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-   -   Blow insulation OVER attic floor? (http://www.diychatroom.com/f103/blow-insulation-over-attic-floor-169312/)

Garblefarf 01-16-2013 04:27 PM

Blow insulation OVER attic floor?
 
I have a simple rectangular attic, about 1,100 sq ft. Construction is 2 x 8 floor joists, 16" o.c. The house was built in 1977 and the existing floor insulation is about 6 inches of either rock wool or cellulose (gray dusty stuff) blown or poured in. The entire attic is floored with 3/8" plywood, mostly just lying there loose and bouncy. Access is by a pull-down stair. This space will never be used for storage or other access, so the floor really is unnecessary.

I figure the existing insulation is roughly R-20, which was state-of-the-art in 1977. Now the recommendation for my location in New Hampshire is R-50 to R-60. The easiest and probably cheapest way for me to upgrade would be to blow in cellulose. Although I could pull up the plywood sheets and blow in, the level would rise considerably above the joist tops, and then, what to do with the plywood? I'm getting old and can't do a lot of heavy work, and really don't want to drag all that plywood out of the attic.

So my big question is, is there anything wrong with simply blowing the cellulose over the top of the plywood floor? It seems like a good method to me, but I seek your advice if there is a problem with doing this. As I said, this attic probably will never be used for anything. Thanks for any tips!

gregzoll 01-16-2013 04:52 PM

You are going to have to rip up all of that flooring, then figure out how to re-frame, so that you have the space for R-50 or R-60. Basically means that to do it correct, you can no longer use that as a storage space in order to get the proper fill levels for insulation.

Fix'n it 01-16-2013 10:35 PM

Quote:

Originally Posted by gregzoll (Post 1095100)
re-frame, so that you have the space for R-50 or R-60.

re frame what ?

joecaption 01-16-2013 10:43 PM

Plywood has to go, I agree, what framing?
Before doing it air seal everything that comes through the tops of the walls and the ceiling with expanding foam.
Add foam or plastic baffles so the soffit vents do not get blocked with insulation.

gregzoll 01-17-2013 10:58 AM

Quote:

Originally Posted by Fix'n it

re frame what ?

Reframe if they wish to use as storage.

Garblefarf 01-17-2013 11:18 AM

Once again, it will not be used for storage. My main question is, what harm is there in leaving the plywood there?

joecaption 01-17-2013 11:27 AM

It may act as a vaper barrier and cause mold to form.

HomeSealed 01-17-2013 11:38 AM

The only way that it would make any sense to leave the plywood would be if it was secured already. You'd then dense pack underneath and blow over it. If it is loose, hire some kid to come pull it all out so that you can add insulation effectively. As Joe mentioned, this would also be the time for proper air sealing up there. Also as mentioned, you'll want to add baffles down to the soffit vents and install dams to prevent wind-wash and/or all of your loose-fill insulation from falling down into the soffit.

AllanJ 01-17-2013 02:20 PM

Somehow I do not think that leaving the plywood there would cause a vapor barrier or moisture problem. Just do not add an additional moisture barrier.

Do not stuff more insulation under the plywood so as to compress either the new insulation or existing insulation significantly. That will actually reduce the R value a tad.

joecaption 01-17-2013 02:23 PM

Also if it really is only 3/8 I sure would not be doing any walking on it. It's to thin for that.

HomeSealed 01-17-2013 05:14 PM

Quote:

Originally Posted by AllanJ (Post 1095693)

Do not stuff more insulation under the plywood so as to compress either the new insulation or existing insulation significantly. That will actually reduce the R value a tad.

That statement requires some extra clarification. Dense packing cellulose will not reduce R-value of the space, nor will it for blown fiberglass that is designed for dense packing. It will increase the R value and slow airflow as well... That said, you don't want to crush batts of fiberglass or regular loose fill.


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