cover the rafters question

[sprint7]

I have an open ceiling in my garage and I would like to cover the rafters on both sides with 0SB or plywood with insulation between. My question is do I need a plastic vapor barrier and if so where does it go? Thanks for the help!

 

[Sokoloff]

Vapor barrier always goes on the warm/humid side of a wall/floor.

In your case, it goes on the bottom of the rafters, before the sheet wood goes on.

 

[D.J.]

When you say "on both sides" do you mean vertically?
________
Ketamine rehab forum

 

[sprint7]

When you say "on both sides" do you mean vertically?

Yes. I wanted to cover the ceiling and insulate but I also want to use the attic area for storage. Therefore I was going to cover it as well. It would stack like this.

OSB
insulation
plastic sheet
OSB

My question is do I need the extra plastic sheet. I am really not sure when it is needed and when it is not.

 

[Sokoloff]

You only want ONE vapor barrier. You want the sandwich to be exactly what you have typed above. (And "one vapor barrier" means you don't want to use faced insulation here, if you already put the plastic sheet on the underside of the rafters.)

 

[sprint7]

do you mean that if i use the faced insulation that I don't need the extra plastic sheet?

 

[Sokoloff]

Well, yes and no. A plastic sheet vapor barrier will be a better vapor barrier than the foil or kraft paper on fiberglass batts. Plus, I think the plastic and unfaced batts are easier to install than the faced batts. But the facing is intended as a vapor barrier. Problem is, lots of moisture can leak past where the paper is stapled, etc.

What you want to be 100% NOT to do is have a vapor barrier on both sides of the insulation, which will trap moisture in the insulation.

If it were me, I'd go with the continuous plastic sheet under the rafters, then install the OSB under, then install un-faced batts (or blown in) above, then OSB on top. The plastic sheet is a better barrier to vapor and air migration.

 

[sprint7]

thanks for the help!

 

[tyrell2004]

Might be some helpful info here.

http://insulation.owenscorning.com/homeowners/easy-weekend-projects/weekend-project-attic.aspx

 

[Falcon67]

I would just staple the kraft faced stuff to the bottom of the joists and cover. I have four 4' square areas on my shop ceiling that are sliding panels so I can access the junk up in the attic. There is no insulation on top of the 1/2" OSB panels. Makes only a small difference in the effort to heat or cool the shop IMHO. In other words - unless you are gonna live in there, don't freak out about a perfect vapor barrier.