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Vapor Retarders and Vapor Barriers

Answers to persistent questions about vapor diffusion

Posted on Mar 12 2010 by Martin Holladay

UPDATED on May 15, 2013

Although building science has evolved rapidly over the last 40 years, one theme has remained constant: builders are still confused about vapor barriers.

Any energy expert who fields questions from builders will tell you that, year after year, the same questions keep coming up: Does this wall need a vapor barrier? Will foam 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. trap moisture in my wall? How do I convince my local building inspector that my walls don’t need interior poly?

To begin a discussion of vapor retarders and vapor barriers, I’ll answer a few of these persistent questions. Since I plan to return to this topic in a future blog, I invite readers to submit further questions.

Q. Why would I want a vapor retarder in my wall or ceiling?

A. Vapor retarders help slow the diffusion of water vapor through a building assembly. During the winter, a vapor retarder on the interior of a wall will slow down the transfer of water vapor from the humid interior of the home into the cool stud bays. During the summer, a vapor retarder on the exterior of a wall will slow down the transfer of water vapor from damp siding towards the cool stud bays.

However, a vapor retarder is a double-edged sword: while under some circumstances it can have the beneficial effect of helping to keep a wall or ceiling dry, under other circumstances it can have the undesirable effect of preventing a damp wall or ceiling from drying out.

Q. How often does water vapor diffusion through walls and ceilings cause problems?

A. Very rarely. In many cases, in fact, an interior vapor retarder does more harm than good. The main mechanisms by which moisture enters a wall are from the exterior (usually due to flashing defects that admit wind-driven rain) and via air leaks that carry “piggy-backing” moisture that condenses in a wall cavity. Vapor diffusion is a relatively insignificant cause of moisture problems in walls. (For more information on why air barriers matter more than vapor retarders, see “Air Barriers vs. Vapor Barriers.”)

Q. Under what circumstances can vapor diffusion cause problems?

A. Although vapor diffusion problems are rare, they can occur. Dangers of vapor diffusion problems are higher:

  • In very humid rooms (for example, greenhouses or rooms with an indoor pool);
  • In homes with humidifiers; and
  • In homes located in extremely cold climates.

Even in a home with one of the characteristics listed above, the mechanism for moisture transport into walls and ceilings is much more likely to be air leakage than vapor diffusion.

That said, there are a few types of building assemblies that merit close attention to risks associated with outward vapor diffusion. These assemblies include cathedral ceilings in cold climates that are insulated with open-cell spray foam, and double-stud walls in cold climates. For more information on these types of building assemblies, see The Return of the Vapor Diffusion Bogeyman.

Q. What’s the difference between a vapor barrier and a vapor retarder?

A. A vapor barrier stops more vapor transmission than a vapor retarder. A vapor barrier is usually defined as a layer with a permeance rating of 0.1 perm or less, while a vapor retarder is usually defined as a layer with permeance greater than 0.1 perm but less than or equal to 1 perm.

Q. What does the code say about vapor retarders?

A. Codes vary; older versions of model building codes often included more sweeping requirements for vapor retarders than more recent versions.

The 2006 International Residential Code (IRC) and the 2006 International Energy Conservation Code (IECC International Energy Conservation Code.) both define a vapor retarder as a material having a permeance of 1 perm or less. This definition includes such materials as polyethylene sheeting, aluminum foil, kraft paper facing, and vapor-retarding paint.

In section R318.1, the 2006 IRC requires: “In all framed walls, floors, and roof/ceilings comprising elements of the building thermal envelope, a vapor retarder shall be installed on the warm-in-winter side of the insulation.” It should be emphasized that this code requirement makes no mention of polyethylene; vapor-retarding paint fulfills this code requirement.

The 2006 IRC includes exceptions to the vapor-retarder requirement. It allows a vapor retarder to be omitted:

  • In Climate Zones 1 through 4 (an area including most of the West coast and the South);
  • In walls, floors and ceilings made of materials (like concrete) that cannot be damaged by moisture or freezing;
  • “Where the framed cavity or space is ventilated to allow moisture to escape” — an apparent (although poorly worded) reference to vented attics and walls with 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. siding.

In section 402.5, the 2006 IECC requires: “Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.”

In the 2006 IECC, the exceptions to the vapor retarder requirement are very similar to the exceptions listed in the 2006 IRC, except for an additional exception: “Where other approved means to avoid condensation are provided.” This last exception gives broad latitude to the building official — and places a heavy burden on any builder intent on convincing a local official that a certain building assembly complies with this exception.

The 2007 Supplement to the IECC and the 2007 Supplement to the International Residential Code (IRC) introduced a new vapor-retarder definition. Vapor retarders are now separated into three classes:

  • Class I: Less than or equal to 0.1 perm [e.g., polyethylene];
  • Class II: Greater than 0.1 perm but less than or equal to 1.0 perm [e.g., kraft facing];
  • Class III: Greater than 1.0 perm but less than or equal to 10 perm [e.g., latex paint].

