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Does Your Air Barrier Work in Both Directions?

An experiment comparing housewrap and taped sheathing under positive and negative pressure

Posted on May 15 2013 by Allison A. Bailes III, PhD

Do you want a good 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. on your house? Of course you do. No one who knows anything at all about building science believes that old myth that a house needs to breathe. We want airtight houses, but then we want mechanical ventilation to bring in fresh air from outside (well, at least as fresh as you can get from your outside).

The air barrier’s job is to minimize the amount of air that crosses the building enclosure between conditioned and unconditioned spaces. Does all of the air moving across the building enclosure always go the same way? No. It goes both ways. The amount of outside air that leaks in is matched by the amount of inside air that leaks out. (Sometimes the air leaking in or out is intentional, and we call that ventilation.)

Once you understand that air can move both ways, it’s clear that you’d want an air barrier that works in both directions. Right? But when we test homes for air leakage, we pretty much always test them in only one direction: from outside to in. We put the house under negative pressure with a blower door and then measure the air flow through the fan, which tells us how much air is leaking in through the building enclosure.

I’ve written in the past about how housewrap is far from ideal as an air barrier. It can be a great 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., however, and that should really be its primary purpose when you use it on the outside of a house. (As some unfortunate homeowners in Kentucky found out recently, trying to build a house without any kind of drainage plane leads to trouble.)

A research project comparing housewrap to taped sheathing

A couple of years ago, I did a little research project for Huber Engineered Woods. (Disclosure: They were Energy Vanguard’s client and paid for this work.) They wanted to find out how air leakage through their Zip System wall sheathing compared to housewrap. Our objective was to test the same home for both infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. with two different air barrier conditions:

  • House sheathed with OSB; housewrap installed according to manufacturer’s instructions to serve as air barrier.
  • House sheathed with Zip System wall sheathing and roof decking. All seams and openings taped as specified in manufacturer’s instructions.

The builder framed the house as normal. They used Zip Wall sheathing and Zip Roof decking but left the walls untaped for the first test. They installed housewrap over the walls and sealed according to the manufacturer’s instructions. They set all windows and doors and sealed the house to the point where we could do the blower door testing. They also taped the Zip Roof decking and sealed all penetrations since the ceiling drywall was not in place.

The air leakage test results

When the house was ready to be tested with housewrap, we ran single-point blower door tests in two configurations – positive pressure and negative pressure, both at 50 Pa relative to outdoors.

After the first test, the builder removed the housewrap and taped the Zip Wall seams. They had the house ready for the taped-sheathing test 10 days after our first test, so we came in and retested the house, as before.

The results for both tests are below. The ACH50 results are based on the house volume of 52,113 cubic feet.

The first test showed an enormous difference between the negative pressure (infiltration) and positive pressure (exfiltration). The exfiltration rate was 77% higher than the infiltration rate. (The results are shown in the “% change 1” column above.)

The second test showed marked improvement in both the infiltration rate (20% lower) and exfiltration rate (56% lower). (The reuslts are shown in the “% change 2” column above.)

Putting the results in perspective

Even with the housewrap, this house was very tight, having infiltration and exfiltration rates of 1.36 and 2.40 ACH50, respectively. As we observed, the builder had taken great care to seal the housewrap completely, and that effort showed in the results.

In a typical installation, the bottom edge of the housewrap is left unsealed. Seams are often taped poorly or left untaped. Numerous rips and tears keep the housewrap from lying flat. Sometimes siding installers cut the housewrap in the corners because the housewrap doesn’t lie flat there. If this house had the typical amount of extra leakage sites that these problems introduce, the initial test would have shown significantly higher leakage.

Also, since this was a large house, much of the wall sheathing did not break over studs or plates (see photo below). Many of the seams that fell over open areas in the framing showed large gaps, as shown in the photos below. These gaps can allow a lot of air through, and that’s probably why the exfiltration result was so much higher than the infiltration result in the housewrap test.

