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Choosing a Cost-Effective Wall System

Those pesky questions about air barriers and vapor barriers just won’t go away

Posted on Jun 18 2012 by Scott Gibson

Erik Olofsson is planning a small house in the Rocky Mountains of British Columbia. Ideally, he’d like to get the walls close to R-40. The question is how.

“Seeing that the received opinion around GBAGreenBuildingAdvisor.com is the tandem of polyethylene sheeting and exterior rigid foam is not ideal, what do the builders on this site recommend?” he asks in a post at the GBA Q&A forum. “Larsen trusses seem fairly labor-intensive and rigid foam is expensive ... Is a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. the answer?”

A complication is a local building code that apparently calls for a polyethylene vapor barrier on the warm side of the insulation. Although once a common building technique, it’s no longer universally accepted by building scientists as the best practice in all climates. Many builders have abandoned the use of interior polyethylene, even as some building inspectors continue to insist on it.

Olofsson’s quest for high performance at a reasonable cost, while solving the riddle of air and vapor barriers, is the topic of this month’s Q&A Spotlight.

Double-stud walls a good option

Double-stud walls are designed to provide lots of exterior wall volume for insulation while sharply reducing thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. . John Klingel and Albert Rooks are among those who think that building double-stud walls is a good approach.

“I built a double-stud in 1980 and have never regretted it,” Klingel writes. “New house will be the same, but thicker, and with dense-packed cellulose instead of fiberglass.”

“A double stud with a plywood exterior and interior poly and ADA [the Airtight Drywall Approach] will work,” Rooks says.

GBA senior editor Martin Holladay doesn’t push the double-stud option, but he does point Olofsson toward a number of GBA articles that have been written on the topic (see the “Related Articles” sidebar).

Or consider 2x6 framing

Rooks also offers an alternative for Olofsson to consider. “Another good quality wall is a 2x6 standard frame with taped plywood or OSB exterior + WRB [water-resistant barrier] + 4 in. to 6 in. of high-density mineral wool,” he says. “It will eliminate cold sheathing and rim-joist bridging while allowing the use of a service cavity in the stud bays.”

A key detail of all of these recommendations is that a polyethylene vapor barrier isn’t necessary. In the case of the wall assembly suggested by Rooks, the OSB or plywood sheathing becomes the “air/vapor control layer.”

“Use a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. detail, good air sealing, and ventilation,” Rooks adds. “I'm a fan of mineral wool because it doesn't settle, doesn't rot even if continually wetted, is fireproof, won't support mold or bugs. It's like a little piece if the Canadian Rockies (since it's made of Canadian Basalt) covering your house, and near as durable.”

Holladay, too, think there are alternatives to the polyethylene required by local codes. “You might want to negotiate with your local building inspector,” he says. “Many inspectors will accept MemBrain or vapor-retarder paint as an alternative to interior poly.”

Deciphering Canadian codes

Malcolm Taylor writes that the British Columbia building code allows an “airtight drywall air barrier” instead of polyethylene. “Illustrated details covering all aspects of the approach can be found in the Building Envelope Guide for Houses published by the Homeowner Protection Office, which is a Provincial Government department,” he says.

But to Holladay, there seems to be some confusion in the codes over the roles that air and vapor barriers play in a wall assembly. “I can't help but shake my head at the inconsistency in the BC code,” Holladay says. “Do code officials think that polyethylene is a vapor retarder or an air barrier? If they think it is a vapor retarder, then the Airtight Drywall Approach is no substitute. To retard the flow of vapor, you need vapor-retarder paint. The airtightness of the drywall is irrelevant.

“If they think it is an air barrier, I wonder whether they require airtight installation details for all installations of poly in the province? For example, do they verify that the poly seams are sealed over framing members with Tremco acoustical sealant? Do they verify that all electrical boxes are airtight boxes? If they do, bless them. However, I doubt that they do. I think it's far more likely that the building inspectors have no idea whether the poly they insist on is supposed to be a vapor retarder or an air barrier.”

Taylor says he can’t speak to how the code is enforced in the province as a whole, but at least on Vancouver Island and the lower mainland, inspectors “insist on meticulously sealed poly.”

“All seams, the joint between poly and lower plate and plate and subfloor all must have generous amounts of acoustic sealant,” he says. “Gasketed electrical boxes are also required.”

In Ontario, adds Lucas Durand, there’s apparently a little more flexibility. “For the house that I am building for myself,” he says, “I have gone with taped 1/2-in. plywood for both air barrier and vapor retarder (and racking resistance). There was some initial skepticism from my inspector but it didn't take that long to convince her — I am lucky she has been open-minded.”

Creating sensible regulations

Some of the material in the Canadian publication gives Holladay reason to wonder whether government officials really understand the science behind code requirements. “As is often the case,” he writes, “this is an example of government bureaucrats enforcing regulations that are based on an incomplete understanding of the relevant scientific principles.”

While that could very well be true, Olofsson’s potential tussle with local officials could have an upside. “The nice thing about your questions is that it is exposing that you can move the envelope quality in your area forward,” Rooks says. “The code seems to allow enough room for well planned modification. Lucas points out a few more methods towards improved assemblies. They are all great when executed correctly.

“There is no shortcut to a ‘quality envelope.’ Plan on it being more cost and work. I think it's the only sensible thing to do.”

Our expert’s opinion

Here’s how GBA technical director Peter Yost sees it:

The reason Martin Holladay and I nearly always agree on building science questions is that we both have studied under/worked with some of the best: Joe Lstiburek, John Straube, Terry Brennan, Bill Rose, Anton TenWolde.

