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Helpful? 1

How Risky Is Cold OSB Wall Sheathing?

The thicker your wall, the colder your sheathing. If you build a very thick wall, will your sheathing stay cold and wet?

Posted on Nov 5 2010 by Martin Holladay, GBA Advisor

During the winter months, wall 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. is usually cold. Cold sheathing is risky, since it tends to accumulate moisture during the winter. Unless the sheathing can dry out during the summer months, damp sheathing can rot.

Cold sheathing can get wet from two directions. It can get wet from the exterior, due to leaks through defective flashing or a poorly detailed 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). It can also get wet on the interior, due to a phenomenon traditionally called “condensation,” but more accurately called sorption. (As building scientist William Rose likes to say, “Capillary materials do not exhibit condensation at the dew point.”)

Most wood-framed walls are somewhat leaky. Interior air can leak into wall cavities through cracks around electrical boxes and cracks between the drywall and the wall’s bottom plate. When the warm air reaches the cold wall sheathing, one of two things usually happens: frost can form on the sheathing, or, at temperatures above freezing, the sheathing (which is hygroscopic or “sorptive”) can gain moisture from the air. (The source of the moisture taken on by sorption can be either interior or exterior moisture; for further details on moisture sources, see Bill Rose's posted comment below.)

Why doesn’t every cold-climate wall have rotten sheathing?

Most cold-climate homes have wall sheathing that gains moisture every winter. Usually, however, the wall sheathing doesn’t rot, because:

  • wood doesn’t rot when it’s cold, and
  • the sheathing dries out every summer.

Building components can survive occasional wetting, as long as the drying potential exceeds the wetting. If, on an annual basis, the wall dries more than it gets wet, it will probably be okay.

Probably — but not necessarily. In many areas of the U.S., OSB-sheathed walls have failed at an alarming rate. A combination of factors — poorly installed WRBs, air leaks through drywall, and the use of claddings (like stucco) that dry very slowly — have caused the OSB on thousands of homes to turn to oatmeal.

Even if the builder gets all the details right, there are still a few reasons to worry about new OSB-sheathed walls, especially if the wall is unusually thick. These days, builders are experimenting with thicker and thicker walls. In some parts of the U.S. and Canada, an increasing percentage of new homes have double 2x4 walls that are 12 inches thick — a design that makes the OSB sheathing colder than ever.

At least in theory, there are two reasons that thick walls are riskier than thin walls:

  • Thicker insulation makes the OSB slightly colder — admittedly, less than 2°F colder in most cases, but colder nevertheless — and therefore slightly wetter.
  • Thicker insulation means that less heat is flowing through the wall to help dry the OSB when it does get wet.

“By doubling the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the wall, we get half the energy available from the interior to drive the evaporation from wet wood,” says building scientist John Straube, a principal at the Building Science Corporation.

Lowering the risk factors

What factors make walls riskier?

Air leaks are bad. Leaks that allow interior air to enter a wall cavity are obviously risky — because these leaks allow moisture to piggyback on the exfiltrating air. So it’s important to create an airtight wall.

Air-permeable insulation is risky. Fiberglass batts do little to slow air movement. Switching from fiberglass batts to dense-packed cellulose raises the risk slightly (by keeping the sheathing colder); but more importantly on balance, it lowers the risk by reducing the chance of air movement through the insulation.

The thicker the wall, the colder the sheathing. The thicker the insulation on the interior side of the OSB, the colder the OSB. If you build a double 2x4 wall with a total thickness of 12 inches, you’ve made your sheathing colder than it used to be.

Sheathing temperature matters. Colder sheathing is at greater risk than warmer sheathing. (To quote Bill Rose again: “Cold, wet. Warm, dry.”) The more insulation you have on the exterior side of your sheathing — and the less insulation you have on the interior side of your sheathing — the warmer (and therefore dryer) your sheathing will stay. So an easy way to reduce the risk that your OSB sheathing will accumulate moisture is to install exterior rigid foam on top of the sheathing. That keeps the sheathing warm. However, the foam also reduces the ability of the sheathing to dry to the exterior, so it’s important to be sure that the foam is thick enough. Thick foam is better than thin foam (see "Calculating the Minimum Thickness of Rigid Foam Sheathing.")

Rainscreen gaps are good. If there’s a reason to believe that your sheathing is getting damp every winter, you want to be sure that it can dry quickly during the summer. One way to encourage faster drying: include a ventilated rainscreen gap between the sheathing and the siding. That reduces the risk of rot.

