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When Sunshine Drives Moisture Into Walls

Because of inward solar vapor drive, vapor diffusion from the outside inward is often more worrisome than vapor diffusion from the inside outward — so you need a good vapor barrier strategy

Posted on Jul 2 2010 by Martin Holladay

Builders have worried about wintertime vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, “Preventing Condensation in Insulated Structures,” included this advice: “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.”

Rogers’ recommendation, which was eventually incorporated into most model building codes, was established dogma for over 40 years. Eventually, though, building scientists discovered that interior vapor barriers were causing more problems than they were solving.

Interior vapor barriers are rarely necessary, since wintertime vapor diffusion rarely leads to problems in walls or ceilings. A different phenomenon — summertime vapor diffusion — turns out to be a far more serious matter.

Something is rotten in Denmark

During the 1990s, summertime vapor diffusion began to wreak havoc with hundreds of North American homes. This epidemic in rotting walls was brought on by two changes in building practice: The first was the widespread adoption of air conditioning, while the second was one unleashed by Rogers himself: the use of interior polyethylene vapor barriers.

Rogers conceived of interior vapor barriers as a defense against the diffusion of water vapor from the interior of a home into cold wall cavities. Rogers failed to foresee that these vapor barriers would eventually be cooled by air conditioning — thereby turning into condensing surfaces that began dripping water into walls during the summer.

Zaring Homes goes bankrupt

As with many scientific discoveries, it took a series of disasters to fully illuminate the phenomenon of summertime vapor diffusion.

One early victim of this type of diffusion was Cincinnati builder Zaring Homes. In the mid-1990s, Zaring Homes was a thriving mid-size builder that completed over 1,500 new homes a year. But the company’s expansion plans came to a screeching halt in 1999 when dozens of its new homes developed mold and extensive rot.

The first signs of the disaster surfaced in July 1999, when homeowners at Zaring’s Parkside development in Mason, Ohio, first began complaining of wet carpets. These moisture problems emerged only ten weeks after the first residents moved in to the new neighborhood. When inspection holes were cut into the drywall, workers discovered 1/4 inch of standing water in the bottom of the stud cavities. “We were able to wring water out of the fiberglass insulation,” said Stephen Vamosi, a consulting architect at Intertech Design in Cincinnati.

Consultants concluded that water vapor was being driven inward from the damp brick veneer through permeable fiberboard 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. (Celotex). During the summer months, when the homes at Parkside were all air conditioned, moisture was condensing on the back of the polyethylene sheeting installed behind the drywall.

“Zaring Homes went out of business because they had a $20 to $50 million liability,” said building scientist Joseph Lstiburek. “Hundreds of homes were potentially involved. To fix the problems would probably cost $60,000 to $70,000 per home. It was a spectacular failure, and they are out of business.” (For more on Listiburek's view of inward solar vapor drive, see Solar-Driven Moisture in Brick Veneer.)

Inward solar vapor drive problems require four elements

The phenomenon that destroyed Zaring’s walls came to be known as inward solar vapor drive. The classic disaster requires four elements:

  • A “reservoir” claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. — that is, siding that can hold significant amounts of water;
  • Permeable wall sheathing like Celotex or Homosote (that is, fiberboard);
  • A polyethylene vapor barrier on the interior of the wall; and
  • An air-conditioned interior.

Reservoir claddings include brick veneer, stucco, manufactured stone, fiber-cement siding, and (to a lesser extent) wood siding. Although wall failures with permeable sidings like Celotex are particularly spectacular, inward solar vapor drive is also a factor in the failure of walls sheathed with less permeable types of sheathing, especially OSB.

Problems with inward solar vapor drive show up first on elevations that get the most sun exposure; north walls are usually immune to the problem.

Whenever a wall separates environments at different temperatures and moisture conditions, the direction of the vapor drive is from the hot, moist side toward the cool, dry side. After a soaking rainstorm, the sun eventually comes out to bake the damp siding. When it comes to driving vapor, the sun is a powerful motor.

The heat of the sun easily drives the moisture in damp siding through housewrap and permeable wall sheathing. The first cold surface that the vapor encounters is usually the polyethylene behind the drywall. That’s where the moisture condenses; it runs down the poly and pools at the bottom of the wall cavity. It doesn’t take long before mold begins to grow and the walls begin to rot.

