Foam panel roof insulation help
I live in the Santa Cruz area of California (lots of rain but no snow). I am re-roofing a small gable roofed house with open ceilings. I want to insulate while I am at it. I am thinking of 4’x8’x2″ foam panels on top of the existing ceiling/roof with a cobra-vent at the peek, but I cannot find any information on how to frame it up. One local guy said to nail down 2x4s on end and 24″oc, put the foam between them (leaves a nice vent space), sheeting on top, and then the shingles. But I thought I saw somewhere a description of just the foam, , 1x4s 24″oc and sheeting on top of that. I figure that the second one would insulate better. Anyone have any sites they can send me too? Maybe I’m just searching with the wrong key words.
Thanks, Mark
Replies
I'm not sure exactly what you mean with the cobra-vent at the top. There is not a lot of point in foaming the surface if you will vent the area underneath the foam. If you mean creating an air path above the foam and under the sheathing, that would work, and probably be the ideal method.
Doing the air path above the foam with a vent is likely ideal since you get a cooler roof surface. Asphalt shingles may last longer when not overheated. The other benefit is that you get a way for the sheathing to dry out if the roof should leak a little.
Another way to do it is considered a hot roof. That is putting the foam down and then screwing the sheathing directly over the foam. You then have an unvented roof. This is like building your own SIPs in place. There is much debate about unvented roofing, and it may not meet codes in your area. As far as doing the edge, you just rip some 2X stock down to the thickness of the foam and install it along the edges.
I chose to do the hot roof method on my own house since it was easier, and I was doing an aluminum roof which doesn't mind getting hotter in the sun. So far the improvement of 2" of foam on the roof has done much more towards keeping my house comfortable than the 2 layers of 6" fiberglass on the attic floor ever did. Which ever way you do it, I do recommend the foam.
I think he means cora (corrugated) vent, its a long continuous ridge vent ..