©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Unless a builder has opted for a Zip System wall  or is willing to ignore building code requirements, a layer of housewrap or building felt typically covers any exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. before the siding is applied. This water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material., or WRB, helps to protect the sheathing from damage in the event that water is driven past the siding.
But as Richard Baumgarten explains in a Q&A post  at GreenBuildingAdvisor, there may be circumstances where the builder would just like to skip it. Under a rainscreen, for example.
"I've avoided the use of a WRB on my roof, and would like to go without on my walls," Baumgarten writes. "Covering an entire house with a WRB seems excessive in a rainscreen system. CapillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. water seems to be the only enemy of a rainscreen wall. Impermeable furring strips, or an impermeable strip at the furring strip layer, would appear adequate at preventing capillary moisture from migrating into the wall from wet siding. Asphalt felt or plastic housewraps do not seem ideal for this task."
Baumgarten presents the question not as a dollars-and-cents issue, but as part of an effort to build a house that minimizes its "toxic imprint."
"A WRB is required by all building codes," GBAGreenBuildingAdvisor.com senior editor Martin Holladay tells Baumgarten. "It is also recommended by all building experts."
If Baumgarden wants to avoid using plastic housewrap or asphalt-saturated felt, he could choose a liquid-applied WRB, Holladay says. "But a WRB is not optional," he adds. "You have to have a WRB so you know where you can tie in the flashing. All flashings must be integrated with your WRB."
Rainscreen Siding 
Baumgarten says he knows what building codes require and what the experts say, but doesn't find it convincing. Using a WRB assumes that bulk water will get past the siding, "which shouldn't happen," and flashing can be integrated directly with the siding.
"I just installed a slate roof," he says. "It doesn't need a WRB to be waterproof, just proper laps. Why can't siding, which isn't nearly as vulnerable as the roof, be detailed the same way? I'm exempt from building to code, thankfully."
But James Morgan agrees with Holladay. "I think you are making two mistakes," he writes. "First, assuming that gravity and capillary action are the only conditions affecting bulk water movement in your wall assembly; second, that you are capable of achieving a perfect lapped siding/trim/flashing assembly that will last indefinitely.
"A WRB is a protection against wind-driven rain or snow working its way past the lapped assembly. It's also an inexpensive insurance against any imperfection in a critical assembly detail which may be compromised by future conditions which you cannot predict. If it never rains and the wind never blows where you live, a WRB-free assembly will be fine. Otherwise, proceed at your own risk."
The discussion prompts Jeff Nelson to ask whether foil-faced polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. board could be used under a rainscreen as the WRB.
Holladay replies that most building codes allow the use of some brands of polyiso as a WRB, "as long as the flashing method for the top of the windows is to use tape — the same type of tape used by the manufacturer during their AC71 testing..." Holladay isn't a big fan of taping over foam as a way of flashing because he worries about the long-term durability of the tape adhesive.
"Of course, you can always go outlaw, and ignore the AC71 details, and instead come up with your own (better) flashing methods," he adds. "After all, very few code officials even understand the AC71 approval process. However, that approach puts you at the mercy of any code official who wakes up one morning and does his homework."
Holladay says that most leaks occur at windows, and that window flashing works best with a WRB. "I'm not saying that it's impossible to flash a window directly to the siding," he says, "but to do it right, you would have quite a bit of visible metal flashing at the window perimeter, and the results would depend strongly on the skill of the installer and the quality of the flashing materials chosen."
Michael Maines, however, isn't convinced Baumgarten is on the wrong track. "With all the right details, well executed, I bet it's possible to build without a WRB," Maines writes. "The building code is the minimum standard as agreed upon by a bunch of experts, but that doesn't make it gospel. I would use large siding overlaps, soldered sill pans and head flashings, vertical shiplap sheathing, an insulation that can take some amount of wetting such as dense-pack or mineral wool, airtight drywall on the inside, and good humidity control. Actually I would just use a WRB but appreciate the desire to think outside the box."
As to Baumgarten's notion that using "impermeable" furring strips in the rainscreen, or inserting an impermeable layer beneath the strips, would prevent moisture from wicking into the sheathing — not so. Holladay tells him that neither housewrap nor felt is a capillary break. "A 3/4-in. air space is a capillary break," he says. "A 4-in. thick pile of crushed stone is a capillary break. Ice & Water Shield is a capillary break."
And in this case, using a WRB looks like a concession that's probably worth making, even if Baumgarten's goals are admirable.
"I think we all understand, admire and empathize [with] your not wanting to use man-made (generally toxic, to some degee) materials," John Klingel says. "But if you or the next cat have to rebuild the house, that has its negative impact, too. Sometimes we just have to lose a battle to win a war. When the house is, eventually, demo'd, the housewrap can go to the recycle center. Put it in and sleep well."
Baumgarten has to agree. "I'm convinced to use a WRB," he says.
Here's what Peter Yost, GBA's technical director, views it:
I don’t disagree with ANY of the perspectives in this discussion; they are all well thought out. And if only all design, specification, and construction were done with this level of understanding and reflection!
My knee-jerk reaction is: of course you need a WRB. But if you asked me which is more important for the long-term hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. performance of building assemblies — the WRB or the rainscreen free-draining airspace — I would have to say the latter not the former!
But I don’t think it is about the choice, it’s about balancing the risk and benefit. ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 160 — “Criteria for Moisture-Control Design Analysis in Buildings” — assumes that 1% of the liquid water reaching the wall claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. gets past the cladding and deposits on the WRB. That number was not arbitrarily selected; it’s based on considerable research and experience from some of our leading building scientists.
Can various wall sheathings handle that 1%? I bet if we did a series of computer simulations using WUFI we would find that indeed many if not most assemblies could tolerate that level of wetting. But my bet is that each of us would sleep a lot better at night knowing that we included a dedicated concealed drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. — the WRB tied into flashings — to manage whatever moisture gets past our claddings.
And when I think of the environmental footprints of all the various materials we include in our buildings, the asphalt-impregnated building papers and spun-bonded polyolefins we can choose for our WRBs are simply not the drivers of environmental impact that I worry about.