And one of the reasons there is still such confusion on basic heat transfer and moisture flow is that too many building inspectors and code officials have not.

Is this silly or what?

  • The highest priority in moisture management is bulk water: how many building inspectors check the connections between the water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. and flashings at penetrations for continuity?
  • The next highest priority in moisture management is capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. water: how many building inspectors check for capillary breaks between porous building components?
  • The next priority is air-transported moisture: how many building inspectors require qualitative and quantitative information from blower-door tests?
  • But by gosh, almost every building inspector insists on and inspects the “warm-in-winter-side” vapor retarder, by far the least important wetting mechanism in nearly all buildings and climates.

And we should not be worried about the vapor permeability of just that one dedicated layer in terms of wetting, but the vapor permeability of ALL layers in terms of drying. (For more information, see my Vapor Profile blog.)

GBA has a series of great resources on vapor retarders and air barriers and their differences. Use your GBA project folders, make one for your building inspector(s), and fill it up with building science reading for them!


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Image Credits:

  1. National Renewable Energy Laboratory

1.
Mon, 06/18/2012 - 08:47

Peter nailed it (of course)
by Allison A. Bailes III, PhD, GBA Advisor

Helpful? 1

Yes, I think we all know that there are plenty of writers and enforcers of the building codes out there - as well as plenty of builders, trade contractors, HERS raters, & others in the home building industry - who don't understand the purposes of the various control layers. That's especially true for air barriers and vapor retarders. There are also some folks who do understand these things and know how to apply them, and their number is growing.

Regarding the control of moisture, Peter nailed it at the end of the article with his hierarchy of the forms and paths of moisture control. Last month when I spoke to the Building Science Discussion Group in Portland, Maine, the discussion turned to the topic of condensation and moisture damage in building assemblies. As I recall, not a single one of the builders, contractors, designers, or anyone else in the room could think of a single case where it was diffusion rather than air leakage that caused moisture damage. And they're in IECC climate zone 6!

We spend a lot of time talking about vapor retarders, and the discussion seems to have evolved from "Make sure you put in a vapor barrier" to "Be careful about putting in materials with permeance ratings that are too low." That's a good thing. Just ask the folks in North Carolina who don't have to put plastic in their walls anymore - and repair the resulting damage.


2.
Mon, 06/18/2012 - 09:02

Diffusion vs. Infiltration
by Allison A. Bailes III, PhD, GBA Advisor

Helpful? 1

For those who may not have seen these yet, here are two diagrams showing how much more water vapor gets into places where it shouldn't by infiltration (second image) than by diffusion (first image). These are from the Builder Guides written by Joe Lstiburek and based on research done over a whole heating season.

water-vapor-research-building-science-diffusion-through-drywall.png water-vapor-research-building-science-infiltration-through-drywall.png


3.
Mon, 06/18/2012 - 14:51

Barriers
by Ron Keagle

Helpful? 0

I have been following this revised thinking concerning the function and need for a warm side vapor retarder to prevent vapor transfer by diffusion. However, I am not quite sure what conclusions to draw for practical application to a new house project in cold climates such as northern U.S.

1) If codes commonly requires a vapor diffusion barrier such as polyethylene film, is it worth it to fight this by omitting and diffusion barrier and only providing a air exfiltration barrier?

2) Is it easier to include a film diffusion barrier than to omit it and provide an exfiltration barrier by meticulously sealing the drywall or other finished surface?

3) Can I believe the science that suggests that drywall is sufficient to prevent condensation problems from diffusion?


4.
Mon, 06/18/2012 - 16:31

Edited Mon, 06/18/2012 - 16:32.

Intelligent walls
by floris keverling buisman

Helpful? 1

Double stud walls are a cost efficient way to build a super insulated house. To ensure a good airtight seal an interior airbarrier is ideal as it can actually be checked an verified by a blowerdoor test. Hence making an uninterrupted airtight layer out of plywood, or even better an intelligent vapor retarder like Intello Plus - which are taped together can assure that your insulation stays free of condensate in the winter in your walls, floor and roof.

The reinforced Intello membrane (disclosure I import this with www.foursevenfive.com) is like membrain - is vapor closed in winter and vapor in summer, however it has as added benefits that it:
- is extremely strong
- can be used as dense pack cellulose mesh
- part of complete airsealing solution line from Pro Clima that includes tapes, gaskets, adhesives and high quality exterior membranes

As shown in this 3d section, a double wall does benefit from a service cavity, which minimizes service penetrations thought the airtight layer and thus avoids vapor issues in the wall.

475 Double stud wall Intello Plus.jpg


5.
Tue, 06/19/2012 - 04:56

Diffusion v. Infiltration
by Peter Hastings 4C

Helpful? 0

To set the matter in context, the 1 square inch hole in Dr Joe's second example is the equivalent of a gap right round the edge of an 8ft x 4ft board which is 3.5 mil wide. Which is, quite literally, a hair's breadth.

This discussion seems to be edging its way, ever so slowly, to the wall construction described as "A New USA Wall" by Greg La Vardera. Disclosure - I have no commercial ties with Greg La Vardera.


6.
Tue, 06/19/2012 - 05:04

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Q. "If codes commonly requires a vapor diffusion barrier such as polyethylene film, is it worth it to fight this by omitting and diffusion barrier and only providing a air exfiltration barrier?"

A. As far as I know, no U.S. building code requires the use of interior polyethylene. In some climates, codes require an interior vapor retarder with a permeance of 1 perm or less. This can be satisfied by the use of kraft-paper facing or (in some cases) vapor retarder paint.

