, the post is still buried at Page 27. Why?