Since 2007, the IECC has required (in section 402.5) that walls in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene.

The exceptions have also been rewritten. Three of the exceptions are listed in section 402.5 of the IECC, which notes that vapor retarders are not required on a basement wall, on the below-grade portion of any wall, or on a wall constructed of materials that cannot be damaged by moisture or freezing.

Further exceptions are allowed in section 402.5.1, which states that in climate zones where a Class I or Class II vapor retarder would normally be required, a less stringent vapor retarder — a Class III retarder like latex paint — can be used under the conditions listed in Table 402.5.1 (see accompanying figure). Only certain types of wall assemblies are worthy of this exception; they must have either an adequate layer of exterior foam sheathing or “vented claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .”

Q. Clearly, I can get in trouble with my building inspector if I omit a vapor retarder in certain climates. Are there any situations where I could get into trouble for including a vapor retarder?

A. Yes. Although it’s perfectly legal to install 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 any climate, these products can lead to moisture and mold problems in most of the U.S. Unless you’re building in Canada, Alaska, or somewhere close to the Canadian border, you don’t want interior polyethylene or vinyl wallpaper — especially in an air-conditioned house.

Interior polyethylene and vinyl wallpaper prevent a wall from drying to the interior during the summer, when inward solar vapor drive (a phenomenon associated with so-called “reservoir claddings” — for example, brick veneer and stucco — that absorb and hold moisture) can cause condensation on the exterior side of the wallpaper or poly. Unless the moisture introduced into the wall by inward solar vapor drive is able to dry to the interior, wall damage can result.

Q. When it comes to vapor retarders, what do the experts recommend?

A. Here’s a sampling of statements by leading building scientists on the subject of vapor retarders:

  • Anton TenWolde: “The calculations show that even with very low air pressures across the assembly, and even with a very good air barrier, sufficient moisture can bypass a poly vapor retarder, degrading its performance. In practice it doesn’t matter what the permeance of the vapor retarder is, because the air leakage will go around it for moisture transfer. I came to the conclusion that the idea that we need a vapor barrier to keep our walls dry doesn’t hold a lot of water, so to speak.”
  • John Straube: “The whole reason we’re talking about vapor barriers is not because vapor diffusion control is so important, but because people believe it is so important. The question comes up, have we seen diffusion-related building failures? And the answer is, very few — maybe in rooms with a swimming pool. Assuming that the vapor came from the inside, you would have to have a very high load before you would see a problem. I think that solar-driven vapor is much more important. The moisture is coming from the other side of the assembly.”
  • Joseph Lstiburek: “In North Carolina, for whatever reason, they build their walls with fiberglass insulation and with poly on the inside. Depending on the cladding — brick and stucco being the worst — the walls rot like crazy.”
  • André Desjarlais: “We can’t assume that the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. is perfect. We have to assume some level of failure: some rain will get into the wall, and there will be imperfections in the air barrier.”
  • Achilles Karagiozis. “It’s all related—the vapor control strategy, airtightness, and whether or not there is a ventilation cavity behind the exterior cladding. If you have a ventilation cavity behind the cladding, it doesn’t matter what kind of vapor retarder strategy you use.”
  • Bill Rose: “In the South, no vapor barrier. In the North, as long as you have insulated sheathing that meets the dew-point test, also no vapor barrier.”
  • Anton TenWolde: “When you put enough foam sheathing on the wall you get away from the cliff rapidly, and there’s no reason to worry about vapor barriers any more.”

To read more questions and answers on this topic, see Do I Need a Vapor Retarder?

Last week’s blog: “The Energy-Efficiency Pyramid.”

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Image Credits:

  1. Martin Holladay / IECC

Mar 12, 2010 11:54 AM ET

More confusion
by Garth Sproule

Thanks for the informative blog. I just wanted to point out that in many places, especially northern climates, that the poly barriers are detailed and sealed because they are not only doing the job of "vapor barrier" but as the primary "air barrier" as well. It is by far the most common method used around here. Almost no one actually calls it an "air barrier" though...almost always referred to as " vapor barrier" which I believe adds to the confusion...

Mar 12, 2010 12:11 PM ET

Good point.
by Lucas Durand - 7A

You're right Garth. It's amazing to me that this confusion still exists. As far as I can tell, the Building Code of Canada actualy does a reasonable job of differentiating between the two.

Mar 12, 2010 12:15 PM ET

Poly air barriers
by Martin Holladay

You're right, of course: in Saskatchewan, builders have used a layer of polyethylene as an air barrier since the late 1970s. The tried-and-true air barrier method involved carefully sealing the seams with Tremco acoustical sealant. The method works well in cold climates.

There are only two unfortunate aspects to this tale:

1. Saskatchewan builders mistakenly referred to this air barrier as a "vapor barrier," which only added to builders' misunderstandings of the poly's primary purpose.

