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Another cathedral question - 2 x 6 rafter:

Appsguy's picture

Foam It Green is saying I can put 3 inches of foam and then attach the drywall to the rafters (leaving 2.5 inches of space).  Is this viable? I don't see a lot of Foam It Green comments.  Looking at investing a significant amount of money (for us) and just don't want to get burned...I presume I'd need to use a good vapor barrier prior to the drywall.

I know there are no shortage of varying opinions on foam use, but thought I'd try to get some (valuable) input.


Clarification... (post #206858, reply #1 of 6)

Location: upstate NY.

Shingled roof, desire to eliminate (block) ridge and soffit vents; rodent/insect infiltration (country living). Trying to do as cheap as possble and stay with the 2x6 with minimal modifications.

Pt2 (post #206858, reply #2 of 6)

Reading other posts lead me to add: I am moving from a 2x6 ceiling joists at 8ft height with rodent destroyed F/G so the 2x6 rafters are bare (facing the living space of course).  There are also skylights which add to the desire to eliminate venting (they span 2 and 3 16-inch OC bays).

Are you intending to remove (post #206858, reply #3 of 6)

Are you intending to remove the joists?  That might not be wise!

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The best you might be getting (post #206858, reply #4 of 6)

The best you might be getting is R19.5 for 3" of R6.5 closed cell foam.  That's a start, but not nearly enough:

You are Zone 5-6, which is calling for levels north of R49.  If you are going all foam to minimize thickness on that cathedral ceiling, you need to get a total of 8" (R52).  You can make some of those inches on the top of your existing roof deck, then lay another deck on top of that.  You would seal the underside with spray foam 1" for air sealing.

As an alternative, you could spray the underside of the deck with 1" of foam to seal it, then fur down from the existing rafters with plywood gussetts to new 2x4's.  In these new deeper rafters, you would blow cellulose to get up to your needed R rating.

YAY!  I love WYSISYG editing!  And Spellcheck!


Piggybacking Paul (post #206858, reply #5 of 6)

If you are looking to increase the R value with minimum space, add this product to the foam that Paul recommends.

So Brandi (post #206858, reply #6 of 6)

What's the increase in r-value and how does that baffle work?

Care to return and explain the benefits other than someone buying it?


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