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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

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 rate of drying exceeds the rate of 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.

Recent research shows that an important factor in moisture accumulation in sheathing on double-stud walls (in addition to the reduced heat flow through the wall assembly compared to walls with less insulation) is wintertime vapor diffusion through the assembly from the interior to the exterior. For more information on this factor, see The Return of the Vapor Diffusion Bogeyman.

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, DensGlass Gold, or diagonal board sheathing over OSB. One other possible (vapor-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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Nov 5, 2010 11:11 AM ET

by Doug McEvers


The state of MN did some test walls in various parts of the state, some with OSB. I asked about the results 10 months ago and have heard nothing. Maybe with your clout you could get the test results.

Nov 5, 2010 11:15 AM ET

Response to Doug
by Martin Holladay

Do you have any idea which agency, university, or researcher was involved?

Nov 5, 2010 1:07 PM ET

MN assembly studies
by J Chesnut

Doug and Martin,
There are researchers at the University of Minnesota who have conducted wall assembly studies both in hygrothermal modeling and physically at the Cloquet Residential Research Facility.
Two names I know are Louise F. Golderg Ph.D and Patrick H. Huelman. Patrick's affiliation at the UofM is the Cold Climate Housing Program in the Department of Bioproducts/Biosystems Engineering. Dr. Goldberg's program was fazed out I believe but I think she still is a researcher/ professor at the University but also heads a consulting firm.
Martin I'll email you a .pdf of a study that I have although it is not specifically about OSB. I would think the researchers of this study could help you track down the OSB study.

Nov 5, 2010 3:51 PM ET

Good topic, good advice
by Bill Rose

Hello Martin,

Interesting question. I found John Straube’s response to be quite good.

OBS is produced under conditions of high temperature and pressure. So I figure that, over time it will relax. I imagine a stack of sheets, stickered, and placed in a park pavilion protected against rain and sun. What would the stack look like after 5 years? Most people say it’ll be just fine. After 500 years? Most say it’ll go to mush. After 50? Most people go hmmm… There are wide differences in the product of course, so there is no single answer on the 50 year question.

I recommend someone conduct a survey of unheated outbuildings sheathed with OSB. Any OSB on an unheated outbuilding, in any climate, assuming no undue moisture loads from the interior, should paint a pretty good picture of how the sheathing will hold up on a well-insulated wall. And it may do well for many decades, I don’t know.

Exterior sheathing gets wet by virtue of getting cold. (You assign the “Cold-wet; warm-dry” quote to Joe Lstiburek. It’s my quote; I’d let Joe have the credit if he wanted it, but I’m not sure he wants it. He’d probably have come up with it long before I did if it accurately captured his thinking.) Your discussion of “condensation” leaves a lingering impression that wetness in cold sheathing materials comes from the inside, that is, from the high-vapor-pressure side (Northern bias throughout). Actually it comes from both sides, and predominantly from the side it’s closest to—the outside. There are ways to tweak both steady-state and transient analysis to answer the question of where the water comes from. We’ll have to leave that as an exercise.

The advice you give here is good. Of course.

Nov 5, 2010 4:04 PM ET

Credit where credit is due
by Martin Holladay

Thanks for your comments, which are always welcome. Your point that moisture absorbed by OSB during the winter is more likely to originate from the exterior than the interior is an important one.

I have corrected the attribution for the "Cold, wet; warm, dry" quote. Thanks for helping set the record straight.

Nov 6, 2010 12:24 AM ET

Would a 'breathable'
by Ray Smith

Would a 'breathable' sheathing solve the problem of moisture forming on the back of the sheathing? I was aware of a product used in UK and Ireland, Panelvent was the name. I do not know the permability but, reading up on the product, it seems that using this type of sheathing would possibly help. To be honest, I have no idea if this product or any similar is available here in Canada or US.
Similarly, would the breathing barrier called Membrain, by Certain Teed, used on the inside also help?

Nov 6, 2010 5:09 AM ET

Response to Ray Smith
by Martin Holladay

I'm not familiar with Panelvent, which is made from wood waste and wax. It is possible that Panelvent shares some of the same durability concerns as OSB.

