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Is OSB Airtight?

Builders and researchers in North America and Europe report that air can leak right through oriented strand board

Posted on Dec 12 2014 by Martin Holladay

UPDATED on August 13, 2015

Most builders assume — and has long reported — that oriented strand board (OSB) is 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.. If a builder uses a high quality tape like Siga Wigluv, Zip System tape, or 3M All Weather flashing tape to seal 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. seams, OSB wall and roof sheathing can act as a building’s primary air barrier. Energy consultant Marc Rosenbaum used the technique at two dormitory buildings at the College of the Atlantic in Maine and several projects on Martha’s Vineyard, and subsequently gave a presentation at a Vermont conference at which he advised builders who wanted low blower-door numbers to tape the seams of their OSB sheathing.

In recent months, however, many building experts have been surprised to read reports that some brands of OSB may not be airtight. The reported leakage isn’t coming from poorly taped OSB seams; instead, the air is allegedly leaking through the OSB panels themselves, even when the seams are flawlessly taped. While this type of air leakage sounds unlikely, evidence is accumulating that the reports must be taken seriously.

Among recent developments:

  • An Irish engineer, Niall Crosson, has reported problems with two OSB-sheathed buildings that were too leaky to meet the Passivhaus airtightness standard.
  • Architect Richard Pedranti and energy consultant Pete Vargo performed job-site testing in 2014 that convinced them that air was leaking through the Weyerhaeuser OSB installed on the walls of a Passivhaus project in Scranton, Pennsylvania.
  • Three Belgian researchers — J. Langmans, R. Klein, and S. Roels — have published the results of laboratory tests showing that some European brands of OSB aren’t very airtight.

Defining acceptable air leakage

What’s the definition of an air barrier? In North America, some building codes have adopted the following definition: an “air barrier material” is defined as a material with a leakage rate of no more than 0.02 liters/sec-m² @75 Pa.

Building scientist Joseph Lstiburek helped develop this definition. “The .02 number was based on a suggestion from Gus Handegord,” Lstiburek told me. “He suggested that an air-barrier material should be defined relative to drywall. We tested drywall and the air leakage rate for drywall was a bit under .02. So that was the basis of the standard.”

The 2009 International Residential Code based its definition of “air-impermeable insulation” on the above definition of an “air barrier material.” The code defines an air-impermeable insulation is “an insulation having an air permeance equal to or less than 0.02 liters/sec-m² at 75 Pa pressure differential tested according to ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E 2178 or E 283.”

The National Building Code of Canada has also incorporated the above definition of an air-barrier material. (Once Canadian codes adopted this definition, DuPont, a major manufacturer of housewrap, was caught flat-footed. According to Lstiburek, Tyvek housewrap, widely promoted as an air barrier material, failed the Canadian standard. “They couldn’t sell Tyvek as an air infiltration barrier in Canada,” Lstiburek said. To address the problem, DuPont developed a new version of Tyvek for the Canadian market — one that was more airtight than the product they had been selling for years. Eventually, this product was branded as “Tyvek CommercialWrap” and was offered to U.S. as well as Canadian purchasers.)

A useful online document, “Air Permeance of Building Materials,” provides the results of laboratory testing in Canada of air leakage rates through a variety of materials. These lab tests showed that the following materials qualify as air-barrier materials: aluminum foil, EPDM roofing, plywood, XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., particleboard, and gypsum wallboard. Asphalt felt (both #15 felt and #30 felt) fails the test, as do asphalt-impregnated fiberboard and 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..

When I asked Lstiburek about air leakage through OSB, he told me that it was his understanding that “OSB has been easily able to meet the .02 requirement.” He added, “The Passivhaus airtightness standard of 0.6 ach50 is very difficult to achieve, but I would be surprised if the weak link [in a building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.] is the air permeability of the OSB.”

In a follow-up phone call, Lstiburek elaborated. “OSB meets the North American definition of an air-impermeable material,” he said. “Anyway, that’s not where buildings leak.”

ASHRAE 90.1 includes an air barrier definition

Section (“Acceptable Materials and Assemblies”) of ASHRAE 90.1, a model building code for commercial buildings, states that the following “continuous air barrier materials” are “acceptable”: “Materials that have an air permeance not exceeding 0.004 cfm/ft2 under a pressure differential of 0.3 in. w.g. (1.57 psf) (0.02 L/s.m2 @ 75 Pa) when tested in accordance with ASTM E 2178.” In other words, this definition is in alignment with the definition in Canada's National Building Code.

This section of ASHRAE 90.1 goes on to state that “Oriented strand board — minimum 3/8 in.” is one of several materials that “meet the requirements of”

According to Sam Glass, a research scientist as the U.S.D.A. Forest Products Laboratory in Madison, Wisconsin, the ASHRAE 90.1 stipulation is based on laboratory measurements of air leakage through three types of North American OSB. The measurements were made by Canadian researchers whose findings were published in a 2006 paper titled “A Thermal and Moisture Transport Property Database for Common Building and Insulating Materials.”

The Canadian researchers tested three types of OSB (3/8 inch, 7/16 inch, and 1/2 inch). Unfortunately, the brands of the OSB were not identified. All measurements of air leakage through these OSB samples were below the 0.2 liters/sec•m² @75 Pa threshold for air barrier materials. (For more information on these tests, see Comment #63 by Sam Glass, below.)

Testing OSB with the job-site “balloon test”

Builders who are worried about air leakage through OSB panels have developed a simple job-site test to demonstrate the problem. A rectangle of polyethylene (measuring about 3 feet by 3 feet) is temporarily taped to the center of a panel of OSB wall sheathing (on the exterior side of the wall). The entire perimeter of the polyethylene is carefully taped to the OSB with a high-quality tape like Siga Wigluv.

Then a blower door is used to pressurize the house to 50 Pascals. If the polyethylene balloons out, that’s a sign that air is leaking through the OSB.

A report from Ireland

Niall Crosson, senior technical engineer at Ecological Building Systems in Dublin, Ireland, posted a report about leakage through OSB sheathing on a LinkedIn Web forum: “I’ve been on two sites in the last two months aiming for Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. airtightness. One had 12.5 mm [1/2 inch] OSB and the other 18 mm [3/4 inch]. We struggled to get to the required N50 [airtightness goal of 0.6 ach @50 Pa] even though I couldn’t fault the application.”

Crosson continued, “I suggested to tape an air barrier membrane over a portion of the OSB.” In other words, the Irish builders decided to perform the balloon test (see Image #5, below).

“In both cases it blew up!" Crosson reported. "We then sealed the OSB fully [with a taped Pro Clima membrane called DA] and this made a dramatic improvement.”

For more information, see the PowerPoint slides from a Crosson's presentation on this topic: Airtightness: When it’s not as airtight as it’s cracked up to be!

A report from Pennsylvania

Richard Pedranti is an architect in Milford, Pennsylvania. Pedranti recently sent me an e-mail describing his headaches with Weyerhaeuser OSB.