Requirements for air barriers are a separate issue from requirements for vapor-diffusion retarders, so it's important not to confuse the issues. All walls need at least one, and in some cases two, air barriers.

Q. "Is it easier to include a film diffusion barrier than to omit it and provide an exfiltration barrier by meticulously sealing the drywall or other finished surface?"

A. Again, you are confusing two issues (code requirements for vapor retarders and the need for an air barrier). Your question is a little like, "What is easier -- installing a window or installing roof flashing?" These are two different issues.

Q. "Can I believe the science that suggests that drywall is sufficient to prevent condensation problems from diffusion?"

A. That science doesn't exist, since drywall is vapor-open. Unpainted drywall does little to limit vapor diffusion. However, your question raises another issue, namely: Do homes generally have any moisture problems that can be traced to diffusion from the interior outwards during the winter? The answer is: No. So don't worry so much about outward diffusion.


7.
Tue, 06/19/2012 - 13:46

Combination air/vapor diffusion barrier
by Ron Keagle

Helpful? 0

Martin,

Thanks for that information. I am very interested in the issues surrounding vapor barriers/retarders, and particularly this new turnabout in thinking about the need to combat vapor diffusion.

Regarding my question:
"Is it easier to include a film diffusion barrier than to omit it and provide an exfiltration barrier by meticulously sealing the drywall or other finished surface?"

What I was getting at was this: Rather than omitting a film vapor barrier under the premise that it is not needed; and then providing the needed air barrier by making sure the drywall is sealed; I would rather include the film vapor barrier to function both as a vapor barrier and as an air barrier. That way, in addition to having an air barrier, I will also have a barrier to vapor diffusion just in case there is any that is worth preventing. By taking this approach, I will not have to air seal the drywall because the film barrier will perform the function of air barrier.

I realize that the mostly impermeable film vapor/air barrier will not let the wall dry to the inside, after being wetted from solar vapor drive from the outside. But I do not want conditions arranged for wet walls to dry to the inside. Moreover, I do not want walls getting wet in the first place, so I would avoid this whole issue by preventing the walls from getting wet from outside sources.

To accomplish that objective, I would avoid the use of reservoir cladding and limit rain exposure by a large roof overhang. I am not fully certain of the exterior detailing, but the point would be to use a house wrap that would prevent the intrusion of rain in conjunction with an air space. So the wall interior would not get wet from rain, or from outward condensing vapor.

However, the wall system would be subject to the same humidity level as the outdoors, and I can see how cooling the building interior with air conditioning might chill the film vapor barrier enough to condense exterior vapor on the exterior facing surface of the vapor barrier. However, in the northern U.S. climate, I would not expect the outside dewpoint to reach the interior air-conditioned temperature very often, if ever.

If any vapor does condense in the wall cavity, it would be cyclic, and I would expect it to dry to the outside in the opposite cycle.


8.
Tue, 06/19/2012 - 14:21

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Plenty of people use what you call an "interior film" as an air barrier. However, I would strongly urge you to avoid the use of polyethylene for this purpose, especially if there is any chance that the building will ever be air conditioned.

If you want to use a membrane that comes in a roll as an interior air barrier, you should use MemBrain, Intello Plus, or Siga Majpell.


9.
Tue, 06/19/2012 - 15:45

Strapped 2x6 wall
by Martin Jelenc

Helpful? 0

After doing many variations on the double wall theme, for my next project I am contemplating a 2x6 wall with 2x4 horizontal straps every 2 feet on the exterior. 1.5" taped foam and housewrap sheathe these straps; 3/8 lath and vertical B&B siding on exterior, drywall on interior, with moisture retarding paint. Cavity insulation is densepack cellulose @7". Rack bracing is let in. R value is over 30, and it is a pretty simple and inexpensive assembly. It also has a redundant air barrier just in case the detailing is less than perfect.


10.
Tue, 06/19/2012 - 16:35

air barriers
by bryan shephard

Helpful? 0

I'm wondering about the placement of the air barrier. On a double stud wall does it matter whether it is on the inside using the drywall or on the exterior using taped plywood? Can both locations be equally effective?


11.
Tue, 06/19/2012 - 16:57

Response to Bryan Shephard
by Martin Holladay, GBA Advisor

Helpful? 0

Bryan,
There is no consensus on the answer to your question; opinions vary.

Here is an article that discusses the relevant issues: One Air Barrier or Two?


12.
Tue, 06/19/2012 - 20:01

Edited Tue, 06/19/2012 - 20:04.

Response to Bryan Shephard
by Lucas Durand - 7A

Helpful? 1

Bryan,
In my (not-so-empirical) opinion, if there is only going to be one air barrier, it should be on the warm-in-winter side of the envelope in heating climates.

And possibly on the exterior side of the envelope in cooling dominated climates.

My reasoning is that there are often interstitial "3D airflow networks" within an envelope...
A good air barrier should be located so as to prevent the air with the higher moisture content from entering the interstitial space (in addition to the more straight forward job of preventing air exchange between the interior and exterior environments).

For example, in a heating climate in winter:
cold, relatively dry air that contacts a warm air barrier via the "3D airflow network" is lower risk than;
warm, relatively moist air that contacts a cold air barrier via a similar "3D network".

I think this is what has prompted some double wall systems to be refered to as "turds" despite possibly having good blower door test scores...


13.
Tue, 06/19/2012 - 23:16

Response to Bryan Shephard
by albert rooks

Helpful? 0

I think that Lucas has it right.

