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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, 2015

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

Jan 4, 2011 7:53 PM ET

Seattle wall assembly
by Green Builder

I'm working on a project in the Seattle area, and the wall assembly is as follows: 5/8" drywall, Certainteed Membrain VR, 5.5" R-21 unfaced fiberglass batts, 7/16" OSB Zip System, 1' rigid xps, 1/2" air cavity and cladding. Do we need the membrain or will latex paint suffice? Also, would cellulose insulation be a better choice due to its hygroscopic capacity, etc? There will be HRVs, and it's possible that some units will opt for mini split heating/cooling systems. Is 1" rigid enough? Is 2" the "right" amount?

Jan 5, 2011 6:02 AM ET

Response to Green Builder
by Martin Holladay

Green Builder,
Many of your questions are answered in an other article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

You are in Climate Zone Marine 4. Since you have 2x6 walls, your foam sheathing should have a minimum R-value of R-3.75. Since you are using XPS, with an R-value of 5, you are fine.

You don't need MemBrain; your interior paint will suffice.

Dense-packed cellulose will perform better than fiberglass batts, because it does a much better job of filling odd-sized cavities.

Jan 11, 2011 7:16 PM ET

Edited Jan 11, 2011 7:25 PM ET.

Vapor Barriers
by tinknocker

I amin the Philadelphia area. I have been ripping apart residential dwellings over the past 30 plus years. Moisture seems to form from obvious condensation and do damage toward the exterior of the building and this is evidence of condensation. I have seen less of a problem on open wall cavities where on these older homes, an almost complete permeance was available and especially toward the exterior. These buildings usually have clapboard and the exterior vents much more than the interior plaster walls. I agree this is rare but it does happen and usually an interior humidity problem is evident.

I have seen many homes where the newer paints are being used, moisture is migrating through to the outside to condense on the high quality paint that forms a seal around the building. Water bags are the result. In the same communities and with the same construction I have seen the same type of home stained with no issues. The stain leaves the venting properties of the wall intact. I have also seen the use of spacers under the clapboard installed before the painting and with no issues. I have opened up walls to find the evidence of condensation on or near the exterior of the building. The "water bags" are very common in this area.

All of these situations and all of these examples (and I have hundreds) lead me to believe that in this climate, a barrier should be present on the interior of the building. I also believe the so-called "permeable" housewraps act as functional vapor barriers on the exterior of the building in this climate (when it is cold).

When the moisture migrates past the interior in the winter and into the walls or attic, mold can be found being fed by the moisture.

Jan 27, 2011 10:51 PM ET

Sprayed polyurethane foam in cold climates
by Philippe

Hi, I live somewhere when it gets very cold and dry in the winter (normal for January is approximately -10 C). and hot and humid in summers. So the vapor can goes both ways. My foundations are not damp proofed actually. What do you think of a material that has 0.3-0.5 perm on the interior side? Can it cause capillarity and potential damages to the sill plate if there is not a capillarity barrier betweed the foundation and the sill plate? Thank you.

Jan 28, 2011 6:27 AM ET

Not enough information, Philippe
by Martin Holladay

Knowing the permeance of a material is not enough information to determine whether its use is appropriate for installation on the interior side of a foundation. So please tell us:

1. What type of foundation is it? Stone and mortar? Brick and mortar? Poured concrete?
2. Is it a crawl space or basement?
3. What is this material? Is it an insulation? A vapor retarder? Wallpaper?

The problem of sill rot due to capillary action is a different problem than determining the best permeance for finish materials installed on the interior side of your foundation wall.

Jan 28, 2011 12:59 PM ET

Concrete and 2x6 exterior wall.
by Gregory Erickson

I have a basement that I am finishing. The bottom portion of the wall(below grade) is concrete with a 2x6 wall9above grade) on top of it. The 2x6 wall is filled with insulation and covered with poly. I have covered the concrete with XPS and built a full 2x4 wall in front of that. I will be putting unfaced batts in the 2x4 studded wall. Should I remove the poly from the 2x6 portion and then make sure that insulation also fills the void that is created between the 2x6 and 2x4 walls. I know that I want fiberglass in front of the XPS but don't know what to do above the concrete. Please advise. I should mention that I am in zone 6, North Dakota.