With the house under positive pressure, air from inside moved out through those gaps, blowing the housewrap out like a balloon. Once outside the sheathing, the air finds holes in the housewrap and escapes. Of course, with claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. installed, the housewrap wouldn’t be able to expand as much. With the house under negative pressure, the housewrap was pulled into the gaps, sealing the sheathing almost as well as the tape did in the second test.

Still, even with such a low starting point for the air leakage rates, the taped Zip Wall sheathing performed significantly better – 20% for infiltration and 56% for exfiltration – than the housewrap used as the air barrier. Because the housewrap installation was better than normally observed in the field, these improvement numbers can be taken as a lower limit of how much better Zip Wall sheathing will perform compared to houses that rely on housewrap, at least when comparing houses of similar wall complexity. In smaller houses where all or most of the sheathing breaks over studs and plates, the effect may be less pronounced.

Further investigation ideas

The really neat thing about the project is that we used the same house to test two different systems. In the end, though, this was only one house, and it wasn’t finished. It would be nice to see this work extended to more houses with more configurations.

One obvious test that would be nice to see is the difference in air leakage for housewrap and taped sheathing after the cladding and drywall are installed and the house is finished. There’s a huge difference between positive and negative pressure air leakage before cladding and drywall, but how big would it be at the end?

Another set of experiments to pursue would be looking at the effect of different cladding types. Then look at the effect of the different methods for putting air gaps between cladding and sheathing: furring strips, wrinkled housewraps...

Takeaways

As I wrote in 2010 (before Huber ever contacted us), housewrap is not an ideal air barrier. If the builder takes great care, housewrap can perform well as an air barrier, but the typical installation leaves a lot of holes. What this research shows, at least for a house without cladding, is that housewrap works better for buildings under negative pressure than for those under positive pressure.

The key takeaway from this initial work is that if you’re looking for really high levels of airtightness, you should seal the sheathing when you use housewrap. Taped sheathing products, like the Zip System or taped foam board, will probably have lower air leakage than homes with housewrap and unsealed sheathing.

Footnotes

I say it was a “little” research project because it wasn't a scientifically rigorous study whose results establish a general truth. It's one quick look at a particular phenomenon and it deserves more study. I worked on the project with Al Landers, PhD (now retired), Allen Sealock, and Danko Davidovic, PhD. Danko is a real building scientist with a degree from one of the best places to study building science: Penn State.

Tape is quite the controversial topic in building science circles, and some people don't trust it. Joe Lstiburek, PhD, PE, is not one of those people. “Residentially we now use tapes on wood sheathings and foam sheathings  to provide water management and air management. So how well do these systems actually work? Now? Real well. In the early days? Not so good.” Read his article, Stuck on You.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


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  1. Energy Vanguard

1.
May 15, 2013 11:11 AM ET

nice info, but predictable ...
by Jin Kazama

how much labor time ($) is usually invested on meticulous taping of sheathing and or housewrap ??

I still believe that using a peel-stick membrane over all of the walls cost alomst the same
( becaus of the simpler installation and almost no details to work on ) including labor
than any taping method, and provides with much tighter buildings.
Why still consider something of lower performance if price is not much different ?
That literally bogs me.


2.
May 15, 2013 1:37 PM ET

What Zip is and isn't
by Randy George

Perhaps it's because Huber funded this study, but it seems to me that there is an impression from these results that the Zip System is the only way of getting taped sheathing really tight. But as I understand it, the deal with Zip is that it's got the WRB applied to it, so when you're done taping, you're all done. But the built-in WRB doesn't improve the air sealing. isn't it true that the same air sealing could be achieved by taping standard plywood sheathing? Then housewrap would be applied over that. Our contractor is planning on doing this with taped AdvanTech (another Huber product) when he builds our house because he is skeptical of the OSB Zip is made with (he thinks it's much less rot resistant than AdvanTech-- can anyone speak to that?) and also because Tyvek has stood up to long term field testing, whereas Zip is still pretty new. Although I know many people who are happy with Zip sheathing, I can't argue with my contractor's skepticism about Zip and I don't see that our house will be any less tight or expensive by going with this system. Am I wrong?