In general, though, you are right: highly permeable sheathings dry faster than impermeable sheathings. That's one reason why diagonal board sheathing -- and to a lesser extent, plywood -- are preferable to OSB.

Although there's nothing wrong with MemBrain, its use on the interior of a wall won't solve the "cold sheathing" problem.

Nov 6, 2010 9:50 AM ET

air barriers
by Joe W

Just curious about the schematic at the top of the article and air barriers.

I'm assuming the sheetrock is intended as ADA.

Is there no exterior air barrier? Or is the plywood taped? What are the two layers of fabric between the sheathing and the rainscreen furring -- housewrap and ....?

Joe W.

Nov 6, 2010 12:28 PM ET

by A non mouse

How long will Advantech sheathings last?

How much formeldihyde is in Advantech?

Why Sheath? use a diagonal brace.

This post points to a Bruce Brownell house being superior to 99% of homes built since Bruce started his Alternative Energy Home buildinng business.

a non mouse & god

Nov 6, 2010 1:40 PM ET

Response to Joe W
by Martin Holladay

Joe W,
Every house needs an air barrier. It is possible to establish the air barrier at the interior drywall, or at the exterior sheathing, or (perhaps) using housewrap. For more information on air barrier options, see, One Air Barrier or Two?

I'm not sure the intention of the artist who drew the detail on this page, but many builders install two layers of WRB -- for example, asphalt felt over housewrap. The idea of two layers of housewrap is to provide more protection against wind-driven rain.

Nov 6, 2010 4:34 PM ET

Drying Out
by Bryan

Thanks. Great article. Here's some more convincing evidence that if you want sheathing that is permeable and will allow your walls to dry out, plywood is definitely better than OSB:

Nov 6, 2010 10:47 PM ET

Brett.... ?
by A non mouse

OSB is perfect for short term use... 20+ year homes that are to be ripped down after that... it's junk imhgo go without wood sheathing it's not needed.

Nov 7, 2010 5:52 PM ET

Ventilation requirements
by Doug

Martin, thanks for the article, this could end up being an important issue.
Could you get John's opinion on how much drain space & ventilation is enough?
Is using a spacer material like Cedar Breather with a 1/4" gap helpful, or not that helpful?
Is it important to create openings at top and bottom of the drain cavity for air to flow through, or is just the gap good enough?
Any suggestions on how to ventilate 20-25' vertical spaces behind masonry veneer or stucco, if it's important to do so?
Thanks for the article, this could end up being an important issue.

Nov 8, 2010 6:34 AM ET

Response to Doug
by Martin Holladay

Q. "Could you get John's opinion on how much drain space & ventilation is enough?"

A. Most rainscreen gaps are between 1/4 in and 3/4 in. Any gap in that range will provide very significant benefits. You'll probably get slightly faster drying with the larger gaps, but I don't want to overemphasize the performance differences between 1/4 in. and 3/4 in. You might want to read an article by Jonh Straube's colleague, Joseph Lstiburek: "Mind the Gap."

Q. "Is using a spacer material like Cedar Breather with a 1/4" gap helpful, or not that helpful?"

A. It's definitely helpful. Cedar Breather counts as a rainscreen gap in my book.

Q. "Is it important to create openings at top and bottom of the drain cavity for air to flow through, or is just the gap good enough?"

A. Ideally you have air inlets on the bottom and air outlets on the top. That encourages ventilation. If you forgot the air inlets and outlets, there are still many benefits to the rainscreen gap, but you won't have the same ventilation drying effect.

Q. "Any suggestions on how to ventilate 20-25' vertical spaces behind masonry veneer or stucco, if it's important to do so?"

A. Any installation of brick veneer is supposed to include an air gap between the bricks and the WRB. Weep holes at the bottom of the wall provide air inlets. As long as you don't block the air gap at the top, you've got a ventilated cavity.

The "Mind the Gap" article has a discussion of stucco details. One of my upcoming blogs will address air gaps behind stucco in more detail.

Nov 8, 2010 8:03 AM ET

barn style homes
by dsayer

would it be best to spray foam insulation right up against the metal or how would you suggest doing it? Or anyone else have a better suggestion of insulating 6" walls with drywall interior.