Avoid OSB. Finally, OSB is more susceptible to rot than plywood. So if you’re worried about the durability of your sheathing, choose plywood (or diagonal board sheathing) over OSB. One other possible (permeable) sheathing choice is structural fiberboard sheathing, which is available from International Bildrite and Georgia-Pacific.

Get your flashing details right. Although it should go without saying, remember to install proper flashing at all wall penetrations, windows, and doors, and remember to integrate your wall flashing with the WRB. Upper courses should be properly lapped over lower courses.

John Straube's advice

After I mulled the issues raised in this blog, I sent an e-mail to John Straube, asking him about the riskiness of using OSB on a very thick double-stud wall, and about whether my advice is on target. Straube’s answer follows.

“We don’t know the full significance of this question, but the basic physics of wood and humidity tells us that OSB sheathing on a thick wall is risky, and experience has shown us that it is risky,” Straube wrote. “By risky, I mean riskier than historical practice.

“Anything we can do to reduce risk is therefore good. A major improvement is adding a ventilated gap over the sheathing: this allows for much better control of rain moisture (biggest concern) and encourages drying of the sheathing (particularly useful for air leakage condensation moisture). Switching to plywood adds more safety by further increasing the ability of interior moisture to dry outward.

“Your advice is good. I would differ by letting people know that it is more important to ventilate than switching from OSB to plywood — but both together are powerful allies.

“The presence of cellulose (rather than fiberglass batts) in the cavity is very helpful as it reduces any air leaks, stops convection loops, and add moisture storage and mold resistance (via borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments.).

“So, when you add the three components: ventilated cladding, plywood, and dense-packed cellulose, you have reduced risk tremendously. Are you back to the same safety that we had in the past? I don’t know. I don’t think so — but you’re likely really close.

“All of this assumes you have done a good job on the 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. (tighter is better of course) and that rain control is managed (window subsill flashing, etc).”

Last week’s blog: “A Conversation With Wolfgang Feist.”


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52.
Wed, 01/19/2011 - 11:52

Response to Richard Clark
by Martin Holladay, GBA Advisor

Helpful? 0

Richard,
Q. "Where should the housewrap go, over the strapping or on the sheathing?"

A. Over the sheathing.

Q. "Where is the air barrier if we introduce air with the rainscreen?"

A. There are several possible locations for the air barrier. It can be at the sheathing, if the sheathing seams are taped. It can be within the wall cavities, if spray foam insulation is used. Or it can be at the drywall level, if the builder follows the Airtight Drywall Approach. For more information on air barriers, see Questions and Answers About Air Barriers.


53.
Wed, 01/19/2011 - 12:01

plywood, OSB, or structural fiberboard
by Rachel Wagner

Helpful? 0

In rereading this post and the comments, I find that no one addresses the option of structural fiberboard sheathing as an alternative to OSB or plywood. It isn't always easy to find, but last I checked at least two U.S. companies made it, International Biltrite, and Georgia-Pacific. It has a permeability much higher than plywood (I believe the perm rating is about 15). Builders I work with don't love the product (think "buffalo board") but it isn't lost on me that when we open up old walls and find buffalo board sheathing it is always dry and I live in a very cold climate with a short drying season. Comments, anyone? Thanks, as always. I find myself going to the "musings of an energy nerd" often, and the topics are current and well researched. Thank you Martin!


54.
Wed, 01/19/2011 - 12:38

Edited Wed, 01/19/2011 - 12:53.

Response to Rachel Wagner
by Martin Holladay, GBA Advisor

Helpful? 0

Rachel,
Thanks for your excellent suggestion. Structural fiberboard sheathing is a less expensive option than diagonal board sheathing. It is a popular sheathing choice for Passivhaus builders.

Here is the full contact information to manufacturers and Web links to the products you mentioned:

International Bildrite
101 Fourth Street East
International Falls, MN 56649
218-283-3900
Manufacturer of Bildrite fiberboard
http://www.bildrite.net/sheathing.htm

Georgia-Pacific
133 Peachtree St. NE
Atlanta, GA 30303
800-839-2588
Manufacturer of Stedi-R structural sheathing
http://www.gp.com/build/product.aspx?pid=6056

I have also edited my article to include your suggestion. Thanks.


55.
Thu, 01/20/2011 - 22:27

Errata
by Riversong

Helpful? 0

at temperatures above freezing, the sheathing (which is hygroscopic or “sorptive”) can gain moisture from the air.

Even below freezing, albeit slower. This is the key principle: hygroscopic materials change their moisture content according to the relative humidity on each side and the driving forces, including vapor pressure and heat flux.

Cold sheathing is risky, since it tends to accumulate moisture during the winter. Unless the sheathing can dry out during the summer months, damp sheathing can rot.