“The project where we experienced OSB air leakage is a new residence that my office designed to meet the Passive House standard,” Pedranti wrote. “The project started construction in June of 2014 and is located in Scranton, Pennsylvania. It’s my first Passive House project, and I was concerned above all else about reaching the aggressive airtightness requirement of 0.6 ach @50 Pa. … Our team spent a great deal of time making sure the construction was tight and sealed. The OSB is taped on the outside with Siga Wigluv. The top floor is a flat ceiling with the OSB fastened to the bottom of the roof trusses and then taped with Siga Wigluv. The contractor took it upon himself to foam every interior stud and plate. … The polyethylene vapor barrier [was] wrapped up from beneath the concrete slab and taped to the OSB with Siga Wigluv.”

So far, so good. Pedranti explained in an e-mail, “After starting the blower door and spending several hours sealing small leaks, it became clear to me the we were not going to get close to 0.6 ach @50 Pa. The lowest we measured was 1.1 ach @50 Pa.”

I telephoned Pedranti for more details, and he explained, “We had been pretty confident that we were going to pass the blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. with flying colors. We got the blower door in, and we started at over 1.5 ach50 early in the day. We were in there for 4 or 5 hours with a fog machine. We were using flashlights, tape, and foam, finding every leak we could. The consultants and contractors were all working hard and sweating. After a few of hours, we were reaching the point of diminishing returns. It was becoming clear that we weren’t going to get below 1 ach50. We were done.”

When the house was pressurized, the poly billowed out

“We were all quite frustrated at that point, and our building performance consultant, Pete Vargo, suggested doing a positive pressure test and looking around outside,” Pedranti wrote in his e-mail. “We reversed the fan and Pete said, ‘Let’s do a balloon test on the OSB just for fun.’ We taped a piece of polyethylene over a portion of the OSB and taped it with Siga Wigluv. The polyethylene immediately ballooned up just like the shroud of the blower door, and Pete declared that we have leaky OSB.”

Pedranti explained over the phone, “I wasn’t very excited to do this balloon test. I didn’t think it was true that the OSB was leaking. I thought it had to be the tape seams or the sill seal at the plate. We taped the poly up there and cranked the fan up, and it just happened instantaneously. We said, ‘This is unbelievable. How did we end up with this leaky OSB?’ I was quite upset.”

A photo of the test (shown at the top of this page) is a bit hard to interpret; according the Pedranti, this photo shows the second of two tests. The first test (using the large rectangle of polyethylene) was performed over a section of sheathing that included taped seams. The photo shows the second test; for this test, the polyethylene was slit horizontally below the OSB seam, and the lower rectangle of OSB was sealed with a new horizontal piece of Siga Wigluv tape to the OSB. For the second test, the tested area did not include any OSB seams.


After observing these tests, Pedranti started asking questions. “When I returned to my office, I e-mailed a number of Passive House colleagues including Adam Cohen, Mike Kernagis, Dan Whitmore, and Chris Corson. They all had the same reaction: ‘Bummer — I’ve heard of this happening before.’ Dan suggested several liquid-applied solutions including Dow Corning Defendair 200, BASF Enershield, Prosoco Cat-5, and elastomeric paints.”

Solving the problem by painting the OSB with elastomeric paint wouldn’t be as easy as it sounds. “One limitation to our solution is that we did not want to remove the Siga Wigluv, which is a water-based tape,” Pedranti wrote. “After a cost and labor analysis, we opted for Siga Majpell [a European air barrier membrane that installs like housewrap] applied over the OSB with Siga Twinet. Twinet is a double-face adhesive tape. The Siga Majpell was also applied to the second floor ceiling.”

In our phone conversation, Pedranti said, “It cost $3,000 to fix the problem, and the contractor didn’t charge that much for labor. The double-sided tape is expensive.” After the new air-barrier membrane was installed over the OSB, a blower-door test showed an air leakage rate of 0.34 ach50 — well below the Passivhaus target of 0.6 ach50.

Pedranti will never again use Weyerhaeuser OSB. “I have changed the sheathing specification in future Passive House projects to Zip sheathing to avoid this issue.”

After hearing Pedranti’s story, I spoke with Alex Kuchar, Weyerhaeuser’s OSB technical manager, and asked him whether Weyerhaeuser has tested the airtightness of their OSB. "No, we have not," Kuchar responded.

Testing in Belgium

An interesting paper on the issue of OSB airtightness was published by three Belgian researchers, J. Langmans, R. Klein, S. Roels. Their paper is titled “Air permeability requirements for air barrier materials in passive houses — Comparison of the air permeability of eight commercial brands of OSB.”

The Belgian researchers wrote, “Based on the recent cases where the OSB was too air-permeable to achieve an n-value [blower-door test result] lower than 0.6 ACH [at 50 Pa], discussion arose whether OSB is a suitable material for air barrier systems in passive houses.”

The three researchers decided to test OSB samples for air leakage. “Eight commercial brands of OSB from the major West European manufacturers were selected. The panels investigated are produced in Ireland, United Kingdom, France, Germany, Belgium or Luxembourg. … The thickness of OSB tested is 18 mm [3/4 inch] which is commonly used for timber framed [stick-built] constructions.”

Only one of the tested brands of OSB met the Canadian standard for an air-barrier material; the other seven tested brands were too leaky to pass the standard (see the graph in Image #9, below). The researchers wrote, “The results … clearly indicate a large difference between the air permeance of the different OSB brands tested. For example, OSB B is at least ten times more airtight then OSB A. Furthermore, although only three specimens were measured for each brand, a large variation is noticed within the same brand.” (The variations in air leakage rates are shown in the graph in Image #7, below.)

The leakiest tested brand of OSB — designated “brand A” by the researchers — had an air leakage rate that was more than 10 times higher than the Canadian standard for an air-barrier material.

The researchers decried the lack of air leakage data on OSB: “Information of the air permeance of materials applied for air barrier systems is relatively rare. In Canada on the other hand, where the National Building Code (NBC) imposes an upper limit of 0.00096 m³/m²/h/Pa [0.02 liters/sec-m² @75 Pa] for materials composing the air barrier system, building materials’ [air] permeabilities are better documented.”

The researchers noted that the leakiest OSB samples were so leaky that it would be difficult for an OSB-sheathed house to meet the Passivhaus standard, even if every seam were perfectly sealed. This conclusion was based on a few assumptions and calculations. They proposed a few typical values for surface-to-volume ratios for Passivhaus buildings, and used these values in their calculations. They also noted that it’s logical to assume that some — probably most — of the air leakage through a tested building envelope will occur in locations other than through the OSB sheathing (since some leakage inevitably occurs through windows, penetrations, and difficult-to-seal cracks).

They wrote, “Assuming that the buffer necessary for unforeseen leakages [for example, leaks at penetrations and seams that are difficult to seal] should be at least 0.5 ACH as mentioned above, the air leakage through the air barrier material [i.e., the OSB] must be lower than 0.1 ACH … [in order to meet] the Passive House Standard. Based on the worst scenario used above, corresponding with a compactness of 0.91 m (A [OSB] /V =1.1), the upper limit for the air permeance results in 0.0018 m³/m²/h/Pa [0.037 liters/sec-m² @75 Pa]. This proposed limit is still less severe than the Canadian requirement [0.02 liters/sec-m² @75 Pa].” (A graph of these scenarios is shown in Image #8, below.)

Recommendations to builders

North American OSB manufacturers do not currently report the results of air leakage testing of their OSB.