Nov 8, 2010 9:09 AM ET

Response to D. Sayer
by Martin Holladay

D. Sayer,
You didn't provide enough information for us to understand your questions. What metal are you talking about? Are you talking about galvanized steel panels used as siding?

Is this a retrofit job or new construction?

Please describe your planned wall assembly in detail. It's also a good idea to tell us your location or climate zone.

Most houses include wall sheathing. Does yours?

Nov 8, 2010 12:09 PM ET

OSB on outbuildings...
by John Semmelhack

Bill Rose wrote:

I recommend someone conduct a survey of unheated outbuildings sheathed with OSB. Any OSB on an unheated outbuilding, in any climate, assuming no undue moisture loads from the interior, should paint a pretty good picture of how the sheathing will hold up on a well-insulated wall.

Wouldn't OSB-sheathed roofs over vented attics provide similar conditions? Surely we have OSB sheathed roofs over vented attics in every climate zone in North America by this point.

Nov 8, 2010 12:59 PM ET

Response to John Semmelhack
by Bill Rose

Yes, OSB roof sheathing over vented attics makes a rather good test case. Roof sheathing goes through wider cycles of temperature than wall sheathing does. On a clear night, the sheathing is very cold--colder than any other part of the building. During the day it is hotter. For central Illinois, the net of these two effects is that roof sheathing is warmer than most other building parts, so it may be somewhat drier overall. The effect of the daily cycling may not penetrate too deeply into the OSB.

Still, it's a pretty good test case. I don't go into that many attics these days, but I don't see much swayback in roofs from the street, suggesting it's holding up pretty well. Other observations are welcome.

Nov 8, 2010 12:59 PM ET

Response to John Semmelhack
by Martin Holladay

You're right that OSB roof sheathing above an unconditioned attic can absorb moisture every winter. However, the OSB roof sheathing will dry much faster than OSB wall sheathing, for two reasons:
1. There is more warming from the sun, and
2. There is usually no insulation on the attic side to slow drying.

Nov 8, 2010 6:54 PM ET

OSB Roof Sheathing
by Doug McEvers

I can spot the butt joints on the OSB roofs when 3 tab shingles are used, this tells me the edges have taken on some moisture.

Nov 9, 2010 9:15 AM ET

cold osb
by David Lanfear

Excellent article! You considered and answered many questions for me. I'm surprised that no one has mentioned SIPs , however. They seem like an excellent example of what you're talking about. I know there have been some problems on SIPs roofs with air exfiltration condensing on the roof deck and rotting. Is this because the roof can't warm enough to overcome the moisture, or more likely that the condensed moisture is trapped? I realize that preventing warm moist air from getting to the deck is the first defense, however, if we're describing frost, it seems that you need to either make sure moisture can't get trapped or put a layer of rigid foam over the osb, this begins to get tedious.
OSB just seems so vulnerable over the long term, it hard to imagine it staying strong over years of repeated wetting/drying, I imagine those fibers just getting looser and looser. One writer mentioned Advantech, which seems much stronger and moisture resistant than standard OSB, but also seems like overkill at 3/4", Also would north walls be particularly vulnerable to this effect due the relative lack of drying potential?

Nov 9, 2010 9:28 AM ET

Response to David Lanfear
by Martin Holladay

The most significant cluster of SIP failures occurred in Juneau, Alaska. There are many resources on the Web describing these SIP failures, including:

The rot in Juneau began at the exterior OSB facing near the seams. There were two mechanisms involved:
1. In some houses, moisture in exfiltrating air condensed on asphalt felt. These houses had seams that were not airtight.

2. In other houses, convection loops through gaps and channels in the SIP seams brought warm humid air in contact with cold materials in the seam area, even when no air was exfiltrating.

I don't know why 3/4-in. sheathing is necessarily overkill. Here in New England, most houses were sheathed with 1-in. or 3/4-in. boards, at least until the late 1940s or early 1950s.

Nov 9, 2010 11:34 AM ET

Letting the moisture out
by Bryan

If plywood is a great improvement over OSB on a thick double stud wall, would
fiberboard be even a better choice?

