I recall reading somewhere recently of a SIP builder (in the NorthEast, I think) who routinely adds a second layer of sheathing to SIP roofs.  They fir out on top of the SIP roof to create a small cavity (1" or less) and then nail sheathing on top.  Their rationale was that it vented the SIPs and allowed them to dry out if the roofing material failed.

I'm building a SIP house and I am trying to find more information on this practice.  Aside from their reason for doing this, I also see a benefit in that it would reduce the temperature of the SIP roof panels - especially if you used dark asphault shingles.  That combination could cause the SIP panels to exceed 190 degrees on a hot day - a temperature where the EPS might start to weaken.

I can't track down that article but I'd appreciate any thoughtful opinions on this practice.  Is anyone else using it?  What are the details of the technique?

Thanks,

Dave

Dave; 

besides the extra cost of labor and materials the only thing I can see you would accomplish is - 2 layers of rotten wood in the event of a roof failure.

We have for may years used steel SIPs with roofing directly to the panel skin. The metal skin is impervious to water and also serves as a secondary water barrier.
On simple gable or hips we have left the panels raw to be used as the roofing finish them selves - where aesthics are not a concern.