Vapor Retarders and Vapor Barriers

More confusion
by Garth Sproule

Martin
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...

Good point.
by Lucas Durand

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.

Poly air barriers
by Martin Holladay, GBA Advisor

Garth,
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.

Two more unfortunate aspects
by Garth Sproule

Martin
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???

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.

Foam sheathing permeance
by Martin Holladay, GBA Advisor

Danny,
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.

Chart question
by Garth Sproule

Martin
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.

Yes, assume that the cavities are insulated
by Martin Holladay, GBA Advisor

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

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.

Don't forget basements and crawl spaces
by Martin Holladay, GBA Advisor

Kevin,
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.

Thanks
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?)

Vapor barriers and vapor retarders
by Martin Holladay, GBA Advisor

Danny,
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.

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.

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?

Unfaced fiberglass batts
by Martin Holladay, GBA Advisor

Greg,
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.

yup, it's confusing all right
by Bill Rose

Martin,

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.

Thanks for your perspective
by Martin Holladay, GBA Advisor

Bill,
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?

prescription
by Bill Rose

Martin,

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.

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.

You might have missed it...
by Martin Holladay, GBA Advisor

Dave,
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.

Brick Buildings
by Mark Hutchinson

Martin,
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?
Regards,
Mark Hutchinson

It sounds like your approach will work
by Martin Holladay, GBA Advisor

Mark,
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.

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.

Response to Adrian
by Martin Holladay, GBA Advisor

Adrian,
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.