November 07 FHB “A New Roof Over the Old One”
Call me Chicken Little
Am I the only one that thinks there may be something wrong with this roof concept?
I do not see how the cavity between the two roofs is going to drain or dry.
I know it is not likely that “much” rain will get in…what about condensation?
Something about a “sealed” cavity between 2 vapor barriers just doesn’t look right to me.
Warning…I am from a Hot Humid Climate and I may not know what I am talking about.
Replies
This is common in Oklahoma and Texas. Evidently in North and South Carolina to, because I got a lot of 2 and 3 layer tear-offs.
Blind plastic all penetrations and walls.
I thought you meant roofing, not roof system. I didn't have the mag when this was posted. If water is getting thru that vertical seam roof he would be in trouble.
Edited 9/30/2007 8:04 pm ET by catfish
Done all the time in New York also.
bump
Isn't the purpose of that foam to prevent water from getting anywhere?
George,
I had a feeling I was asking a dumb question. It looked to me from the photo that the cavity(between the 2 roofs) was not being completely filled with foam. Maybe they went back and added more foam?
John
It also looked to me that the cavity wasn't being completely filled, but I'm still not sure there is a problem. Certainly, if rainwater gets in, there's a problem, but condensation is only a problem if fresh, humid air gets into the cavity. If the cavity is air-tight (i.e. sealed with foam), then whatever moisture is in there when it was sealed is all there ever will be in there. My understanding of the foam is that it does exactly that. It creates an air-tight cavity or seal.In a typical "cathedral" ceiling application, for example, the top portion of the cavity is filled and sealed with foam, but there is bound to be a gap between the bottom of the foam and the sheet rock. I can't see how that differs significantly from having a small gap between the foam and the roofing layer.George Patterson
In a typical "cathedral" ceiling application, for example, the top portion of the cavity is filled and sealed with foam, but there is bound to be a gap between the bottom of the foam and the sheet rock. I can't see how that differs significantly from having a small gap between the foam and the roofing layer.
George,
The difference I see is that the cavity that you are speaking of is inside the thermal envelope.
Is there a chance that humid air can get into that cavity and then the roof deck can become colder than the cavity?
I dunno....I am asking because I am trying to learn.
John B
Thanks George,
I did find this link that says air spaces without vents may be ok under certain conditions.
http://www.taunton.com/finehomebuilding/how-to/qa/roof-always-need-venting.aspx?ac=fp
Which is pretty much what you said earlier.
I am guessing that the cavity is well air sealed and that very little moist air is being exchanged?
Edited 10/3/2007 10:28 am by homedesign
> I am guessing that the cavity is well air sealed and that very little
> moist air is being exchanged?That's my understanding. Of course, I got that from reading articles such as the one you just found.George Patterson