Jan 28, 2011 1:04 PM ET

Response to Gregory Erickson
by Martin Holladay

Q. "Should I remove the poly from the 2x6 portion and then make sure that insulation also fills the void that is created between the 2x6 and 2x4 walls?"

A. Yes.

Q. "I know that I want fiberglass in front of the XPS."

A. I usually advise people to avoid the use of fiberglass below grade. In general, it's almost always better to add a thicker layer of XPS (or another type of rigid foam) and to skip the fiberglass completely.

In your case, since the wall is already built, it's probably OK to proceed as you have planned.

Jan 28, 2011 1:09 PM ET

Will I need a vapor retarder
by Gregory Erickson

That is what I will do. When the wall is finished should I use a vapor retarding paint of will a normal paint and primer suffice? Thank you!!!!

Jan 28, 2011 1:18 PM ET

Paint question
by Martin Holladay

In your climate, which is quite cold, it's probably best to use vapor-retarder paint. Be sure to pay attention to air sealing when installing your drywall, especially around electrical boxes. Air tightness is even more important than the vapor-retarder paint.

Jan 28, 2011 9:36 PM ET

air sealing
by Gregory Erickson

Thank you Martin!

What extra methods do I have to do when hanging the drywall to make it as air sealed as possible.?

Jan 29, 2011 6:52 AM ET

Airtight drywall
by Martin Holladay

Follow the Airtight Drywall Approach. You can Google those three words for more information, or you can read an article I read on the topic. Here's the link:

Jan 7, 2013 2:44 AM ET

West coast marine climate vapor barrier in crawl space
by L Buser

The walls in my crawl space are 2 feet of concrete block below and just a bit above grade, with two feet of wood framed wall above. There is a concrete floor and the area is considered part of the house. The exterior is painted cedar over 'tar paper' and then plywood. I have caulked the interior around the frame and siding, and insulated with Roxul bats. The house is on Vancouver Island: maritime cool wet winters, warm summers. Do I need to use a vapor barrier (polyethylene) and/or air retarder? Which or both? (I found after reading this article and others that I wasn't sure of what would be best.)

Also, has anyone used double sided tape to apply polyethylene vapor barrier to wood framing? I don't like the petrochemical smell of acoustical sealant and since it does not dry, adds to indoor pollution (unless covered with gyproc, which I don't plan to use in a crawl space).

Jan 7, 2013 6:01 AM ET

Edited Jan 7, 2013 6:06 AM ET.

Response to L. Buser
by Martin Holladay

L Buser,
You don't want to install an interior polyethylene vapor barrier, for the reasons mentioned in this article. Instead, use vapor retarder paint or a smart retarder like MemBrain.

You always need at least one air barrier. In your case, I recommend that you follow the Airtight Drywall Approach. For more information, see:

Airtigtht Drywall

Questions and Answers About Air Barriers

Feb 1, 2013 12:05 AM ET

Is it the same for a concrete block wall?
by L Buser

Thanks for your reply to my earlier question.
My basement has a 3.5-foot high concrete block wall, with a standard framed wall above. The concrete block is currently finished with 2x4 framing, fiberglass insulation and poly vapor barrier covered with gyproc, which I know is not what is recommended but that is how we bought the house. I want to change it to solid foam insulation as recommended, with slats or 2x3 and gyproc.
Since concrete blocks are more porous than poured concrete, and since there are the indented spaces along the mortar joints , would those pose any problem with using solid foam, taped and sealed? My concern is the highly porous nature of concrete blocks and the air gaps. The exterior has black sealant and good drainage.
Also would using the metal fasteners possibly crack the concrete blocks or mortar joints and should I avoid using those and just use adhesive?

Feb 1, 2013 8:17 AM ET

Edited Feb 1, 2013 8:18 AM ET.

