Sandwiching OSB with Fiberglass insulation


Old 11-08-16, 04:29 PM
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Sandwiching OSB with Fiberglass insulation

We want to add more insulation from the outside to an area currently insulated and covered outwardly with 3/8" OSB. In other words, we have from inside to outside: interior structure, kraft paper, fiberglass batts, then OSB. Due to several issues (such as problems accessing screws attaching the OSB) huge effort would be saved if we could add unfaced insulation over the OSB - in effect having insulation + OSB + more insulation + another OSB. Thus, we'll be sandwiching the existing OSB between two layers of fiberglass insulation.

I believe I can do this without moisture concern because the OSB is 3/8", which I think is 2.5 perms. I'd appreciate someone confirming my understanding.
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Old 11-08-16, 04:48 PM
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Well that's interesting?
What's "interior structure"?
Who would use screws to attach OSB?
Never once heard of anyone using 3/8 OSB as sheathing, far more likely it's 1/2".
No mention of how much insulation is in the wall now, in your area at least R19 is suggested.
Not a big fan but you may be better off with using blue foam on the outside wall but that would require removing all the windows and door to pad them out.
Old 11-09-16, 04:08 AM
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Adding fiberglas insulation to the outside of the existing OSB would not be a good idea, IMO. You would not have anything to attach your next layer of OSB to, if I am reading your post correctly, and you can't compress insulation. It loses its insulative qualities if you do. You also run into the problem of window and door protrusions that will need to be addressed as Joe mentioned. Most likely you have 7/16" OSB on the original exterior as opposed to 3/8". It is considered 1/2" in the trade.

What is the depth of your existing walls? Hopefully 6" to accommodate more insulation to achieve what is required locally.
Old 11-09-16, 04:47 AM
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Thank you both for responding. I agree with all the comments but I suspect getting into many of these side questions will only complicate matters.

I'm not adding insulation to a wall, it's under a bump out floor. We are able to move a piece and are certain it's 3/8" but removing it all would be a problem for a variety of reasons. Instead, we want to add fiberglass insulation around exposed plumbing (which is under the joists and has fiberglass insulation plus OSB above it) and can do so well - with neither attaching it nor compressing insulation as issues.

So, the question is, can I leave the existing OSB and add insulation, creating a sandwich of OSB with insulation above and below it or would vapor permanence be a problem?
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