I have a question about using this product Grancrete   http://www.grancrete.net/videos/videopopup.cfm?v=5 direct over the exterior and interior of the SIP's panels. They claim that this product is waterproof so it is a problem using and seling the whole structure ? I am thinking that if the OSB is dry and this product is used as a finish it should be no concern about any moisture going in and out. I want to know what the professionals think about this , because it will save  a lot of labor and materials if this is the case.

Do you know why wood buildings built in the late 1800's in the West have lasted so long? It's not because they were waterproof. It's because they did a good job of shedding water, i.e., guiding water harmlessly from above down to the ground. And when water did get inside the walls, roof, or whatever, there was always enough air movement to quickly dry it out. Air movement is the key. It's a whole lot easier to make an enclosed volume water tight than it is to make it vapor tight. Grancrete very well may cause most all of rain water to flow down the outside of the wall, but it will also trap air, and therefore water vapor, behind the coating. My concern is that it will create a coating that is almost water tight, and definitely largely vapor tight, and so if and when moisture does get behind it, it can't get out.

It's nigh impossible to keep water vapor from creeping into the space behind the Grancrete, especially as the surface heats and cools due to daily temperature swings. But the surface will be sufficiently tight that air cannot freely move in and out from behind it. As a result there's a risk of vapor building up to the point of condensing due to the lack of drying air and thus rotting the wood behind the Grancrete.

If the shed they show in the video is in the humid South, I'd be willing to bet that in 5 to 10 years time the OSB will be rotted mush under a solid layer of Grancrete. Especially since they coated both sides of the OSB. Had they coated the side exposed to rain, and left the other side uncoated, vapor could escape through the wood to the air. If you coat a SIP with Grancrete you seal the outer surface with Grancrete and the inner surface is already sealed by the foam. There's nowhere for vapor to escape.

Go to buildingscience.com and study up on water proofing roofs and walls. What you'l find is don't fight the rain, guide it away from where you don't want it to where it can flow away harmlessly. Since you can't keep moisture out 100%, give it a safe path to get out.

The key to keeping a building dry is don't waterproof it, water shed it.