There is no reason to believe that Weyerhaeser OSB is any more or less leaky than most other brands of North American OSB. In the absence of good testing data, it's hard to choose among available brands of OSB.

It's worth emphasizing that Weyerhaeser OSB is not defective. North American OSB is not marketed as an air barrier material, and current manufacturing standards for OSB do not require OSB to resist air leakage.

If the range of OSB performance among North Americian brands is similar to the range of OSB performance in Europe, it's possible that some brands of North American OSB are as leaky as the Weyerhaeuser product that plagued Richard Pedranti, while other brands (as Joseph Lstiburek and Sam Glass report) are tight enough to meet the code definition of an air barrier material.

In the absence of data from manufacturers, however, builders are forced to make decisions based on the best available information and the anecdotes of their colleagues. At this time, builders who want to use OSB sheathing as an air barrier should probably avoid OSB manufactured by Weyerhaeuser and should instead specify Huber Zip sheathing. (Huber Zip sheathing is a brand of coated OSB; many builders have reported good success using taped Zip sheathing as an air barrier.)

A second way to address this issue is to specify plywood sheathing rather than OSB sheathing.

A third approach is to create an air barrier using different materials — for example, a taped European air-barrier membrane like Solitex Mento, Intesana, Pro Clima DA, or Siga Majvest; or a liquid-applied air barrier like StoGuard, Grace Perm-A-Barrier VP, Henry Company’s Air Bloc 31, DuPont Fluid Applied WB System, or Tremco Enviro-Dri.

Is the 50 pascal depressurization test relevant?

It would be interesting to know how often the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. and wind speeds are powerful enough to cause enough air leakage through OSB sheathing to matter very much.

I discussed this question with Vladimir Kochkin, the division director at Home Innovation Research Labs (formerly known as the NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center). Kochkin pointed out, “This becomes a question of conversion of air leakage at 50 Pa to air leakage at natural pressures. Even if the OSB is leaking during the test, it does not mean it will leak substantially under natural pressures.”

Fifty Pascals of depressurization is roughly equivalent to subjecting a house to a 20 mph wind. While buildings regularly experience such winds, there are few locations in the world where such a high wind speed occurs very often.

Postcript: Inspired by this article, researchers at Prosoco, a manufacturer of flashings, tested air leakage through samples of OSB and confirmed the fact that OSB can leak air. The results of their tests are shown in Images #10 and #11, below.

Martin Holladay’s previous blog: “How to Use the Psychrometric Chart.”

Click here to follow Martin Holladay on Twitter.

Tags: , , , , , ,

Image Credits:

  1. Images #1, 2, 3, and 4: Richard Pedranti
  2. Image #5: Niall Crosson
  3. Images #6, 7, 8 and 9: J. Langmans, R. Klein, and S. Roels
  4. Images #10 and #11: Tom Schneider / Prosoco

Dec 12, 2014 9:53 AM ET

Insane to think cheap is not cheap.
by aj builder, Upstate NY Zone 6a

This blog s a mile too long for this farmer taught boy.

Premise; Let's build an expensive high tech PH. Let's build it with crap. What's wrong, oh the crap is bad? Those lousy manufacturers!!! How can they be so bad! I want top notch cheap not!!

Cheap cheap! That is my short message instead of a thousand word blog and million dollars of future studies. Give me a break.

PH folks... here's the lesson to learn. You get what you pay for!

Buy Zip, even my nearby home being built horribly has Zip!!! It also has 1/2" dow foam going over the Zip. I guess 2" is too much money.

Money, budgets drive us all to make stupid decisions and choices.

If I were to build PH I would try the liquid coatings mentioned. and not frame in December! There are two homes going up where I travel daily and both could care less that the Zip tape is peeling off.

Good luck trying to convince a homeowner that since the permit process had delays we should not start the project now on January 1st! Twice for me they just said push ahead. Nuts. I just signed a job for a civil engineer who easily saw the merit of going with a finish date not a start date of Jan 1. Thank you sir for your sensibilities.

Dec 12, 2014 10:02 AM ET

Without the fan. How many
by aj builder, Upstate NY Zone 6a

Without the fan.

How many homes have blower doors in them while they are occupied? NONE.

My question is... what would have been the added fuel cost or whatever if this home in Scranton had been finished and they gave up on the PH plaque for the wall?

$3,000 saved right away.
Toss the PH Pro off the project and end payments.
A couple weeks of fidgeting and fretting and paying carpenters to do the same. $$$$

I bet the fuel cost wouldn't go up $10/yr. and

Dec 12, 2014 10:07 AM ET

Edited Dec 12, 2014 10:08 AM ET.

Martin, get a hold of the
by aj builder, Upstate NY Zone 6a

Martin, get a hold of the testing experts and ask them how leaky the crappy OSB is if it is not under 50 pascal pressure more like a normal everyday pressure like the pressure of the stack effect?

stack effect.JPG stack effect formula.JPG

Dec 12, 2014 10:11 AM ET

Martin this is a serious
by aj builder, Upstate NY Zone 6a

Martin this is a serious request. What pascal number is normal average for a home to be put at over the course of a year averaged?

A PGH home with 1.2 number like these over the top boys got at the test would be heaven for this farmer taught gent.

Dec 12, 2014 10:43 AM ET

4 PA verses 100,000 PA
by aj builder, Upstate NY Zone 6a

natural air pressure difference is 4 PA

14psi is 100,000 PA

Can't believe 4/100,000 is something shoving air molecules through crappy OSB.

Dec 12, 2014 10:46 AM ET

50 PA is used to obviate the
by aj builder, Upstate NY Zone 6a

50 PA is used to obviate the effects of stack and wind.

It also must make OSB do something it most likely does not do at 4 PA which is said to be the natural pressure difference.

Somebody tell me I'm crazy or they are.

Dec 12, 2014 10:56 AM ET

Response to AJ Builder
by Martin Holladay

The questions you are asking are the same questions that I raised in the last three paragraphs of my article.

I look forward to seeing the results of air leakage testing on North American brands of OSB. At this time, the data do not exist.

You're correct that it would be interesting to compare leakage rates at 50 Pa to leakage rates a lower pressures -- perhaps 25 Pa, or 10 Pa, or 4 Pa, as you suggest. We need labs and manufacturers to perform the tests so that these questions can be answered.

In the meantime, anyone who builds a Passivhaus has to meet the 0.6 ach50 requirement. Meeting this target is not optional. For these builders, the performance of OSB at 50 Pa of pressurization and 50 Pa of depressurization is absolutely relevant.

Dec 12, 2014 11:16 AM ET

I'd look at the glues
by Antonio Oliver

Having done some work on the micro adhesion properties of soft polymers some time ago, I'd take a close look at the glues used to make this material. Even the same generic polymer can have quite different properties depending on how it was processed and its temperature history. It wouldn't surprise me to find that the polymerization of glues used by a particular brand is considered a trade secret though. The next logical thing to think about along these same lines is how well the OSB glue performs after 5 to 10 years of winter/summer cycles--especially in cold climates. I'm guessing manufacturers were more concerned about strength properties than airtight-ness when deciding what to test.

