Blown In Insulation *over attic floor*?

[3PedalMINI]

A few years ago we had an addition put on, Then life took over and the economy took a dump so we put the project on hold. When it was put on we decided it would be smart to add a floor to the attic. We are finally at the point that we are getting ready to finish it, Finishing electrical next week! Now we are completely stumped on insulation and how we want to go about doing it. We are sealing every single opening, and the lengths of Studs/Joist's where they meet the sheathing to prevent any air or drafts.

The attic is large enough that we can get 20" or more of the Blown in insulation all over the living space. So we were wondering if we make sure that we get that much all over and all spots where air can come in are sealed do we need to use batts to fill in the joist cavities?

Just so im making sense there would be 7 1/2" of dead air space between the drywall and attic floor then with 20" + of blown in insulation. Can i do this? does anyone see any issue in this?

Thanks for any help or advice!

 

[Cryptic1911]

I don't see an issue with it, but if you want it between the joists, you could always drill some holes between and let them blow it in that way

 

[boiler7904]

I would fill the void with blown fiberglass or cellulose after installing the ceiling drywall - installer can drill 2" holes at each end of each joist bay to ensure complete filling to proper density.

I have 2 questions though:

Cellulose has a R-Value of 3.2 to 3.8 per inch giving your instance a total R-Value of 64-76 for 20 inches of material. Unless you built wall and floor systems that are insulated comparably, this is overkill (by a lot) since typical installs are closer to R-38 (code required minimum). R-50 (13-15" of cellulose) is starting to work it's way into codes in various locations - what does your location mandate?

You mentioned that have an attic floor - are you able / do you want to use it for storage? If so, pack the joist bays with cellulose and then use layers of rigid foam over the plywood to meet your intended R-Value and another layer of 1/2" plywood as a wear surface. Not necessarily the cheapest way to insulate an attic but it lets you use the space.

 

[Steevo]

Take a 4" hole saw and make a bunch of holes in the plywood floor, a couple or more per joist bay, and the insulation guys can blow under there, too.
Otherwise, it won't hurt anything, as long as you can get insulation out to the edges of the attic space on top of the walls.
If you have vented soffits, be sure to use baffles and blocks to keep insulation out of the soffits and to let the air flow up past the insulation into the attic area.

 

[3PedalMINI]

Thanks Guys!

Minimum code here is R19 for the attic (believe it or not) Figured pack it as much as we can but thats a really good point about the walls. We have used this for storage since we built it (why there is a floor down) But decided i rather have a really nice insulated attic over storage. These past couple of weeks weve tossed out a dumpster full of ****

I really didnt feel like pulling up all of the plywood so thought if i could just leave a dead airspace. But 4" hole saw would work really well, thanks for the IDEA!

Also we figured we would do the Owens Corning Attic Cat system from Home Depot to save a little bit of money by dedicating a weekend to Blowing insulation!