Is there any reason you can't have a SIP roof on an unvented attic (with asphalt roof shingles)?  Does the attic have to be vented if you use SIPS?  If the attic is not vented, where does the moisture go?
I understand that you need to properly tape the seams on the inside to protect against moisture leaking through the panel seams.

Thanks.

judorock'

it would be rare to vent a sips roof, they are by in large unvented sytems. So you do not have moist unconditioned air in the space above you, it is all conditioned same as the living space.
any interior moisture baths/kitchen/dryer vents are exhausted to exterior. Depending on what part of the country you live in it may be adviseable to install a HRV or ERV to help with any additional moisture build up and fresh air intake.
I have found in our area (SE. US) it is not a necessity, butwould be more so in the northern US

an unvented shigle roof is predicted to have about 10% less life ,although more recent studies are not sure that is true either.

I think there is probably a little less longevity, but the benefits of SIPs far outweigh the minor drawbacks

Shingle lifespan will be affected far more by external conditions (weather, environment, etc.) than by whether the roof is vented or unvented.

Shingle lifespan is reduced only with elevated temps. MANY variables will affect peak & average temps, the least of which is venting the roof deck.  In a non-SIP vented attic there's about ~R1 of roofdeck & felt between the hot shingles and (none-too-cool) attic air, whereas there's ~ R0.1-0.25 of boundary air insulating it from the cooler exterior. 

If the pitch of the roof is above about 2:12 most of the shingle cooling is from exterior  air convection. As the temps rise the emissivity of the shingles begins to provide ever higher radiation cooling. "Cool roof" materials keep themselves cool via a combination of low solar-absorption and high infra-red emissivity.

The primary benefit of roof ventilation isn't shingle cooling, but rather providing DRYING capacity for the roof deck to minimize rot potential.  With a SIP roof you have almost zero drying capacity toward the interior, and highly variable drying capacity toward the exterior.  There are also periods of high vapor drive from a superheated sun-exposed wet roof into the outer skin of the SIP.  Whether the SIP will have sufficient drying capacity to always avoid rot on the exterior skin with a typical felt & composite shingle layup is highly local & seasonal-climate dependent.  The all-climate solution to this is to apply a secondary ventilated nailer deck on furring or purlins screwed into the SIP, or steel roofing on purlins with a venting space provided by the furring/purlins. 

Some of the iso & eps panelized roof insulation vendors are onto this, and have ventilated roofing panels (basically a half-SIP, with no interior-side skin) with a structural OSB skin on the insulation plus a ventilated gap between an OSB nailer-deck & structural skin, eg Hunter Cool Vent  or Atlas Crossvent.  Copying this dual-deck approach with a SIP roof would be advisable in most of the US, since swapping out a failed SIP is far more complicated than a failed 4x8 panel above a structural roof deck.