Dec 12, 2014 12:28 PM ET

Antonio in the real world this debate is about nothing.
by aj builder, Upstate NY Zone 6a

Say you build a PH, then the glue performs less so in a decade so now your natural ACH is what? It wouldn't change enough to be measurable is what! Change in heating degree days due to solar flares does more to change an annual fuel cost by a $10 bill. Islamic Estate is changing your annual costs way more than your crappy OSB folks.

Get real. We in the USA live in a glass house. Martin knows much of this with his world travels and my two visits had me in tears upon seeing swimming pools in every back yard flying back into Miami.

If you want the PH plaque the simple answer is to enhance your sheathing, ZIp comes enhanced and there's liquid applied and all the rest.

It's not worth improving OSB for air leaks IMO. The aim of OSB production is low cost low cost low cost. That is the main objective given to the engineering group no matter what the marketing folks are saying as they are given the task to make an ugly duck smell like roses or something like that. LOL

Dec 12, 2014 1:53 PM ET

Edited Dec 12, 2014 1:57 PM ET.

50Pa vs. real world
by Nick Welch

Air pressure is air pressure... intuitively (and I know intuition can be wrong), it seems very unlikely that OSB somehow leaks at high pressure but does not leak at normal pressures. Sure, its leakage is low compared by any but Passive House standards, but this is all about PH testing. If you're worried about gaps and cracks, you might as well be worried about leaky OSB too.

I don't pretend to know enough to not possibly be taking this out of context, but I found an interesting statement in this paper:

"In addition, extrapolation of results from tests at high pressures to those typically experienced by a building envelope does not introduce a bias in infiltration predictions."

Dec 12, 2014 3:07 PM ET

I'd look at the glues by Antonio Oliver
by Richard Beyer

"Fiberboard and particleboard lumber are built-up from wood particles bonded together by an adhesive, the adhesive being selected according to the intended use of and the properties desired for the lumber. Often times, the adhesive is combined with other additives to impart additional properties to the lumber. Common additives are fire retardants, insect repellants, moisture resistants, fungus resistants and color dyes. In some fiberboard and particleboard lumber products, wood particles have been combined with other cellulosic materials, such as vegetable fibers, pulp and the like. A significant advantage of fiberboard and particleboard lumber products is that they have many of the properties of plywood, but can be made from lower grade wood species and waste from other wood product production,

< Resins used to manufacture OSB >

Commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins..

(P-MDI) Polymeric and (E-MDI) emulsion type Isocyanate Resins and Acetone (used to even out the resins during manufacture) are also used in High Performance OSB.

Dec 12, 2014 3:34 PM ET

Is it air-tight?
by Dan Kolbert

As Martin describes, "air barrier" is just a line that got drawn, with thought but still somewhat arbitrarily, not a bright line dividing one side and the other. If a material is good enough to get you to 1.0 but not 0.6, what does that mean? I don't know. Some material is always going to be your weak link no matter how tight you get it.

Dec 12, 2014 3:59 PM ET

Edited Dec 12, 2014 5:55 PM ET.

Perm of OSB?
by Nick T - 6A (MN)

Is this air leaking OSB different then the OSB of old? A push for less glue, due to weight? Glue costs? Or is it just a batch issue - kind of hit or miss - depending on humidity/glue/compression/dryng speed...etc?

Is it maybe intentional to increase the drying ability of the boards or walls?

I know that Perm and Air barriers are different - but have to imagine something riddled with holes will also have a higher vapor/moisture transfer ability. Isn't perm of sheathing a spec on lumber?

Dec 12, 2014 4:14 PM ET

Edited Dec 12, 2014 4:21 PM ET.

Response to Nick T
by Martin Holladay

Q. "Is this air leaking OSB different then the OSB of old?"

A. There are no data on this issue whatsoever, other than a couple of European studies. So the answer to your question is: we don't know.

Q. "Is it maybe intentional to increase the drying ability of the boards or walls?"

A. There is absolutely no evidence of that. Certainly none of the OSB product specialists at any of the manufacturers I contacted suggested that.

Q. "Isn't perm of sheathing a spec on lumber?"

A. While many laboratories have tested the vapor permeance of different brands of OSB, I don't think there are any regulations requiring a specific vapor permeance range.

Dec 12, 2014 4:48 PM ET

Edited Dec 12, 2014 4:48 PM ET.

I had posted a question
by Chris Barnes

I had posted a question about this subject here in the Q&A section about a year ago and the general wisdom at the time was that OSB was air tight (a search could probably find this question). At the time I had done some internet searches and found some forums were builders from outside of NA were finding that their OSB was leaky....

Anyways, my question is what evidence do we have that plywood is not leaky?

Dec 12, 2014 5:11 PM ET

Edited Dec 12, 2014 5:29 PM ET.

Response to Chris Barnes
by Martin Holladay

Q. "What evidence do we have that plywood is not leaky?"

A. The evidence comes from laboratory testing performed by Canadian researchers in 1998. My article includes a link to a report of their findings: Air Permeance of Building Materials.

The rate of air leakage through 8 mm plywood sheathing was found to be 0.0067 l/s-m2@75 Pa.

The rate of air leakage through 9.5 mm plywood sheathing was so low it was called "non-measurable."

Dec 12, 2014 5:53 PM ET

by Jin Kazama

AJ ur the man lately!
You seem to be "on fire" as of late!!

Cheap arse products are used on some PH projects because the "budget" was spent elsewhere.
No other reasons.

Plywood is my friend.
I like it alot.
It looks like actual wood, not a compressed puzzle of particles.

Never found numbers,
but as i stated before, i am quite sure that plywood has more wood and less glue to it
than OSB, thus it should be greener.

There are some very cheap painting products that could be used to coat plywoods right before being applied ( like coated in a shop ) that would make them fully water resistant.
I bet we could find some natural oil ,wax mix or something similar that woudl be green.

Something i'll try and look out for in the next years,
before spending hundreds on peel sticks and wraping buildings in bad for health adhesives
and plastics .

Dec 12, 2014 7:20 PM ET

by christian corson

The Langmans paper dates back to 2010. I have been having this conversation with The Euros' since about '08. The reality is that some OSB products are airtight and some are not. When dealing with PH levels of airtightness it can and does make a huge difference. Having designed, built and tested many homes using OSB as an air barrier ranging in airtightness from .95 (lame) to .18 ACH 50(beyond robust) I can attest to the real world in situ performance of OSB as an airtight later.We have also used fabrics and bituminous membranes as well as fluid applied. I have opinions on all of them. Not all fluid applied membranes are created equal and not all OSB is created equal. To label OSB as crap because of its expense is ridiculous. The cost of an item does not affect its performance.
When it come's to leaky OSB I am less inclined to look to the glues and more inclined to think of the 'strands' themselves. OSB manufactured in the north is fabricated from hardwood; predominantly Aspen, and in some cases I have heard of poplar,oak and and other hardwoods being thrown into the mix but I have never been able to verify ( or tried to hard for that matter).
In the south OSB is fabricated predominantly with SYP. Softwood. The same phenomenon occurs in the EU. For what it is worth stepping up to 5/8 and especially 3/4 closes the air tightness gap.
I have personally tested both Arbec ( Quebec and NB. Canada) and LP ( Holden Maine) OSB's and found them to both be airtight at 75-125 PA. The Langmans' study poses an important result:
1- not all OSB is equal
2- OSB ranges from airtight to not airtight
3- Nobody knows who the hell is what in the study, so none of us are wiser consumers (gotta love European govt. funded social graces)

Air tightness is a quality thing as well as the best bang for the buck to save energy, but the quality thing to me is much more important. The less the infiltration and exfiltration, the less vapor is traveling through the assembly. The more robust the assembly.

