I’ve seen several posts on the perils and difficulty of adequately air sealing SIP roofs and wanted to ask a few questions and get some input.  Our SIP roof was installed recently and because of our roof structure, we ended up with 2×12 rafter framing that supports the SIPs.  They did not install SIP tape on the interior but spray foamed & installed SIP tape on the exterior.  I’m not really trusting of their seal, however, as the next day, the crew installing the underlayment had to reapply some of the SIP tape and I had them roll it down but was not on the roof to supervise it.  Anyhow, it is a fairly complicated hip roof and difficult to completely seal and because of all the rafter bays, they did not seal the underside of the SIPs.  

That being said, knowing how important it is to be sealed well to prevent rot in the future, I’m considering a few options.  A) Use a product called Aeroseal to seal any hidden gaps we can’t see in the SIPS,  B)  Spray open cell foam 3-4 inches thick at all my hips / valleys / ridges & zip tape or prosoco fast flash other seams I can get to on underside, C) Spray open cell foam 3-4 inches on entire underside of SIPs, D) spray 1-2″ of closed cell foam on entire underside of SIPS.   All the options are somewhat pricey but the bottom 2 add additional R-value to the roof if I’m spending the money anyhow.    Just not sure if spray foam under the SIPS will trap moisture & cause further issues & not wild about the air quality issues spray foam can sometimes present.

One final note – thankfully we used a synthetic roof paper and metal shingles by TEK with a batten system.  There is some 1/2 EPS foam under the shingles (part of their system) but still should be some airflow under the roof to help with drying potential.  We are in SE Kansas or Zone 4a, so we get shorter winters but can get fairly cold.