I like to think of it this way:  An air barrier will keep the wind from blowing in and out of your envelope.  For that simple barrier,  placing it at the exterior sheathing or the interior will do the trick. However,  life is full of improvements... Placing said barrier on the warm side of the insulation in a heating climate will keep the water vapor that travels with the interior air -inside the habitable space and out of the wall.

On a thick wall in a cold climate, the exterior sheathing is cold since it is well removed from the warm interior. By keeping warm air out of the wall, it is argued that you are reducing chances of condensation on the interior of the exterior sheathing. Hence, an upgraded air barrier ( if you will).


14.
Wed, 06/20/2012 - 07:11

Location, Location, Location ...Location matters
by John Brooks

Helpful? 0

Opinions about Best Single Air Barrier Location Vary.
I agree with Lucas & Albert

IF...IF...IF there must only be ONE Air Barrier in a Heating Climate...
The Best location is on the Interior side of the Cavity (3-D Network)

Attached is my version of a John Straube Illustration ...
Showing that even a perfect Exterior Air Barrier can be a "Fail" IF there are any voids in the cavity.

I think a Better solution for walls (and Vaulted Ceilings) is TWO Air Barriers
One Interior + One Exterior

convection.jpg


15.
Wed, 06/20/2012 - 11:37

Water Vapor Control Fundamentals:
by Ron Keagle

Helpful? 0

This is my analysis of the entire water vapor issue for buildings:

Water vapor can enter the wall from either side, either by diffusion or as a component of moving air. If that vapor encounters a cool enough surface inside of the wall, it can condense into water. This problem can be prevented by preventing vapor from entering the wall; or by making sure that entering vapor cannot contact cool enough surfaces to condense.

To prevent water from entering a wall by diffusion, a relatively non-permeable barrier is needed. Such a barrier will also prevent water from entering a wall as a component of air. However if the objective is only to prevent water from entering as a component of air, a less impermeable barrier will suffice.

If vapor does condense into water inside of a wall, it may be removed by evaporation if vapor is permitted to escape to either the interior or exterior, either by diffusion or as a component of moving air.

Barriers that prevent the passage of vapor into a wall by diffusion will not permit drying evaporation of the wall interior by diffusion.

Barriers that prevent the passage of vapor into a wall as a component of air will not permit drying evaporation of the wall interior by moving vapor as a component of air; however, such barriers will permit the drying evaporation of the wall interior by diffusion.


16.
Wed, 06/20/2012 - 12:18

Edited Wed, 06/20/2012 - 12:19.

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Sorry -- you're still not quite there.

You wrote, "To prevent water from entering a wall by diffusion, a relatively non-permeable barrier is needed. Such a barrier will also prevent water from entering a wall as a component of air."

There are many perfectly acceptable vapor retarders that are lousy air barriers. So it's simply untrue to write that a barrier chosen to limit vapor diffusion will "prevent water from entering a wall as a component of air."

If you want to limit vapor diffusion, there are paints that do a perfectly acceptable job. The kraft facing on fiberglass batts also works well as a vapor retarder. However, neither product will "prevent water from entering a wall as a component of air." For that, you need an air barrier.


17.
Wed, 06/20/2012 - 12:52

Martin, I do not follow your
by Ron Keagle

Helpful? 0

Martin,

I do not follow your comment. How can something be a lousy air barrier while being a perfectly acceptable vapor retarder? A lousy air barrier would permit vapor transmission as a component of air. And it would also offer no resistance to vapor diffusion. Diffusion could simply occur directly through air, even if air was not moving. So how can a lousy air barrier possibly be a “perfectly acceptable vapor retarder”?


18.
Wed, 06/20/2012 - 13:18

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Q. "How can something be a lousy air barrier while being a perfectly acceptable vapor retarder?"

A. It's easy. Vapor diffusion is a totally separate phenomenon from air leakage. When exfiltrating air carries interior moisture into a cold wall cavity, it can lead to moisture problems. But this moisture transport mechanism has nothing to do with vapor diffusion.

Q. " A lousy air barrier would permit vapor transmission as a component of air."

A. That's true.

Q. "And it would also offer no resistance to vapor diffusion."

A. On the contrary: bad air barriers can be effective vapor diffusion retarders. A wall finished with painted gypsum wallboard might leak a lot of air, especially if electrical boxes aren't air sealed. However, if there are several coats of oil-based paint on the drywall, you'll get almost no vapor diffusion.

Q. "Diffusion could simply occur directly through air, even if air was not moving."

A. I'm not sure what you mean. Vapor diffusion is a different phenomenon from air leakage. For example, vapor can diffuse through unpainted drywall, even if the drywall is airtight.

Q. "So how can a lousy air barrier possibly be a 'perfectly acceptable vapor retarder'?"

A. It's easy -- all it has to do is stop diffusion.

I think you are hung up because you think that vapor diffusion matters. It actually doesn't matter very much -- especially vapor diffusion outward during the winter. What matters is air leakage.


19.
Wed, 06/20/2012 - 16:07

Martin, I see what you mean
by Ron Keagle

Helpful? 0

Martin,

I see what you mean by the possibility of a lousy air barrier being a good vapor retarder. By vapor retarder, you are referring to retarding vapor that is moving only by diffusion. So you could have a near perfect vapor retarder/barrier, even if it were full of holes because transmission through the holes would not be diffusion. According to that definition, a membrane could be nearly perfect as a vapor barrier, but nearly worthless for stopping the flow of vapor.


20.
Wed, 06/20/2012 - 16:36

Edited Wed, 06/20/2012 - 18:08.

