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Choosing the Right Water-Resistive Barrier

Are there times when a rubberized asphalt membrane makes more sense than ordinary plastic housewrap?

Posted on Dec 21 2015 by Scott Gibson

Yes, James Timmerberg's new house will have a 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. (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 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. , serving as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., 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 (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) 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 2x4 walls upstairs, and R-25 batts in the 2x6 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 1x4'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.

Tags: , , , ,

Image Credits:

  1. Adam Emter

Dec 21, 2015 9:57 AM ET

Further Questions.
by Dillon Vautrin

One of the benefits of housewraps is that they are vapor permeable. I don't remember the original Q&A post but if James is using a non-permeable exterior insulation board that would pretty much negate this positive attribute of the housewrap's vapor permeability. Also it is widely stated that ice and water shield membranes are self healing. Is this true even if they are covered by thick insulation? Does the membrane only self heal or seal itself when it remelts due to higher exterior temperatures or direct sunlight to the exterior, or will it do this at lower temperatures as well? Or am I missing the point and the self healing occurs during installation?

Dec 21, 2015 10:54 AM ET

Response to Dillon Vautrin
by Martin Holladay

When authors say that a rubberized asphalt membrane (like Ice & Water Shield) is "self-healing," they simply mean that it is rubbery and stretchy. When you penetrate the membrane with a nail or screw, the membrane pinches around the fastener, reducing the chance of air leakage or water leakage. (Similarly, if you get a nail in your automobile tire, sometimes you don't get much leakage until you pull the nail out of the tire.)

This characteristic of a rubberized asphalt membrane is what some people call "self-healing." It doesn't depend on the sun, and it happens at cold temperatures as well as warm temperatures.

Dec 22, 2015 10:50 AM ET

Returning to Dillon's question
by James Timmerberg

If you use foil-faced foam over the sheathing, does that permit the sheathing to dry to the exterior, such that it is significantly less risky than a rubberized asphalt membrane?

Dec 22, 2015 11:01 AM ET

Response to James Timmerberg
by Martin Holladay

Q. "If you use foil-faced foam over the sheathing, does that permit the sheathing to dry to the exterior?"

A. No. Like rubberized asphalt membranes, foil-faced rigid foam is a vapor barrier. If you are designing a wall assembly with either rubberized asphalt membrane or exterior foil-faced rigid foam, the wall needs to be designed to allow the sheathing to dry to the interior.

Q. "Is a wall with foil-faced foam on the exterior side of the sheathing significantly less risky than one with a rubberized asphalt membrane?"

A. The answer depends on other wall details. For either type of wall assembly to work, you need to be sure that enough insulation has been installed on the exterior side of the sheathing to make sure that the sheathing stays above the dew point during the winter.

Dec 22, 2015 12:13 PM ET

Percentage exterior insulation to keep sheathing above dew point
by James Timmerberg

The Remote Manual, if I am reading it correctly, recommends that a minimum of 43% of the insulation be on the exterior of the sheathing for climate zone 6. Three questions: (1) Are there any studies, stories, rumors, or wild speculation suggesting that 43% exterior insulation would not keep the sheathing above the dew point in zone 6? (2) If 43% works for zone 6, wouldn't it also work for zone 5? (3) Does anyone have a more specific percentage recommendation for zone 5? (The building code is silent on this issue other than mandating a minimum of either R-20 cavity insulation or 13+5.)

Dec 22, 2015 12:26 PM ET

The IRC gives guidance on that.
by Dana Dorsett

The IRC is not silent on this. See the relevant portion of chapter 7:

For zone 6 the IRC specifies R7.5 exterior insulation for 2x4 framing, R11.25 for 2x6 framing. The most you're going to get with fiber insulation in a 2x4 cavity is R15-ish, the most in 2x6 framing is R23-ish, so per the IRC prescriptive 33% of the total R would be the minimum allowable in zone 6.

For zone 5 they specify R5 for 2x4 framing, R7.5 for 2x6, so anything over 25% of the total R would be good enough.

Dec 22, 2015 1:01 PM ET

Response to James Timmerberg
by Martin Holladay

Here is a link to an article that should answer your questions: Calculating the Minimum Thickness of Rigid Foam Sheathing.

