vapor barrier placement withn a built-up enclosure

A sample enclosure that I am considering, all applied to exterior of frame: 2x6 t&g, rigid foam sheets, (EPS or urethane, don't know yet), 2x3 vertical strapping 16"oc, screwed through the foam to the t&g, 15lb felt as a drain plane, then clapboards, with end grain and backside sealed.
My questions are centered around vapor barrier placement/necessity, foam types, and breathable walls and roofs. Conventional building practice says "vapor barrier to warm side of wall". Warm side of the wall, when? In winter or summer? In my area of N.E. Ohio, we have summers with many 90 degree days, with matching humidity. The temp in an air conditioned house stays at 70 degrees. Winters are cold and dry. Again, temp stays at 70 degrees. So, the "warm side" of the wall in July is not the same "warm side" of the wall as in January. So, where does my vapor barrier go? And, depending what type of foam insulation I use, do I need one? Should internal and external water vapor be allowed to slowly diffuse through the walls via the use of permeable EPS?
Or does one use a closed cell foam, urethane, and expect that to function as your vapor barrier? Am I correct to think that vapor drive, (internal and external), is then stopped if I use urethane?....I know, lots of questions, but I'm wondering how some of you guys in climates similar to mine handle this situation. Oh, and in my sample enclosure, what about those screw punctures/vapor breaches, in my insulation? Thanks in advance for any and all input. Ron Mansour

It is my understanding urethane will be the vapor barrier, the thicker the better. I am now working on a project where we are using 6" of urethane 2"x4'x8'sheets, 3 staggered layers, than strapped, on the walls and roof. I am not using any added vapor barriers inside or outside. Tim

Timberpeg sells wall packages that are very similar to what you describe.
They recommend a 6 ml plastic vapor barrier between the t&g and the foam insulation, with all seams lapped and sealed. In my own TF project, I used 6 ml plastic on the walls, but Grace Ice and Water Shield as a vapor barrier above the ceiling boards. This was much more expensive, and overkill really, but I needed a temporary roof and found the plastic was nearly impossible to walk on.

I was not too concerned about trapping condensation against the back of the t&g because I figure it still can dry to one side (the inside) and the foam will keep the vb warm. We are not talking about a stud frame cavity (with its many opportunities for air infiltration) in which collected moisture trapped by the vb could cause unseen rot. In a high moisture area (such as a bathroom), in some climates, maybe a plastic vb against the t&g could cause problems especially if the foam insulation is not staggered and well sealed at the seams.

By the way, this "wrap and strap" system can be hellishly labor intensive.
Unless it's a small DIY project on a tight budget, I'd try to avoid it and go with SIPs.

Why use the 6 ml plastic vapor barrier at all, leave it out. It has to be a nuisance to apply, and does nothing more than what the foam is doing, it is a redundancy. As for the labor part, I would rather see the money go to the local builder than "big panel". Panels are quick though. Tim

You're right about it being a nuisance to apply.
I think if you are only using a single layer of foam, the plastic is desirable in case there are any little gaps in the insulation seams. If you are using two staggered layers of foam, and are careful to seal the seams, you may be right about eliminating the plastic (especially if you can close everything in quickly.)

While it may be possible to get by without installing a poly vapor retarder, it is important to be sure the type and thickness of rigid insulation being used meets the perm rating for a vapor retarder. Check the perm rating with the specific manufacturer that you are using. Also, my experience has been that some building inspectors still just won't accept it. They see the installation as problematic(difficulty taping/sealing of seams) and don't feel it will properly do the job of vapor retarder. Make sure they accept it before getting too far.

Good advice, no code issues in my area. Vapor retarder? Is that the same thing as a vapor barrier? It sounds more appropriate. What is the accepted proper term? Tim

Vapor retarder (or vapor diffusion retarder) is the same thing as a vapor barrier. Vapor barrier is probably more commonly used (that is what it used to be called) but vapor retarder is more accurate. Vapor barrier is misleading because it implies it stops all vapor transfer when in fact it does not. It does allow some vapor to pass through to some degree (you can't completely stop it).