Once the phenomenon of inward solar vapor drive was well understood, it was identified as one of the main mechanisms causing a cluster of wall-rot problems in EIFS-clad homes in North Carolina. Inward solar vapor drive is also blamed for many of the “leaky condo” problems in stucco-clad multifamily buildings in Vancouver, British Columbia.

Moisture and temperature probes confirm the phenomenon

Data from a 2003-2004 wall-drying study by building scientists John Straube, Eric Burnett, and Randy Van Straaten confirmed the phenomenon of inward solar vapor drive.

“Inward vapor drive resdistributes moisture quite dramatically,” said Straube. “Some people have said, ‘Summer condensation on the interior does not occur.’ But summer condensation does happen, even in Ottawa.”

Worry about diffusion from the outside in, not the inside out

For decades, builders have worried about vapor diffusion into walls from the indoors during the winter. But if a home has air conditioning, vapor diffusion into walls from the outdoors is a much bigger problem.

According to Straube, “Solar-driven vapor is much more important” than winter diffusion from the interior. He continued, “The moisture is coming from the other side of the assembly.”

Avoiding problems caused by inward solar vapor drive

If the components of a wall assembly are poorly chosen, as they clearly were at the Parkside development built by Zaring Homes, there may be no faster mechanism for destroying a house than inward solar vapor drive. After only 10 weeks of occupancy, some of the Zaring homes were so wet that most of the brick veneer, sheathing, insulation, and drywall had to be removed and demolished.

But once you understand inward solar vapor drive, it’s relatively easy to choose building details to avoid problems. Here are a variety of ways to reduce risks; of course, you’ll probably only need to adopt one or at most two of the following measures to avoid problems.

  • Never include interior polyethylene or vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). wallpaper in an air-conditioned home. If your building inspector insists on a vapor retarder that comes in a roll, choose a smart retarder like MemBrain.
  • Avoid high-permeance sheathings like fiberboard behind reservoir claddings. Instead, specify foam sheathing — especially behind brick veneer, stucco, or manufactured stone.
  • Homes with asphalt felt experience fewer problems with inward solar vapor drive than homes with plastic housewrap.
  • Consider the use of a water-resistant barrier (WRB) that is impermeable to water vapor. The best-known vapor-impermeable WRB is Delta-Dry. Delta-Dry is made of stiff high-density polyethylene formed into a 5/16-inch-thick egg-carton configuration. The three-dimensional WRB creates two air spaces: one between the siding and the WRB, and the other between the WRB and the sheathing. Unlike high-permeance housewraps, Delta-Dry depends on air movement (ventilation) to dry the gap between the Delta-Dry and the sheathing.
  • Walls with a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap between the siding and the sheathing experience much less inward moisture transfer than walls without a gap.
  • Ventilated rainscreen gaps are more effective at limiting inward moisture transfer than unventilated rainscreen gaps.
  • More vapor drive problems occur in homes with dark-colored siding than light-colored siding.
  • When specifying stucco, choose a traditional stucco formulation without modern polymeric admixtures, since stuccos with these admixtures dry much more slowly than traditional stucco formulations.
  • Choose a siding (like vinyl siding) that is not a moisture reservoir.

I'd like to thank architect Steve Bostwick, one of the consultants who investigated the Zaring Homes disaster, for graciously sharing his photos. I’d also like to thank William Rose, whose historical research has highlighted Tyler Stewart Rogers’ role in establishing the idea that the warm-in-winter side of wall insulation should be protected by a vapor barrier. Rose is a building researcher at the University of Illinois in Urbana-Champaign.

Last week’s blog: “Using Ceiling Fans to Keep Cool Without AC.”

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Image Credits:

  1. Steve Bostwick

Jul 2, 2010 8:59 AM ET

much appreciated
by j chesnut

I’ve heard many times the concept of ‘inward solar vapor drive’ but this is the first time I’ve been given a thorough explanation with examples of failures from the field to boot. This fills many gaps in my knowledge.

This article extends the argument that winter time vapor transference is much less of an issue than previously thought. If I understand correctly its the power of the sun to ‘drive’ moisture caught in a reservoir that’s a real culprit, not so much the temperature differential between inside and outside of an effective air barrier (because the temperature differential can be as great and greater in winter cold climate conditions.)