2. The method was adopted in warmer climates where interior polyethylene is inappropriate.

Mar 12, 2010 12:28 PM ET

Two more unfortunate aspects
by Garth Sproule

Add these to the unfortunate aspects
3. Poly actually makes a poor air barrier because it is easily damaged and flexes to much (pumping)
4. Most new homes up here also use AC and have stucco on the exterior....more trouble???

Mar 12, 2010 10:46 PM ET

Thanks Dr. Joe
by Danny Kelly

For the shoutout to North Carolina!

Martin - good article - you did not discuss the foam sheathing much which is also a vapor barrier - getting conflicting opinions on this site. In a mixed-humid climate - they have always told us no VB - allow the wall to dry in both directions - now most seem to recommend foam exterior sheathing - seems like we are breaking our own rules. I assume it is ok during the summer - will stop the vapor drive from the exterior. During the winter - vapor is moving from inside to outside so the foam will keep us above our dewpoint. Does this pretty much have us covered - is there anything else we should be thinking about? If we are going to install foam on the exterior - how can we calculate the dew point to make sure it is think enough? Do you think 1/2" is enough in NC or do we need to go up to 1"?

For future articles, would like some information on roof vapor retarders/barriers - seems like most of the new "high performance felts" are vapor barriers - having a hard time understanding when we can use them - vented vs. unvented attic, etc. Thanks.

Mar 13, 2010 5:15 AM ET

Foam sheathing permeance
by Martin Holladay

Your statement that foam sheathing is a vapor barrier is only true for foil-faced polyisocyanurate. At typical thicknesses, EPS and XPS are vapor retarders. One inch of EPS has a permeance that ranges from 2.0 to 5.8 perms, while one inch of XPS has a permeance that ranges from 0.4 to 1.2 perm. In general, permeance decreases as foam density increases — and, of course, as thickness increases.

If you are building in North Carolina, it's important to consider the possible effects of inward solar vapor drive. By limiting this phenomenon, foam sheathing is your friend, not your enemy. And, as you correctly point out, foam sheathing reduces the chance of winter condensation in wall cavities.

In climate zones 3 and 4 — the climate zones of North Carolina — the likelihood of problems arising from winter condensation in walls is quite low, especially if your wall includes an air barrier. Although 1/2-inch-thick foam sheathing won't get you into trouble in your climate, it also won't provide much of a thermal benefit. To reduce thermal bridging through studs, I would think you would want your foam sheathing to be 1" thick or thicker.

Mar 13, 2010 10:35 AM ET

Chart question
by Garth Sproule

When using the values on the chart that you provided, is it assumed that the cavity in the wall is insulated or not? I'm guessing yes, but would appreciate confirmation.

Mar 13, 2010 10:49 AM ET

Yes, assume that the cavities are insulated
by Martin Holladay

As you surmised, the chart assumes that the wall's stud bays are insulated.

Mar 13, 2010 11:21 AM ET

What about removing the high RH?
by Kevin Hanlon

I appreciate all that's been said, and agree that a very effective air barrier is what's required in northern climes, to minimize moisture laden air into cold wall cavities. But the real culprit is moisture / high RH. Homeowners need to recognize that it's their responsibilty to keep RH low and stable in winter. Using bath fans, kitchen exhaust and HRV's / ERVs and whatever else to \ keep RH in the 30% - 40% range, AND a great air barrier should minimize or eliminate all threats to wet walls and roof assemblies. 30%RH in a 68F house is pretty comfortable, not dry feeling. It's the builders /renovators responsibility to provide the mechanicals and the A/B, and the homeowners responsibility to maintain them. The vapor barrier idea has shown it's only in the the right place half the year.

Mar 13, 2010 11:27 AM ET

Don't forget basements and crawl spaces
by Martin Holladay

You're right. But don't forget the contribution of basements and crawl spaces to indoor humidity levels. If these spaces are designed and built to stay dry, the indoor air during the winter is likely to be dry as well. If these spaces are poorly detailed and built, they can contribute tremendous quantities of moisture to the indoor air.

Mar 13, 2010 4:24 PM ET

by Danny Kelly

Thanks Martin - very helpful. I think the DOW SIS has a perm rating of .3 so thought that was a vapor barrier (anything under 1 perm?)

Mar 13, 2010 4:36 PM ET

Vapor barriers and vapor retarders
by Martin Holladay

Depending on your definition, anything under 1 perm is usually called a vapor retarder, while the term "vapor barrier" is usually reserved for Class I retarders (under 0.1 perm). At 0.3 perm, Dow Structural Insulated Sheathing is a Class II retarder.

Mar 13, 2010 8:38 PM ET

The decimal demon gets me again
by Danny Kelly

Under 0.1 not 1.0 - got it. Well - that's good news for me and solves another one of my confusion mysteries. Thanks Martin.

