Yes, James Timmerberg’s new house will have a water-resistive barrier (WRB) on the exterior walls. The question is which of two possible options will be best choice: Grace Ice & Water Shield, a self-adhering membrane typically used to waterproof roof decking, or Tyvek Drainwrap, a housewrap designed for areas prone to wind-driven rain.
In a Q&A post at Green Building Advisor, Timmerberg asks, “If applying a WRB under rigid foam insulation, what are the pros and cons of using Grace Ice & Water Shield compared to Tyvek Drainwrap, in terms of protecting the sheathing, serving as an air barrier, difficulty of installation, flashing details, and any other reason you might prefer one product over the other to serve as a WRB?”
Timmerberg assumes that the Ice & Water Shield will be more expensive. Is it worth the extra investment, he asks, for a 2,000-square-foot-house that will be built in Climate Zone 5? That’s the topic for this Q&A Spotlight.
These products are apples and oranges
Ice & Water Shield, as David Meiland points out, is considered a roofing product, not something that’s typically applied to exterior walls.
“These products aren’t in the same category,” he says. “If there are particular areas of your project that might benefit from a self-adhering membrane, then I would look at Vycor, but it should only be on small areas of the wall the are still vulnerable with correctly overlapped layers of more usual materials.”
Ice & Water Shield is expensive, he adds, and a “major ordeal” to apply on the entire exterior wall surface of a two-story house.
Timmerberg understands that a membrane such as Grace Ice &Water Shield is used in two construction techniques, called PERSIST and REMOTE, but Meiland points out that those building approaches are meant for extremely cold climates.
If you follow the PERSIST script, adds GBA senior editor Martin Holladay, using the membrane on outside walls will be just fine.
“The PERSIST wall system (as originally conceived) places all of the insulation on the exterior side of the wall sheathing,” Holladay writes. “Stud bays stay empty. If you follow this approach, you can safely use a rubberized membrane like Grace Ice & Water Shield on the exterior side of the sheathing.
“If you deviate from the PERSIST method by switching from R-20 or R-24 foam to just a thin layer of foam, or by adding insulation between your studs, then installing a rubberized membrane becomes risky,” he adds.
When there’s insulation in the stud cavities, and some insulation on the outside of the sheathing, what’s needed is a vapor-permeable WRB rather than an impermeable barrier such as Grace Ice & Water Shield.
Holladay continues, “The PERSIST approach was developed in the 1960s before high-quality European tapes were available. The first approach to installing a PERSIST air barrier was to use torch-down membranes. After a few disastrous fires, PERSIST builders switched to rubberized asphalt.
“Now that it’s easy to buy European tapes designed to tape sheathing seams, builders have many more ways to create a good air barrier at the sheathing level than they did in the 1960s. I would advise you to tape your sheathing and use a conventional housewrap.”
Holladay also points out that the PERSIST approach works in all climate zones, not just very cold ones, and it could be used by Timmerberg as long as he was careful in all of its details.
Exterior membrane is a ‘robust’ solution
An endorsement for using a self-adhering membrane instead of housewrap comes from Adam Emter, who used it on the house he built in North Dakota. Emter used an Owens-Corning product called HydroSeal rather than Grace’s Ice & Water Shield, but the idea is the same.
“I got it on sale and did my whole house (approximately 2,000 sq. ft.) for approximately $1,000,” Emter says, “so it only really cost me about $700 more than using a traditional WRB.”
Emter placed the membrane over his OSB sheathing, followed by 6 inches of expanded polystyrene (EPS) foam, furring strips, and then siding.
“I opted for this method over taping OSB seams or taping foam seams simply because it seems like a more robust system in the long term,” he adds. “I’m planning to live in this home for many decades and I wanted to have as airtight construction as possible.”
Emter adds, however, the membrane was difficult to install: too cold, and it doesn’t want to stick; too hot and it “just turns into a huge asphalt mess.” He suggests putting the smooth face of the OSB facing outward, and wait for days when the outdoor temperature is about 70 degrees.
