Natural Plaster on the Exterior of a Standard Construction Home

Well! Last time I came here to ask the collective Permies oracle a question, I was stumped about what insulation I should use in my walls. After much dreaming on my side, and much practicality on my family's side, we met in the middle at denim insulation.

It was really easy to work with (a bit fluffy when you rip it, but no prickles- oh, and the blue snot it gave me was pretty fun too), but I could not get around putting a plastic vapour barrier over it. I know I don't feel comfortable about sealing the walls, but I can't really make a decent argument as to why when I'm speaking with people used to building to code. Anywho- so we've now got to-code insulated walls complete with plastic vapour barrier and more tuck tape than you can shake a stick at, and instead of drywall we have plywood walls (for strength, mostly).

I'm going to plaster the interior with American Clay Loma, but the exterior is where I'm stuck. I know that I would rather work with a natural plaster than a cement stucco, but I'm worried that a breathable finish like earthen plaster over tar paper might allow moisture to enter and create condensation? I'm really stuck on all this. What I get from the conventional builders is, "well, you just need vapour barrier! Without it your house will be draughty and your walls with rot!" But now that there's a vapour seal on the inside, should I have a tight one on the outside, too? Does this mean I have to say goodbye to my natural-ish house all together? Any advice would be greatly appreciated! Any other exterior finish ideas would be wonderful too : )

Kat

Hi Kat, congrats on your fun project and blue boogers. My main question is what climate are you in? Usually, vapor barriers VBs are only required/recommended in the coldest N.American climates.

Your question gets into some pretty heavy building science issues. Tight is great because it means more control of your indoor, conditioned air. VBs are an old-school way of getting tight (air barrier) and they have largely fallen out of favor. Most building code departments will accept a simple latex paint on drywall if there is still something in the code that requires them.

The problem with VBs is that they can create a condensing surface. Breathable (like gore-tex, tyveck, OSB, plywood, #15 felt) is a desirable attribute when it comes to air barriers because it allows moisture in the wall to escape. Depending on who you ask however, one VB is usually OK. Iam assuming your VB is on the interior (between studs and plywood in place of drywall). It would be very dangerous adding a VB on the exterior because moisture might get trapped in the wall.

I think your 15# felt (tar paper) would be fine and personally, I would go for at least two layers and ideally have an airspace between the stucco/plaster coat and felt. The true air gap is a tough detail and the two layers is an easier way of achieving it. Most important are your flashing details. Moisture rarely gets in the wall from vapor drive although its much higher with porous claddings like stucco (hence the air gap recommendation). Moisture damage is usually the result of poor flashing and when it accumulates faster than it can dry out, the results are bad.

Doing it over, and if climate/code allowed, I would skip the VB and tape the plywood using it as the secondary air-barrier. The danger of interior VBs are usually in the Winter when warm, humid interior air finds its ways to the colder VB. My advice is to be meticulous eliminating these air paths and consider some exterior, insulative sheathing which will help raise the temperature of the condensing surfaces inside your building envelope. Your ground up wranglers will do a great job insulating the cavities but the studs and framing members (thermal bridges) will be working hard to rob your heat and lower the temp of the VB.

Hope this helped more than it hurt!

Thanks Brian! Your advice and information really does help. I wish we could go back and not put in the plastic VB, but it's in there now and the tip on using 2 layers of felt below the plaster is a very useful one! We live on Vancouver Island, so the climate is mild but often damp, so we'll definitely pay extra attention to flashing. Thank you again! Where else but this site can a person get such a helpful and friendly response?