Mar 14, 2010 3:17 PM ET

Kraft Paper
by Greg Miller

I will be insulating the interior of my basement walls with foam board, then building a 2x4 stud wall over it. I know the fiberglass insulation I put in that assembly needs to have no vapor barrier to allow the wall to dry inward, but I am having a difficult time finding the proper material - kraft paper faced batts and rolls is the norm. (When I can find it the cost is two or more times that of faced insulation). What are your thoughts on my attempting to destroy the barrier by making numerous cuts with a razor knife after installation?

Mar 14, 2010 4:14 PM ET

Unfaced fiberglass batts
by Martin Holladay

I'm surprised that you are having difficulty finding unfaced fiberglass batts.

If faced fiberglass batts are cheaper, simply peel the kraft facing off the fiberglass and discard the kraft facing.

Even better: determine the desired R-value of the wall -- in northern climate zones the minimum code requirement now calls for R-15 basement wall insulation -- and buy enough rigid foam (for example, 3 inches of XPS) to meet your needs. Keep your stud bays empty except for electrical wiring.

Mar 15, 2010 9:51 AM ET

yup, it's confusing all right
by Bill Rose


So much of the vapor barrier discussion seems like medieval Scholasticism (“How many angels can dance on the head of a pin?”), which sought to integrate a theology with a natural philosophy, and failed at both. This discussion mixes two very different matters: performance outcomes and prescriptive requirements. There is no assurance whatsoever that prescriptive compliance delivers performance. It occurred to me this weekend that putting in a vapor barrier to prevent “condensation” is much like taking off your shoes at the airport to prevent “terrorism”. (Yes, I removed my shoes.)

Everyone wants good performance. No problem there. Prescriptive requirements are intended as shortcuts to good performance, and they facilitate commerce. They should be allowed to remain in effect only if: 1) their subject is critically important, 2) they are necessary, 3) they are sufficient, and 4) if the link between the prescription and the performance outcome is continually policed. In my opinion, all four are open to question. That said, we might imagine a future in which the building code sections that address the vapor barrier would all go blank. I bet most readers would be able to design excellent buildings that perform well and are quite durable, without using the word “vapor barrier” at any point in the process.

Performance benefits clients and the public. Prescriptions benefit practitioners. Once the benefits of compliance with a prescription (to the practitioner) outweigh the benefits of performance (to the client), an ethical line is crossed.

Mar 15, 2010 10:04 AM ET

Thanks for your perspective
by Martin Holladay

As you remember — you were there — I attended several of Joe and Betsy's "vapor barrier summits" a few years back, as they were drumming up support for their proposed code changes.

Here's the dilemma: many of the experts that Joe and Betsy assembled were trying to fix something that was broken (vapor retarder requirements in the building code). The experts weren't really starting with a blank slate -- weren't really answering basic questions like, "Do we need to discuss vapor diffusion in the code at all?" or "What's a building code for?"

Ultimately these code changes were political, not technical or scientific. (I know that you don't think the word "science" should ever be applied to building detail recommendations.) The questions were, "What are the worst aspects of this code that need to be changed first?", followed by, "What's politically possible?"

Your posting here is very philosophical, so I'm going to tug at your balloon string and bring you back down to earth. Answer me this: What aspects of the current IRC or IECC can get builders into trouble? How can adhering to prescriptive requirements still lead to screw ups?

Mar 15, 2010 2:44 PM ET

by Bill Rose


You recall correctly—I have been in support of Joe Lstiburek’s efforts to tweak the prescriptive requirement so that it at least permitted good performance outcomes, even if it didn’t force them. Joe, incidentally, doesn't support taking the next step which I suggest above, of calling the prescriptive measure itself into question. It is primarily in retrofit insulation where a prescriptive requirement for a vapor barrier becomes a horror (and where Joe might well agree). Retrofit really should be done with performance analysis and estimates. Face it, we're prescription junkies.

Mar 22, 2010 10:06 PM ET

vapor barriers
by Dave Barber

did I miss anyone talking about vapor barrier primer paint? I like it because it eliminates a step in the builiding envelope contruction, is an unbroken barrier and is inexpensive- I use it as the pre texture primer step.
It keeps the moisture on the surface of the rock, not allowing it to stop at insulation or plastic.

Mar 23, 2010 8:25 AM ET

You might have missed it...
by Martin Holladay

In the answer to the question about building codes, I wrote, "This [code] definition [of a vapor retarder] includes such materials as polyethylene sheeting, aluminum foil, kraft paper facing, and vapor-retarding paint."

You're right, paint works fine. Benjamin Moor Moorcraft SuperSpec Latex vapor retarder primer 260 is one brand. Most vapor-retarding paints produce a dried film with a permeance of about 0.45 to 0.90 perm.

By the way, don't worry about whether or not your vapor retarder is "unbroken" -- a vapor retarder that covers only 90% of a surface (in other words, with 10% defects) is still 90% effective.