Cold sheathing is not risky, since neither mold nor decay organisms can grow in the cold. And, if sheathing has to wait until the warm summer months to begin to dry, then it's likely vulnerable to mold, if not decay, as soon as it warms up. Sheathing in a cold climate should be able to dry out continuously to the outside through permeable claddings.

The thicker the insulation on the interior side of the OSB, the colder the OSB.

If the sheathing is separated from the outside air by clapboards or some other thin cladding, its temperature will closely track the outdoor temperature regardless of how much insulation is behind it (unless there is little to none or its entirely air permeable and vulnerable to convective currents).

Switching from fiberglass batts to dense-packed cellulose raises the risk slightly (by keeping the sheathing colder); but more importantly on balance, it lowers the risk by reducing the chance of air movement through the insulation.

As Straube points out, dense-pack cellulose dramatically lowers the risk to sheathing, mostly because of it's high moisture storage capacity and its high moisture diffusivity, which quickly redistributes moisture to reduce local concentrations.

Straube: "The presence of cellulose (rather than fiberglass batts) in the cavity is very helpful as it reduces any air leaks, stops convection loops, and add moisture storage and mold resistance (via borate)."

And because today's cellulose contains a very effective fungicide which is non-toxic to humans and pets, and which also repels both the insects and rodents which tend to create air channels in insulation.

So an easy way to reduce the risk that your OSB sheathing will accumulate moisture is to install exterior rigid foam on top of the sheathing.

Since the primary wintertime drying potential comes from heat flux through the sheathing, adding impermeable exterior insulation reduces both the heat flux and the vapor flux in the dominant wintertime direction.

However, the foam also reduces the ability of the sheathing to dry to the exterior, so it’s important to be sure that the foam is thick enough. Thick foam is better than thin foam

Thick foam will further reduce outward (or inward) heat flux as well as vapor flow and thereby reduce drying potential in both winter and summer seasons.

One way to encourage fast drying: include a ventilated rainscreen gap between the sheathing and the siding. That reduces the risk of rot.

An exterior ventilated gap will aid in drying of the outer surface of the sheathing, which is more likely to be wetted by exterior rain or humidity, but it also brings the sheathing into more direct contact with cold, damp outside air (winter air in most of the US has higher RH than summer air). A vented gap will also reduce inward heat flux and summer drying potential.

Moisture flow and its consequences can no more be stopped than the US Army Corps of Engineers can dam up a river and prevent flooding. The dam (or exterior foam) may prevent routine seasonal flooding (or moisture damage) but is more than likely to eventually result in catastrophic flooding (or moisture damage).

Since we cannot fool Mother Nature nor push a river, we would be wise to design and build houses to cooperate with nature's forces. This means structural and thermal envelopes that are reasonably air-tight, insulated with dense hygrosocopic moisture-buffering material (like cellulose), and breatheable in both directions but most importantly to the outside in a cold climate. A good rule of thumb is to keep the outer skin at least five times as permeable as the inside skin.

Such a structure, built with moisture tolerant materials, like sawn lumber rather than either plywood or OSB, will be a durable and healthy structure (assuming proper flashing and rain shedding details). Then extra complexities, such as rainscreen gaps, are not necessary (except with reservoir claddings such as stucco or masonry).


56.
Tue, 03/15/2011 - 01:27

Is my wall ok ?
by Jim Wright

Helpful? 0

In the article, "Calculating the minimum thickness of rigid foam sheathing", one paragraph states,

"What if I live in one of the warmer climate zones?
If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem."

Would that "any foam thickness" also include no foam at all ?

I am at the point of needing to install my windows and then put on the vinyl siding, and I'm not sure if I can get by without using any foam over the sheathing (don't want to foam it if I don't really need to).

I am in zone 3 (mid-western Arkansas) and my walls currently are 2x6 with 1/2" Zip wall panels which are OSB coated with whatever that water repellent coating is that Huber puts on the panels; the panel seams are taped with the Zip tape. I didn't plan to use a housewrap because Huber says that the Zip panels after being taped make a sufficient air barrier. They also said that a layer of building felt over the sheathing was not necessary because of the coating on the panels.

Question 1:

Do the above two statements seem logical ?

I will be using Roxul 5.5" bats between the studs and 5/8" drywall on the inside with latex paint.

Question 2:

For my climate, can I get by with no foam without risking condensation on the inside of the sheathing ?

(The Roxul is supposed to hold and release moisture better than fiberglass, if that makes a difference.)

(The house will be heated with a wood stove, if that makes any difference either. I know wood stoves generally make for dryer inside air than central heat.)


57.
Tue, 03/15/2011 - 04:06

Edited Tue, 03/15/2011 - 04:36.