Nov 9, 2010 11:36 AM ET

Response to Bryan
by Martin Holladay

Fiberboard can be a very good option. As you point out, it is very vapor-permeable, and it provides some racking resistance (bracing).

However, it doesn't hold a nail like plywood.

Nov 11, 2010 5:09 PM ET

OSB sheathing
by Anders Lewendal

Great discussion. Thank you. What do you think about a 2x6 stud wall with 1" closed cell foam on the outside as sheathing, taped, two inches of closed cell foam sprayed against the inside of the foam sheathing, 3 1/2" of cellulose, and either 5/8" sheetrock for sheer or 1/2" OSB sheer with 1/2" sheetrock on the inside of the stud wall? The idea is to keep the insulation on the outside with the dew point somewhere in the insulation and the sheer on the inside. My engineer is letting me experiment with this idea on a simple custom.

Nov 11, 2010 5:24 PM ET

Response to Anders Lewendal
by Martin Holladay

You don't mention your climate, but your suggested R-31 wall should work in just about any climate, so I think you are fine. (I'm not sure what type of foam sheathing you intend to use; all three types of foam sheathing -- EPS, XPS, and polyiso -- are closed-cell foams.)

Nov 11, 2010 9:34 PM ET

SIS instead of OSB
by Vic

In our most recent project we used the DOW Structural Insulated Sheathing product, taped at the seams, instead of OSB/Tyvek. We built with 2 x 6 exterior walls and used dense pack cellulose insulation. Our exterior consists of thin cut stone/efis up front and hardieplank around the rest of the house. Does anyone foresee any long-term moisture issues? If so, any remedies?

Nov 11, 2010 10:08 PM ET

SIP walls
by Roger Lin

So, does this mean a house build with SIPs is vulnerable to OSB failure? From the discussion, it seems that one solution is to put rigid foam on the outside of the SIPs to move the dew point. How thick does that layer need to be in the mid-atlantic area to be enough to prevent the OSB from turning to mush?

I asked a question on here a couple of weeks ago about adding rigid foam on the exterior of SIPs and the recommendations I received were to not to sandwich OSB between two layers of relatively impermeable foam, due to the slow drying potential of EPS.

So, which one is right?

Nov 12, 2010 6:35 AM ET

Response to Vic
by Martin Holladay

It's impossible to answer your question without knowing your climate. It's also important to know the thickness of the Dow SIS that you used.

Dow SIS comes in two thicknesses. The 1/2 inch panels are R-3, while the 1-inch panels are R-5.5.

To answer your question, you need to refer to the table I included in a different blog, "Calculating the Minimum Thickness of Rigid Foam Sheathing." From that table, you will see that R-3 foam sheathing is insufficient to keep you out of trouble with a 2x6 wall unless you are in a warm climate zone -- Climate Zone 1 through 4 (but not Marine 4).

If you have the thicker SIS, with an R-value of 5.5, your 2x6 wall will be OK in Marine Zone 4, but not in a colder climate.

Dow SIS is a two layer product consisting of polyisocyanurate foam bonded to a composite material made out of recycled cardboard. We don't have enough experience yet to determine how that fibrous layer holds up if it gets wet on a regular basis.

Nov 12, 2010 6:39 AM ET

Response to Roger Lin
by Martin Holladay

Q. "Does this mean a house build with SIPs is vulnerable to OSB failure?"

A. Some experts are worried about the long-term durability of OSB facings on SIPs, especially the exterior facing. (The interior facing stays warm and dry, and is therefore at less risk.) There have already been SIP failures in Juneau, Alaska.

If I were building a SIP home in a cold climate, I would certainly include a ventilated rainscreen gap between the SIP and the siding, to encourage rapid drying. And I would avoid any siding (like stucco) that dries slowly.

Nov 12, 2010 10:32 AM ET

RE: SIS instead of OSB
by Vic

Martin, thanks for the reply. We are in the mid-atlantic, DC area. I believe its the northern part of climate zone 4. We used the 1/2" R-3 SIS.

Nov 12, 2010 4:05 PM ET

Follow up question
by Roger Lin

Thanks Martin! Yes, I did hear about the Alaska failures. The other question I had was whether sandwiching OSB between two EPS foams is risky. Thanks

Nov 12, 2010 4:14 PM ET

Second response to Roger Lin
by Martin Holladay

That question is hard to answer.

