It is OK to blow cellulose over a plywood attic floor?
I am preparing my attic for some additional insulation and was wondering what the consensus is on this subject. I currently have a fair amount of plywood flooring in my attic, installed over R19 fiberglass batts that were laid in between 2X6 ceiling joists. The batts are faced, installed with the paper side down (towards the ceiling sheetrock). I was planning to blow cellulose over these floored areas, leaving the plywood in place, but I thought I better be sure there wouldn’t be issues with moisture, etc. Does the plywood need to come up or can I go over it with the celullose?
I have done some research and gotten mixed reports on this subject, so I wanted to ask the buildingsciencemeisters here! I have learned so much in my lurking these past few weeks. What a great place!
BTW I am near San Antonio, Texas (Zone 2).
Thanks in advance!
Tommy
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Replies
Tommy,
Q. "It is OK to blow cellulose over a plywood attic floor?"
A. Yes.
Air seal the floor if you can, before insulating. Tape the seams and joints. An attic floor air barrier interrupts the attic's particpation in the house's stack effect , exfiltration and the natural upward moisture drive.
Thanks Martin!
Yes I do plan to seal the attic, at least as much as I can. The plywood does complicate that some, as it covers the some of the top plates in one room. Although in fact that room has a doubled top plate, with a 2x6 that laps over a 2x4 and covers the drywall gap that normally would require sealing. So I guess there's really no getting to that gap anyway, even if the plywood wasn't there.
I would welcome any suggestions about this. It would be too easy to assume that the lapping of the 2x6 over the 2x4 and the drywall gap would seal leakage, but I would suspect that's not totally the case...
UPDATE: Definitely not the case! The 2x6 doesn't come in contact with the ceiling drywall in many of the areas I was able to inspect. So air has a pathway for sure.
Which raises the question: If I'm not able to completely seal the entire attic, is some (say 75%) better than none? Seems I read somewhere that incomplete sealing just causes more air to traverse the pathways that are left open.
Tommy,
Air sealing efforts are always worth it, even if cracks in some inaccessible areas can't be sealed.
Air sealing, particularly DIY, has incredible bang for the buck, even if it is not a comprehensive job.
"Air Sealing Existing Homes with Foam-in-a-Can Rivals Energy Cost Benefits of CFLs"
http://web.ornl.gov/sci/buildings/2012/2010%20B11%20papers/80_Bibee.pdf
Seal the joists at the perimeter of the plywood with polyiso blocking and single part foam, then cover the joints in the plywood with strips of self-adhering membrane or caulking.
Thanks so much to everyone! Encouraging news!
In thinking some more, and going up there to look, I think it would be fairly easy to partially pry up the plywood over the plate areas and wedge it open long enough to get a gun in there to seal at least the bottom edge of that 2x6 plate. Worth a shot for sure!
Thanks Nick!