I cringe when I see reasons to use vinyl siding. I like how GBA’s companion website Environmental Building News (EBN) provides reports on the impacts of material manufacture, disposal, toxicity concerns, etc. Content on Green Building Advisor on the other hand tends to consider on site performance only. While this makes sense in terms of energy efficiency and creating a durable structure and assembly I think the two pools of knowledge could integrate to create product recommendations that both work in the field and include more considerations of the full life cycle impacts of manufactured components.

Jul 2, 2010 9:16 AM ET

On vinyl siding
by Martin Holladay

J. Chesnut,
In a discussion of the phenomenon of inward solar vapor drive, it's important to be scientific and dispassionate. Although plenty of green builders prefer stucco to vinyl siding, it's important to report evidence that walls with stucco are more prone to wet-wall problems than walls with vinyl siding. That's a fact. Understanding that fact helps us make better decisions and helps our homes last longer.

It's up to each reader to weigh all the pluses and minuses of different materials when selecting products like siding. Of course, the manufacture of PVC is nasty. However, PVC insulation is routinely used for Romex wiring in most homes.

I'm not an absolutist when it comes to the use of vinyl. Try to buy a car without vinyl! Perhaps you don't ever drive -- perhaps you ride your bicycle everywhere. If so, good for you. The rest of us make compromises every day.

Jul 2, 2010 10:19 AM ET

Old and new research
by Bill Rose


ASHRAE funded a major study on solar vapor drive a few years ago. The final reports are probably only a few weeks away from being publicly available through ASHRAE. Should be interesting and very helpful. Dominique Derome was the PI. She has moved from Concordia University in Montreal to ETH in Zurich. The work was conducted in labs in Montreal and Leuven, Belgium as well as in the field.

I think the first reference to this condition was A. G. Wilson "Condensation in insulated masonry walls in summer" in Moisture Problems in Buildings, RILEM/CIB Symposium, Helsinki, August 1965. Grant Wilson was Head of the Building Services Section, Division of Building Research, NRC Ottawa Canada. (h/t Joe Lstiburek) This work was done in heated and humidified test huts in Ottawa that had insulation and interior vapoUr barriers. The abstract notes "No moisture problems were encountered in winter, but some undesirable wetting conditions developed during the summer period." The conclusion of the paper reads:

Moisture contained in masonry walls exposed to the sun is transferred by diffusion toward the inside of the building in summer. This vapour movement can result in condensation of water in the insulation and on the vapour barrier of walls incorporating permeable insulation on the inside of the masonry. Under adverse conditions, the amount of condensation can be sufficient to encourage deterioration of inner components of the wall. Problems arising from this source can be avoided by using an exterior cladding that is not readily wetted by rain, or where absorptive mansory is used, by ventilating with outside air between the masonry and inner components. Such venting may also result in less wetting of masonry during driving rain conditions, by equalizing air pressures across it, and in more rapid drying of wetted masonry during dry weather.


Jul 2, 2010 10:28 AM ET

Response to Bill Rose
by Martin Holladay

I was unaware of A.G. Wilson's 1965 report and am grateful that you pointed it out.

Jul 3, 2010 12:34 AM ET

reservoir claddings
by lucas dupuis

I really appreciate articles such as this on GBA. It's an excellent example of why we need building "science". But here's my question, Many of the components of these buildings experiencing catastrophic failures are more or less essential to the process of homebuilding in this day and age. AC, although problematic, is here to stay, Likewise with modified stucco, fibercement, and lick'stick stone. (-no excuse when building with brick IMHO) What is the best way to accommodate these types of materials? They're all more durable, lower maintenance, and more aesthetically pleasing than vinyl, LP, Canexel, etc.... and are thus chosen by many builders and designers who are looking to build 100 year + houses. Even if a builder isn't hitting the mark on energy efficiency they ought, at least to have the option of building something durable with on the shelf materials. I personally am a big fan of rain screens, but here in the dry inter mountain west its rare to find a builder who is familiar enough with the detailing to use them in a cost effective manner. In the vein of, "above all else, do no harm" I would appreciate your thoughts.

BTW, rant warning ,

I refuse to accept that vinyl siding is the only cost effective solution to solve this problem. We're stuck with PVC cladding on Romex, but as responsible builders and designers we cannot justify the continual use of PVC in every part of buildings, especially when the Europeans are phasing it out of buildings entirely.