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
It's not my definition; it's the definition established by building codes. For decades, U.S. building codes defined a vapor retarder as "a material having a permeance rating of 1.0 or less when tested in accordance with ASTM E 96." There is no mention whatsoever of whether or not the material is a good air barrier. (More recent building codes have complicated the definition by breaking vapor retarders into three classes -- Class I, Class II, and Class III -- but they are all based on permeance testing, not air leakage testing.)

Plenty of materials can meet the definition of a vapor retarder without being effective air barriers.

When you write, "According to that definition, a membrane could be nearly perfect as a vapor barrier, but nearly worthless for stopping the flow of vapor," you are not quite correct, but you are getting closer. Actually, any membrane that is nearly perfect as a vapor barrier is, by definition, nearly perfect at stopping the movement of vapor by diffusion.

I would express your idea this way: A membrane can be a nearly perfect vapor barrier but nearly worthless at stopping the flow of air.

It's also possible to say: A membrane can be a nearly perfect vapor barrier but nearly worthless at stopping the transmission of water that accompanies air moving through defects in the membrane.


21.
Wed, 06/20/2012 - 17:09

Martin, Setting aside the
by Ron Keagle

Helpful? 0

Martin,

Setting aside the issue of a good vapor barrier that has holes in it, when we speak only of continuous material, what materials would be a good vapor retarder/barrier while being a poor air barrier? I can’t picture a material that would be impermeable enough to stop the diffusion of vapor, and yet be porous enough to permit airflow.


22.
Wed, 06/20/2012 - 17:36

Edited Wed, 06/20/2012 - 18:09.

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Materials used to build buildings always have seams, so your theoretical question is interesting but possibly irrelevant.

A wall of metal shingles, installed like cedar shingles, would probably be vapor-impermeable but air-leaky. So would a section of wall with lapped kraft paper.

Many painted materials could pass the permeance test without passing an air leakage test.

In any case, building codes require vapor barrier materials to pass a permeance test. Once installed, however, all of these materials have seams and laps (as well as penetrations), and all of these materials, even those that pass the permeance testing with flying colors, are air-leaky as usually installed.

Here's a sample job-site dialog to help you understand what I'm saying:

Martin: What are you using as your vapor retarder?

Ron: I'm using vapor-retarder paint.

Martin: That makes sense. What are you using as your air barrier?

Ron: I'm using vapor-retarder paint, because as far as I know, any material that passes the vapor retarder test should be a good air barrier.

Martin: But...


23.
Wed, 06/20/2012 - 18:36

My vote is for closed cell spray foam
by Frank Bovio

Helpful? 0

Without starting the "R" value debate, I say closed cell SPF is the best choice. 2 inches is a vapor barrier, while spraying and quoting spray foam you base it on an average, so I always recommend 3 inches. The perfect situation and budget would be all closed cell, but we all know in the real world thats not always in the budget. 3 inches of closed cell, than cover with open cell for R value.
Personally not a fan of flash and batt, and mineral wool is pretty exspensive as well, and if the cavities are not perfectly sealed we all know air will flow through it, and where there us air there will be moisture.
Spray foam in my eyes is the solution.


24.
Wed, 06/20/2012 - 19:09

Martin, You mentioned that
by Ron Keagle

Helpful? 0

Martin,

You mentioned that U.S. building codes defined a vapor retarder as "a material having a permeance rating of 1.0 or less when tested in accordance with ASTM E 96." And you went on to say that “Plenty of materials can meet the definition of a vapor retarder without being effective air barriers.”

When I asked what materials would have those characteristics, you say that materials having those characteristics would be materials meeting the definition of vapor retarders, but being installed with open seams and other defects. I don’t see that as being a material that qualifies as a vapor barrier but is not a good air barrier. The material is one thing, and the quality of installation is another thing.

So, I am standing by my assertion that materials meeting the definition of vapor retarder/barrier would also be effective as an air barrier. I don’t see how possiblities of leaks or other installation defects changes that fundamental truth.

Can’t a vapor retarder also be an air barrier, so one membrane performs both functions? That was my original point in this discussion.


25.
Wed, 06/20/2012 - 22:47

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron.
You wrote, "I am standing by my assertion that materials meeting the definition of vapor retarder/barrier would also be effective as an air barrier."

OK -- then go ahead and use paint as an air barrier. I'm going to choose different materials for my air barrier.


26.
Wed, 06/20/2012 - 23:48

Martin, Doesn't vapor
by Ron Keagle

Helpful? 0

Martin,

Doesn't vapor retarder paint need to be used in conjunction with a substrate such as sheetrock? Wouldn't the sheetrock be a suitable air barrier with or without the paint?

Actually, what I am considering for use is DURA-SCRIM 6WW 6 mil four-layer reinforced extrusion laminate with a perm rating of .07. I would install it with no leaks, and expect it to function as a vapor retarder and air barrier combined.


27.
Thu, 06/21/2012 - 05:37

Edited Thu, 06/21/2012 - 05:38.

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Q. "Doesn't vapor retarder paint need to be used in conjunction with a substrate such as sheetrock?"

A. Yes. But the drywall can't meet the permeance requirement of the vapor retarder provisions of the code. The paint is a vapor retarder -- not the drywall.

Q. "Wouldn't the sheetrock be a suitable air barrier with or without the paint?"

A. Yes, if the drywall is installed with the necessary gaskets and with airtight electrical boxes, following the details required by the Airtight Drywall Approach. If that's done, the drywall and gaskets are your air barrier, and the paint is your vapor retarder.