Thanks to Richard Pedranti for publishing his results with leaky OSB. Not only proving me wrong in a 5 year disagreement with some other plugged in dudes, but also for 'possibly'? edifying the notion that this is an issue of materials (Hardwood v. softwood) in the production process.

Who knows???????????

Dec 12, 2014 8:21 PM ET

Just use ICF
by Peter L

Simple solution: Just build with ICF (2.5" EPS x 6" concrete x 2.5" EPS) and that is about as airtight as you can get.

Dec 13, 2014 12:52 AM ET

by Jin Kazama

2.5" ICF forms are sooooo yesterday!
You gotta up to 4" +4 " if u wanna get some respect around these parts mate! :p

I do agree with all of my ICF walls ( my house ) that they tend not to leak much.
The window bucks area is another problem though and need to be detailed very carefully!

Dec 13, 2014 6:25 AM ET

Edited Dec 13, 2014 7:46 AM ET.

by Terry Lee

I didn't read the article, no need. Out of all the composites I have designed for decades OSB makes the least sense. If you have ever been part of the manufacturing processes the battle is always voids and delaminations. Take a porous material wood throw it in with some phenolic resign and catylist in hopes that is seals the wood without a ultra sound or x-ray test most manufactures do not perform results in any of number of permability to the consumer. The ASTM does not call out a void or delam size nor inspection criteria and corrective action. Having to find the them and fill them with injections is costly, OSB would not be the cheap. There is definitely a relationship between cost and quality.

One could take some wood chips and lime mixed in a mortar mixer and have a MUCH better product. In this case, the casting over framing would seal as it dried, and breath (voids are good). You run some test like the manufactures to determine a ratio, r-value, perm rating, easy! No siding, insulation, drywall, vapor protection, needed.

A sheet of phenol resin and catalyst with glass fibers would make more sense. If structural, woven glass fibers, or better graphite, embeded in resin....when this day arrives we will not need framing, siding. "Marine Board" plastic sheet does much better and handles moisture, you get at a local plastic manufacture without the wood chips. It cost a little more, you don't need siding.

The resin is the moisture and air barrier in this laminated assy, the wood is to cut cost of the resign an create voids that the resign needs to infiltrate and seal.

OSB-Glue-OSB, SIPS, same issues, junk!

The best ply's are "pre-pregs" Here a manufacture takes glass or graphite cloth and impregnates it with resign to reduce voids. You stack up plies to a desired thickness, the only voids are between plies if the manufacture does not get the temperature and pressure correct. If they don't then we get dry areas or resign starved. In that case, a disposition to fill the area, costly. The edge in another area of bleed out from the pressure plates used to put the assembly under pressure.

It won't be long as cost drops carbon fiber reinforced plastic (CFRP) sheeting pre-preg replaces junk OSB and is SIPS.

Lots of varaibles manufactures can screw up including Huberd and GP the big fish.

Taping seams won't last, not room temp cure adhesives with low lap tensile-shear of less than 100 psi, after so many thermal cycles they will leak. I have validated this time and time again. Take a blower door after 5-10-15 years see for yourself. If you are going to do this DO NOT design it, use a ZIP systems where Engineers have tested the type and content of phenol-cat ratio's that mate to their tapes adhesive type and properties. You could always ask for lap tensile and shear of any tape against the specific OSB you are attempting to design, and ask the tape manufacture for fatigue life testing against the OSB you are using. I doubt you find that, it is an unknown you chance. Noone should advise others to use something there is no data for or they do not understand completely!

ZIP is an improvement since if I remember right they came back with a co-cured glass ply in a second stage bond operation to add a WRB and seal any voids, however there is always the possibility of a delamination.

Dec 13, 2014 7:06 AM ET

Edited Dec 13, 2014 12:57 PM ET.

Response to Christian Corson (Comment #18)
by Martin Holladay

Thanks for your comments. Almost all of your observations are consistent with what I wrote.

I appreciate the fact that you have added two brands of OSB to the list of "probably OK" brands: Arbec OSB from Quebec or New Brunswick, and LP OSB from Houlton, Maine. Thanks.

I'd like to point out that one of your statements -- "stepping up to 5/8 and especially 3/4 closes the airtightness gap" -- is an unproven hunch, especially in light of the fact that all of the samples of OSB tested by the Belgian researchers were 3/4 inch thick.

For what it's worth (if I am reading the OSB grade stamp correctly), the OSB on the house in Scranton, Pennyslvania was 418/1000 inch thick -- a little less than 7/16 inch.

Dec 13, 2014 7:16 AM ET

Edited Dec 13, 2014 7:19 AM ET.

Remember the topic of this article
by Martin Holladay

A note to readers: the topic of this article is air leakage through OSB. I'm grateful to Christian Corson (and several other posters) who submitted comments that were relevant to the topic.

A few readers (one of whom admits he hasn't read the article, and another of whom says he doesn't have to read the article) have posted off-topic comments. We've learned that OSB is cheap, for example, or that ICF walls are great, or that in the future we will all be sheathing our homes with carbon fiber panels.

All comments are welcome -- but I am especially interested in comments on the topic of air leakage through OSB.

Dec 13, 2014 7:49 AM ET

Edited Dec 13, 2014 8:01 AM ET.

Air leakage through OSB
by Terry Lee

Marin, read my post again. I discussed it in great detail. I don't need to read the article since I have 30 years experience manufacturing composites at companies like Georgia Pacific GP and Huber and larger. Last I checked direct experience speaks volumes.

If anyone has any question as to why exactly they leak you will find that in my last post along with tape properties that cause leakage. If you have never worked for the manufacture or have direct experience producing OSB you probably won't get it.

Also, feel free to call the engineers at Hurber and GP to validate the accuracy of my post, I have discussed this with them many times.

Dec 13, 2014 9:46 AM ET

by stephen sheehy

My contractor and I discussed using cdx sheathing, but his recent experience here in Maine is that cdx available locally more often than not is badly warped, has too many voids, etc. We opted for Advantech instead. Joints are taped with 3M 8067 tape. The tape sticks really well to cold, damp surfaces.

Dec 13, 2014 10:44 AM ET

Thanks Martin
by christian corson

Your comment : I'd like to point out that one of your statements -- "stepping up to 5/8 and especially 3/4 closes the airtightness gap" -- is an unproven hunch, especially in light of the fact that all of the samples of OSB tested by the Belgian researchers were 3/4 inch thick. - You are absolutely right, and I apologize for the context getting lost in brevity

This comment was made in regards to conversations about the potential for leaky OSB in the US market and whether a thicker specimen would make a difference. 7/16 being the standard thickness used in in the US as sheathing.Again I reiterate that my thought is that the difference is in the material fibers. The reality is that the thickness is mute point. If one were to step up to 3/4 in OSB per say than the cost delta to zip sheathing would make that particular product a more attractive option.