Jul 3, 2010 5:48 AM ET

Some materials are here to stay
by Martin Holladay

I'll address vinyl siding first: I never said that vinyl is the only cost-effective solution to this problem. I don't have vinyl siding on my own house and I'm no fan of vinyl. But if you and I choose to avoid vinyl, it's important to understand why we are avoiding it. It's not because of performance issues. Vinyl siding performs very well. We're avoiding it for other reasons.

Back in the early 1990s, I used to hear project managers and experts from Vermont's Dept. of Historical Preservation announce in public meetings that the use of vinyl siding was dangerous because wrapping a building in plastic would trap moisture. This was utter nonsense, of course. It's important to clear the table of misconceptions and establish a few facts. Then you can choose any siding you prefer.

You say that air conditioning, modified stucco, fiber-cement, and adhered manufactured stone are here to stay. (You may be right, but I certainly would never shed a tear if the manufactured stone industry withered and died.) All of these can be used in a home without the catastrophic failures experienced by Zaring Homes. The key is to include details to prevent inward solar vapor drive.

If you install adhered manufactured stone over OSB without an air gap, your risks of failure are extremely high. If you install adhered manufactured stone over rigid foam sheathing, or in an installation that includes an air gap, you should be able to sleep at night without those fretful nightmares experienced by all the GBA readers who are now muttering, "Gee, I installed adhered manufactured stone over OSB on my last house. I wonder if the OSB has turned to oatmeal yet."

Jul 3, 2010 6:30 AM ET

Unintended Consequences
by Kevin Dickson, MSME

It's fascinating to me how incremental changes in methods, starting in 1938, can add up to disaster by the 1990's.

The lesson for today's builders is to pick a wall section from the BSC-approved list, and be very careful when changing anything or substituting anything.

Jul 3, 2010 10:00 AM ET

Down South
by Daniel Ernst

Martin - You're correct when you say that this disaster requires four elements.

Down here in the south, there are plenty of production builders that use brick w/ air gap, a combination of OSB and Celotex sheathing, fiberglass batts, and drywall.

Some use housewrap, some do not . . . and I'm not sure how they get away with it. Of course, the housewrap doesn't help with solar driven moisture, but it does inhibit bulk water flow.

Although I would never want to own such a house, they work. At least they aren't the disasters as seen with Zaring . The walls get moist (or wet during wind driven rain events, at least those without the WRB). Then they dry out.

I question their long term durability. And their energy efficiency is obviously poor. But it goes to show you how a simple change can wreak havoc on a wall system (interior poly).

Jul 3, 2010 1:54 PM ET

mildew stink plus...
by aj builder, Upstate NY Zone 6a

Just stopped at a hotel... open door to lobby and the musty mildew smell almost knocked me off my feet.

So needed a pitstop.. worse in bathroom, looked down to baseboard of vinyl wallpapered wall to find black line of yuk oozing.

Imagine what's behind ?

Jul 3, 2010 5:27 PM ET

by Phil M.

Where does that leave the ICF (Insulated Concrete Foam) blocks that have Styrofoam on the inside and outside of a 6" concrete slab? Is there going to be a moisture problem with condensation in the deep south of Alabama and Florida with the HIGH moisture and heat/cool changes? All of the ICF sales people say there is no problem. Thank you.

Jul 4, 2010 5:11 AM ET

Reponse to Phil
by Martin Holladay

The two layers of EPS included in most ICF walls are vapor retarders. Even if an ICF home has a reservoir cladding, any inward solar vapor drive is stopped by the EPS foam.

Jul 4, 2010 5:40 AM ET

Response to Daniel Ernst
by Martin Holladay

I think you're right -- these houses are saved by the lack of polyethylene. Inward solar vapor drive is undoubtedly occurring in these homes, but the moisture isn't condensing in the wall cavities. Instead, the damp drywall is drying to the interior.

Such homes are risky, if only because all it takes is a homeowner's decision to install vinyl wallpaper to push the homes into failure.

Jul 4, 2010 6:09 AM ET

vapour barrier
by mikey

would you use an internal vapour barrier for a building in the UK? This would be solid brick wall (no cavity), with internal insulation behind. No ventilation gap and no air con. Internal insulation would likely be fiberglass in stud wall.

Jul 4, 2010 4:00 PM ET

Response to Mikey
by Martin Holladay

1. No, I would not use an interior vapor barrier. I would use an interior vapor retarder (for example, vapor-retarding paint, kraft paper, or a "smart" retarder like MemBrain).

