Sure, exterior rigid foam when blowing wall cavities might be a good idea, but is it NECESSARY?
My 1920 platform framed home is on the border of climate zone 5&6.
The empty wall cavities are framed with true 2x4s, 16 OC. Walls are covered with lathe and horsehair plaster on the inside and 1″ pine sheathing boards outside. The current siding is a horrendous installation of vinyl over the original clapboards. The cavities are now empty.
We intend to remove the vinyl (carefully, for reuse), strip the clap boards, dense pack the wall cavities, wrap the house in tar paper, and rehang the vinyl.
Well, what about adding exterior foam? Do we HAVE to, in order to avoid condensation? Yeah, I get it…. adding foam either inside or out will lower thermal bridging, and provide better overall performance. And I understand that once you add ANY foam, you introduce a whole new set of factors in terms of moisture-control and key once you add any is to make sure you add ENOUGH. If we add foam, we’ll go with 1.5″ XPS, which I think gives the required R 7.5 for zone 6.
But what about just living with the thermal bridging and lower performance and SKIPPING the foam? Is that a recipe for soaking our sheathing and cellulose, assuming the interior and exterior planes are doing their respective water & moisture jobs?
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First, you never actually get condensation, the moisture enters the wood in the form of adsorb, not liquid.
Pine planking has a higher vapor permeance than OSB or plywood, so it dries more quickly than those materials. With inherently back-ventilated vinyl siding, there is no question that it is adequately protected from interior moisture drives in zone 5, even with just latex paint as the interior vapor retarder. On the warmer edge of zone 6 it's not particularly dangerous either. If the dense-pack is cellulose, and not fiberglass, it should be fine anywhere in zone 6, though it's technically a code violation unless the interior paint has a vapor permeance of less than 1 perm.
But does the interior paint have a vapor permeance less than 1 perm?
If it's old enough to have horsehair plaster, it probably has multiple layers of oil based paint, which would be WELL under 1 perm! And that may be a whole other issue in your wall stack-up decisions.
The key factor for moisture control would be the condition & lapping of the window & door flashing to the drain-plane layer, which is something you can assess & fix if you're stripping it down to the plank sheathing.
But then there's the low thermal performance issue too. The additional R of the true 4" depth is offset by the larger framing fraction of the full-width 2" wide studs- it's usually slightly lower whole-wall performance than a 3.5" deep milled timber due to the higher framing fraction, unless it's a balloon framed multi story and the window & door headers are single rather than doubled-up lumber.
If you are thickness constrained, an R6-ish 1.5" rigid rock wool solution would be better than an R7.5-ish XPS solution, since at 1.5" the XPS may be too vapor-tight for a wall with multiple layers of oil paint on the interior side. Rigid rock wool panels are over 25 perms, and would not inhibit drying to the exterior in any meaningful way- there are housewraps more vapor-tight than that(!).
If you're not thickness constrained you'd still be OK with up to 2.5" of unfaced Type-II EPS, which would have more than 1-perm of drying capacity into the air behind the vinyl siding.
The 1" rock wool solution would make IRC 2012 code-min for whole-wall performance in zone 5 but not zone 6. With 2.5" of EPS (or 3" of rock wool) it would meet IRC 2012 code min for zone 6.
If you put 1.5" of Type-II EPS out there (cheaper than rock wool or XPS) it would run R6.3 nominal, but R6.8-ish during the winter outdoor averages, which is still adequate for dew point control at the warm edge of zone 6 (though technically a code violation, unless the interior is under 1 perm) , but it would have 1.5-2 perms of drying capacity, which is still fine. With 1.5" XPS you'd be looking at 0.6-0.8 perms, which is a bit tight.
My answer will be briefer than Dana's.
No, exterior rigid foam is not necessary in this application.
There is no reason to worry about moisture problems in your sheathing, due to the fact that the board sheathing is vapor-permeable, and due to the fact that vinyl siding is so well-ventilated. Don't worry about the multiple layers of interior paint; I have never seen a house like the one you describe with diffusion-related moisture problems at the sheathing level. (If your house ever gets moisture problems at the sheathing, it will be due to flashing errors, not vapor diffusion.)
Thank you very much, Dana and Martin