We are planning to build an insulated and heated 30x40x14 post-frame RV garage/shop (Zone 6B) on an existing insulated mono-slab that is plumbed for in-floor radiant heat.  We looked at a conventional stick-framed building, but the quotes we got were more than twice as much, hence the “pole-barn” approach.  I have read all of Martin’s excellent pieces on the problems and limitations inherent with insulating this type of structure, particularly without OSB sheathing between the wood frame and the metal siding/roof.

The builder proposes installing house wrap between the wood framing and the steel siding.  Our thought was to either fill the wall cavities completely with closed cell foam, or use at least 2″ of foam in combination with batt insulation, covered by interior steel liner.  The ceiling would consist of a poly vapor barrier and steel liner attached to the underside of the scissor trusses, with the appropriate amount of blown-in fiberglass or cellulose insulation on top.  The attic will be vented with baffled soffit vents and ridge vents.

Martin, your primary concern seems to be creating a tight air barrier.  Wouldn’t the closed-cell foam effectively accomplish this in the walls while also helping to seal any leaks in the wall to slab area?  It seems to me that having the house wrap between the foam and the steel siding would allow us to achieve a decent air and vapor barrier with the foam while also avoiding permanently “gluing” the siding in place.  I would also be open to spray foaming the upper side of the ceiling deck before blowing in insulation if that would be useful in creating a better air barrier between conditioned space and unconditioned attic. 

I would very much appreciate your opinion on whether this approach would be “good enough” even if it’s maybe not your optimal solution.  Cost is definitely a factor and we’re just trying to balance the future expense of heating the building with the (currently very expensive) price of building materials.

Thank you,
Dave