2. I wouldn't use fiberglass in contact with a solid brick wall. Instead I would use closed-cell spray polyurethane foam or rigid foam insulation.

Jul 4, 2010 4:13 PM ET

that's interesting, I don't
by mikey

that's interesting, I don't think membrain is available in the UK, I think we have an equivalent - intello plus but it's expensive. Why would you avoid fiberglass in this situation? Presumably polyurethane would not allow the brick to dry to the inside, perhaps EPS under a certain thickness would.

Jul 4, 2010 7:44 PM ET

Retrofit in Arkansas
by Chris Ladner

Martin, as a long time subscriber to Building Green and a practitioner of commercial energy consulting and commissioning, I have really enjoyed the blog and resulting conversations. I am in the process of practicing what I preach through my own "deep green" renovation of a 1968 brick home. I have been researching cost-effective roof and wall insulation retrofits. The house is fairly typical for our area; brick, air gap, black fiberboard, poorly installed r-11 insulation and gyp board. Interior insulation, using rigid polyiso, has been my focus but your recent blog has scared the smarts out of me. It seems that many of us in the south want airtight homes, good wall insulation, levels, and NO water-related issues. Any ideas for a retrofit strategy?

Jul 5, 2010 4:40 AM ET

Response to Mikey
by Martin Holladay

It would probably be best for you to get advice from a consultant who is familiar with British construction methods -- something I'm not familiar with.

Fiberglass can dry out after it gets wet, but it performs poorly in basements where it is in contact with damp concrete. I'm not sure how it would perform in your house, but without an air gap, I'd be nervous.

Jul 5, 2010 4:42 AM ET

Response to Chris Ladner
by Martin Holladay

Were you planning to remove the existing gypsum wallboard and fiberglass? You might be better off with open-cell spray polyurethane foam.

Jul 5, 2010 8:35 AM ET

Arkansas Retrofit
by Daniel Ernst

Chris / Martin - I just saw something in the Rural Arkansas (July issue) that had me shaking my head.

Doug Rye is a licensed architect in Saline County, host of the the "Home Remedies" radio show, and a columnist for the Rural Arkansas (Arkansas Electric Cooperative publication).

In his latest column he was touting the benefits of Reftrofoam, a tri-polymer insulation product.

Here's a quote from Doug Rye's article:

"I have also learned that this (Retrofoam) can be a great application for brick veneer houses. In this case, small holes are drilled in the mortar joints and foam is injected into th entire airspace behind the brick. I am anxious to see this done soon."

And that is "expert" advice!

I don't have any solutions to share with you. You have a difficult task ahead. I guess my point is that you should carefully research any advice you get . . .

Is the house a ranch, or a two-story?

Jul 5, 2010 8:37 AM ET

Retrofit in Arkansas
by Chris Ladner

Yes and no. We are trying to be frugal with the amount of waste we generate so i was going to maintain the existing walls that were not affected by the remodel (60%?). That being said energy trumps waste on our priority list. We are also trying to balance the costs of wall sealing/insulation/drywall with the attic sealing/insulation and new windows (currently looking at r-5 minimum). The ductwork and evaporator are in the attic spaces and the benefits/costs of sealing the attic adds another variable to the list.

Jul 5, 2010 8:46 AM ET

Retrofit in Arkansas
by Chris Ladner

Daniel, That solution was presented to me by a local insulator. I asked why they thought the air gap (drainage plane) was there in the first place????? This "solution" will be a problem for many homeowners in the future.

No comment on Doug. He is heavily sponsored by the elec Co-Op's.

It is a 2-story colonial with a 1-story kitchen/mud room (25% of the total).

Jul 8, 2010 12:58 PM ET

Retrofit in northern California
by Cliff McAuliffe

I am in the process of replacing the cedar shingle siding on my home in northern California. I am replacing it with OSB sheathing, Tyvek house wrap and fiber-cement siding. This meets the local code requirements. My home is not air conditioned. Even with high efficiency windows, the temperature difference on the hottest summer days is 12-16 degrees.

The existing cedar siding has not shown any moisture problems during its removal and there is no evidence of moisture problems existing inside. With this new exterior construction, would you think that I am in danger of a moisture problem?

Thank you.

