What happens to the vapor permeability over the long term when the interior side of the exterior walls are repainted.  Obviously vinyl wallcovering should be avoided, but what about multiple layers of paint?  Any studies?

Bruce

Much like painting stucco I suspect.

As I understand it, a coat of latex paint has a permeability of 3-4 perms. Adding layers of material decreases permeability of the assembly. If the permeability of all layers is the same, then the relation is simply inverse with thickness; doubling the thickness cuts the permeability in half. For layers of differing permeability, the same math applies to vapor transmission and heat transmission. Permeability is like U (=1/R) of insulation. To get U of an assembly, take the inverse of the sum of the layer R values. To get permeability of an assembly, take the inverse of the sum of the 1/permeability numbers of the layers.

Edit: I thought I ought to expand on the reasoning. Both heat and vapor transmission by diffusion are area based; doubling the area doubles the rate of movement of heat or water vapor. Steady state heat transmission goes linearly with temperature difference; steady state vapor diffusion rate goes linearly with water concentration or partial pressure difference across the assembly. Heat conduction varies with thermal conductivity of the materials in the path, and the U (BTU/ft2/hr/degree) is the measure of how readily heat passes through. Water diffusion varies with the permeability of the material and has thickness in its definition; dividing by thickness cancels that out of the dimensions, leaving something like grains of moisture per sq.ft. per psi of vapor pressure difference.

Quality and craftsmanship is always in the details. This starts with proper surface preparation and any needed repairs. Then continues with choosing and applying the best primer and paint. It can be virtually maintenance free for many years!

It takes a lot of latex or acrylic layers to bring the permeance under 1.0, but alkyds can hit that in one application. In most heating dominated climates interior vapor retardency between 0.5-1perm is desirable when theres a bare-minimum of exterior foam, and often even lower permeance is called for (as in an old school studwall in Zone6, with no exterior foam over the sheathing), and not harmful in most cooling dominated climate stackups.

Going deeply under 0.5 perms can make the assembly less resilient when there's a strong vapor barrier on the exterior (eg, foil faced iso, or unperforated radiant barrier), but you'd need a nearly structural paint thickness to get there with standard latex. ;-) Kraft facers on batts are typically ~ 0.4-0.5 perms, and often not quite low enough to be protective in cold climates unless other wall details such as vented rainscreened cladding &/or exterior foam get built in.

IRC is somewhat broader in where a vapor retarder (I believe < 1.0 perm) is required. Since an extra air barrier is a good thing, I'd lean towards a smart film vs kraft paper and/or oil based paint to achieve this. In zones 1 or 2, I'd avoid oil based paint.


SECTION R318.1 MOISTURE CONTROL

R3181. Moisture Control. In all framed walls, floors and roof/ceilings comprising
elements of the building thermal envelope, a vapor retarder shall be installed on the
warm-in-winter side of the insulation. Exceptions:
1. In construction where moisture or freezing will not damage the materials.
2. Where the framed cavity or space is ventilated to allow moisture to escape.
3. In counties identified as in climate zones 1 through 4 in Table N1101.

Exception 1 per the IRC list includes exterior foam sufficient to keep the average temp of the structural wood above the dew point of the interior air. (A reliable approach, relatively easy to implement for most of the US, but difficult/expensive in cooler parts of Canada.)

Exception 2 describes a rainscreen gap/cavity between the exterior and structural sheathing. (Required by local code for timber frame construction in all of Canada, and in a few places in the US.)

Employing both 1 & 2, using semi-permeable foam and a rainscreen gap cures a world of ills on interior vapor retardency and interior air tightness faults.

Yes, multiple air barriers are a definite plus. But like housewrap, sheet poly or smart-polymer films aren't air barriers unless detailed to be air barriers. Airtight wallboard is also an option.

All interior vapor retarders and air barriers are fairly easily damaged over time (in fact, almost guaranteed to fail), which is why it's better to design the assembly to be resilient to minor air leakage rather than relying too much on the absolute interior vapor retardency value & air tightness. IIRC Natural Resources Canada did a nice study of the effect of a single picture-hanger nail hole in a studbay penetrating the code-required 6-mil interior poly a couple years ago. It didn't inspire confidence in the lowest possible perm interior vapor barrier approach to moisture control. Interior poly in Canada has been responsible for many rot issues in brick-clad buildings due to summertime moisture drives condensing in the walls of air-conditioned buildings.

Blanket rules are easily mis-applied.

No, those interpretations of exceptions 1 and 2 go far beyond what it says. Note that neither say "part of the wall assembly" or discuss dew points. Either build the wall completely out of moisture proof materials, vent the entire thing, or install a vapor retarder. Poly not required (not sure why that straw man was used).

Really? Do tell!

Standard latex paint is a class-III vapor retarder.

Looks like my analysis of what I quoted was correct, but it has been replaced by a newer IRC with different wording, conditions and exceptions.

The IRC is simply prescriptive. The dew point analysis etc, is the theoretical basis behind the prescription, (and empirically proven to work, in these cases.)

The right amount of exterior foam &/or rainscreen (or other vented cladding, such as vinyl siding) removes the need for sub-1 perm interior vapor retarders to protect against moisture accumulation in the sheathing. Keeping the assembly as high-perm as possible while still avoiding excessive winter moisture accumulation maximizes it's overall drying capacity, making it more resilient to construction faults, and to summer-season moisture drives (particularly with masonry claddings.)

More is always better with exterior foam- with a more-permeable interior the structural wood then stays much closer to the conditioned space temperature & moisture environment. The IRC prescriptions are minimums, and have some (but not huge) margin built-in.