Vapor-open exterior sheathing

Untreated, unfinished wood sheathing is more than permeable enough to maintain the 5:1 ratio, assuming a 1 perm vapor retarder primer on the interior to meet IRC standards.

In addition to its vapor permeability, wood is also hygroscopic so that it can absorb, diffuse and release liquid moisture as well. Which means that occasional condensation is not problematic as long as the moisture can diffuse outward and is not trapped by another impermeable or non-hygroscopic layer.

I dunno, Jesse - I think you could make the opposite argument - the T&G would be less permeable than similar width boards butted together, since the entire assembly would be have less air flowing thru.

Well, you can figure the average u-factor for a wall assembly (stud, then insulation, then stud etc.) so there's got to be a way to figure the same for vapor permeance. 5.5" of pine, then tiny gap, then more pine etc. A small hole = a big difference applies more to air infiltration than permeance as i understand it, so there must be some way to average it out. If you have an air-sealed drywall interior then you should be able to take the "leakage at joints" out of the equation and look purely at permeance. I thnk the bldg sci note about the siding is because there's inherently a gap and not air sealed.

Robert, sure, all that is true, but to be able to compare the performance of T&G pine against miracle engineered all wood fiber high-perm structural sheathing like Agepan (http://www.peakbp.net/products.html) we need some numbers, and using solid pine numbers place it at a disadvantage to a material that is in the 30-40 perm range, comes as a sheet good with quick installation, and has marketing brochures.

Like so many non-industrial materials, it's fascinating to see the perm curve of wood change with relative humidity. It's fairly impermeable at low humidity (0-3), but once you cross the 60-70% humidity range it ramps up in a logarithmic curve to 10+. It's just what you want in a building material.

Dan, I'm just looking for vapor diffusion quantities separated from air permeability, cause you're right, not good as an assembly at blocking air, but that's what an interior air barrier would take care of.

Hunter, you're probably right, it should just be calculated as a minor difference in thickness, t&g doesn't really have open grooves that would act as a pure air gap like square boards would have. Probably lost in the noise.

Part of the beauty of wood is its infinite variability. Because of this, it's not possible to determine a precise permeance for a type or species of wood (or structural strength, for that matter, which is why a 6:1 safety factor is built into design values).

But, when a little of something is good (like permeance), a lot is not necessarily better. 5 perm is sufficient for a breathable building assembly. More than that and it too quickly adsorbs environmental moisture, such as on sunny, humid days when there is a strong inward moisture drive.

For this reason (and others), asphalt-impregnated felt makes a better weather barrier than high-perm polymeric housewraps. And, when choosing a polymeric housewrap, I will prefer a moderate-perm Typar over a high-perm Tyvek.

It has been repeated often that variable permeance is good. Since we are not discussing diode like one way permeance then to me the product let's in as much as it then has to let out.

Explain why this is any advantage over a constant permeance?

I.E. High vapor drive and high perm let's much more h2o into the wall as well as letting it out.

Anonymous, put a name to your question and I'll be glad to answer it.

Martin, Bill Rose, Robert... the idea of variable permeance being a good attribute needs some explaining in regard to how for example Membrain would function any better than a constant high perm barrier.

I would think the high permeance would let in a wall what one is trying to keep out. (humid unairconditioned home in the summer)

As has been stated repeatedly air movement transports much more vapor than permeable substrata.

So... if I answer this question myself, vapor is not getting in a well sealed wall easily through air movement, but when it does and creates a high RH inside a wall, then the Membrain layer or whatever will switch to a higher perm aiding the vapor getting out of the wall buit not until summer high humidity is done with... And poly... would trap it causing problems with moisture/rot. Am I close?

I think it should be noted too that according to Martin's other postings a very low perm externally foam board insulated wall is a well functioning build for all climates in the US.

Personally I do not favor the use of oil based products like foam but will build with and with out them for now. And if indeed high external foam insulation is the absolute safest wall, may have to learn to love oil based foam till the RPI mushroom foam is stocked everywhere. (I do tennis court work for the advisor to the team that came up with the mushroom foam. Great people, great school)

No need to yell at me now... post a nice polite response if possible... thank you!

Membrain is not effected by the dampness of the drywall it is against which is effected by the moisture in the home or room as in a bathroom where severa steamy showers are taken daily?

I guess I don't know how the product can know that it is getting damp from inside a wall verses inside the home via damp drywall?

In fact, MemBrain is very smart. It has a woven-in computer which monitors the occupant's behavior and not only knows when you're showering and for how long but with whom.

