Replies

1. Expert Member
   Dana Dorsett | Aug 08, 2016 02:05pm | #1

   The short answer: 20"

   Cellulose would perform at ~R60 at settled depth of 16-16.5", but the initial depth has to allow for ~10-15% of settling, ergo 18-20". The 20" number the recommended initial depth for R60 some of the better fiberglass blowing wools too.

   http://www2.owenscorning.com/literature/pdfs/10011287AttiCatManufacturersFactSheet.pdf

   http://nationalfiber.com/docs/CelPakSpecSheet1209.pdf

2. 
   RMaglad | Aug 08, 2016 02:17pm | #2

   R60/3.5/in = 17.14", go 20" heel to allow for venting baffles.

3. 
   whitenack | Aug 08, 2016 02:45pm | #3

   For blown cellulose, you need 18". So you'd need at least that much clearance above the conditioned space, plus a gap for airflow just below the roof.. Your truss design depends on roof pitch, eave depth, etc.

4. 
   JC72 | Aug 08, 2016 03:03pm | #4

   Loose fill cellulous has an R-value of 3.7/inch so you'll need approx. 17" to hit R-60

   http://www.bpiexamacademy.com/1-appropriate-insulation-applications-based-on-existing-conditions.html

5. GBA Editor
   Martin Holladay | Aug 09, 2016 05:19am | #5

   Adam,
   The answer depends on the type of insulation you will be installing.

   Here is a link to an article with more information on the issue: How to Insulate an Attic Floor.