As far as I know RP's experience is the first documented case of leaky OSB that I know of in the US. This deficiency is well known in Europe and one reason why dedicated control layers are intrinsic to resiliency in buildings.

OSB was designed as a racking layer, It is sheathing. Not as an air barrier. The structural integrity of OSB is certified by TECO and the APA so to blatantly call it "crap" borders on trolling.

Martin, thanks for a great article and bringing this discussion forward. It is disheartening the see the quality of these articles sullied by some on the comments.

Huber ZIP and LP OSB are manufactured in Houlton NOT Holden Me. Typo on my part.

Dec 13, 2014 12:24 PM ET

Edited Dec 13, 2014 12:26 PM ET.

by Terry Lee

Thickness is not the issue, do not be fooled into thinking that there is a direct relationship to it and air-tightness, more is better. More thickness, more material, more temp, more pressure, more potential for voids. It is not a problem to design and produce air tightness in any thickness if the quality and manufacturing process is right.

Since the so called topic is "air tightness" I won't get into too far into "structural integrity" since I'm the one Martin will call out on going "off topic", hummm! ;) but let me tell you, OSB does not provide THAT much 'racking" resistance, or a better definition is resistance to rotational moments, or 2nd moments of inertia. Actually, OSB differs in it's ability to take out loads unilaterally since the strands and grains are multi-directional....this is nothing new, laying down multi-directional plys or, adding wood or glass fibers to resign systems has been around for decades and is the basis for the design. Plywood has better directional properties along the grain direction and is not as unidirectional, it is stronger, however, at resisting racking or bending, and/or shear plane loads at the multiple bond lines. The bond lines, depending on voids, are the structure and gives it "integrety" or strength. If there are voids or delams the structural properties and knocked down. Most failures occur or should occur at the bond lines. There are better choices than both.

I advise manufactures to use a oil additive to help with resign flow and saturation of the wood chips along with pre-preg soaking. Air tightness has is very dependent on bonding temps and pressures, FOD (foreign objects damage) along with the ratio of resign-to-catalyst-to-oil that can vary drastically between manufactures, not thickness. I believe advantec due to the glass second stage bond is air and vapor impermeable, and very strong......structurally it combines the best of both worlds, multi- and uni-directional strength.....again not new, that came from laying down fiberglass plies at different orientations (0, 45, 90, 180, 270) degrees and either room temp of elevated temp and pressure cures)....

Tapes will never see the mechanical and thermal air sealing or strength at room temp cures that high temp and pressure bonds have. As I said, it is a system stick with the same manufacture or look at properties if you are a designer.

Dec 13, 2014 12:57 PM ET

Edited Dec 13, 2014 1:00 PM ET.

Engineered Sheet Good
by Daniel Ernst


I worked in a high-end cabinet shop for a couple of years. During that time we upgraded our CNC equipment. With the older model we secured the machining substrate (MDF) and the hardwood parts with fasteners. We fastened the MDF to the CNC frame with screws, countersunk from the top; we then screwed the hardwood parts to the MDF with screws, from the back. It was a cumbersome process.

The newer model used a vacuum manifold to hold the MDF substrate in place. To our amazement, the vacuum also held the hardwood parts in place--through the 3/4" MDF substrate! As the manufacturer described the phenomenon, the MDF was extremely porous. The tiny gaps between the wood particles acted as a filter, but did not significantly inhibit the level of vaccum achieved at the MDF surface.

I'm sure there are significant differences between MDF and OSB (and the rate of depressurization between a CNC vacuum and a blower door test). However, there are a lot of similarities. Both are engineered sheet goods. They are both manufactured with wood strands or wood flours, resins, heat, and pressure. They are both porous products--look at these products closely and you will see the gaps between the particles. And they are both produced in various densities.

I recenty had a lengthy discussion with a Huber sales representative. He clearly described the differences between their product line-up (AdvanTech flooring, AdvanTech sheathing, and ZIP sheathing) and other OSB manufacturers. He stated that the strand size, strand orientation, resin type, resin content, and overall board density all affected product performance. He emphasized that they used a more costly and difficult-to-use resin to manufacturer their boards: MDI, methylene diphenyl di-isocyanate. He said that the resin was very important in determining their OSB quality.

Interestingly, their AdvanTech products are tested and rated according to ESR-1785, which qualifies their product as having greater stiffness and nail holding power. Handling a sheet will reveal the density difference between various OSB products. And their ZIP sheathing is tested according to ASTM E 2178 (<0.02 L/s.m2). I believe they are the only OSB manufacturer that has actually tested and rated their product as an air barrier. Whether this product parameter is a result of the sheathing overlay or the actual OSB board . . .who knows?

This is not a plug for Huber. We are using GP's Plytanium plywood for our sheathing layer and air barrier. Plywood has its own faults, which are well documented, but we still lean in that direction.

As a final thought, perhaps a call to Huber's technical department might shed some light on Chris Corson's theory about hardwood vs. softwood strands. They have a plant in Easton, ME, but most of their manufacturing takes place in the south (presumably with SYP strands). Personally I think the gaps between strands--and how they communicate with each other--are more important. In any case, they are probably best positioned to address the issue of OSB leakage since they have studied the issue enough to figure out what it takes to make a board capable of meeting the ASTM standard.

Dec 13, 2014 1:13 PM ET

Response to Daniel Ernst
by Martin Holladay

Thanks very much for your useful comments. The anecdote about the air permeance of MDF is quite interesting.

Advantech seems to be an excellent air barrier; if you look closely at the photos of the OSB on the College of the Atlantic in Maine -- the project where Marc Rosenbaum first used taped OSB sheathing as an air barrier -- you can see that Advantech was used on that project. That probably accounts for the project's success.

I've never heard of any air barrier problems attributable to Huber products (Advantech or Zip).

Dec 13, 2014 2:01 PM ET

by christian corson

For non specific lateral forces on frame, OSB gets the job done. If you want to use CF more power to you. I appreciate your comments, and to the greater extent agree with most of them, but we are talking about building building's here. When GBA changes its name to Green Aerospace Advisor everything you just said will become much more relevant.
Just Sayin. ( I dont use emoticons but if I did, I would insert a winky,smiley here.)

Dec 13, 2014 2:28 PM ET

Edited Dec 13, 2014 2:29 PM ET.

by Terry Lee

I'm laughing! I do tend to over think, can't help my background. I sit back and look at the titles now, no need to read what I have been a part of solving for the past decades.

"Green Aerospace Advisory", did you know they are green when they come off the line prior to paint? I was thinking more along the line, Green Foam Advisors....since it is the solution to most problems out here ;)

OSB-FOAM-OSB ....Now we talking GBA talk ;) SIP it up, or ZIP it up, your call ;)

Dec 13, 2014 2:48 PM ET

Edited Dec 14, 2014 5:44 PM ET.

by christian corson

Daniel, Interesting stuff.

