In another thread the issue of permeability on outside surfaces came up. Jonr made a distinction between a vapor barrier and a vapor retarder. This seems like an important distinction that I hope to better understand. I have several questions. 1.How much, if any, difference is there between vapor, moisture and air barriers? What is the difference between permeability that stops air and that that stops moisture? Intuitively it would seem that air should move through material more easily than water, so, if we do not create a complete barrier to moisture, then we are not going to create a complete barrier to air. 2.I guess these measurements are in perms. Is there some snazzy heuristic that can help a layperson wrap their head around this measurement? 3.How much difference is there in the spacing between permeability? Basically the difference in a permeable sheet of material as opposed to an impermeable sheet that has occasional breaks. This is especially relevant to foam applications. Rot caused by an inability for small amounts of moisture to dissipate seems to be pretty localized. So moisture that has made its way to the mid section of a piece of foam panel is unlikely to be aided by small gap between panels two feet way. And of course this issue is exacerbated by spray foam applications. 4.Speaking of spray foam applications, open cell is said to be vapor permeable or something to that effect. But, of course, permeability is a continuum with an arbitrary point of demarcation applied to it. My sense is that, in most wall assemblies, a small amount of water between a wood member and spray foam insulation would not dissipate fast enough to avoid rot. 5.It seems that there are a lot of variables that impact both air and water permeability of a wall assembly including, thickness of material, sequence of materials, time of exposure, pressure, temperature and probably lots more. What percentage of this equation is permeability?

Water molecules are two tiny hydrogens glued the outer electron shell of a big fat oxygen. Nitrogen, oxygen, & carbon dioxide are roughly 2x the size, and won't pass freely through many materials that water-vapor can. Liquid water has surface tension forces on it that keep it from moving through many materials that both air and water vapor can. "Vapor retarder" and "vapor barrier" are strictly about the ease of water vapor diffusion, not air or liquid water rejection.

The "perm" value describes the rate at which moisture will cross a given amount of surface area at a given difference in vapor-pressure, not to be confused with air-pressure.

Materials that are vapor retarders aren't necessarily air-barriers or conversely. A ripped sheet of foil is a lousy air-barrier, since air pressure differences can move air through it. But it's still a very powerful vapor retarder/barrier since without the movement of air, large differences in humidity can exist on either side of the boundary without moving much, if any moisture.

Spray foams are somewhat permeable, but the permeance goes down with increased thickness, and with increased density. Most half-pound open cell foam is still 10+ perms at 6" thickness, but most 2lb foam is about 1.2-perms at 1" thickness, and about 0.6 perms at 2".

The idea behind using vapor retardent materials in air-tight building structures it to limit the rate at which seasonal moisture accumulates in susceptible wood, while still allowing the structure to purge moisture during other seasons. The classic cold-weather scenario is limiting wintertime moisture from the interior accumulating in sheathing and studs. This happens wherever the average temp of the wood is below the dew point of the interior air. (In practical terms that's a dew point of 38-40F, for 68-70F, 30-35% RH conditioned space air.) The amount of vapor retarder required depends a lot on just how many hours the sheathing stays below 40F over a handful of winter weeks, and how much drying capacity there is to the exterior. If you put sufficient exterior foam sheathing to add R-value, the sheathing stays warmer, for fewer condensing hours, and you can raise the interior perm rating dramatically without fear. Similarly, if there is a vented "rainscreen" gap between the siding and the sheathing, the drying capacity toward the exterior is much enhanced, and the interior perm rating can be raised.

A decent short-hand explanation of the IRC exceptions for interior vapor barriers (and some definitions) can be found here:

http://www.buildingscience.com/docu...quirements

Permeability on the outside surface of a wall can cause damage as well. Proper maintaince of walls is required in ever such area where there is high permeability of Water Molecules.......

Permeability is only about water vapor pressure and moisture transfer as water vapor, not liquid water. It's not the mere presence of water molecules, it's the vapor pressure of the water vapor in the air on the outside compared to the vapor pressure of the moisture of the air inside the wall assembly that determines the direction of moisture transfer. At higher temps the moisture content of the air CAN be higher than cooler air, but it isn't necessarily the case.

Some materials are highly vapor permeable, but also waterproof, which is useful on the exterior layers of the assembly where some amount of liquid water penetration is likely or inevitable. For example, Tyvek housewrap is ~30 perms, but also waterproof to liquid water, and a preferred weather-resistant barrier for the drain plane in many applications. Old school 15# felt (still a fine product when used appropriately) varies with actual humidity levels between 2-5 perms. Typar housewrap is ~10 perms, and still HIGHLY permeable compared to an inch of XPS (~1.2 perms). XPS and 2lb spray polyurethane are also waterproof to liquid water, yet remain semi-permeable to water vapor at inch or less of thickness.

Where higher than 2 perms on the exterior of the structural sheathing runs into problems is usually with masonry claddings without adequate rainscreen/cavity gaps & venting, keeping the air in the gap much higher than the average exterior air humidity, punctuated with periods of extremely high vapor pressures when the sun bakes the moisture out of the brick/stone/stucco. But even 1 perm is good enough to moderate the rate of moisture transfer from the exterior of a masonry-clad wall into the cavity, so long as the interior side is a half-perm or greater, and the conditioned space humidity is reasonably controlled. With interior poly or foil it's often a problem, since whenever the interior surface is below the dew point of the cavity-air liquid water condenses on the foil, and gets absorbed by any wood in the cavity that's at or below the dew point.

Controlling moisture is a science into itself, and even with a solid understanding of the science, the vagaries of weather conditions, building conditions and operation make protecting against all troublesome conditions difficult at best. The operative rule is to provide as much safety margin as possibly without incurring much extra cost.