Rigid foam for rafter vents?
Well the deafening silence on my question about who’s using Clip-Vent told me what I needed to know. It does look like there are a fair number of people who use rigid foam set 1 1/2″ from the sheathing for rafter vents rather than buying the premade chutes. I can see how 1″ foam might improve the R value and keep the insulation from being washed by air, but isn’t there a risk of trapping moisture? What about it being a fire hazard? I’d be using it in southern New England, which is mostly cold or dry, but also has several weeks worth of high temp, high humidity days in summer.
Pete
Replies
Hey Pete, are these what you're talking about?
View Image
If so, I know there are several folks using them hereabouts. This pic is from one of Mike Smith's threads from a year or so back. Here it is from the start:
http://forums.taunton.com/tp-breaktime/messages?msg=44500.1
If you can get Mike's attention, he can probably tell you all you want to know about them.
jt8
"All the knowledge I possess everyone else can acquire, but my heart is all my own." -- Goethe
Nope. My understanding is it's actually 1" foamboard supported by furring strips to take the place of the premade chutes.Pete
No, I meant were those the chutes you were asking about in the first place.
Either way, Mike Smith has done 'em both ways.
jt8
"All the knowledge I possess everyone else can acquire, but my heart is all my own." -- Goethe
Guess I wasn't clear. I'm interested in the pros/cons of using foam board chutes instead of the premade ones, and what the best way to do them is. It just so happens my rafters are 10.5" deep, which would allow for 1" of foam plus R30C and still leave 1 1/2" of vent space.
Pete
I put in the premade thin chutes. Two years later
went back in to do something and found the
thin chutes somehow very fragile, you could
poke a finger through them easily. My question
is will the foam board approach be better?
Will they tend to buckle or warp? I would
think the boards would offer a better air
path.
I did it, I'll be honest. Only I was required to provide more than 1.5" of spacing- a total of 2.5" was insisted upon by the inspector. I didn't use the pre-made foam baffles because I needed to put 2x2s as furring strips on top of my rafters to provide the vent space and adequate insulation R value. Without full support to sheathing, these pre-moulded vents would seem to be far too flimsy to survive the pressure exerted by spray foam or dense-packed cellulose.
Unfortunately, 2.5" was also enough room to fit a squirrel, or more properly, a couple of them. They got in before the soffits and fascia were complete. They burrowed through the 1" EPS into the icynene and made nests. The rest is "history" I would love to forget, and hope nobody else repeats!.