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cmlarsen 12-21-2011 02:30 PM

Insulating/Vapor barrier/Venting a 2/12 pitch Shed Roof with Cathedral Ceilings
 
I am building a cabin in Northern Michigan (hot summers, cold snowy winters). I have a low pitch (2/12) shed room framed with 2x12 douglas fir rafters on 16" centers with 3/4 OSB decking and GAF Liberty SA modified bitumen roof.

I've vented each rafter bay with a 1.5" air space with openings at the low side and high side of the roof in the soffits. I built the channel from 1" extruded foam that fits into the bay providing insulation and clear venting for the roof after that there are unfaced r30 fiberglass batts

Now the problem: I am about to install the vapor barrier on the interior then realized that with the foam vent channel against the roof, the fiberglass in the middle and the plastic on the inside, I would be creating a double vapor barrier situation.

At this point, I am a little lost as to what to do.

Here is a picture of the foam vents. They are glued up against 1.5" tall battens so there is a 1.5"x13.5" airspace above the foam:
http://dl.dropbox.com/u/1539673/IMG_0333.JPG

shazapple 12-21-2011 03:15 PM

What kind of extruded polystyrene is it? Is it faced? A 1" thick unfaced XPS typically wouldn't be classified as a vapour barrier, and is probably in the "vapour semi-permeable" range. The manufacturer has data sheets that will tell you the permeance.

cmlarsen 12-21-2011 03:25 PM

It is unfaced. I am not certain of the brand but is similar to the DOW or Owens Corning™ FOAMULAR®250. It claims to have "Exceptional moisture resistance".

I think what has made me most nervous is that we have been heating the space for the past few weeks (its quite cold now). We've insulted the ceiling, but the other day I had a hot fire going (inside was around 70F, outside was around 15F) was moving some wires around and removed one of the ceiling bats. Behind the bat was a surprising amount of moisture between the bat at the foam. It could be that that piece of insulation had gotten wet at some point before it was installed, but my thinking is that water is that moisture is condensing on the foam.

shazapple 12-21-2011 03:36 PM

Where the foam is only 1" thick the inner surface is probably cold enough to have the moisture condense. The same thing would happen if it was any other material (plywood, plastic vent baffles, or whatever). Once you install your vapour barrier the moisture wont be able to get to the cold foam to condense, and the foam will be colder anyway. Any moisture that does get between your vapour barrier and foam should be able to escape since the foam is unfaced and relatively thin (which makes it 'semi permeable'). I wouldn't worry.

cmlarsen 12-21-2011 06:13 PM

OK. That is what i was hoping, but I am still bit concerned that the water that does make it past the vapor barrier will get trapped and leave more slowly than it comes in and slowly i will end up with a giant sponge for insulation.

One solution I was thinking about would be cut out a 4" section of the foam at the highest point to allow that vapor to escape more rapidly than if it had to fight its way through the XPS.

At the least it would be some extra work, at the most it would prevent my rafters from rotting. Think its worth the effort?

Windows on Wash 12-21-2011 10:03 PM

XPS at 1" should still have a perm rating at or around 1 (good enough to allow the diffused moisture to dry to outside.

Don't both with cutting the hole. Moisture diffusion isn't like air movement.

The venting should be fine. Make sure the air barrier details is fine and you should be okay.

Bulk air transfer will carry +50X the moisture with it than diffusion as compared to painted drywall.

Your rafters are still the weak point in this equation and could condense on the side of the vapor barrier/drywall. I would prefer to see a rigid foam across that prior to the drywall and break up the thermal bridge. The foam could double as your vapor barrier although air barrier detailing is more critical.

Gary in WA 12-21-2011 10:50 PM

I hope you don't have a problem with only 1-1/2" venting on that low a slope: http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/57_e.pdf

Gary


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