Response to L. Buser
by Martin Holladay

L. Buser,
Q. "Since concrete blocks are more porous than poured concrete, and since there are the indented spaces along the mortar joints, would those pose any problem with using solid foam, taped and sealed?"

A. No. Just be sure that each piece of rigid foam has a strong bead of adhesive around its perimeter, to provide an air seal. In addition, you should tape the joints of the rigid foam with a high-quality tape. The easiest type of foam to tape is foil-faced polyiso.

Q. "Would using the metal fasteners possibly crack the concrete blocks or mortar joints and should I avoid using those and just use adhesive?"

A. As far as I know, you can use Tapcon fasteners on concrete block walls. These require pre-drilling. But frankly, I've never used Tapcons on concrete blocks; so if I were you, I would verify this information with your fastener supplier.

If you want to use adhesive only, you can. However, be prepared with a pile of plywood scraps, 2x4s and braces to provide pressure for several hours after the foam is installed.

Mar 16, 2013 6:24 PM ET

Edited Mar 16, 2013 6:31 PM ET.

Vapor barrier in garage recording studio
by Mark Nelson

Hello--this is a wonderful forum--thank you!

I have what I think is an unusual situation, and I hope that I haven't made an error that I can't recover from.

Here is a quick description of my building: it is a garage that I 'm converting to a recording studio, and one of the unusual aspects of the construction is that it is a "room within a room" design, meaning that there is an entire building structure built inside the outer shell of the building, with a 2" gap separating them. The outer and inner shells do not touch anywhere, and the reason for this is that the sound isolation is much more effective that way.

The outer shell has cedar siding, 15# siding felt, yellow Dens Glas sheathing over the 1910 era 1x8 sheathing, and then finally 5/8" drywall attached to the inside, between the studs. All of this additional sheathing is necessary to give the structure more mass, and make it less prone to transferring noise. Between the studs of the outer shell is R13 fiberglass unfaced insulation.

The inner shell is unusual in that it is framed inside-out. in other words, the drywall is facing the outside of the building, while the studs are facing inside. It is built this way to save space in the narrow room, as necessary acoustical treatments can be installed between the studs, making the room "sound" better without sacrificing interior space. The walls are built on the floor and then tipped into place.

The vaulted ceilings of the two shells are built as follows: The outer shell ceiling has two layers of 5/8" drywall covering r-21 fiberglass unfaced insulation. The roof framing is scissors trusses, and there are soffit vents running the length of the building, along with a ridge vent. The inner shell ceiling has two layers of 5/8" drywall over r-19 unfaced fiberglass insulation with a poly vapor barrier.

So finally, here's my question: as you can see, this construction method in unusual, and I got confused about where the vapor barrier for the walls should go. I built the inner shell without a VB behind the drywall and I have no way of getting it in there now. After reading this forum, I'm not even sure if I should have it. Would it make sense to paint the entire inner wall, framing, drywall, and all, with a Vapor barrier paint? I should mention that all the seams, holes, etc, have been caulked prodigiously with acoustical caulk. The inner space, while sealed extremely well, will have an air intake with an in-line fan and silencer along with an exhaust vent with a silencer at the opposite end of the room, high on the wall. This system moves fresh air into the room quite efficiently, and is necessary, since the construction is so air-tight. I will also have a mini-split heat pump in this space that will be used for heating and ac. I'm in Seattle, so ac doesn't get used a lot, but with a room full of hot, sweaty musicians, it will.

Thank you in advance for reading this lengthy post. I hope you don't tell me I have to tear walls down!!

Mar 17, 2013 6:33 AM ET

Response to Mark Nelson
by Martin Holladay

Your post was long, and it took a while to glean the important information. As I was reading it, I was thinking, "What's the climate?" You finally told me, near the end: Seattle.

As you probably know, airtight details matter more than vapor barriers. It appears that you have paid close attention to air sealing, so that's good.