Q. "What I am considering for use is DURA-SCRIM 6WW 6 mil four-layer reinforced extrusion laminate with a perm rating of .07."

A. The product you are describing is a type of polyethylene. It is not a "smart" retarder. Your approach has been successful in many Canadian buildings for 30 years, but interior poly can still lead to disasters. Interior poly has been blamed for many of the "leaky condo" problems in Vancouver, since it prevented wet walls from drying inward. You can also get into trouble using interior poly in Ontario or Quebec if the house is air conditioned.

In short, good luck.


28.
Thu, 06/21/2012 - 10:37

Location, Location, Location
by Ken Levenson

Helpful? 0

John Brooks, we wholeheartedly agree with your analysis - and John Straube's diagram is very informative.
To reiterate and expand a bit on the idea: First priority should be an air barrier at the interior, preventing possibility of bulk vapor carried by air leakage from inside. Second, particularly if one is using cellulose or mineralwool or fiberglass or other fibrous insulation, is an air-barrier on the outside, to optimize the insulation - like a windbreaker over a sweater.

I would add to this that airtight drywall is an unreliable air barrier and there should be a dedicated air barrier (either rigid or fabric) protected by a service cavity, behind the finish drywall. We have an informative blog post on the use of service cavities here: http://bit.ly/LDMUR4
But also protect the exterior air barrier with a vented rainscreen/roof, as we describe the benefits here: http://bit.ly/MvD4lM
Finally, as my colleague Floris noted above, if you are doing an interior air barrier in a cold climate it may make sense (particularly at roofs) to use an intelligent vapor retarding air-tight membrane that is essentially vapor closed in the winter and vapor open in the summer, thereby minimizing the wetting and maximizing the drying potential.


29.
Thu, 06/21/2012 - 13:06

Reply to Martin Holladay
by Ron Keagle

Helpful? 0

Thanks Martin. I understand your points about the vapor barrier, and have been considering all of them as my project develops. For the exterior, I would use a vapor permeable air and water barrier combined with flashing that would prevent any water intrusion.

I would try hard to avoid the issue of inward vapor drive by using non-reservoir siding and a large roof overhang that would substantially reduce rain contacting the siding, and even reduce the storm saturation of the ground right next to the house. My objective would be to limit the in-wall humidity to the level of the average outdoor ambient.

The poly vapor barrier on the warm side would not permit drying the wall cavity to the interior, but I don’t anticipate much need for drying. The only need for wall cavity drying that I can see is the possibility of condensation on the exterior side of the vapor barrier if the outdoor dewpoint exceeds the vapor barrier temperature during days of the highest humidity and maximum air conditioning. But I don’t anticipate that overlap occurring very often, and when it does occur, it might be with a dewpoint of 78 degrees and an interior temperature of 76 degrees, for example.

I understand the new thinking that vapor diffusion is not a problem in buildings. Nevertheless, I cannot verify that for my application, so I would still want to protect against diffusion condensation as insurance. That is my reason for using the polyethylene vapor barrier on the warm side. I would expect the vapor barrier to perform as an air barrier as well.

Any condensation that does happen to occur within the wall cavity during air conditioning during high humidity would dry to the exterior. I am aware of smart vapor barriers that would permit this potential moisture to dry to the interior while preventing outward vapor flow during the heating season. That performance would ideal because the inward drying would be into the air-conditioned space, which will be very receptive because the air conditioning would be removing water vapor from the interior. On the contrary, drying the in-wall moisture to the exterior will have to wait until after the air conditioning/high humidity phase ends. But again, I do not see much need for drying the wall cavity because condensed moisture will be relatively unlikely to be in the cavity.

The only problem I see with these smart vapor retarders is the possibility that they will not do what is claimed for them. And there is no practical way to verify their performance. Basically, you have a membrane that is acting as an automatically operating valve. I simply would not take that on blind faith that this type of barrier would continue to cycle functions effectively as it was intended and claimed. Once installed, there would be no reasonable possibility of correcting it if it failed, and it would also be very difficult to detect a failure.


30.
Thu, 06/21/2012 - 13:45

No blind faith required.
by Ken Levenson

Helpful? 0

Ron,
The science behind the intelligent vapor barriers is fairly straight forward, well established and tested. I encourage everyone to take a closer look at a Pro Clima study, here (pdf download): http://download.proclima.com/en/int/study.pdf

Note: While we are in the process of translating all the units in this document to "American", (Perm = 3.28/Sd value) the physics remain unchanged.


31.
Thu, 06/21/2012 - 14:24

Edited Thu, 06/21/2012 - 14:28.

Keeping It Away From A Cold Surface
by Leigha Dickens

Helpful? 0

I have the understanding that the science allows (even if the building code doesn't always) one to forgo a warm-side vapor barrier in a cold climate if there is a certain amount of insulation exterior to the sheathing. The idea being that the exterior insulation raises the temperature of the sheathing enough to keep any vapor that does get into the wall from the interior from feeling the need to condense on it. The number of inches of exterior XPS/EPS/Mineral Wool/etc that you would need for this depends on the coldness of the climate and on the thickness of the wall in question--but the requirements go up if you have a darn thick wall like a double stud wall. Am I understanding this correctly? Is avoiding condensation on colder exterior surfaces the only reason for vapor control in a heating climate?

If I am understanding that correctly, then I wonder if one could substitute a flash-and-fill style insulation system into a double stud wall in place of adding many inches of exterior insulation. For instance, if design conditions say you need 2" of XPS on the exterior of the wall to negate the need for an interior vapor barrier, but you instead install a certain number of inches of spray foam on the inside of the wall and fill the rest of the cavity with something fibrous, are you not accomplishing the same purpose with that interior spray foam of warming up the sheathing? Further, would it create a bad "vapor sandwich" problem for the sheathing to have some exterior insulation AND a flash-and-fill system in the cavity?