Jul 8, 2010 1:07 PM ET

Response to Cliff
by Martin Holladay

The short answer is no.

The long answer is, any wall can be subject to moisture problems if there are problems with the details -- for example, flashing details.

I don't know how cold it gets in winter in your location, but I usually advise anyone who is replacing siding to seriously consider the installation of exterior rigid foam insulation. It will never be easier than it is now.

Jul 8, 2010 1:49 PM ET

Inward Solar Vapor Drive
by Carl Mezoff


A fascinating article -- particularly for an Architect who has specified 6 mil poly under the gypboard regularly for years here in lower Connecticut! Perhaps I have been lucky, but I have not seen any problems yet. Perhaps because I regularly also call for wide roof overhangs, I have dodged the bullet by keeping the walls relatively dry in the first place.

Another detail that may have saved me is the use of a 2" layer of foil-faced iso foam under the interior gypsum board. Presumably the foam prevents inwardly driven summer water vapor from reaching the gypboard's cold surface during the cooling season, and keeps the vapor in the wall cavity above the dewpoint.

Jul 8, 2010 2:12 PM ET

by steve mindel

Interesting article. Would typar over plywood help stop vapor drive? We use fiber cement clapboards almost exclusively. We've also used felt paper and would continue to do so, more labor, hard to get it to lay flat on the sunny sides. We've used cedar breather behind the fiber cement but it creates a ripply wall as it pulls in where the nails are. We've done rain screens with wood strapping and plastic products but it creates a lot of detailing problems around windows, corners and other trim. I would be interested in comments on the Delta dry product.

Jul 8, 2010 2:18 PM ET

Response to Steve
by Martin Holladay

Typar does not stop inward solar vapor drive. Like all housewraps, Typar is designed to be vapor-permeable. When it's time for a damp wall to dry to the exterior, the high vapor permeability of housewrap is a useful characteristic. However, when moisture in the siding is being driven inward, its vapor permeability is a liability.

Plywood is a vapor retarder, so it's more effective than Celotex or Homosote at resisting inward solar vapor drive. When dry, plywood has a permeance of about 0.5. The permeance of plywood rises as its moisture content rises, up to a maximum of about 20 perms -- making plywood a "smart retarder."

Jul 8, 2010 2:26 PM ET

reply to Martin
by steve mindel

So, would felt paper work better? Is a rain screen still necessary? Better I'm sure, but necessary behind fiber cement? Would taping the joints of the plywood gain anything?


Jul 8, 2010 2:49 PM ET

Second reply to Steve
by Martin Holladay

Q. Would felt paper work better?

A. As I wrote in my article, "Homes with asphalt felt experience fewer problems with inward solar vapor drive than homes with plastic housewrap." Asphalt felt is less permeable than most plastic housewraps.

Q. Is a rain screen still necessary?

A. It depends on the details of your wall assembly -- your siding type, your choice of WRB, your sheathing type, your insulation material, and your choice of an interior vapor retarder -- as well as your climate and how much roof overhang you have. The less risky choices are provided in the bulleted list at the end of my article.

A. Would taping the joints of the plywood gain anything?

Q. Taping the joints of plywood sheathing improves air tightness but doesn't affect vapor permeance.

Jul 9, 2010 11:38 AM ET

Carl Mezoff's post is worth discussing further
by aj builder, Upstate NY Zone 6a

Martin could you discuss Carl's theory to do with foam sheeting under drywall?

Jul 9, 2010 12:00 PM ET

Response to Adkac
by Martin Holladay

Moisture condenses on cold surfaces. In an air-conditioned home, interior poly is a cold surface. If there is no interior foam under the drywall, fiberglass batts allow the exterior moisture to contact the cold poly.

If a builder installs a layer of rigid foam between the studs and the interior drywall (or between the studs and a layer of interior poly), and if the seams of the rigid foam are taped, then the first surface encountered by the inwardly driven moisture is the back of the rigid foam. But it's not a cold surface like the poly; it's warm. (The foam insulation separates the warm air between the studs from the cold air-conditioned air on the other side of the foam). With no cold surface, there is no condensation.

Interior rigid foam addresses the problem of summertime condensation, just as exterior foam addresses the problem of wintertime condensation.

Jul 9, 2010 4:12 PM ET

Rigid foam in attics
by Dan

Could rigid foam be safely installed between the rafters, spray foamed in place and then recovered with loose celulose?

