Posted By kschweitzer69 on 20 Dec 2010 05:24 PM
Great! that gives me some piece of mind. I did just learn that space is actually 2x12" instead of 2x10" so that gives us a little more insulation value. For the open blow area should I demand a depth more than 14" to achieve settled R-values? Any other detail I should be weary of for the open blow. I thought I would spray foam beads around can lights, anything else I should look for? In my attached garage area is it worth doing cellulose or would fiberglass batts be sufficient?
Tell them what your settled-depth goals are, and let them do the calc on lbs and inches. If you measure it yourself later (or put some depth markers every 2" or so on trusses etc. for later verification), if it's 14" or less, ask them what the rated R per inch is of the stuff. It varies a little bit from vendor to vendor, but R3.2/inch at initial blown density would be on the very low side, R3.4-3.5 would be more typical, and at settled density you can be in the R3.6-3.7 range. Nationall Fiber specs 14.5" initial depth for an R49 at a settled depth of 13.1" (see:
http://www.greendepot.com/greendepot/assets/images/docs/CelPak_SpecSheet.pdf ) So for a target-R of R49 using their goods you'd blow it to 14.5" (~ R3.4/inch) and expect it to settle to 13.1" (~ R3.7/inch) over the next decade or two. In fact it'd start out at about R52-53-ish if you actually measured it, to settle in at R49.
If you use somebody who has been in the business for quite awhile and has good references you'll probably be starting out at 15-16" to hit R50-settled. If it's under 14" or less, pull the spec for the goods.
I'm not sure fiberglass batts would be any cheaper than doing it in cellulose in an open blow. The bigger the project, the better the price per square foot x R. The performance of R49 batts isn't so great with bigger delta-Ts and the installation has to be perfect-fit & gap-free to hit the performance of even R38-ish levels with higher-density blown goods. If you're less concerned with the semi-conditioned garage area and want to save money, blow it to R40 with cellulose i
When they're done blowing the attic it should look fairly planar & smooth on the top surface of the cellulose, not huge waves or lumps:
http://t0.gstatic.com/images?q=tbn:...TUFFw_u1DA