32.
Thu, 06/21/2012 - 15:55

Smart vapor retarders work
by Dick Russell

Helpful? 1

Ron, if you doubt reports supporting the performance of smart vapor retarders, go on Certainteed's web site and order a sample kit of their MemBrain product. At least when I got mine several years ago, it came with two plastic pouches, one of MemBrain and the other regular old polyethylene, and two thin wood strips. The instructions said to soak the wood strips to saturate them, then seal them inside the two pouches and leave them around for a number of days. Initially you see beads of condensation on the inside of each, but less so in the case of MemBrain after a few days. After perhaps a week, the MemBrain sample was totally dry inside, while the polyethylene pouch allowed no drying of its wood piece at all. I was happy, and I used it under the drywall in my own house, a double frame, cellulose-filled wall structure. I detailed it properly, to tie into the floor system and everything else so as to form a continuous air barrier as well as vapor retarder.


33.
Thu, 06/21/2012 - 16:14

Response to Leigha Dickens
by Martin Holladay, GBA Advisor

Helpful? 0

Leigha,
Q. "The number of inches of exterior XPS/EPS/Mineral Wool/etc that you would need for this depends on the coldness of the climate and on the thickness of the wall in question--but the requirements go up if you have a darn thick wall like a double stud wall. Am I understanding this correctly?"

A. Yes. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

Q. " Is avoiding condensation on colder exterior surfaces the only reason for vapor control in a heating climate?"

A. I don't think there is much reason for vapor diffusion control on the interior of a house. However, houses with reservoir sidings that have air conditioning need vapor diffusion control on the exterior of the walls, especially if the interior finishes have a low vapor permeance.

Q. "If I am understanding that correctly, then I wonder if one could substitute a flash-and-fill style insulation system into a double stud wall in place of adding many inches of exterior insulation. For instance, if design conditions say you need 2 inches of XPS on the exterior of the wall to negate the need for an interior vapor barrier, but you instead install a certain number of inches of spray foam on the inside of the wall and fill the rest of the cavity with something fibrous, are you not accomplishing the same purpose with that interior spray foam of warming up the sheathing?"

A. Yes.

Q. "Further, would it create a bad 'vapor sandwich' problem for the sheathing to have some exterior insulation AND a flash-and-fill system in the cavity?"

A. Yes. I would advise the use of exterior mineral wool in this case rather than rigid foam.


34.
Thu, 06/21/2012 - 18:15

Smart Vapor Barriers
by Ron Keagle

Helpful? 0

Yes, I understand the science behind smart vapor barriers, and I don’t doubt that it has been have proven and demonstrated to perform as promised. Really, the only question I have is how long they will perform. I simply do not know the answer to that question. When products are promised to perform on a molecular level, I think it is an act of faith to accept the manufacturer’s promises. I see it as an issue like paint and stain performance promises.

There is one interesting point that I noticed about the Certainteed smart vapor barrier information. The barrier opens up in high humidity under the assumption that high humidity means it is summertime, and drying to the interior would be needed. Any yet, it is humidity that the barrier is intended to stop as the humidity moves outward during the winter. The product information cautions not to use the product if there are continuous high humidity conditions inside the house because the smart vapor barrier would open up and vent the humidity during the wintertime when it is supposed to stop the vapor flow.

The information says that temporary high humidity would not be sufficient to trick the smart vapor barrier into opening when it is needed to be closed. But that seems like somewhat of a fine line to walk.

But my main reason for not using the smart vapor barrier is that I do not believe I will need that type of performance, so I would choose a fixed phase vapor diffusion barrier. I am in Minnesota, so I am not much concerned with air conditioning promotion of in-wall vapor. I have built one house here with the same type of superinsulation and poly vapor barrier system as the new one that I am planning now.


35.
Sat, 06/23/2012 - 20:19

cost of double wall w 2" rigid rock wool vs 2x6 w 4" rockwool
by Patrick Walshe

Helpful? 0

I'm curious if anyone has worked out the cost difference between doing a double stud cellulose wall with 2" of rigid rock wool on outside versus a 2x6 wall with rockwool batts and 4" of rigid rockwool. Let's say the first wall assembly is a staggered 2x4 stud wall on a 9.25 " plate so it all goes together at the same time. Both walls have airtight drywall on inside and plywood sheathing, house wrap, rainscreen, and fiber cement siding ( this is Vancouver Island BC).


36.
Sat, 06/23/2012 - 22:23

Fiberglass Batts & Minneral Wool Batts
by Ron Keagle

Helpful? 0

Has there been any detailed discussion of the pros and cons of fiberglass batts and mineral wool batts that I could link to? I am interested in the pros and cons of each material relative to all other types of insulation, plus a direct comparison of the two materials.


37.
Sun, 06/24/2012 - 05:45

38.
Sun, 06/24/2012 - 18:18

Fiberglass Batts Fair Shake
by Ron Keagle

Helpful? 0

Martin,

With great interest, I read your piece on fiberglass batts. In the comments, I am stunned that people call for a ban on fiberglass batts because people fail to install them correctly.

When you state, “Of all the commonly used types of insulation—including cellulose, rigid foam, and spray polyurethane foam-- fiberglass batts perform the worst,” the clearest implication is that you are comparing performance of the insulation materials per se.