Jul 9, 2010 7:36 PM ET

Response to Dan
by Martin Holladay

Are you talking about rafters or joists? If you are talking about rafters, I'm not sure how you intend to install the cellulose.

If you are insulating between your rafters, you need to decide whether to go with an unvented or a vented roof. It's possible to install rigid foam between the rafters, foamed in place as you describe, to define the ventilation channel. Ordinarily, 1.5 inch x 1.5 inch spacers are installed first, to keep the foam away from the roof sheathing.

If you fill the rafters with cellulose, you'll need to install netting to hold the cellulose in place.

If you're talking about joists, not rafters, the answer is yes.

Jul 12, 2010 1:40 PM ET

Would felt paper work better?
by David M

I would like to add a couple of comments with respect to the paper vs housewrap issue.....

1) The MVTR of asphalt saturated felts and papers increases in hot/humid environments. So in the exterior moisture drive situation, they are "dumb" membranes.

2) Not all housewrap manufacturers believe that higher is better when it comes to MVTR. For theresevoir cladding/air conditioned interior combination, it is best to use a housewrap with an MVTR near the building code minimum.

Thank you opening up the discussion.

Jul 15, 2010 3:35 PM ET

Rigid foam in attics 2
by Dan

yes, Joists, not rafters. confusing my terms. I experience alot of heat in the attic (vented) that is transmitted to the second floor rooms. I also have a valted ceiling facing south that is packed full of cellulose to the point that the foam stand offs on the bottom of the roof deck has collapsed. My thinking to correct this problem is to add several inches of sheet foam to the roof deck, apply a second layer of sheathing (basically making a SIP panel) and then add shingles or standing seam panels. Would I need to correct the failed air channel in the rafters if I created an air channel between the new sheet foam and new sheathing? what would be a good method to pursue?

Jul 15, 2010 4:13 PM ET

Response to Dan
by Martin Holladay

It's certainly possible to insulate a roof deck from above with one or two layers of rigid foam.

To avoid moisture problems on the lowest level of roof sheathing, you need to be sure that your rigid foam has a high enough R-value to keep the sheathing above the dew point during the winter. It's also a good idea, as you propose, to include a vent channel between to top surface of the new rigid foam and the top layer of roof sheathing.

Aug 31, 2010 12:46 AM ET

Use of polyethylene vapor barrier
by bruce


I am building a house in Iowa with the following wall system: Vertical 12" wide steel siding (board & batten), Tyvek, OSB, 2x6 studs, R-19 fiberglass insulation (asphatic inpregnanted kraft paper on one side of the insulation), sheetrock.. The house will be air conditioned. Would you use a polyethylene vapor barrier under the the sheetrock if was your house? What are the pros (if any) and cons of using a polyethylene vapor barrier in this situation?

If I don't use a poly barrier, what are your comments regarding the use of the kraft paper faced R-19 insulation verse using an unfaced insulation?

I am constructing the house now so I would appreciate your comments soon. Thanks.

Aug 31, 2010 1:30 AM ET

Effectiveness of polyethylene vapor barriers
by bruce


Many years ago, I read some research regarding vapor barriers which I think was conducted at the University of Minnesota. If my memory is correct, I believe the researchers found that a polyethylene vapor barrier was ineffective if there was small tears or punctures in it. I think they said two pencil punctures in the 16" space between the studs was a problem.

If that is true, then my concern with using a poly barrier in the midwest has been this: If the
the vapor barrier allows passage of vapor into the wall cavity due to relatively small damage and condensation occurs, then the poly sheeting would hinder the drying of the wetted materials. Therefore, I have thought that it is better to not use it. What are your thoughts regarding this observation?

In a stud wall, the poly sheet laps over the vertical edge of the bottom and top 2x6 sill plates and is held tightly in place by the sheetrock. Can vapor get into the wall cavity behind the poly barrier through the horizontal faces of the 2x6 material?

Aug 31, 2010 5:04 AM ET

Response to Bruce's first post
by Martin Holladay

Interior polyethylene should never be used in an air-conditioned house.

You have designed a wall with very poor performance. Depending on where you are building in Iowa, it may even be illegal.

I assume you have visited the GreenBuildingAdvisor Web site because you want to build a green home. If I'm guessing correctly, then I strongly advise you to choose a wall system with a much higher R-value. Fiberglass batts perform poorly, and you will have a lot of thermal bridging through your studs.