Your understanding of the function of a vapor retarder isn't quite accurate. Vapor-retarding paint doesn't "keep the moisture on the surface of the rock, not allowing it to stop at insulation or plastic." All it does is slow down the rate of diffusion. But that's all you need. There is no moisture "on the surface of the rock." However, the moisture content of the drywall will reflect the indoor relative humidity. Since "the surface of the rock" should always be at the indoor temperature, there shouldn't be any concerns about high moisture or condensation on that surface anyway.

Like kraft paper, vapor-retarding paint will allow moisture to diffuse through the drywall. If it encounters a cold impermeable surface inside the wall, the moisture could still condense. However, as the article points out, not much moisture moves by diffusion anyway, so the entire discussion is moot. All we really need to worry about are air leaks.

Apr 5, 2010 1:16 PM ET

Brick Buildings
by Mark Hutchinson

Most of the information that I have found addresses wall assemblies for wood buildings but how, if at all, would your recommendations change if you are dealing with a brick building? I am doing a retrofit of a brick row home in Philadelphia, pretty representative of your typical Mid-Atlantic and North-East row home, and am currently thinking of insulating the walls with 2" exterior XPS (tape all seams and add a rain screen and cladding on the exterior, we can only extend the facade 4") and then 5" interior cellulose (frame with 2"x4" studs 1" off of the wall, covered by drywall on the inside and re-framing and installing new high-performance windows and doors) then 8" exterior ISO on the roof and 8" of cellulose in the basement ceiling. We are getting some pushback by architects and tradespeople that don't like the idea of sandwiching the brick but if we are removing interior moisture through a ducted ERV and properly seal and caulk the exterior, do you see any problems with this approach? Do you have any alternative recommendations?
Mark Hutchinson

Apr 5, 2010 1:34 PM ET

It sounds like your approach will work
by Martin Holladay

It sounds like your approach can work, but there are many details that could potentially trip you up if they are not executed well. Exterior flashing details and rainscreeen details must be carefully thought out, as well as potential thermal bridging at the wall / roof intersection.

I strongly advise you to read "Insulating Residential Masonry Buildings In Cold Climates" by Chris Benedict (in the the current edition of Home Energy magazine, March / April 2010). You may also want to consult with Chris Benedict.

Jul 17, 2010 10:23 AM ET

interior poly and "old-house" smell
by Adrian

An interior poly seems to prevent the diffusion of smells already existing in the wood structure.
When renovating an old home (gypsum board replacement) should I use poly? Is there another method to prevent the diffusion of smells? House is in zone 4 border with 3 (Southern PA).
My dilemma is that If I also use an outside insulating sheathing I may create a vapor trap inside the wall resulting in the wall not being able to dry in any direction.

Jul 19, 2010 12:41 PM ET

Response to Adrian
by Martin Holladay

As far as I know, if you create an airtight barrier, you won't smell anything on the other side of the barrier.

Interior polyethylene is not recommended in your climate. I suggest you install your new drywall using the Airtight Drywall Approach (ADA).

If your home has a forced-air heat distribution system (a furnace), you might consider installing a central-fan-integrated supply ventilation system. Such a ventilation system will slightly pressurize your house, reducing the likelihood that smelly air from your wall cavities will ever reach your nose.

Aug 11, 2010 12:19 PM ET

Vapor Woes
by Miles

I can't decide what I need for a vapor barrier in my walls. Conflicting information is everywhere. I am re insulating my older home from the inside (2x4 walls, R13 craft faced batts).

I am in zone 4, close to zone 5 (Baltimore). I don't cool my house more than 79F in the summer if I use the AC at all.

Coming from Canada, I am tempted to use polyethylene on the interior. Is this a bad idea?

Aug 11, 2010 12:41 PM ET

Response to Miles
by Martin Holladay

Yes, it's a bad idea. If you (or a future resident) runs an air conditioner, moisture can condense on the outside of the polyethylene and rot your walls.

1. I advise you to consider installing a more effective insulation than fiberglass; for example, cellulose.

2. Whatever insulation you choose, install your drywall following the Airtight Drywall Approach (ADA). This requires gaskets or caulk between the drywall and the bottom plates, as well as between the drywall and the top plates. All electrical boxes should be carefully air sealed.

Aug 11, 2010 2:40 PM ET

If I have airtight walls,
by Miles

If I have airtight walls, won't the moisture condense on the cold side, regardless of whether it is plastic, drywall, foam, or other? Since there is no vapor barrier on the exterior of the house, shouldn't the moisture evaporate to the outside?

Aug 11, 2010 3:42 PM ET

Response to Miles
by Martin Holladay

You wrote, "If I have airtight walls, won't the moisture condense on the cold side?" What makes you think that airtight construction promotes condensation? The opposite is true. Leaky walls promote condensation, since exfiltrating air carries moisture that can condense on cold sheathing. The solution to this problem is air sealing.