Response to Jim Wright
by Martin Holladay, GBA Advisor

Helpful? 0

Jim,
Q. "Would that [statement that] 'any foam thickness' [will be safe in a warm climate] also include no foam at all ?"

A. Yes. You don't have to worry about moisture accumulation in OSB during the winter in Arkansas. However, a wall without any foam sheathing will not perform as well as a wall with foam sheathing (even in Arkansas), because of thermal bridging through your studs. To put it simply, a 2x6 wall without foam sheathing isn't as well insulated as a 2x6 wall with foam sheathing.

Q. "Do the above two statements [about the Huber Zip system] seem logical ?"

A. Yes, the Huber Zip system creates an effective air barrier (although you still have to worry about air leakage at the bottom and top edges of your wall, as well as air leakage through your floor system and ceiling). And yes, Huber has demonstrated that you can skip the installation of housewrap if you want. (However, conservative builders worry about the longevity of the Zip system tape, and often install housewrap as cheap insurance, in case the tape gives out in 30 years.)

Q. "For my climate, can I get by with no foam without risking condensation on the inside of the sheathing?"

A. Yes.


58.
Thu, 02/16/2012 - 14:27

Will OSB dry to the outside?
by Doug Pulsifer

Helpful? 0

I am building in Nova Scotia and have sheathed my house with osb. The exterior has been covered with house wrap and sided with vinyl siding ( but not strapped ). Now I am in the process of installing my 6' fibreglass batts in the 2x6 walls which I will then cover with 1" high density foil faced foam as my vapour barrier under my gyproc. My problem is, because it's winter here now, the inside of my sheathing and wall cavities are getting wet from the condensation of all the moisture within which freezes once it touches the osb. I've has some heat on from a wood stove I've installed, but that seems to make it worse. My question/concern is; once I install the batts and cover with my foam and seal all the interior joints, will the osb sheathing dry to the outside atmosphere? The builders gapped some of the osb sheathing, but not all. Should I drill holes within the wall cavities to the outside to assist in drying? I'm not sure how I could do that from within without breaking my house wrap.


59.
Thu, 02/16/2012 - 14:41

Edited Thu, 02/14/2013 - 11:28.

Response to Doug Pulsifer
by Martin Holladay, GBA Advisor

Helpful? 0

Doug,
Before I address your questions, here is some advice to GBA readers: don't do what Doug did. Remember, rigid foam belongs on the outside of the wall, where it can help keep the OSB warm and dry, not on the inside of the wall.

Don't worry, Doug, everything will probably be OK.

Ideally, you will wait until spring to install your insulation, so that the OSB can warm up and the walls can dry out. If that's not possible, crank up your wood stove, and at least wait for a warm, sunny day to get the OSB as warm and dry was you can before you enclose it.

Once your walls are insulated, you need to pay meticulous attention to air sealing to be sure that more interior moisture doesn't migrate into your wall cavities. That means using airtight electrical boxes (your electrician installed those, I hope -- the ones with flanges) and sealing the drywall or the rigid foam at the top plates and bottom plates with gaskets or caulk.

More information on the Airtight Drywall Approach can be found here:
http://www.finehomebuilding.com/how-to/departments/energy-smart-details/...

If you follow this advice, your OSB will dry to the exterior next summer.


60.
Thu, 02/14/2013 - 11:09

Blown fiberglass
by michael scannell

Helpful? 0

In all these articles I've read, the talk is to change from Fiberglas to blown cellulose. I assume you mean fiberglass batts. We have been using blown fiberglass , BIB, blown in blanket . It won't settle like cellulose, but it doesn't have the sorptive factor either .
What are your thoughts on blown fiberglass vs blown cellulose?
i build in the high desert of Central Oregon. Zone five . High, dry and cold. The best , affordable solution I've developed from research for this cold climate construction is; Air Drywall application , air barrier at the dry wall to keep conditioned air out of the wall cavity. ACH 1-11/2,50Psc. Double wall 7 1/2" BIB fiberglass wall. Ply wood wall sheeting. And using a vented air gap with reservoir sidings. We dont get much rain here, 10"/annual, so my concern with outside moisture is from solar driven moisture. I think the Hardie lap will provide an air gap with the laps eliminating the need for a vented air gap. I think if I'm incorporating a well vented attic, I can get by with OSB roof sheeting. Any suggestions ?


61.
Thu, 02/14/2013 - 11:32

Edited Thu, 02/14/2013 - 11:33.