You don't need any polyethylene vapor barriers in your climate. Polyethylene vapor barriers can cause problems, especially in buildings that are air conditioned. As far as I can tell, you installed a polyethylene vapor barrier in your roof assembly, but not your wall assembly.

Was that polyethylene vapor barrier a good idea? Probably not. Will it cause any problems? Probably not (especially because you have ventilation from your soffits to your ridge vent).

You didn't mention whether you installed any baffles to create an air barrier above the unfaced fiberglass batts in your roof assembly. (These baffles would separate the insulation from the ventilation channels.) Such baffles would have been a good idea.

Now that everything is built, I wouldn't worry about anything. My guess is that everything will be fine.

Mar 18, 2013 1:08 AM ET

Edited Mar 18, 2013 1:03 PM ET.

Response to Martin Holladay
by Mark Nelson

Thanks so much for your response. Yeah, I know it was long--sorry about that--I just thought there was a lot of information I needed to impart to give an accurate picture of my unusual construction.

To answer your question about baffles, since the roof has scissors trusses, the only area where there would be danger of blockage of the vents would be right near the soffits at the bottom of the roofline. I installed chicken wire (not very elegant, but effective) to ensure that the insulation didn't block the vents, so I'm confident that the air will move freely.

I wish I had learned about this site before I installed the vapor barrier in the ceiling of the inner shell, but I guess there's not much I can do about it now, short of tearing out a double layer of drywall. My reasoning with putting the barrier in the ceiling of the inner shell was that any moisture that got through the inner shell would also have to get through the outer shell above it before it was vented through the ridge vent above that. I thought with the vb and the fresh air circulation through the inner room I could keep moisture from collecting between the two shells. That was my reasoning, anyway.

Anyway, unless you have any additional opinions about this, I guess I'll stop losing sleep over it.

Thank you, Martin, I really appreciate your input!

Mar 18, 2013 1:05 PM ET

And one last question for Martin...
by Mark Nelson

I have one section of the inner ceiling shell that I have not yet covered, which is about 45 square feet out of 250 square feet of total roof area. I'm assuming I should just leave the VB off of this section, unless there's are reason I should install it since the rest of the inner shell does have a VB. any thoughts on this?

Thank you!

Mar 18, 2013 1:08 PM ET

Response to Mark Nelson
by Martin Holladay

Don't install any polyethylene.

Mar 18, 2013 2:52 PM ET

Response to Martin Holladay
by Mark Nelson

Ok--I will steer clear of the poly from here on out. Thank you for the advice.

Mar 28, 2013 6:01 PM ET

by Andrew Gross

I have been researching like mad to find an answer to my specific room problem. I just added a small bathroom to a house in Iowa that was built in the late 80's. Plastic vapor barrier was installed before drywall in outside walls and ceilings when the house was built new. My question is this: Would it be better to omit the vapor barrier in the remodel, especially the ceiling of the new bathroom so as to keep water from permeating the ceiling sheetrock, getting stuck between the sheetrock and vapor barrier and staying wet forever? This house has a vented attic space. Is the above blog/article suggesting that vapor barriers in my climate are now not preferable? I am dying to know the answer.

Mar 29, 2013 8:07 AM ET

Response to Andrew Gross
by Martin Holladay

Q. "Would it be better to omit the vapor barrier in the remodel?"

A. Yes. There is no need for interior polyethylene in your climate. Of course, it is vitally important that you pay attention to airtightness when you build the addition.

Q. "... especially the ceiling of the new bathroom so as to keep water from permeating the ceiling sheetrock, getting stuck between the sheetrock and vapor barrier and staying wet forever?"

A. Moisture can't get "stuck" between your ceiling drywall and the polyethylene. During the winter, your indoor environment will be different from the outdoor environment. The interior will be warm and humid, while the exterior air will be cold and dry. That's normal. The moisture contained in the interior air should stay put. As long as you pay attention to airtightness when you build your ceiling, there won't be any air leaks through the ceiling that can carry moisture. The amount of moisture that moves through the ceiling by diffusion won't be enough to worry about, as long as you have an adequate layer of insulation, installed carefully. If the insulation is air-permeable (like fiberglass), you'll need ventilation above the insulation layer. If the insulation isn't air-permeable (for example, spray polyurethane foam), then you won't need any ventilation above the insulation layer.

