Wondering what experiences anyone has had with different water resistant barriers. I a talking about coating the SIP OSB with one of several air barrier products that is water resistant but yet reasonable vapor permeance to allow the OSB and EPS to dry to the exterior, sort of like a "Goretex" coat for the exterior both on the SIP walls and the SIP roof. Most systems put some emphasis on the seams and either use a mesh fabric or a special caulk. There have been a number of new products in the past few years to do this. There is an Air Barrier Association and there are products by Tremco, Prosoco, STO, and many others. Anybody have any experience with some of these products? I plan on using them as a WRB under a rainscreen type wall exterior as well as for a cold roof installation. They seem pretty important and I haven't seen any topics really address the current state of the art wrt SIPS. Moisture will migrate as a vapor thru the wall especially in cold weather areas and condense on the outer surface (OSB). We all know that OSB doesn't like to get wet so a rainscreen or cold roof will allow cold air to carry this condensation away.

I have used several "liquid applied" WRBs. However, only on vertical surfaces. The one I currently recommend is made by Sustant. It applies easily and creates a great barrier to moisture absorption. I helped the West Virginia University Solar Decathlon team with their SIP package and Sustant donated enough material to coat all the wall and roof panels. The majority of SIP structures still rely on either a synthetic house-wrap or a asphalt impregnated felt. The key to a successful assembly is to understand permeability, airtightness, and (MOST IMPORTANTLY)drying. And all this is dictated by your climate zone. A WRB is a must because ALL sidings leak! All SIP assemblies should be installed properly sealed and thus an individual air-barrier is not required. If an air-barrier is installed and it has little or no permeability, it is classified as a vapor barrier and must be installed on the correct side. All this applies to walls and roofs. Improper specification and installation of vapor barriers prevents drying. If your climate or cladding require back-venting, (Or, as you called, a "rain screen")the introduction of air will promote drying and improve durability of the assembly regardless of the type of SIP or cladding. Up on top, the term cold roof is used to describe the back-ventilating that is exactly the same as a rainscreen in vertical assemblies. I believe the term cold roof should be replaced with "Dry roof" I understand how it relates to ice dams, but the reason we back-ventilate a roof cladding has more to do with moisture than heat! Agin, the roof's geographical location and type of cladding will dictate the need for back-venting. In short, if you're north of the Mason Dixon, I strongly recommend either a self-ventilating cladding and/or a back-ventilating (Dry-roof) detail. Al

Thank you for your reply. I hadn't heard of Sustant before. It appears that they have only been around since 2011. I agree that drying and back venting are pretty important. OSB by its nature needs some important detailing that I do not see happening like it should. Perhaps I should post in the Design forum area to get more response.