Dec 22, 2015 1:34 PM ET

Edited Dec 22, 2015 1:35 PM ET.

Is everyone comfortable with the IRC recommendation?
by James Timmerberg

Thanks Martin and Dana.

Is everyone comfortable that a minimum of 33% exterior insulation is sufficient in a climate zone 5 wall assembly that has no ability to dry to the exterior? Having grown up in a house built to code in Chicago, with four aluminum sliding glass doors, I am receptive to the idea that code minimum might not always be good enough. I also realize that, at the end of the day, it's my sheathing that will rot if the IRC has this wrong (or wasn't intended to address the use of foil-faced polyiso), so it's my decision. But I would be very interested to hear other opinions on this issue, if there are any.

Dec 22, 2015 1:47 PM ET

Edited Dec 22, 2015 1:57 PM ET.

Response to James Timmerberg
by Martin Holladay

The issue has been extensively studied. These walls have been modeled. They have also been studied in test huts in a wide variety of climates. The test huts have lots of sensors to record the moisture content of the sheathing.

Of all the wall types studied, walls with exterior rigid foam (meeting the minimum R-value requirements shown in the IRC) have the driest wall sheathing. These walls win. The wall sheathing under the rigid foam stays dryer than the sheathing on ordinary walls with no exterior foam, and it also stays dryer than the sheathing on double-stud walls.

You should really be asking, "Is it safe to build a wood-framed wall without exterior rigid foam?" Those are the walls with damp sheathing.

Dec 22, 2015 1:49 PM ET

One more thing
by Martin Holladay

I should also point out that these walls dry in both directions. Everything on the exterior side of the rigid foam (the furring strips and the siding) dries outward. Everything on the interior side of the rigid foam (the wall sheathing, studs, fluffy insulation, and drywall) dries inward.

Dec 22, 2015 4:55 PM ET

Edited Dec 22, 2015 6:47 PM ET.

2 products or 1?
by Curtis Sachse

This is an issue I have thought a lot about, but have no practical experience. What are the durability/longevity issues involved with using high-density SPF vs (whichever) WRB, and then rigid foam. I would think a single product and contractor would make the entire project less expensive, and although the SPF has negatives in terms of off-gassing, maybe the reduction in transportation for materials and labor could offset these negatives. Is there a big durability issue with being forced into outie windows instead of the option for innie?
I am referring to using SPF in an exterior application as 1 product for both wrb and exterior insulation.

Dec 22, 2015 5:26 PM ET

Response to Curtis Sachse
by Martin Holladay

Q. "What are the durability/longevity issues involved with using high-density SPF vs (whichever) WRB, and then rigid foam?"

A. If you are talking about installing spray polyurethane foam between the studs, the answer is simple: installing rigid foam on the exterior side of the wall sheathing is far preferable to installing spray foam between the studs, because (a) the sheathing stays drier, and (b) the rigid foam greatly reduces thermal bridging through the studs.

Since the sheathing is likely to be more damp with the spray foam option, the rigid foam option is likely to win the durability contest.

Dec 22, 2015 6:50 PM ET

Second response to Curtis Sachse
by Martin Holladay

I see that you have recently edited your question to indicate that you are talking about closed-cell spray foam installed on the exterior side of the wall sheathing.

Such applications of spray foam are possible, of course, but relatively rare, because of the need to install studs and stand-offs, or furring strips and stand-offs, before the spray foam is installed.

Dec 23, 2015 12:21 PM ET

Zip Wall Sys
by steve stemper

Im going to be building in zone 4 with a double 2x4 wall using the R-6 Zip Wall Sheathing(foam factory applied to inside zip). My veneer is going to be brick and I want to use cellulose between the double studs. Does anyone have opinions on this. Thank you.

Dec 23, 2015 1:15 PM ET

Response to Steve Stemper
by Martin Holladay

You never want to install any rigid foam or Zip-R sheathing on the exterior side of a double-stud wall.

Here is a link to an article that explains why: Exterior Rigid Foam on Double-Stud Walls Is a No-No.

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