Mar 29, 2013 4:56 PM ET

by Andrew Gross

Thanks for the above thoughtful response. You stated that I need to "pay attention to airtightness when you build your ceiling..." I have a bath fan and 4 IC can lights in this bathroom remodel. Are you suggesting I do something to seal those???

Also, I have already applied two coats of vapor retarding paint/primer to the ceiling of this bathroom just as additional info.

Thanks again. I feel like I am finally on a path to get this bathroom done. I have already installed the plastic vapor barrier and the drywall is finished. Based on your comment above, I plan to remove it from the attic side. I only installed it because that's how it had been done before (construction in the late 80's).

I just need to know if I need to do anything with the bath fan and can lights.


Thanks again!

Apr 3, 2013 9:03 AM ET

more info
by Andrew Gross


Did you have any thoughts on my last post in response to your reply to me?


Apr 3, 2013 9:26 AM ET

Edited Apr 3, 2013 9:28 AM ET.

Response to Andrew Gross
by Martin Holladay

Most green builders who pay attention to airtightness avoid the use of recessed can lights in an insulated ceilings. These fixtures are notoriously leaky; they take up room in the ceiling assembly that should be filled with insulation; and they create "hot spots" which accelerate the stack effect and ice damming problems.

You have two choices:
1. You can remove the can lights now, before it's too late, and repair the holes in your drywall ceiling. Then install surface-mounted fixtures instead.

2. You can learn to live with the recessed can lights, and their energy penalty, and you can do your best to repair the damage from the attic side. If you go this route, you should encapsulate each can light in an airtight box made of drywall or rigid foam. These boxes must have all seams carefully sealed with high-quality tape or canned spray foam. Each box needs to be sealed to the drywall from above. The boxes need to be large enough to provide several inches of air between the box and the recessed can. Then pile insulation on top of the boxes.

Carefully seal the crack between the drywall and the exhaust fan housing with high-quality caulk.

Aug 23, 2013 1:28 PM ET

Insulation in Zone 5
by Harry Shoe

I am located at the northern end of NJ. Zone 5 according to the map.

For a upstairs add-a-level project (new 2nd floor and attic), is the following flash and batt ok?

3 in of closed cell spray foam in attic, 1inch of closed cell spray foam in walls, with fiberglass batting

Additional information:
* wood construction
* standard shingles
* standard siding - though a question is, do we still need rigid foam board on the outside with the closed cell foam on the inside?
* this would result in an unvented & insulated attic space. The floor of the attic would be left uninsulated so that it is within the rest of the building envelope.

I have a constructor lined up but I am concerned because the insulator would not provide any sort of guarantee. I couldn't figure out what the dew point would be.

Alternative is to go with standard vented attic with fiberglass in the floor of the attic. Tried and true and crappy...

Thank you!

Aug 23, 2013 1:51 PM ET

Response to Harry Shoe
by Martin Holladay

Your plan will work. According to the building code, the type of unvented roof assembly that you are planning to install will only work (in your climate zone) if you have a minimum of R-20 of foam above your air-permeable insulation. Three inches of closed cell spray foam will give you an R-value of close to R-20, so you'll be OK.

As far as I know, most states impose guarantees on the work that contractors perform in homes. Even if your contractor says that he can't provide a guarantee, state law probably requires him to do so.

For more information on this type of insulated roof assembly, see Creating a Conditioned Attic.

Aug 25, 2013 3:55 PM ET

Is polyethylene vapour retarder needed in northwest coast home?
by C Calderwood

Lots of interesting comments here. Thanks folks. I have a stick frame old house (over 100 years old) with wood shakes and siding. The walls are in great shape after over a century of no vapour barrier and no insulation. We are renovating. We have installed fibreglass insulation in the exterior walls. Given our moist (living close to the ocean) and temperate climate (never gets very hot or very cold) and not needing air conditioning, With lots of cooking and showers, we have condensation inside the double glazed windows during the winter. Is it healthy for our house to have polyethylene on over the insulation installed over the fibreglass insulation on the interior side of the wall? I'm wondering about rot and mold in the walls.

