We are doing an exposed rafter/t+g roof system. There is contiuous VB on the top of
the decking in the form of Ice and water shield that ties in directly with the wall poly
VB. For insulation and overhangs we are adding 10″ SIPS to the deck that will get
another layer of breathable of paper and then strapped with 2x4s and then metal
roofing.
I would like to put a layer of 2×4’s under the SIPS to facilitate the
fastening as the rafters I need to hit with a 12″ screw are only 1 1/2″ wide. Is the
airspace a bad idea as it could be a moisture trap? Is it a better idea as it could add
a drainage layer? Does anyone have experience with this detail?
All help much appreciated
Replies
Greetings truehaven,
This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
thanks rez
What is your climate/location?
I'm curious about the I&W shield UNDER the SIP panels. I'm not saying this is a problem, but your proposed detail caught my eye. Are you sure you need a vapor barrier. The perm rating of the SIP panels will be somewhat low.
Having said that, I don't see a problem with the 2x strapping between the I&W shield and SIP panels, as long as you have a place for any moisture that does collect there to get out. Since it's a roof, any condensation or bulk water that collects would run down and tend to concentrate at the lowest point (obviously). If you are sealing the bottom of the rafter bays up, this could cause problems.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
J.,
We are in Kingston, Ontario. Lots of heating days this year! My research suggests
that SIPS without a vapour barrier can deform as the vapour likes to find its way
between the panels. Also, the ice and water keeps the roof in pristine condition as
we lay a course as soon as we get enough decking on (3'). And it keeps the building
department happy as they prefer things by the (ontario building code) book.
Your climate is far enough north of me to defer to others with more experience in a cold climate such as yours. And, if your building code really does want and intermediate drainage plane I would follow the code unless there is a really good reason not to.I am interested in your comment about the air leakage around the SIP panel edges. I would think that the manufacturer recommended sealing techniques would eliminate this leakage. Do you have any more info on this phenomenon?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Jon
Most of my info comes from googling SIPS and failed installations. There was one
case in particular from a site in Alaska I think that was a multi unit place that was
a real horror show. I also have a book
by Joeseph Lstiburek 'Bulder's Guide to Cold Climates' that deatils some issues with
SIPS and vapour and deformation. I have some experience with them on the roof and I can see how
the joints are subject to leakage as I am never 100% certain that the spray foam is
continuous. My SIPS rep supplied me with an installation sheet that included as one
of three sealing options using poly under the SIPS at the joints. It is not a fuss to
do it either with poly or ice and water so I figure why risk it.
The vapour barrier is really what the code is after, the drainage plane is more of a
result of me making the install easier by giving a wide suface to hit.