The use of plywood and OSB sheathing is a fairly recent phenomenon. Before these sheet goods became readily available, builders nailed wood boards to the frame of a house for sheathing, and it is a house with this type of sheathing that Nick Welch is trying to update.
His 900-square-foot house in Climate Zone 4C is sheathed with 1×8 boards, apparently over a layer of asphalt felt. There is apparently no insulation in the wall cavities behind the sheathing. His plan of attack is to air-seal the house, then install foil-faced polyisocyanurate insulation over that. The question is how.
“I’m having a hard time finding much information about how to air seal old horizontal 1×8 board sheathing,” Welch writes in a post at GreenBuildingAdvisor’s Q&A forum. “Housewrap seemed like the clear choice until I realized that you aren’t supposed to tape the bottom seam.”
Should he use flashing tape to seal each every seam between boards? Could he skip the housewrap under the exterior foam and use polyethylene sheeting instead? And if he applies foam over the sheathing, will the layer of asphalt felt present a problem?
Those issues are at the heart of this Q&A Spotlight.
Tape the polyiso seams
A layer of foil-faced polyiso will be the air barrier, GBA Senior Editor Martin Holladay tells Welch. “Caulk the perimeter of the wall when you install your polyiso,” he writes, “and tape the polyiso seams.”
Welch is correct in assuming that the permeance of housewrap beneath the foam is irrelevant, Holladay says. “The key will be making sure there’s enough foam on the outside of the house to keep the stud cavities above the dew point so moisture from the interior of the house doesn’t condense inside the walls.” (Holladay has written an article with more information on this topic: “Calculating the Minimum Thickness of Rigid Foam Sheathing.”)
The asphalt felt won’t be a problem, Holladay adds, but it would be a good idea to fill the empty stud cavities with dense-packed cellulose insulation.
“I don’t think you need to tape every sheathing seam,” adds Dan Kolbert. “You’re creating a new enclosure with the foam, and if you tape the seams in the field there you should have it well sealed in the middle.”
What Kolbert would be more concerned with are the top and bottom of the walls and the areas around door and window openings. “They are tough to seal under the best of circumstances,” Kolbert says. “If you can arrange it, get a blower-door test done before you start siding to see if you’ve got any major leaks at the perimeter that you can seal with tape, gaskets, foam, caulk, etc.”
Will water collect behind the housewrap?
Welch still wrestles with a key detail: Is it a good idea to seal housewrap at the bottom of the wall?
“It seems that it is commonly advised against, so that any errant water can drain out,” Welch says. “Should the foam be caulked to the housewrap at the bottom? This presents a similar concern. If neither are sealed at the bottom, then essentially I am relying on the “squish” of the foam against the house to reduce air leakage through this area. The drainage plane will be at the sheathing and the windows will be innie-style.”
This confusion apparently originated with an article by Allison Bailes in The Journal of Light Construction in which Bailes says housewrap isn’t installed as an air barrier. As relayed by Rick Russell, Bailes writes that housewrap should never be sealed at the bottom edge because any water that gets behind the housewrap won’t be able to drain out.
“This is contrary to your comments,” Russell tells Holladay. “Perhaps you and Dr. Bailes can sort this out for us. In doing so, it occurs to me that which approach to take (seal the bottom of the wrap or not) might depend on whether or not the siding is installed over a rainscreen gap and how carefully any tears in the wrap were taped prior to the application of siding.”
It’s a non-issue, Holladay replies.
“Housewrap is often taped at the bottom,” he says. “That’s what’s recommended in the Tyvek installation instructions. “
“Can I imagine a failure mechanism or flashing error so egregious that it allows liquid water to pool behind housewrap at the bottom of a wall?” Holladay adds. “I guess. Is such an egregious flashing error likely? No.
“Now, the situation anticipated by Allison Bailes requires a reverse flashing lap, an enormous quantity of water, and enough of an air gap between the housewrap and the sheathing to allow liquid water to reach the bottom of the wall,” he says. “Here’s my opinion: it’s not going to happen.”
Should the windows be removed so that the rough openings can be flashed?
Welch’s house has vinyl windows, which he assumes replaced the original windows.
“Everything I’ve read says it’s best to remove them and completely re-flash the rough opening, but this seems like a massive addition of work, adding disruption to the interior of the home, and I wonder if there is some way around it,” Welch writes. “It seems that plenty of people re-side without removing their windows, but apparently none of them have decided to explain how it’s done on the internet.”
The two most vulnerable spots in a window installation are the bottom corners of the rough opening, Holladay replies, and whether that is a problem in Welch’s house depends on a variety of factors, including how much rainfall the area gets, the size of the roof overhang, and how well the windows were installed in the first place.
“First of all, you need to examine your walls for signs of water entry when you have the siding removed and are installing your foam,” Holladay says. “If you see signs of moisture, you know that you have to start from scratch with the window flashing.”
If the walls are dry and Welch decides to leave them windows in place, he’ll have to install new exterior window sills, Holladay adds. Sills must be sloped so they direct water to the exterior.
“Of course, these new sills must cover the new rigid foam,” he says. “The easiest material to use is painted aluminum flashing. Use a heavy gauge, and try to get the sloped flashing under the windows. Pay attention to water-sealing details at the corners, where the sill meets the jambs, because those are your most vulnerable areas.
“Do a good job and keep your fingers crossed.”
Our expert’s opinion
We asked GBA Technical Director Peter Yost for his thoughts. Here’s how he replied:
By removing the existing exterior cladding and adding exterior rigid insulation, you have the opportunity for the rigid insulation to be your continuous air, thermal, and bulk water barrier (or weather-resistive barrier — WRB). For these barriers to be continuous, they must be sealed at all penetrations and margins. And then for the WRB, that continuity is further supported by a weatherlap at penetrations, like windows.
Martin is right to advise checking to see if the existing assembly shows signs of repeated wetting, but remember, you are adding insulation which will significantly reduce drying potential, so I think it is imperative that you either re-install the windows or at least make sure that you have the proper weatherlap with a new sloped sill flashing.
And now that you have added a Class I vapor barrier to the exterior of your wall assembly (that would be the foil facings of the rigid polyiso insulation), you should make sure that your wall assembly can dry to the interior. In an old building, vapor permeability of interior layers is certainly likely, but not a guarantee (and by the way, that existing building paper is vapor permeable, especially when or if it gets wet).
If the only insulation you add to the wall assembly is the exterior rigid, then you won’t need to worry about wintertime interstitial condensation, but as Martin points out, you will need to have the right ratio of exterior to cavity R-value should you add cavity insulation, OR you should consider an interior vapor retarder (for example, vapor retarder paint or a smart retarder like MemBrain — not a vapor barrier like polyethylene). The former is much preferred because that added interior vapor retarder to control wintertime movement of vapor into the wall assembly will restrict inward drying during the rest of the year.
I also strongly agree with an in-process blower-door test; make sure you have that continuous air barrier (in more than just your walls) before you complete the exterior and interior of the walls and rest of the building.
Finally, I again agree with Martin regarding the taping of the bottom margin of the housewrap: if you have to worry about that much bulk water getting behind your new continuous WRB and collecting at this junction, you have bigger and more immediate problems than that taped bottom edge acting as a stopper.