Aug 26, 2013 6:43 AM ET

Edited Aug 26, 2013 7:30 AM ET.

Response to C. Calderwood
by Martin Holladay

Q. "We have condensation inside the double-glazed windows during the winter. Is it healthy for our house to have polyethylene on over the insulation installed over the fibreglass insulation on the interior side of the wall?"

A. The condensation on the interior side of your windows during the winter is a symptom of a problem. The solution has nothing to do with polyethylene in your walls.

Your interior relative humidity is too high. That's why you are seeing condensation. You need to lower the indoor relative humidity. The easiest way to do this during the winter is to increase your ventilation rate. If your home doesn't have an HRV, you can begin by running one or two of your bathroom exhaust fans for 24 hours a day until the condensation goes away. Then you can ventilate for fewer hours.

Once you've done that, it's time to find out why you have so much indoor moisture. The usual culprits include a wet basement or crawl space, too many houseplants, wet hobbies (like having many fish tanks), storing firewood indoors, drying clothes on indoor racks, taking lots of showers, and doing lots of cooking.

Some of these issues can be addressed by fixing problems (especially a wet basement or crawl space). Others involve habits that you may not want to change. If you are unwilling to reduce the indoor moisture load, you need to increase your ventilation rate.

High indoor humidity can definitely cause moisture problems in your walls, so you will be doing your house a favor by getting your interior RH under control. In your climate, I do not recommend the use of interior polyethylene, for all of the reasons stated in the article on this page.

Aug 26, 2013 5:56 PM ET

Edited Aug 26, 2013 5:56 PM ET.

Healthy Home Humidity
by C Calderwood

Hi, We have a humidistat connected to our ventilation. We can set the percent of humidity we want to try to maintain. What percent humidity is healthy for a wood frame house with fibre glass insulation in the exterior walls and painted wood exterior siding?

Aug 27, 2013 6:03 AM ET

Response to C Calderwood
by Martin Holladay

C Calderwood,
It makes sense to aim for 20% to 40% relative humidity during the winter, and 60% during the summer. Remember:

1. You can't use ventilation to lower the indoor relative humidity during the summer. Ventilation only works for that purpose during the winter.

2. Many humidistats and humidity meters are poorly calibrated. If you still have condensation on your windows, your indoor RH is still too high, no matter what your meter says.

3. You still need to identify the source of the moisture in your house and rectify the underlying problem.

Mar 2, 2015 3:45 PM ET

Edited Mar 2, 2015 3:47 PM ET.

Bathroom in Sunny South Carolina Vapor Barrier
by Tom Juchum

Great information! I live in South Carolina. I am re-doing my bathroom tile. I have Kraft faced insulation on the exterior inside wall, and am going to put up Hardie backer board over it.

There is a lot of conflicting information on the Internet. Most tiling forums suggest putting up a vapor barrier behind cement board, with the requirement to slit the Kraft faced insulation. Reading this posting I get the idea I should not put plastic 6 mil up on the wall.

No plastic vapor barrier? And if I may ask, why is it so encouraged to do so if it is the wrong thing to do. Would I have the same problem if I used a product like RedGuard instead of plastic?

Thank you!

Mar 2, 2015 4:06 PM ET

Response to Tom Juchum
by Martin Holladay

There are two issues here. If the wall tiles are part of a shower surround or tub surround, it makes sense to ensure that the installation is waterproof. A variety of products are available for this purpose.

If the wall is not part of a shower surround or tub surround, ordinary wall construction advice applies. In all climates, it makes sense for a wall to include an air barrier. In South Carolina, walls should never include an interior vapor barrier (polyethylene), for the reasons explained in this article.

