I installed some new OSB flooring in my attic. The area it covers is about 160 sq. ft. The total area of the attic is about 750 sq. ft. There is just blown fiberglass up there, and there is no vapor barrier between the fiberglass and the drywall, or between the fiberglass and the OSB flooring I installed.

Will I have problems with moisture collecting under the flooring I installed? If so, what would you suggest to prevent that from happening?

1 Answer 1


Nope, it shouldn't cause problems with moisture buildup -- but how much blown in fiberglass do you have up there? If it's less than your roof joists, it's not enough, and you should think about adding some (or a lot, actually) to bring you up to R-30 at the least in your attic.

If you do have that much up there, but you moved some to the side to put down the OSB, you committed a minor home improvement sin and you should pull up the OSB, put down some 2x6's crosswise to your rafters, and then put the OSB on top of that.

  • My joists are 2x6s, and each has a 2x4 on top of it. I have at least 10 inches of blown fiberglass where I laid the OSB, and more in other parts of the attic. Is that enough fiberglass? Jan 17, 2012 at 17:16
  • You get between R-2.5–R-3.7 per inch of loose-fill fiberglass. At the low end, that means you have R-25. It's OK, but not ideal. Jan 17, 2012 at 17:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.