Attaching thick layers of foam
He used two layers of 3-inch-thick foam. Where gaps between sheets of EPS were large enough, Emter used canned foam to seal them. As he applied furring strips over the second layer of foam, Emter removed the screws that had been holding the foam in place and filled the cavities with foam.
As for insulation, Emter plans on using R-11 fiberglass batts in the 2×4 walls upstairs, and R-25 batts in the 2×6 basement walls. In the end, he will have about 55% of his wall insulation on the exterior, enough to keep the sheathing moisture free. On the inside, he plans on using the Airtight Drywall Approach to keep moisture out of wall and roof assemblies.
“Attaching the furring strips was difficult to get the hang of,” he adds. “Eight-inch timber screws would have only penetrated my studs by about an inch so I chose to go with 10-inch timber screws. I make my measurements as accurate as possible on the exterior, but I did miss a few studs here and there. The problem when you miss is that it’s often hard to tell if you missed left or right of the stud.
“I used approximately 850 screws total and maybe missed about 40 or so. Like anything, it takes time to get the hang of. I predrilled the screw holes in my 1×4’s. I highly recommend also drilling a countersink so that no part of the screw will be proud of the furring. That way your siding install will be nice and flat.”
Timmerberg will be using 2 inches of foam, which will made the total assembly 2 3/4 inches when the furring strip is included. In this case, Holladay writes, either a 4 1/2-inch or 5-inch screw would be the right choice.
Dana Dorsett recommends the 5-inch screw, “and sleep well at night during major windstorms.”
Our expert’s opinion
Peter Yost, GBA’s technical director, had this to add to the conversation:
I do like the approach of exterior rigid insulation protecting the dedicated WRB; all WRB materials become more robust when they are sheltered from temperature and moisture extremes in this way.
And combining this moisture control layer with your air barrier also makes a lot of sense; I like air barriers both interior and exterior but if given just one, the exterior makes it easier to get continuity.
In comparing a flexible housewrap like StuccoWrap to a rubberized asphalt membrane like Vycor, two properties jump out:
- Most housewraps (many of which are spun-bonded polyolefins or SBPOs) have a relatively high vapor permeance, permitting good drying potential to the exterior of the assembly. Most rubberized asphalt membranes like Vycor are Class I vapor retarders, permitting little if any drying potential to the exterior.
- Most of the rubberized asphalt membranes are self-healing, so maintaining water and air performance around fastener penetrations represents a significant performance advantage compared to housewraps like StuccoWrap.
Two approaches not mentioned are liquid-applied WRBs and taped structural sheathing like Zip sheathing. And while each of these may be more expensive per square foot of material than either a rubberized asphalt membrane or a housewrap like StuccoWrap, getting the same quality and continuity of these layers, particularly at penetrations, is very difficult to do. So when we compare the costs of each approach, it’s hard to account for how much more time and effort and frustration is associated with each system.
And no matter which system you choose, it’s the margins, transitions, and penetrations that are the most demanding and the most important. I must confess that I have not actually used any liquid-applied products, but the systems offered by Prosoco, Dow, and Tremco are impressive — just pricey.
I have to add that the Wingnut Test Facility is engaged in yet another round of testing of high-performance acrylic pressure-sensitive adhesive (PSA) tapes (see the photos below).
We are currently testing acrylic PSA construction tapes adhered to OSB on half their width and either PVC or an aluminum flange on the other. We are wetting the tapes front and back (we routed a groove in the OSB that the tapes span so water “spritzed” from the back gets to the tape) to mimic bulk water getting past the WRB on the exterior and condensation getting to the window install from the interior.
It’s pretty early to be talking about results, but the only acrylic PSA tape that has not failed in this round of testing — wetted or not wetted — on the aluminum flange is Zip System wall tape. Zip wall tape on just about any substrate, not to mention the one for which Zip tape is designed, is pretty remarkable in its adhesion under tough application and in-situ field conditions.
We are far from issuing conclusions on our testing, but based on our testing to date and anecdotes from a growing number of high-performance builders and architects, the Zip System — a system that incorporates wall sheathing, WRB, and an air barrier — would be at the top of my list.