Sep 14, 2015 10:33 AM ET

Exterior foam shower vapor
by Stan Smith

Hey Martin....I'm getting ready to put up hardi backer on the shower walls but am confused about what to do with a vapor barrier behind it. Two of the walls are exterior which has a few inches of exterior ISO. Any advice on what to do for a vapor barrier behind the tile walls? Thanks

Sep 14, 2015 10:46 AM ET

Response to Stan Smith
by Martin Holladay

You posted your question in two locations. I have posted my answer on the other page where you posted the same question:

Jun 13, 2016 1:56 PM ET

Utterly confused
by Tony Tibbar

The wall I'm planning to build looks like this: Aluminium siding, 2" EPS, plywood, 2x4 frame with Roxul batts, drywall, non-vinyl wallpaper.

I've not yet decided it will be air-conditioned. (60% it does)
What air/moisture retarder/barrier you advise where in the wall?
I'm living in zone 5A near the border of zone 4.

Jun 13, 2016 2:11 PM ET

Response to Tony Tibbar
by Martin Holladay

I assume that we are talking about a new house that has not yet been framed. Right? If that's what you are designing, you may want to read this article: How to Design a Wall.

In your case, the easiest location for the air barrier is at the plywood sheathing. If you tape the seams of the plywood sheathing with a high-quality tape like Siga Wigluv, and seal all air leaks at penetrations (windows, plumbing penetrations, and wiring penetrations), then the plywood will be a high-quality air barrier.

Your water-resistive barrier (WRB) can either be installed between the plywood and the EPS, or it can be installed between the EPS and the siding. This article explains more: Where Does the Housewrap Go?

It's even possible to use EPS as a WRB, but I don't recommend that approach.

Your EPS is a vapor retarder, so you don't have to worry about inward solar vapor drive. If you live somewhere where there is a building inspector who wants you to install an interior vapor retarder, you should install a layer of vapor-retarder paint on your drywall.

For more information on the building science principles underlying walls with exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Sep 13, 2017 5:50 PM ET

Fiberglass Wall Batt Kraft Paper on Exterior Side in North GA
by Tom Gotschall

I just heard of a home builder installing the batt insulation with the kraft paper on the exterior side. Now, even in "Hotlanta", there is more cooling degree days than heating degree days. However, depending on the year and lifestyle, it is often a toss-up as to whether there are more cooling days than heating days.

So given this climate situation, if this home is reasonably well sealed (passes the blower door test as required by Georgia), if the home builder placed the batt insulation on the "cold in winter side", what would be the down-side? The home already has drywall and the home builder is not willing to remove and replace according to the prescription.

Sep 13, 2017 6:03 PM ET

Edited Sep 13, 2017 6:05 PM ET.

No down side at all. @ Tom Gotschall
by Dana Dorsett

Asphalted kraft facers are "smart" vapor retarders, and become substantially vapor open when there is enough moisture in the cavity to become a mold issue. Asphalted kraft runs about a half-perm when bone dry, but over 5 perms under high humidity (more vapor open than standard latex interior paint.) If they were FOIL facers there is a potential condensation problem during peak cooling season, but with kraft there is no issue.

If it has plywood or OSB sheathing, the sheathing is itself a smart vapor retarder, limiting the rate at which outdoor moisture can get in via vapor diffusion.

Unlike cooler climates kraft facers offer almost no resilience upgrade (on either side of the assembly) in Atlanta's climate. Facers only make it harder to inspect the installation quality.

[edited to add]

I just remembered Allison Bailes' blog on this very topic a few years back:

Sep 14, 2017 5:15 AM ET

Response to Tom Gotschall (Comment #89)
by Martin Holladay

Dana Dorsett is correct. That said, builders should remember that they are required by building codes to install materials according to manufacturer's instructions. If the manufacturer of the kraft-faced batts requires that the insulation be installed with the kraft facing on the "warm-in-winter" side, then that's the way that the batts have to be installed -- or the builder is at risk of a contract dispute that could land him in court.

In this case, it makes no sense to make any changes. But builders should be aware of the risks of violating manufacturers' instructions.

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