Unvented vs. vented roof
Hello,
I live in the Pacific north west, and will be building a home off the cost of Vancouver island this fall. The house is small, 700 square feet, with a 1 n 12 pitched roof as the design. We have not started construction on the house as yet and we want to deal with the roof in a way that will, of course, provide the best of all worlds.
This is our issue: we want to create an open joists ceiling in our home. Right now the house is 26 feet wide, with a supporting wall located about the 12 foot mark that runs the entire length of the home, that length being 28 feet. The roof joists will sit along the shorter part of the home, meaning width as opposed to length. I want to expose the joists, designed currently at 2 x 10 at 16″ o.c.
Since we want to have exposed joists, and we don’t want to have a really thick roof above that we were looking at creating an unvented roof, or warm roof system. I have done extensive research and in Europe the warm roof system involves: joists, sheathing, membrane, insulation (rigid) sheathing again, and roof material – in our case standing seam metal roof.
The insulation currently is slated at a polysio (sp) rigid insulation, 6″ to attain R-32. The information out there is very contradictory. Some say yes to this system, some say you still need to vent, our engineer told us we can’t have insulation next to organic material unless it’s vented, that is so contrary to warm roof systems I’m wondering what is happening.
If anyone has some information to add to this to clarify that would be great.
I guess the biggest concern is getting the permits to build this kind of roof and if is even doable. Has anyone heard of the warm roof system used in Europe? Why is it so popular there and so hard for me to find information on it’s use here in North America. I seriously must be doing something wrong in my research.
Many thanks
J
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Replies
Jeanette,
The type of roof you are describing is routinely installed in North America. Every low-slope commercial building on the continent uses this system -- every WalMart, every supermarket, every warehouse, and every factory. Any roofer who does commercial work will be familiar with it.
These roofs are usually unvented.
My only suggestion would be to beef up your R-value from R-32 to something in the range of R-38 to R-49.
Hi Martin,
Thanks so much for responding so fast. Yes, I am familiar with this system on commercial buildings. But my concern was doing it on a house. There is a lot of information for using this system on commercial buildings, you are absolutely right, but I'm having trouble finding consistent information for this system for a house using wood based products for sheathing.... The engineer i spoke to said you can't do it on a house because the insulation cannot touch anything organic and that we would most definitely need venting with this system, or any other type of roof system, which is contrary to the European perspective. What I'm having trouble finding is consistent information for construction here in North America. I was hoping that someone in this online community might be able to let me know that yes this is completely doable for a house here on the Pacific North West, of course it needs to be done right.
Many thanks
Jeannette
Jeanette,
The engineer is wrong. Polyisocyanurate insulation is routinely installed in contact with plywood or OSB. If it makes your engineer feel better, you can inform him or her that the usual assembly consists of roof sheathing (typically plywood or OSB), followed by a roofing underlayment, followed by at least two layers of rigid foam installed with staggered seams.
Talk to your roofer to determine whether your roofer prefers to install the standing-seam metal on furring strips (usually 1x4 or 2x4 purlins, installed 16 inches or 24 inches on center, extending from rake to rake) or on solid sheathing (a second layer of plywood or OSB).
Hi Martin,
That is what I thought. I also just spoke to the rep at Raycore SIP products and he said the same thing as you. Really appreciate the feed back and the information.
Jeannette