We're about to use an OSB SIP roof (fairly simple design....a "T" shape so 2 peaks, 2 valleys) but are still concerned about the long term integrity of the product.  We live in the Adirondack Mountains of NY.  I just read the long article about the horror stories associated with SIP roofs in Juneau, AK.  Granted it appears to a great extent that many of the issues has to do with poor installation techniques rather than SIP itself.  I'm looking for feed back on 1)longevity and 2)recommendation for vapor barrier/felt/grace ice shield and the like for use under a standing seam metal roof.  In addition, has anyone used Cedar Breather or Stoguard spray on vapor barrier?   Thanks in advance.  Cathy

The most fool-proof design uses a ventilated drainage space between the cladding and the SIP.  This can be accomplished by putting 1-2" furring strips over the SIP roof (running vertically), and then attaching your metal roof to those strips.  (You may need another layer of OSB on top of the furring strips, depending on the metal roofing manufacturer's recommendations.)  The space between the metal roof and the SIPs should then be open at the top and bottom (with a ridge vent at the top to keep the rain out, and possibly a Cor-A-Vent type product at the bottom to keep the bugs out).  Underneath the furring strips you should have a vapor permeable membrane, to allow the OSB SIPs to breathe into the ventilated space, while draining away any actual water that makes it through your cladding through space.  In this way whatever water or water vapor makes it through the metal cladding will not be trapped, and will rapidly drain or evaporate away.  Exterior-grade OSB can handle being wet briefly; it just doesn't like being wet continuously.

Of course, this can be an expensive solution.  You will probably need the second layer of OSB for most types of standing seam roofing, though I gather there are some types that will install directly over the furring strips.  It also adds height to the roof which may cause design problems.  But you are pretty safe from moisture problems this way.

(Note that the furring strips may instead be called purlins by roofing guys.  When you do it to a wall, it's furring.  For roofs, they'd use purlins.  Same basic idea.)

You wouldn't use a self-adhesive water and ice shield?  Just something like Tyvek under the strips?  Thanks.  Cathy

You want a breathable (vapor permeable) barrier under the furring/purlin strips, to shed water but still allow moisture to escape from the OSB.  Tyvek works.  Building felt works (better?).  Many (most?) self-adhered membranes are not very breathable -- they stop water and also water vapor, which will prevent the OSB from drying when it inevitably gets wet somehow (leaks, condensation, initial construction, etc.).

There is a new underlayment for metal clad roofs that I found at a trade show recently that looks like a great solution to haveing a cold roof over your sips roof. It is a breather membrane that has what amounts to plastic piles on it that the manufacturer says gives you the airspace and the breathable membrane in one material. It is manufactured by Delta and is called Delta-Trela. The web address is DELTATRELA.com I don't remember the cost but I thought he said around $1.00-$1.50 a square foot. I will be using it on my next cold roof application.
KR