Thanks for the anecdote. For what it is worth, I have spoken at great lengths with engineering at Huber and pretty much every other company that make sheet goods. Not only about the wood products used in the northern v. southern mills but also about MDI resins, PUF's, changing permeance of different products relative to RH, saturation rates,structural properties, and the reality that they misguided consumers by implying on their website at one time that zip sheathing has a PERM rate of 16. ( they have since added an asterisk to the statement to specifically denote that the coating Green and red is 16 PERMS) I like Huber products. Advantec is simply a superior product to regular OSB, no doubt about it. It is also 3 times the cost and isocyanates are toxic. This has always been a clean material choice v. cost decision for us.

1 ATM = 14.7 psi = 103000 Pa. 50 pascals or KPa is not a lot of pressure. It simulates maybe 20 times exterior ambient conditions. It is really not a lot of pressure. The bottom of a swimming pool exerts more pressure. So does the wind.

That being said, in the 10+ PH that we have site built and the 4 that we have manufactured we have never had a blower door test come back higher that .44 ACH 50. That is also using the EIN 13829 testing standard which is less forgiving that the ASTM volumetric calcs. Some of these homes used OSB as the sole airtight layer, but most used a combination of OSB and fabrics such as proclima's Intello.

I have often wondered if we should just switch to zip sheathing or advantec but I have just not been able to justify the cost. I will continue to re-think.

This is an conversation that I have been having for a long time with a lot of different folks.

Dec 13, 2014 2:56 PM ET

by christian corson

Thanks for appreciating my sense of humor. For what it is worth, I am a cellulose guy. I know, I know, I could use aerogel, but then what we would have to talk about ?

Dec 13, 2014 3:16 PM ET

Killing two birds
by Andrew Michler

This is such a great conversation as using your sheathing layer as an air barrier is one of the few places we can kill two birds with one stone in our wall assemblies. I can only add some anecdotal information. We have had two houses sheathed and taped with OSB (LP 15/32 I believe) and one came in at .35 ACH 50p and the other .25 ACH 50p, and one house using CDX which came in at .45 ACH 50p.

At these levels of airtightness its not possible to isolate a single product but bodes well in my book for taping a stock sheathing material. In Europe they have 4 grades of OSB, and this issue came up in conversation I had with Dr. Benjamin Kirck at PHI in Darmstadt. They have definitely seen a big difference in the air permeability between the grades. Since we are not using graded OSB it is more about learning from experienced folks what is working and what is not. Resorting to a ZIP wall, or heaven forbid ICF seems like a big leap in cost that may not be necessary. It also shows how important doing an air pressure test at sheathing stage is. I think that if there is doubt then use a four ply CDX and hope the knots don't line up!

Dec 13, 2014 3:57 PM ET

Edited Dec 13, 2014 5:58 PM ET.

ZIP and Engineering
by Terry Lee

ZIP has not passed the test of time yet, we don't know what it's life cycle cost is. We do not see life cycle test on the data sheets. The tape will more than likley be the first failure in time, when depends on the temperature and pressure cycles. It's not if, it's when. Concrete in much stronger, especially in winds greater than 90 mph, or better hail, 150-250 mph, and large temp swings, acids, salts, rot, decay, fungi.

Call an engineering dept have fun, lol! You'll more than likely get a college kid since the seniors are doing real work besides answering every joe blows questions. Ask a "sales rep" oh my, I'll leave that one alone....The new term is "sales engineeeeer' same ol, same ol, business degrees all they know is what they think they heard some Engineer say in a staff meeting or a manager that has no idea. Take ten different senior engineers, do not be surprised if you get ten different answers.....your best bet is to be an Engineer that knows what questions to ask and can tell how knowledgeable the person is you are talking to. Or better yet look at test data or lack of it (unknowns) on the site.

A blower door test is a snap shot in time usually taken during or after new construction, not to be confused with life cycle. If you need several, you may want to look for a better lower cost solution that is clearer and reliable over long periods of time.

Dec 13, 2014 5:49 PM ET

Edited Dec 13, 2014 6:40 PM ET.

Vacuum Pressure
by Terry Lee

' I love big tools, but my sense is that there is a big a%# compressor lurking behind that CNC hold down table, and I like it. '

Christian, it's not a compressor, it is a vacuum pump that pulls around an 1-3 inches of mercury to suck down plys. OSB is pressurized by caul plates, or pneumatic, hydraulic, mandrels. Autoclave cure temps around 200-350 F.

Porous materials under sealed vacuum pressure have no effect on the required pressure, that is a well known fact anyone that has done this before would know.

You have made some bad statements in your post that Martin seems to value because he obviously has no experience with it, I know are wrong. I'd pull out more but no need, I know.

BTW: Most industries follow and market aircraft level technology which in most cases in sales hype BS! This industry is far behind it, but will take on alot of it's technology as it already has, I pointed out, to include graphite composites...stupid not to when the properties far exceed wood, concrete, e-s glass, hybrids.

Dec 13, 2014 6:45 PM ET

This is a porous conversation ;-)
by Daniel Ernst


It sounds like you have studied the issue at great depth. The decision to use one sheathing over another is largely dictated by cost. Most of the industry uses commodity OSB because it has the lowest price point. Regardless of the manufacturer, if it has the stamp, then it meets the APA standards for sheathing. Like you said, they're not making it or advertising it as an air barrier.

Builders of high-performance, energy-efficient, or "green" houses evaluate the products on very different levels. That's a small, but growing percentage.

FWIW - Currently I can purchase a high quality CDX plywood for less than ZIP sheathing. Whatever helps you sleep at night, huh?

It's funny that you mentioned the vapor permeance of the ZIP product. The Huber rep. quoted that figure to me also; I was quick to challenge the number. He clarified by stating that the overlay had a rating of 12-16 perms, not the sheet.

Huber also likes to demonstrate the low wicking potential of the AdvanTech product (you've probably seen AdvanTech vs. plywood soaking in the red dye). I told the rep. that one of the reasons I choose plywood is the very fact that it does wick so well. He looked at me like I was crazy.

There are several OOMs difference between the pressure of a CNC vacuum and a blower door (and yes, that big a%# pump is quite awesome!). That said, I guess I was trying to make the point that this is not a question of if, but how much. All of these engineered sheet goods are porous and air permeable. Whether or not a particular brand of OSB is porous enough to negatively affect a blower door result . . . that's the million dollar question. It sounds like most PH builders are winning that lottery--and unfortunate that Pedranti lost.

Perhaps the attention that Martin is bringing to the conversation will lead to improved industry standards. Me, I'm still waiting for a company to start producing a plywood version of Huber's ZIP sheathing . . .

Dec 13, 2014 6:54 PM ET

"Duclos method of airtightness?"
by Daniel Ernst

From the pictures it appears that Pedranti sheathed the entire structure before conducting the first blower door test (what Adam Cohen calls the "Duclos method"). Do you know if this was the case?

Dec 13, 2014 7:25 PM ET

What makes me
by Keith Gustafson

What makes me think.............

If there is that much variance in airtightness, what is the variance in strength?

I mean, air barrier is a secondary consideration, OSB is supposed to be a structural material, if the manufacturing varies that much......

Dec 13, 2014 8:46 PM ET

by Peter L


You are being unnecessarily harsh on SIPs. The SIPs that were made back in the 1970's are still standing and still doing OK. They were OSB-EPS-OSB. Millions of homes are sheathed with OSB and they are still standing.

I think people are making a mountain out of a molehill on this OSB issue.

