This isn’t an ordinary wall“Humor me,” Minneapolis replies. “What’s stopping the assembly from drying to the interior? Inside the poly [on the walls] is plywood, fiberglass, drywall, paint.”He also points to comments Thorsten has made about using poly as a substitute for peel-and-stick membranes in REMOTE walls with extremely low rates of air infiltration.“Not exactly trying to ‘win’ here, just asking if, based on these points, it might actually be an acceptable approach — and that (counter-productively) thinking of it as an antiquated approach might mean I miss an opportunity to seal well at one layer while that opportunity is still available to me,” Minneapolis writes. “It certainly seems much more straightforward than later scrambling to create that barrier at the drywall.”Maybe so, but John Klingel points out that Thorsten is no longer using poly unless he’s required to do so. Instead, he uses a double-stud wall assembly and air-seals plywood on the outside of the inner wall. Tyvek housewrap goes on the outside of the outerwall, but there’s no poly involved. RELATED ARTICLES It’s a PERSIST-like wallHolladay, too, has come around to see Minneapolis’s point of view. Holladay explained that his first response was based on a misunderstanding of Minneapolis’s question. “Assuming he is using foil-faced polyiso, the foam is already a vapor barrier, so adding polyethylene beside the polyisocyanurate doesn’t change the drying characteristics of the wall,” Holladay writes. “I’ll admit that my first response misunderstood the planned location for the polyethylene; that’s why I originally raised a warning flag.“In fact, his plan is similar to a PERSIST wall, except he wants to use polyethylene instead of peel-and-stick. I’ll add my usual advice to those building a PERSIST wall — it’s usually better to resist the temptation to fill the stud bays with fluffy insulation. The fluffy insulation just makes your plywood sheathing colder. Empty stud bays are better from a building science perspective.” Polyethylene sheeting has had its ups and downs as a preferred building material over the last 20 years.At one time, it was routinely used in wall assemblies as a vapor barrier. As building scientists learned more about air and moisture movement through walls and ceilings, however, they began to advise builders that an interior vapor retarder is better than an interior vapor barrier, and the perceived usefulness of poly plummeted.In most climates, air movement, not vapor diffusion, came to be recognized as a bigger threat to buildings. Air barriers, which can be vapor-permeable, became a more important priority. Builders also realized that because of its very low permeability, polyethylene had the potential of trapping moisture inside walls.Even though poly has gradually lost its luster, a GBA reader nicknamed Minneapolis Disaster wonders whether it could have a place in an outbuilding he’s putting up. In a a Q&A post at GreenBuildingAdvisor, Minneapolis lays out plans for incorporating a 6-mil poly air barrier in both the walls and ceiling of his planned building. In the exterior walls, the poly will be sandwiched between the exterior plywood sheathing and 5 1/2 in. of layered and taped polyisocyanurate foam insulation. In the ceiling, Minneapolis plans on adding poly between two thick layers of polyiso. RELATED MULTIMEDIA The ceiling is the big questionJ Chesnut agrees with Minneapolis that poly might be used in exterior walls.“Because I’ve seen this method of air sealing used (on GBA) when installing Larsen trusses to retrofit existing structures it seemed like a valid approach for the walls specifically,” Chesnut writes. “Does a 20-year-old approach necessarily mean it won’t work?“It’s the ceiling/roof area that’s in question, Chesnut adds.“The roof/ceiling condition is the gray area,” Chesnut writes. “MD is sandwiching his air barrier poly to the outside of R-45 with only R-32.5 to the outside of the poly. This concerns me because it can’t be determined if the poly will always be warm enough not to act as a condensation plane. This is the heart of his question.