In the article on this page, I wrote, "The main mechanisms by which moisture enters a wall are from the exterior (usually due to flashing defects that admit wind-driven rain) and via air leaks that carry “piggy-backing” moisture that condenses in a wall cavity. Vapor diffusion is a relatively insignificant cause of moisture problems in walls."

You are worried about vapor diffusion, but your worry is unnecessary. Vapor diffusion from the interior toward the exterior is not a problem -- especially in your climate. Air leakage is, so try to address air leakage.

If any moisture does accumulate in your wall (and it shouldn't), it will dry readily -- to the exterior in winter and to the interior in summer.

Aug 11, 2010 5:23 PM ET

Thanks Martin. I must admit
by Miles

Thanks Martin. I must admit that this is a rather confusing issue. If I understand correctly, condensation on the exterior side of air conditioned drywall is less of an issue that we've been lead to believe. If that's true then it makes sense that simply reducing or eliminating air movement through the walls would be beneficial.

Aug 12, 2010 6:28 AM ET

Condensation on cold drywall
by Martin Holladay

It's impossible for moisture to condense against cold drywall, as you fear, since drywall is hygroscopic. If drywall gets damp during the summer because of inward solar vapor drive, the drywall will dry readily to the interior.

While it's impossible for condensation to form on cold drywall, it is possible for condensation to form on cold polyethylene. Hence the rule: Never install interior polyethylene or vinyl wallpaper in an air-conditioned house.

To learn more about summertime condensation in air-conditioned homes, read When Sunshine Drives Moisture Into Walls.

Aug 27, 2010 10:30 AM ET

All the talk is about walls but I assume this holds for ceilings
by Blue Fan from MI

I have to agree with Miles, every time I think I understand the issue of vapor barriers I read more that makes me question what I read before. We are on border of zone 4and 5. While all the conversation keeps mentioning walls, sheathing, rigid insulation, rain shields, etc. I assume the message is the same for ceilings. My project is a masonry load bearing structure with masonry veneer and a wood truss roof structure. We are proposing to use a spray vapor barrier on the outside face of the block then 3 inches of rigid (c.i.) to avoid moisture hitting dew point in wall, air space and then brick veneer. At the ceiling, I was going to put polyethylene behind drywall covered with 15" of blown in cellulose. To deal with ceiling to wall issue we were going to use peel and stick vapor barrier (ice and water shield) to top of wall over wood plate and on to bottom of trusses then stick poly to it. Sounds like I should pull the poly out and avoid the peel and stick. Right?

Aug 27, 2010 10:38 AM ET

Response to Blue Fan
by Martin Holladay

Blue Fan,
If MI means Michigan, I'm not sure how you can be between Climate Zones 4 and 5. According to the DOE Climate Zone map, Michigan is divided into two climates zones: zone 6 to the north and zone 5 to the south.

In Michigan, you don't need ceiling poly, although the poly is unlikely to cause any problems if your attic is well ventilated. One problem with ceiling poly is that it can hold a lot of water in the event of a roof leak. You would probably be fine with vapor-retarding ceiling paint. Of course, your ceiling drywall should be installed in an airtight manner -- no can lights allowed!

You don't need a complicated vapor barrier connection (liek peel-and-stick) between the rigid foam on your wall and your ceiling -- but you DO need insulation over the top of your block wall. You need to be sure you have a continuous thermal barrier connecting the top of your wall foam with your cellulose layer in the attic. I think this might be a good place for spray polyurethane foam -- perhaps a HandiPack or two.

Aug 29, 2010 8:11 PM ET

Mixing Barrier Free Walls with Barrier Inclusive Walls
by Mark Benson

Hello Martin.

I appreciated this timely discussion as I am faced with a rather unique project right now that needs some vapor barrier solutions. We are building a 4000 square foot, traditional Japanese timber frame (that part already done - see for pictures) with light straw/clay walls a la Econest system and conventionally framed gables and roof right now in Southern Alberta, Canada.
We have found straw clay walls to be brilliant because they are completely breathable and the clay coating on each strand of straw protects the straw from moisture as well as giving the wall incredible moisture storage and capillary action when needed and resolves so many problems talked about in this forum. They are hard to build for gables though and the same system can't be used for roofs. So..., we need to come up with a congruent wall system for the gables and roof using conventional insulation products. The gable walls will match the straw clay walls at 1 foot thickness and the roof is being built upwards from a 2x6 T&G decking on top of the timber beams. In the wall, we are planning to use Rock Wool Batt at about R40 value and a heavy poly air barrier with acoustic sealant on the interior side (as per Canadian Code) which will be dry walled on the interior and sheeted with 1/2" plywood on the outside which will then receive a lime plaster. I am worried about inward solar vapor drive though, especially with a lime plaster stucco on the outside. There is no one around here that even considers this however. What are your thoughts? I could leave a sizeable air gap/ventilation gap on the exterior of the wall being I have 1 foot to work with.
Regarding the roof, we are looking at insulating with Icynene open cell insulation (like the breath-ability of the product - thoughts please!) at about R30-40 value. This will be sprayed directly onto either a poly eth air barrier (or "vapour barrier" as even the building inspector calls it) or peel and stick membrane that will be applied directly to the T&G decking, filling in between the 2x10 sleeper rafters part way up and then there will be a 4+ inch air gap left on top of that with cross ventilation between the cavities facilitated by the 2x4 strapping on top of the sleeper rafters on top of which 22 gauge standing seam roofing will be applied. The roof will be ridge vented and we are searching for a breathable yet water shedding membrane to be applied under the metal roof to keep condensation and/or perish the thought, a roof leak, from going into the open cell insulation below. What are your thoughts on this system? We have huge temperature swings here in the winter and summer. We can go from -30 degrees C to +10 degrees C in one day here in the winter and almost have the same variance in the summer.
The building has in-floor hot water radiant heat and an HRV unit but no air conditioning.
Any thoughts or advice would be GREATLY appreciated.