If the exterior OSB is dry when it is covered with another layer of EPS, then adding the extra foam should keep the OSB warmer and dryer than it would otherwise have been. However, it also makes it hard for it to dry out in the future.

My guess is that adding another layer of EPS will make the OSB last longer than it would have otherwise. But the bottom line is, we don't really know how long OSB is going to last. So I guess my conclusion is, "We really don't have enough data to answer your question."

Nov 12, 2010 8:40 PM ET

Thanks Martin. I guess only
by Roger Lin

Thanks Martin. I guess only time can tell.

Nov 15, 2010 12:23 PM ET

Exterior foam situation
by Jeff Patrick

I've presently got foil faced fiberglass insulation in the stud wall and I want to add foam to the exterior. I'm in Zone 4, marine... the PNW. Will the foil facing cause me problems by inhibiting drying to the interior? Should I remove the foil? That would be a hassle, but doable.

Nov 15, 2010 12:44 PM ET

Response to Jeff Patrick
by Martin Holladay

My usual response to questions like yours is that you should keep your eyes peeled for any moisture problems when you remove your siding. Inspect your sheathing carefully. If you sheathing is dry and sound, with no signs of dampness or rot, proceed with your plan to add exterior foam.

If there are any signs of dampness or rot in your sheathing, though, it's time to put on the brakes. At that point you'll need to do a forensic investigation and determine the source of the moisture.

Nov 15, 2010 2:06 PM ET

Seal the OSB?
by Dan Russell

I'm not a builder but I'm interested in the subject. I have an unheated workshop above my garage with fiberglass bats on the north side of the roof and rigid foam on the south side of the roof. The bats are covered (rather poorly) with plastic. I'm not concerned about the south side because it gets really hot between the foam and the OSB thanks to the abundance of sunshine here in Colorado. In fact I have two fans that draw that heat out during the day and blow it into the shop area. At night in the winter the temps in the shop may be in the 40s and daytime temps in the 50s or 60s.

Here is my question: would it be worth it to remove the fiberglass bats on the north side of the roof and coat the OSB with marine varnish or some other type of sealer? Thanks!

Nov 15, 2010 2:24 PM ET

Response to Dan Russell
by Martin Holladay

The short answer to your question is No.

A couple of comments:

1. My blog is about walls, not roofs.

2. Are you worried about your OSB roof sheathing getting damp? There is probably not much reason for concern, since most garages and workshops don't generate a lot of moisture. (However, I don't know what activities you perform in your workshop.)

3. In general, the best way to address a concern about moisture buildup in roof sheathing in an insulated cathedral ceiling like the one you describe is to be sure there is a ventilated cavity between the top of the insulation and the bottom of the roof sheathing. You could also install an air barrier (gypsum wallboard) under your insulation. That will improve safety considerably compared to exposed polyethylene, which is a fire hazard.

Nov 17, 2010 1:44 AM ET

About the Insulation
by smalld

Have you ever considered rock-wool insulation. It has been used for many decades in Europe and more recently here in Canada with superior results. It is also a recycled content material from the waste of generations of industrial production - products such as Roxul are an example. I am not a rep simply a designer/builder/teacher who was fully aware of the product but had to wait some 18 years to finally have the product available in NA after researching it back in the 1980's.
In the Pacific Northwest we describe your furring space as a rainscreen and insist that it be used if we have any integrity at all.

Nov 17, 2010 6:23 AM ET

Response to Small D
by Martin Holladay

Small D,
I call it a rainscreen too.

Nov 26, 2010 7:38 PM ET

condensation in wall assemblies

I'm surprised John Straube didn't mention the report he wrote for CMHC.

Nov 27, 2010 6:53 AM ET

Response to EcoRalph
by Martin Holladay

Thanks for the link. The information in the CMHC report is consistent with advice promulgated on the GBA Web site. Some interesting points from the report:

"Regardless of the vapour permeance of the materials used in a wall assembly, control of air flow through the enclosure must be provided by an effective air barrier system. Inadequate control of airflow remains by far the most significant source of condensation for both above- and below-grade assemblies. ...