P.S. Using one of a half dozen aliases is no different than posting anonymously. Using a real name means standing by one's words. Using a fake name is a way to avoid accountability.

For those interested in an honest and open exchange: Membrain is largely a scam.

While it may offer some advantage in a AC dominated climate in which the inward vapor drive predominates and in which an interior vapor barrier would present a significant condensation potential, it is largely useless is a heating-dominated climate in which the dominant vapor drive is to the exterior.

If it's used at all, it should be avoided in bathrooms and probably kitchens and laundry rooms as well, because high indoor RH will open the Membrain and drive moisture into the thermal envelope. As far as summertime inward drying, its >10 wet perm will offer no advantage if the paint on the drywall is only 3-5 perm, since that will be the limiting factor for inward vapor diffusion.

Where naturally-variable perm is advantageous is in hygroscopic materials (i.e. wood) used as exterior sheathing. If water vapor condenses behind plywood sheathing, for instance, its perm will rise and allow an accelerated vapor flux to the outside, slowly reducing its water content. In winter, it's only water vapor that is driven outward by vapor pressure (absolute humidity) differential, while liquid water can be driven inward by RH differential (most regions in the US have high winter RH).

Of course that variable vapor openness is useless with a low-perm exterior insulation that becomes the limiting factor for outward vapor drive. And, while sufficient exterior insulation might mitigate condensation, the moisture content of wood framing and sheathing is determined by the RH. It doesn't require 100% RH (the dew point) to cause problems. 80% RH is sufficient to create and sustain mold activity, and 90% RH is sufficient to support decay fungi.

So Martin's statement, "That's why you want to design a wall for which drying potential exceeds wetting potential", would argue against exterior insulation in a cold climate or the wasteful practice of burying an expensive variable-perm membrane behind a lower perm layer like painted drywall.

Martin,
I am not surprised to learn that MemBrain outperforms polyethylene, but we already knew that interior polyethylene is a poor choice in an air-conditioned building. If I'm forced by a code official to install an interior vapor retarder, then MemBrain looks like a good option. But outside of code requirements, is there any reason to spec MemBrain?

Robert,
Would you mind clarifying which types of sheathing you would classify as "wood"? T&G white pine is wood, obviously. You mention plywood in your own example, but we know how you feel about other wood-based sheet materials like OSB and fiberboard. Does the lamination process in which plywood is made maintain its "wood" qualities?

Martin,

I don't doubt that MemBrain would improve the performance of the worst-performing wall assembly, such as the one in the Wisconsin study: vinyl siding (highly permeable due to loose fit), Tyvek (too high a permeance), OSB (too moisture vulnerable), fiberglass (too open to convection and far too permeable with no buffering ability), and poly VB (traps moisture). This is like saying my Toyota Tundra gets great mileage - compared to a Hummer.

We were once forced to use poly vapor barriers because we commonly built houses with the worst possible materials and methods. Now we know better (perhaps). In a well-designed thermal envelope, MemBrain is an unnecessary extravagance. It's a "solution" only for those who don't build intelligently.

And, of course, MemBrain isn't brainy enough to differentiate between moisture entering the wall assembly and moisture leaving the wall assembly.

According to ToolBase.org, "Another design, not yet commercial, is a spray-on, cellulose film reinforced with glass fiber, polyethylene, and latex. This latter design is intended as a one-way vapor retarder to limit transmission from one direction but allows transmission from the other direction." At least this approach is truly "brainy" and can allow inward drying while excluding outward wetting. But this sounds like a much too complicated and expensive a "solution" to poor hygro-thermal design.

Ben,

As Lstiburek has pointed out, the more processed the wood materials the less it behaves like wood and the more vulnerable to moisture it becomes. So there is no sharp line, but rather a continuum.

Natural wood has a low vapor permeance at low RH and a rapidly increasing permeance as the RH increases. Nature is pretty smart without our interference. Plywood is mostly natural wood with thin layers of resin which tend to limit vapor flow, but it has a similar tendency to increase its permeance with increased humidity. OSB has far too much resin and too little contiguity between wood fibers to allow either much dry permeance or much increase in permeance as humidity rises. Additionally, because it doesn't allow much liquid or vapor diffusion, it tends to maintain a higher surface moisture content and hence a higher proclivity to mold growth.

“Wood is a fantastic material because it’s a hygric buffer,” says Joseph Lstiburek. It's hard to beat Mother Nature. But, being so headstrong, arrogant and foolish, we keep trying - with "better solutions through chemisty".