Mark Benson

Aug 30, 2010 8:05 AM ET

Response to Mark Benson
by Martin Holladay

Inward solar vapor drive shouldn't lead to problems if there is no air conditioning. However, your house will outlast the current residents, and air conditioning might be added in the future. In your climate, I don't think interior poly on the walls will lead to problems, but the climate is changing. You would be safer with MemBrain instead of interior poly.

Your roof sounds fine.

Aug 30, 2010 12:11 PM ET

Straw/clay and Conventional Mix
by Mark Benson

Thanks for the comments and consideration Miles.
After posting, my brain was turning. While there is no air conditioning, speaking from the experience of our straw/clay home we built last year, it stays almost air-conditioned cool in the summer on its own and thus my concern about inward solar vapour drive. In contemplating the 1 foot wall cavity available in the gables, my brother and I came up with a plan to vent the conventional wall cavity in through a hip roof that joins the building at the junction of the straw/clay top plate and where the conventional insulation starts, which will vent up and into the soffit and ridge venting on the roof, ... just in case.
In reading all your dialogue, if I may say, it seems that straw/clay construction (where the walls are more simple) solves all the problems discussed! I guess the centuries of anecdotal examples of such structures surviving mold free in a variety of climates bears witness of that as well. For anyone out there interested in knowing more about that option, you can look at our farm's website where we are slowly recording the building process and what we are learning at .

Thanks again,


Sep 8, 2010 12:06 PM ET

Vapor Retarder?
by Miles

Hi Martin,

I'm about ready to re insulate and am convinced by your article that no vapor barrier should be present however, what about craft faced insulation? Is there any real harm there? The ADA guide says even polyethylene vapor barrier can be used on exterior walls.

Sep 8, 2010 12:17 PM ET

Response to Miles latest question
by Martin Holladay

1. You can use kraft-faced fiberglass insulation if you want. The kraft facing won't cause any problems.

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

Sep 8, 2010 12:34 PM ET

Thanks Martin ^_^. This site
by Miles

Thanks Martin ^_^. This site has been invaluable.

Oct 6, 2010 11:04 PM ET

vapor barrier
by marge

We are renovating an existing room, installing new windows, have taken out the old fiberglass insulation because of previous moisture and mold issues. We are thinking of using ROXUL insulation. The room is over a crawl space, there is whole house AC and an April Air dehumidifer in the basement. The house is in Eastern Long Island in New York which can be a damp enviornment. The exterior is board and batten with plywood sheathing and 2x4 studs. Do you think a vapor barrier is needed and or is a good idea on the interior over the studs in use with the ROXUL?

Oct 7, 2010 4:49 AM ET

Response to Marge
by Martin Holladay

Vapor-retarder paint is all you need. Don't install interior polyethylene. If your building inspector insists on a vapor retarder that comes in a roll, use MemBrain.

If possible, install the drywall following the Airtight Drywall Approach to limit the flow of interior air into your stud cavities.

Nov 24, 2010 7:19 AM ET

New house in South Carolina
by jim

I am building a new house in SC and am curious about vapor barriers. Most of the discussion above seemed to focus on northern zones. Our external sheathing is OSB, tyvec, and Hardieplank. The attic has ridge vents. If I understand the threads correctly, it sounds like I should not use vapor retardants/barriers on the inside, but it would be ok (and probably advisable) to use the ADA. Any good refs to the ADA? Thanks.

Nov 24, 2010 7:27 AM ET

Response to Jim
by Martin Holladay

You are correct that in your climate, ordinary latex paint applied to your drywall is enough of a vapor retarder to meet your needs. You should certainly not install any polyethylene on the interior of your walls.