Response to Michael Scannell
by Martin Holladay, GBA Advisor

Helpful? 0

Michael,
Blown-in fiberglass isn't quite as good as dense-packed cellulose at reducing air leakage, and it is less able to provide a hygric buffer to lessen the impacts of moisture changes in the wall assembly.

In spite of those two disadvantages compared to cellulose, blown-in fiberglass can work well.

You asked, "Any suggestions?" That's a pretty wide-open question.

Your wall has an R-value of about R-27 or R-28. That's pretty good, but some builders are aiming higher. Other than that, your wall sounds fine.


62.
Fri, 02/22/2013 - 21:44

Insulation options overwhelming
by keith miller

Helpful? 0

Martin,
I have been a framing contractor for 19 years and now I am able to build my own home.
I am paying attention to all the options and ROI out there. I am almost to the point that the old ways are best, such as plywood not Zip or OSB and felt paper not synthetic for breath ability.
I am framing with traditional techniques because I like the strength of a good framed building, just not into the new sparce spaced framing right now.

My question to you is with insulation, I am putting in a geothermal system ,so am I crazy to just put in fiberglass batts in 2x6 walls and then spray foam the roof so to not run into any moisture issues. Also would I not need to vent the roof and everything under would be conditioned space?

I don't want the thick foam outside for the flashing and deep well details. Allowing the sheathing to dry out still seems best. Also flash and batt is risky because of human error.
Any opinion advice would be much appreciated
Thanks Keith


63.
Sat, 02/23/2013 - 06:12

Response to Keith Miller
by Martin Holladay, GBA Advisor

Helpful? 0

Keith,
1. If you install spray foam on the underside of your roof sheathing, it's clear that your roof assembly won't be ventilated. However, there are plenty of ways to build a ventilated roof if you prefer to have it ventilated. For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

2. You asked, "I am putting in a geothermal system, so am I crazy to just put in fiberglass batts in 2x6 walls?" The answer is probably, "Yes." You didn't mention where you live, but that sounds like a lousy wall. In most parts of the country, a ground-source heat pump system costs $18,000 to $30,000. Why spend so much money on your heating and cooling system if you aren't even willing to invest in a high-R wall? There are plenty of ways to build a high-R wall that will avoid moisture problems.


64.
Sat, 02/23/2013 - 15:29

Edited Sun, 02/24/2013 - 11:21.

Follow up to Martin
by keith miller

Helpful? 0

I am in zone 5, so I know the fiberglass batts are lousy, what is a good option without any moisture issues . With staying inside a 2x6 cavity without spray foam?

I am thinking only option is closed cell spray foam in the walls and roof, bite the bullet and get the cost savings over the long haul.
Or option 2 possibly the flash and batt with closed cell but after all I've read that seems risky with getting the right thickness
of foam

Your thoughts?
Thanks Keith


65.
Mon, 03/04/2013 - 17:43

Response to Keith Miller
by Martin Holladay, GBA Advisor

Helpful? 0

Keith,
Q. "What is a good option without any moisture issues, staying inside a 2x6 cavity, without spray foam?"

A. C'mon, Keith -- that's not fair. You're asking for a unicorn. Insulation between the studs doesn't make much sense. The best location for insulation is outside the studs, on the exterior side of the wall sheathing.


66.
Tue, 11/05/2013 - 22:27

reply to keith miller and osb thoughts
by john c

Helpful? 0

As an diy'er and rental owner I also have an interested in doing things right/better. I saw (TOH) newer ways to insulate walls which was to spray the inside wall cavities with a latex sealer (to address convection) and finish the bay with regular glass batts. The intent that it should be less costly than a full spray foam fill.
I took real interest in seeing a room addition method in a rather low income neighborhood being executed. The roofers painted the osb roof sheathing seams with asphalt lap cement before applying the tar paper and shingles. Amazingly, they were addressing the tendency for the cut (and non cut) edges prone to absorb moisture and it was a practice I never saw being done in the multi million dollar home nearby neighborhoods! So maybe just like asphalt lap cement is used to waterproof cmu block walls, maybe that or even gloss latex paint might be used to waterproof the exterior walls - yeah not very economical or practical though.
I am still amazed at the building products that don't stand the test of time: compressed wood siding, aluminum wiring, cpvc water supply, pex fittings that leak with hard water, vermiculite insulation, lead paint, clay sewer pipe that lets roots in at the rubber gasket or won't take water jetting, steel drain pipe rusts through at the thinner threaded sections, steel water pipe that constricts (also used in the earliest radiant floor systems), and here's one you probably didn't know: abs drain pipe that rats chew through. In a vacancy, sewer rats chewed through the "Y" conjunction to build a nest in the walls - a rare downside, but obviously wouldn't have happened in steel or cast iron systems.


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