3.
May 15, 2013 2:44 PM ET

Response to Randy George
by Allison A. Bailes III, PhD

If there's "an impression from these results that the Zip System is the only way of getting taped sheathing really tight," I don't think it's because of what I wrote. In the last paragraph I wrote, "Taped sheathing‡ products, like the Zip System or taped foam board..." and tried to use the term "taped sheathing" in other places where I was talking generally.

I don't know much about the difference between the OSB in Zip and Advantech, but maybe someone else, possibly even someone from Huber, will jump in and tell us.


4.
May 15, 2013 3:06 PM ET

Can we really trust tapes to last 25-50-100 years ??
by Jin Kazama

Aren't we all ( here ) designing toward green ?
Having to rip the finish apart in 40 years because some of the tape on the sheating tacked off ,
or having a building that leaks much more than it used to for 25 years beause of a similar issue.


5.
May 16, 2013 2:00 PM ET

Response To Randy George
by Allen Sealock

Randy, I am the Technical Director for ZIP System and designed the study highlighted in Dr. Bailes article. AdvanTech and ZIP System Sheathing are manufactured in the same plants using the same process and the same raw materials. The amount of raw materials (wood, wax, and resin), differ slightly between products due to the differences in their end use. ZIP System has an integrated water resistive barrier that is vapor permeable yet bulk water resistant. Therefore it does not require the same level of adhesives as AdvanTech which doesn’t have the weather barrier.


6.
May 16, 2013 2:59 PM ET

So use a WRB over Taped Sheathing
by B McKay

From what I gather from this article is that to have the best, efficient system builders should tape the sheathing and then use a wrap, like Tyvek, over it. ZIP is a good OSB but the integrated material is equivalent to Grade D building paper (like Randy George states: " the built-in WRB doesn't improve the air sealing"). So, if you use ZIP, or some other foam board, tape the seams, then wrap it with a product like Tyvek. But, don't just tape your sheathing with no additional wrap over it. Like Jin Kazama wrote above "Can we really trust tapes to last?" I think we can all agree on the answer to that.


7.
May 16, 2013 3:18 PM ET

Response to Jin Kazama
by Allison A. Bailes III, PhD

There are a lot of different systems for building homes, and Zip System sheathing is one choice among many. Just as someone choosing it over peel-and-stick WRBs boggles your mind, stick-building instead of using SIPs or ICFs or another construction method seems insane to others. If you don't like it, you have other choices.


8.
May 16, 2013 3:22 PM ET

Edited May 16, 2013 3:27 PM ET.

Response to Jin Kazama and B McKay
by Allison A. Bailes III, PhD

Tape works and it can last a long time. Perhaps I should have put the link in the main article rather than a footnote, but you should read Joe Lstiburek's article, Stuck on You, to see what tests they've put tape through and how well it is fared.

If it's not done properly, yeah, of course it'll fail. Here's what Dr. Joe says:

"But with all of the advances in materials science we have still not figured out how to get things to stick to mud, dirt and frozen surfaces. Let me repeat this because as good as things are there is no way to stick stuff to muddy, dirty, cold, wet and frozen surfaces. I should repeat it again, but you folks should just stop here, re-read the previous sentence and think about it. Re-read it again. OK, now lets move on."


9.
May 16, 2013 4:15 PM ET

Response to Allison
by B McKay

Right, Joe states "With tapes the biggest single problem is folks thinking they can stick them to muddy, dirty, cold, wet and frozen surfaces. You would think this is obvious. Again, amazing as it seems, folks keep trying to do it." So, I would still cover my taped sheathing as I've never seen a clean job site. No harm in providing that extra protection from air and water.


10.
May 21, 2013 2:31 PM ET

Quality is in the details
by Tony Calistro

The comments about applying tapes over muddy, frozen, dirty, wet, etc. building products reminds me of the insulation contractor that wanted more money to insure that the batt insulation fully filled the cavity and left no voids. In other words, if you want it done right he needs to charge more. Quality is in the details, if it is okay to apply tape over faulty conditions would it also be okay to apply paint over dirty drywall or to install windows without shims or flashings?


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