Aug 31, 2010 5:09 AM ET

Response to Bruce's second post
by Martin Holladay

Your question shows a confusion between air barriers and vapor retarders. If you are designing an air barrier, small holes matter.

When it comes to a vapor retarder or vapor barrier, small holes are irrelevant. Interior polyethylene with many holes still works as an effective vapor retarder -- in fact, in most homes it is too effective.

In your climate, the problem with interior polyethylene is not that it might not stop the diffusion of vapor. The problem is that it WILL stop the diffusion of vapor -- making it hard for a damp wall to dry to the interior during the summer.

To learn about the difference between air barriers and vapor retarders, I suggest you read:

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Aug 31, 2010 5:35 PM ET

Response to Bruce's first post by Martin Holladay
by bruce

Martin, Two questions: 1) Other than the R value and thermal bridging at the studs, what problems does wall have? 2) I believe there is foam insulation that "fits" behind the steel board and batten steel siding, would that be something to consider adding?

Aug 31, 2010 7:38 PM ET

Another response to Bruce
by Martin Holladay

You're right that the main problem with the wall is its low R-value.

In addition, you will need a good plan to create an air barrier for your wall, since fiberglass batts are particularly susceptible to air leakage.

If you can install continuous rigid foam sheathing on the outside of your studs, that would definitely help.

Aug 31, 2010 10:39 PM ET

Another response to Bruce by Martin Holladay
by bruce

Martin, Tyvek will be used over the OSB. Is that sufficient for the air barrier? You stated ".....since the fiberglass batts are particularly susceptible to air leakage." What is meant by "susceptible to air leakage"? Thanks.

Sep 1, 2010 5:41 AM ET

Response to Bruce
by Martin Holladay

I'm beginning to suspect that you aren't reading any of the articles that I have been suggesting. You really need to start getting a foundation in the basics.

No, Tyvek is not sufficient as an air barrier, although it can be one component in an air barrier system.

To answer your second question: fiberglass batts do almost nothing to slow air movement (which is why fiberglass is often used as a furnace filter). Other types of insulation, like dense-packed cellulose, are much more effective at slowing air movement.

Sep 1, 2010 1:58 PM ET

Response to Bruce by Martin Holladay
by bruce

Martin, I have read the articles that you suggested and others articles. Please bear with me as I try to understand the basics of the problem.

From the various artices and discussions between respondents, it appears to me that a polyethylene vapor barrier should not be used in a wall in a house in the midwest (say Iowa, Illinois, South Dakota) if it is air conditioned, even though in the winter some vapor will (might) enter the wall and probably condense because the wall without the poly can dry. Is this a correct conclusion on my part?

Sep 1, 2010 3:12 PM ET

Response to Bruce's latest post
by Martin Holladay

Yes, that's correct.

You can do two things to reduce the chance of condensation:
1. Install your drywall using the Airtight Drywall Approach.
2. Install rigid foam sheathing (sized according to the needs of your climate) on the exterior of your wall.

Sep 1, 2010 10:23 PM ET

Response to Bruce's latest post by Martin Hooaday
by bruce

Martin, Thank you. I appreciate your help. Where can I read about the Air Drywall Approach?

Sep 1, 2010 11:14 PM ET

Kraft Paper Backed Fiberglass Insulation (asphat coated)
by bruce

Martin, Another question in trying to understand the problem of solar driven vapor. Would the use of kraft paper backed fiberglass insulation (asphat coated) in the walls of a air conditioned house present the same problem as the polyethylene sheet.

Sep 1, 2010 11:18 PM ET

Dew Point Temperatures
by bruce

Martin, Where can i get information regarding the dew point temperature for various levels of humidity?

Sep 2, 2010 5:40 AM ET

Response to Bruce's latest three questions
by Martin Holladay

Some of your questions can be answered by using Google or the "Search" function on the GBA Web site.

1. Here are links to more information on the Airtight Drywall Approach:

2. No, kraft facing will not cause the same problems as interior polyethylene. Kraft facing is more permeable than polyethylene.

3. Here are two links to articles about dew-point calculations:

Sep 2, 2010 10:44 AM ET

E-mail address
by bruce

Martin, Thank you again. Could you send me your e-mail address?

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