“Unlike the wall assembly, the sheathing of the roof assembly has no outboard insulation. It will be a low slope shed roof with potential for a perimeter soffit vent but this method a venting may not establish air movement. Because the sheathing will be cold a vapor retarder I believe is needed at the ceiling plane.”He suggests that Minneapolis move the poly to the bottom of the ceiling joists. It could be lapped down interior walls and caulked to the top plate of the wall to form an effective air barrier and vapor retarder. Air Barrier ComponentsHe’s encouraged by the practices of Alaska builder Thorsten Chlupp, who has successfully used poly in cold-climate buildings. In addition, the poly is available in 12-ft. wide rolls, which will make it easier to lap and seal than 10-ft. wide housewrap. “I’ve come to the conclusion that for the small building that’s underway in my yard, I’ll see good air-sealing results from 6-mil poly, adhered at edges and openings, around the walls and another layer lapped over, across the ceiling,” Minneapolis writes.But he has a nagging concern the ceiling assembly may be a problem. “Condensation problem in the ceiling?” he asks. “Or am I in the safe zone?”That’s the focus of this week’s Q&A Spotlight. GREEN PRODUCT GUIDE Questions and Answers About Air BarriersForget About Vapor Diffusion — Stop the Air Leaks!Air Leaks Waste Energy and Rot HousesGetting Insulation Out of Your Walls and Ceilings Podcast:Air Barriers vs. Vapor Barriers Will it Really Trap Moisture?GBA senior editor Martin Holladay’s initial reaction is to counsel against using poly.First, vapor diffusion isn’t going to be a problem because the inside surface of the exterior foam will be above the dew point. Hence, no condensation.“If you ever have a flashing problem, however,” Holladay adds, “and rain gets into your assembly from the exterior, you want your wall assembly to dry towards the interior. That’s why the basic rule is: whenever you have exterior rigid foam, never install interior poly.“If you’re installing poly as an air barrier, you are about 20 years behind the times,” he adds. “Establish your air barrier at the plywood sheathing, or use the Airtight Drywall Approach.”Dick Russell agrees with Holladay, suggesting that Minneapolis substitute CertainTeed’s MemBrain for the poly. Its permeability is designed to change with seasonal conditions so moisture inside walls can dry to the inside.Philipp Gross also warns against poly, writing: “I do not think these assemblies (neither roof nor wall) are good ideas. I checked those quickly with an old fashion German diffusion analysis called ‘Glaser diagram.’ I might be over-careful with this analysis because it does not account for any drying potential. To my knowledge the rule of thumb is: If the ‘Glaser’ analysis works, no further analysis are necessary for the diffusion problem. For the mentioned assemblies this is not the case.“It is also not just about keeping the sheathing warm but also about the permeability profile of the assembly,” Gross adds. “For example the mentioned assemblies are fine in the ’Glaser’ analysis without the poly (the way [Building Science Corp.] recommends them).” Our expert’s opinionHere’s what Peter Yost, GBA’s technical director, has to say:Referencing the REMOTE and the PERSIST systems puts this poly question in the right context.In the PERSIST system, a peel-and-stick membrane in between the exterior structural sheathing and the exterior rigid insulation functions as both the air barrier and the weather resistive barrier. It just so happens to be vapor impermeable, requiring the PERSIST assembly to dry to the interior. The beauty of the membrane location is that the exterior insulation “warms” the membrane, shielding it from wild temperature and relative humidity swings that stress the membrane. This exterior location also generally means fewer penetrations, making it easier to achieve continuous air and water barriers.In the REMOTE system, the poly replaces the membrane as the air barrier and weather resistive barrier mostly because poly is so much cheaper than any peel-and-stick membrane. Poly also comes in larger sheets, speeding its installation quite a bit in comparison to any membrane product. And the REMOTE system, like the PERSIST system, must dry to the interior because it too is essentially vapor impermeable.We think of polyethylene sheeting as a dedicated interior vapor and/or air barrier because that is how many of us initially learned to use it. But the way a building material performs can change as we move it around and change its function. That is the case with poly in the REMOTE system. When we use poly under a slab, it also functions as a capillary break, even if that is not the reason we installed it!