I have a turn-of-the-century home in a registered historic district that I am planning on re-siding. I originally had grand plans to build out the walls with up to 2" of rigid insulation, but the historic landmarks commission won't let me build out the walls (understandably so). So, I am working on a build-up that will replace the 13/16" board sheathing with a comparable thickness of sheet bracing plus rigid insulation w/ a drainage plane. Here is my proposed build up (the interior was remodeled about 20 years ago--hence the modern existing materials) from interior to exterior: 1/2" drywall (no vapor barrier) with 10+ estimated coats of latex paint , R13 unfaced fiberglass batt insulation in 2x4 stud cavity, 1/2" Dow Styrofoam Structural Insulated Sheathing, 5/16" marine plywood ripped into furring strips to created a vented drainage plane, 1/2" cedar clapboard siding with 3 coats latex paint. My concerns/questions relate to condensation buildup in the interior cavity and vapor permeance. The 1/2" Dow SIS is R-3 with <.3 perms. I thought I had my head around preventing condensation issues, but I don't. I live in Wisconsin. Can I use this buildup without creating condensation buildup issues in the inner wall cavity? Thanks for your help!

The answer is high density foam. It is an effective vapor barrier, is structural and the only way to meet most minimum code R-values (R-21+). Here in Minneapolis we keep our foam guy busy nearly full time on our HVAC renovation work.

2# foam makes it easier to heat old homes with sub-floor radiant panels.

I can't afford to hire someone to reinsulate the wall cavity with urethane foam, so I'm restricting my project to the plan noted above. The Dow product I mentioned is rigid foam attached to a structural board that will meet code for sheathing and air/water barrier, as well as adding R-3 for 1/2". My concern is that at <.3 perms, it is nearly vapor impermeable. Am I asking for trouble in my Wisconsin climate?

Without effective vapor barrier on the inside, I would treat your walls as we do basements when fiberglass is the only option...that is leave off the interior barrier so the wall can breathe. We are mostly concerned with winter time migration and in a older home with no foam, you can't get it that tight.

Dana can get more technical, I'm sure.

It looks like the Dow product I mentioned isn't available in my area (should have checked that BEFORE posting--sorry). So, now it looks like my best option to improve the home's performance is inside the wall cavity. Badger's got me thinking about spray foam again, and I think I can afford it if I do it myself with a spray kit (FWIW, I considered dense-pack cellulose, but installers are few-and-far between in my area, and I don't want to attempt a DIY dense-pack job).

Once again, I am stumped with the vapor permeability aspect of the project. I sent Dana a private message and will hopefully get a response, but if anyone else has advice, please feel free. Am I under the correct assumption that in a mixed climate like mine (Madison, WI), the wall buildup should be vapor permeable throughout? We can get sustained 90 plus degree heat in the summer and negative temps in the winter If a vapor-permeable buildup is best for my climate, open-cell foam with no vapor barriers is what I need (open-cell foam is 30 perms an inch). Would using closed-cell foam (very low permeability) cause moisture and mold potential in the cavity? The more articles I read, the more confused I get, so I really need someone to set me straight.

2# foam will not "trap" appreciable moisture and is the answer "I grew up in La Crosse". A vapor barrier always goes on the inside first. It is only when the inside is in place that the outer layers need be considered.

I think nearly every foam, fiber and fill is available in Madison as it is a progressive town. DIY foam takes a bit of practice and as Dana will surely point out, you will want a modicum of solid foam on the outside, under the siding for that insidious "thermal bridging".