Venting valley rafter channels
I recently rebuilt (from the first floor up) the roof on my story and a half house, raising the height significantly, adding dormers, etc. I put in continuous soffit vents as well as ridge vents. My question is about the rafter channels that run from a very long (26′) valley rafter to the ridge on the side of a gable dormer. The dormer is assymetrical, but I have the same issue with a shorter valley rafter on the other side. I have racked my brain trying to come up with a way to get intake air into the channels that dead end on the valley rafter so that the roof can be properly vented, but I’m stumped. Do I just not bother trying to vent these sections of roof and instead insulate the piss out of them? In that case, what’s my best bet for an insulating material? The rafters are a mix of 11 1/4″ TJI’s and LVL’s.
Replies
I think isonene insulation is the best solution. Mass. inspectors don't require venting w/isonene. It cost is off set by the carpentry labor to build venting systems. Since I started using it, I haven't gone back to fiberglass. Cellulose is an alternative also.
If the roof isn't framed yet and you don't like the isonene idea, I'd recommend raising the jack rafters up (or dropping the valley down). This makes a nice venting channel over your thick LVL valley. For a supply you could make an upside down snorkle to a wall register or...
Thanks mark...Ok, I've done a thread search and found lots of venting info, but nothing relating to my current dilema. I'll throw a picture up of the inside and maybe that will spark some ideas. This could also pass as a picture of the worlds most over-built roof. I was planning on putting on a slate roof (very heavy + minnesota snow loads), but then got scared off by the time it was going to take me pretty much by myself. The structure went up in a long weekend with friends and family... a good old fashioned barn raising... but now I'm on my own.
It is too late to set the jack rafters above the valley, so icynene is one option. You could cut holes so the rafter bays cross ventilate, but that typically doesn't work very well.
Since it sounds like your roofing is not on yet, you could install Cool-Vent Hunter panels above the sheathing, and insulate with fiberglass. If you go this route, get the 2" air space. Refer to http://www.hpanels.com/pages/proinfo.html
Venting is required with fiberglass insulation to allow moisture to escape. The sprayed in foams are closed cell, and do not permit moisture to migrate through it.
I had a similar situation on my house (built 8 years ago). I did set the jack rafters higher than the valley, and insulated with fiberglass. That is on my list of "Things to do differently next time". I live in Central NY snow country, and get a lot of wind. The fiberglass cannot provide a tight enough seal against the angled valley rafters, etc., and I get too much air infiltration through my tongue and groove ceiling. Also, I had planned on putting a cupola on, so did not have ridge venting until this summer. There was plenty of moist air trapped in that roof system when I cut it open.
Buck up for the sprayed in foam insulation, and you won't regret it.
BMan