It's always advisable to minimize air leakage in your home, and the Airthight Drywall Approach (ADA) is one way to do that. Here are some links to get you started:

Nov 26, 2010 7:20 PM ET

tyvec and external sheathing
by jim

Thanks for the links. I learned a lot from the gov site. From inside the house, I can see that the external 7/16" sheathing has horizontal gaps of 1/16-1/4" between the sheets. These gaps are covered with a continuous layer of tyvec on the exterior. Do I need to caulk these gaps to establish an airtight wall, or does the tyvec accomplish that?

Nov 27, 2010 6:44 AM ET

Response to Jim
by Martin Holladay

Q. "Do I need to caulk these gaps to establish an airtight wall?"

A. No, you don't need to caulk the gaps.

Q. "Does the Tyvek establish an airtight wall?"

A. No, just because you have Tyvek doesn't mean you have an airtight wall.

If you want to establish an air barrier at your exterior sheathing, it's best to tape the seams between your OSB or plywood panels. This work has to be done from the exterior, at the time of construction, before housewrap or siding are installed. Read more here:

Airtight Wall and Roof Sheathing

One Air Barrier or Two?

Although some builders have experimented with trying to use Tyvek as an air barrier, the results are not as good as when the air barrier is created at the sheathing level.

Dec 15, 2010 6:22 PM ET

Foam in Michigan
by Wally

I am curently building a 3300 sq ft home in Michigan. I put 2 inches of Extruded on the outside, then tyvek, then an air space, then face brick. My question is what type of insulation should I use on the inside, and do I need a vapor barrier. Any help would be appreciated.

Dec 15, 2010 7:29 PM ET

Edited Dec 16, 2010 7:27 AM ET.

Response to Wally
by Martin Holladay

Your R-10 foam insulation may or may not be enough to keep your sheathing above the dew point; it depends on whether you are in Zone 5 or 6, and whether you have 2x4 or 2x6 walls. As long as you don't have 2x6 walls in Zone 6, you should be OK. See Calculating the Minimum Thickness of Rigid Foam Sheathing for more information.

I would recommend that you install dense-packed cellulose, not fiberglass batts, in your stud bays. Cellulose does a better job of filling gaps and odd-shaped cavities than fiberglass batts, and does a better job of reducing air leakage rates.

No, you don't want a vapor barrier. All you need is a vapor retarder.

Dec 22, 2010 12:47 AM ET

Stuck in indecisiveness mode
by ken

I have a old home (1923) in Portland, OR with a basement that was finished by a previous owner. The basement doesn't have the complete footprint of the main floor. Under one of my main floor bedrooms there is an unvented crawlspace which until last week has not been insulated. I used unfaced fiberglass bats (R-30) in between the joists. I am now wondering if I should lay a layer of vapor barrier (i.e. 6 mil plastic) on the floor of the crawlspace to further insulate. How about a vapor barrier over the concrete walls of the crawlspace (i believe the walls are technically called the footing or the foundation of the house)? The bedroom always felt cool and damp to me so i thought it would be a good idea. Would using a faced insulation draped over the walls be a better idea for controlling moisture?

Dec 22, 2010 5:53 AM ET

Response to Ken
by Martin Holladay

The best way to detail a crawl space is to close all vents permanently, to install a vapor barrier on the dirt floor, and to insulate the crawl space walls. This creates a sealed conditioned crawl space. You can read more here:

However, you have chosen to insulate between the joists, so it's a little late. I hope you installed a rigid material under the joists (plywood, OSB, or rigid foam) to protect the fiberglass batts.

Yes, you should certainly install a layer of 6-mil plastic on the floor of your crawl space. This poly won't help insulate, but it will reduce the evaporation of moisture into your crawl space.

If you choose to insulate your crawl space walls, don't use fiberglass batts. Use rigid foam (XPS, EPS, or polyisocyanurate).

Dec 22, 2010 2:23 PM ET

Thanks Martin for the reply.
by ken

Thanks Martin for the reply. Im curious what purpose the "rigid material under the joists" serve? And when you say "under the joists", you mean between the floor boards and the fiberglass bats correct in which case, yes it would be too late as the insulation is hung, but not a huge job to take it down and re-do since its only a 13'x8' area.
What means is available to hang rigid foam to concrete walls?
This website has been a great find! Thanks for all your help.

Dec 22, 2010 2:37 PM ET

Edited Dec 22, 2010 2:38 PM ET.

Second response to Ken
by Martin Holladay

If you installed fiberglass batts between the floor joists that form the ceiling of your crawl space, then the batts are exposed on the bottom. Over time, some of the batts will fall out or hang down; some will begin to get damp and grow mold; and others will be damaged by rodents.

Home inspectors see such conditions all the time in existing crawl spaces.

If you install a layer of plywood, OSB, or foil-faced rigid foam across the underside of the floor joists, you will protect the fiberglass batts and make it less likely that they will fall down.

If you decide to install rigid foam on the walls of your crawl space, the foam can be attached to concrete walls with foam-compatible adhesive. It can also be attached with concrete fasteners like Tapcons.

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