Dec 13, 2014 9:11 PM ET

by Terry Lee

Peter L, I agree SIPs has some great benefits I will expand on in great detail after I see the answers to some of the questions above, can't wait for Martin and perhaps AJ builder follower to spread some light here.....oh my, LMAO! :) I'm too afraid to go, well, you know, "off topic" although AJ builder does on just about ever thread I see him on which is most, go figure! :)

Love it! :)

Dec 14, 2014 6:43 AM ET

Edited Dec 15, 2014 7:30 AM ET.

Response to Terry Lee
by Martin Holladay

While your comments frequently note that your knowledge exceeds that of everyone else who writes for GBA, and that most building materials are much worse than the materials used to build aircraft, I'm sorry to say that these observations aren't very relevant to the day-to-day problems faced by residential builders.

It is perfectly possible to build a house with poured concrete walls and a carbon-fiber roof -- as well as a $50,000 engineering budget --- and you are free to do so. Such a house is likely to last a very long time. That said, many builders need to advise their clients on whether to sheathe their walls with OSB, plywood, or Zip sheathing. This article attempts to provide information that will help such builders understand the issues behind that choice.

Dec 14, 2014 6:50 AM ET

Response to Daniel Ernst
by Martin Holladay

Yes, Pedranti was following the Duclos method. In his e-mail to me about his approach to air sealing and blower-door testing, he noted that he was following the three-test Duclos method.

Dec 14, 2014 10:44 AM ET

Edited Dec 14, 2014 10:49 AM ET.

PH builders
by aj builder, Upstate NY Zone 6a

PH builders, I would think all of you would purchase your own blower doors and learn how to use them. Do you all do this? And now that it seems you need to use the lowest cost sheathing as a true air barrier then next on the list would be to build a simple sheathing tester. Build your box, and test a sheet from your supplier.

You build expensive high tech homes. To test your sheathing seems like the simple next step.

My earlier posts referred to OSB as cheap, where I come from any time the lowest cost item is preferred or purchased we call that the cheapest and call each other cheapskates. For example, go buy the lowest cost cell phone or a high cost cell phone. I bet most of you do not buy the lowest cost cell phone if you are a PH home builder. I make sense to me, you do not make sense to me, but there is room on the planet for both of us.

Buy the lowest cost anything and good for you, you saved. You get what you pay for most of the time folks.

Do some extra work, build a test box on a Saturday and you can rest easy that the low cost OSB you need is going to do the job. You don't need the industry to do a million dollar study, do a $100 dollar project for fun too in half a day. Even Terry Lee could do this for $100 and no team of Boeing engineers.

[Inappropriate comments deleted by GBA editor.]

Martin, why do we capitalize days of the week? Are they that important? sunday Sunday... sunday is fine for me... like aj... what am I a king? King Aj? Terry Lee, what do you think? terry lee or King Terry Lee?

Dec 14, 2014 12:16 PM ET

Edited Dec 14, 2014 12:19 PM ET.

Very surprising issue
by albert rooks

Hi all,

To date I had only heard rumors of this issue. It's interesting to read about a well documented occurrence. For my part, this does not seem to have come up in the West, or at least the Pacific Northwest. It's true that many builders have moved to Huber Zip, but that aside, there have been many projects since 2010 that performed well below 0.60 ACH 50 using OSB (Including Dan Whitmores 1st project).

Artisan group has reported that the last 2 project results at 0.06 & 0.075 ACH 50. This is a factor of 10x past the PH requirement. The typical build is with OSB. I'll check to see if that is still the case.

People occasionally express concern about plywood since when taping it you can miss lateral voids at the top veneer layer. One way to deal with this is (if you like to tape) is to roll on a primer. It will fill any lateral void, nail tear out etc... Another good solution is to use Prosoco Joint and Seam Fill (Note that J&S should always be top-coated with Fast Flash in exterior flashing applications).

I like Chris Corsons Hardwood/Softwood theory. Out here in the west we are mostly softwood in OSB. However an additional improvement to lowering the air permeance of a composite is to add "fillers" into the adhesive. While softwoods may compress more readily and "fill gaps", it's not hard to add a "filler" to the adhesive at the production line. The filler would also help keep water out of the "stands" making up the OSB by closing some gaps for moisture to travel. While I don't know this to be the case, it does make sense in a "soggy" climate.

Dec 14, 2014 12:58 PM ET

Edited Dec 14, 2014 4:28 PM ET.

Albert if ACH approaches zero
by aj builder, Upstate NY Zone 6a

Albert if ACH approaches zero and the HRV malfunctions ...? That's not good right? Are there alarm systems employed? CO2 monitors well placed? VOC concerns?

Sounds like too low an ACH could and possibly should be corrected with passive air inlets or?

Albert, no way does a sealed box = a 1970s home. How the heck are you in the PH business and say that? One is ACH 10-20 and one is ACH 0. One has whole house natural air changes in minutes or hours and one has no air change until a door is opened and left open? And I am talking doors closed, for example sleeping. Are the owners to wake up, feel like they are drowsy and then open doors till they figure out why? IF the HRV stops functioning a 0 ACH home I would think is in trouble sooner than later if well occupied and small.

I'm not talking about imminent death but am talking rather super high C02.

Dec 14, 2014 1:12 PM ET

only slightly off topic
by Bill Rose

Well, we don’t know as much as we’d like to about OSB. Which reminds me of a story from a decade or so ago…
The Fire Service Institute at the University of Illinois produced a video of a fire test of engineered lumber joists with OSB webs. In their setup, a weighted floor system with a fire below collapsed in 4 minutes with engineered joists, and lasted 30 minutes with dimension lumber. The Wood Products Association called a meeting. I was invited as a friendly outsider.
The wood people complained that the engineered joists were overly dry and the dimension lumber was overly wet—good points. The Fire Service Institute complained that the burn properties were unknown for the engineered wood. I was asked to weigh in, and mentioned that my field is moisture and we do not know the vapor permeance or the change in structural properties when wet. At which point a young participant from a wood products manufacturer chimed in saying “Oh, we don’t have any mold or water problems with our products, we provide a wax coating to protect the panels…” at which point all eyes turned to him, and one of the firefighters asked “You slather your products with WHICH petroleum distillate?”
Afterwards at the bar it turned out that all the firefighters build homes in their spare time and they all prefer using engineered joists to dimension lumber.

Dec 14, 2014 3:01 PM ET

Reply to aj builder comment 46
by albert rooks

aj, If it gets stuffy, occupants can open a door. There are studies that talk about the volume of O2 in homes and how long it can take to be dangerous. The process is so slow that you have time to react. It's not like burning charcoal where occupants are replacing O2 with CO and unaware of the quick change.

If the HRV fails then occupants are still safe, yet they now live in the 1970's. Not bad on a summer weekend.

Dec 14, 2014 3:05 PM ET

Bill that was on topic.
by albert rooks

Was the wax coating covered in the above posts? I did not see it but might have missed it.

Nice story Bill.

Dec 14, 2014 5:13 PM ET

Another anecdote...
by John Semmelhack understanding from Gary Nelson at The Energy Conservatory (air-tightness testing equipment manufacturer) is that their calibration chamber is made from.....

(drumroll please)...


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