"The addition of insulating sheathing controls both cold weather and warm weather condensation without the use of plastic sheeting, but the R-value of the insulation must be sufficient to control winter-time condensation for the interior humidity expected. ...

"Warm weather condensation on plastic sheet vapour barriers can occur in air-conditioned buildings with a rain-wetted absorbent cladding exposed to the sun. However, this specific and persistent combination of factors is required before condensation occurs. ...

"The study shows that above-grade walls with either polyethylene or vapour barrier paint can be made to work in Canadian environments. It also shows that the performance of either can be compromised under severe environments or without proper attention to details."

Dec 19, 2010 4:04 PM ET

osb vs. plywood
by snyder

I'm very confused by the contractors differing opinions. I have purchased a balloon frame building in New Orleans that, because of years of water intrusion needs the siding replaced and structural components replaced on both sides of the structure. Plan is to systematically remove the wood siding, make the internal repairs, sheet with OSB, install Drainwrap Housewrap with flashing system around existing windows, and install vinyl siding. Any comments are welcomed. Snyder

Dec 19, 2010 4:54 PM ET

Response to Snyder
by Martin Holladay

You didn't ask a question; you just invited comments.

What differing opinions are you encountering?

Dec 27, 2010 2:31 PM ET

I used craft faced insulation
by Chris Wooster

I used craft faced insulation instead of unfaced insulation and a polyethelene vapor retarder. I am located in montana. I am now finding that the craft facing is only a class II vapor retarder and I feel like I should have used the poly sheeting as it is a class 1 vapor retarder. As well as the fact that evert time I had to cut the insulation to fit stud spaces around windows and doors I now have a spot that is unfaced. I have already sheetrocked and sided and I am now wishing I had used the poly sheeting. my questions are: would a vapor retarding paint seal the sheetrock enough to eliminate issues, would installing an air exchanger remove enough moisture from the air, or am I just being overly paranoid? This is a shop space, I do heat it and like to have it at about 60 degrees. the exterior temp is regularly between 20F and -20F between november and april. most builders around here use the poly, however I was told that the faced fiberglass was the same as unfaced and poly. Any insight is appreciated in advance.

Dec 27, 2010 3:58 PM ET

Response to Chris Wooster
by Martin Holladay

You have nothing to worry about. Vapor diffusion from the interior toward the exterior very rarely causes problems, and kraft facing is an adequate vapor retarder in any case.

It's always a good idea to seal any air leaks in your wall, especially around electrical boxes. But don't worry; your wall should be fine.

To learn more on this topic, see:
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!

Dec 28, 2010 11:09 AM ET

gypsum a good air barrier?
by Chris Wooster

Thank you for responding! . I have 1 more question for you: I was also under the impression that gypsum wallboard was not at all an effective air barrier. If you look at it under a microscope would it not look like swiss cheese? Thank you again!

Dec 28, 2010 11:16 AM ET

Second response to Chris Wooster
by Martin Holladay

Gypsum drywall is an excellent air barrier. To use drywall as part of an air barrier system, it is of course necessary to prevent air leaks at the edges of drywall areas and at electrical boxes. To learn more about the Airtight Drywall Approach, see Airtight Drywall.

If you want to know whether a material makes a good air barrier, hold a piece of the material up to your mouth and try to blow through it. If you can blow through it, it isn't an air barrier. Examples of good air barriers include gypsum drywall, poured concrete, glass, plywood, OSB, and peel-and-stick rubber membrane.

Dec 28, 2010 11:39 AM ET

Thanks again
by Chris Wooster

Thank you I now am satisfied. You are a great resource and I appreciate you taking time out of your day to respond to me!

Jan 19, 2011 11:48 AM ET

Edited Jan 19, 2011 11:56 AM ET.

Gap over sheathing
by Richard Clark

A gap to the exterior side of the sheathing has been recommended (to allow air movement, with rainscreen, to allow sheathing to dry), using 1/4"' to 3/4" vertical strapping.

My question is where should the housewrap go, over the strapping or on the sheathing?
Where is the air barrier if we introduce air with the rainscreen?

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