Insulation advise

[ktm010]

I have a portion of my ceiling to insulate, below my attic area. The space of the bottom cord of the truss is 5 1/2", I could use fiberglass insulation R-21, or 3 layers of Dow blue board R-27. The ceiling will be finished in metal so I'am worried I might have water vapor that can't migrate up thru the foam, but will with fiberglass. I would like the better R value, any thoughts on this would be great.

 

[danfromsyr]

you should have a vapor barrier on the WARM side of the insulation.
so if you go with fiberglass you should use a plastic vapor barrier under the rafter between it and the ceiling material.
if you use foam panels just seal the edges with a thin line of expanding foam.. (use minimal expanding used around windows to not pop out your panels.
I'd even go so much as using a sheet of the reflective foam insualtion on the ceiling under the ceiling material unless your ceiling is low to begin with.

 

[dirttracker18]

You don't need the expanding foam as sheathing tape is cheaper and easier to use. It also serves the same purpose

As noted a vapor barrier is required with batts.

 

[stingry]

Just use kraft-faced batts and be done with it.

Cheers
Steve