However, you then base this conclusion that fiberglass batts are the worst performer by assessing their actual performance after they are installed on a number of actual jobs, and therefore include poor installation quality in your assessment of insulation performance. I believe it is unfair to conclude the material performs poorly because it is often improperly installed.

You mention the problem of leakiness, but again, you link it to poor quality installation. You also link the problem of leakiness to the insulation material itself by saying that the material is permeable to airflow, which can degrade the insulation performance. But how does that discredit the fiberglass batt? It is intrinsically air permeable, so an air barrier has to be part of the insulation scheme as it does with any other air-permeable insulation.


39.
Mon, 06/25/2012 - 05:37

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
I have never called for fiberglass batts to be banned.

My statement — “Of all the commonly used types of insulation — including cellulose, rigid foam, and spray polyurethane foam — fiberglass batts perform the worst” — has been amply documented by many research studies. As typically installed, fiberglass-insulated homes are leaky. Whole-wall R-values for fiberglass-insulated walls are significantly lower than the R-value on the fiberglass package.

As I wrote in the article, it's possible to get fiberglass batts to perform close to the R-value shown on the package, but it takes a lot of work. It is extremely rare to see anyone perform that work. In fact, it takes so much work to try to get fiberglass batts to work the way they are supposed to that almost every builder who has looked into the issue closely has decided that it's easier, faster, and cheaper to use cellulose, rigid foam, or spray foam insulation than it is to carefully detail a fiberglass batt job in hopes of achieving the R-value on the package label.


40.
Mon, 06/25/2012 - 13:45

Fiberglass batts
by Ron Keagle

Helpful? 0

Martin,

I understand that you mean to say that, in an average sample of actual installations of the common insulation materials, installations using fiberglass batts perform the worst. I suspect that would be true. And I did not mean to suggest that you were calling for a ban on fiberglass batts.

I think that you have fairly covered the difficulties of installing fiberglass batts. It is an interesting problem. Perhaps an underlying reason for poor installation quality is influenced by the physical irritation caused by fiberglass unless a person is wearing full body protection. Most people installing it want to get done as quickly as possible.

Perhaps because the material is so soft and compliant, it encourages deforming and using torn pieces to fill gaps. Fiberglass batts need to be measured, cut, and fitted as though they were as solid as wood. Installation truly does require a fully professional, and conscientious attitude. Apparently, this is lacking in most cases of installation. In regard to the fiberglass batt insulation material alone, I see nothing indicating that it performs poorer than other types of insulation.

I found the comments on fiberglass compression to be very interesting. The golden rule to never compress fiberglass has been long shouted from the mountaintop. But the truth is that limited compression can raise the R-value of the cavity albeit using more insulation than the cavity calls for. But another value of compression is elimination of voids.

The manufacturers finally began to clarify the advantages of compression, however, they worry that too much compression will interfere with the sheetrock work. Now that they offer a high-density fiberglass batt, they seem to no longer promote compression. They will tell you that the high-density material performs the compression for you.

I do not understand what you mean when you say that “whole-wall R-values for fiberglass-insulated walls are significantly lower than the R-values on the fiberglass package.” The package only stipulates the intrinsic R-value of the insulation. The package labeling does not account for thermal bridging, insulation gaps, excessive packing, or other installation defects that altogether will affect the R-value of a whole wall.


41.
Mon, 06/25/2012 - 14:15

Edited Mon, 06/25/2012 - 14:19.

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Several factors explain the low performance of fiberglass batts:

- The presence of insulation voids in most framing bays, due to the presence of wiring and plumbing vents, as well as installer sloppiness (careless cutting and haphazard installation).

- The fact that the insulation is very air-permeable, which explains why, on average, homes insulated with fiberglass batts have worse blower-door results that homes insulated with cellulose or homes insulated with spray foam.

- The fact that fiberglass batts can only achieve the R-value on the package when they are surrounded on all six sides by an air barrier -- an installation method that very few builders honor.


42.
Mon, 06/25/2012 - 16:52

Sealing penetrations in walls that use exterior mineral wool
by c talwalkar

Helpful? 0

Thanks to everyone for this very informative thread.

I am left wondering how people who decide to use mineral wool outside housewrap and sheathing, e.g., as specified in the USA New Wall design, will seal window and door penetrations?

I don't have any experience working with Roxul, but the number of enthusiastic posts about it suggests there must be fairly straightforward solutions to this problem.


43.
Mon, 06/25/2012 - 17:13

Response to C Talwalkar
by Martin Holladay, GBA Advisor

Helpful? 0

C Talwalkar,
There are two kinds of sealing required at a window penetration: sealing against water entry and air sealing. If the wall has exterior Roxul, neither type of sealing differs from ordinary practice.

Typically, the air seal is established between the window frame and the window rough opening.

The water-entry sealing occurs at the WRB and related flashing.


44.
Tue, 06/26/2012 - 21:01

Edited Tue, 06/26/2012 - 21:12.

Drying To the Interior Question
by Ron Keagle

Helpful? 0

Consider a design where you want to stop outward migrating vapor at the interior wall. The insulation will be air permeable, and outer wall will be allowed to breathe with house wrap.

Suppose you forego the use of a polyethylene vapor diffusion barrier under the drywall, and use the airtight drywall to create an air barrier. The theory is that vapor diffusion is so slow and limited that its role in creating condensation of outward migrating vapor in the wintertime is so small that it need not be addressed.

However, if airflow is allowed to pass outward through the interior wall, it will carry water vapor into the insulation cavity and pose significant condensation wetting problems in the wall. So, while the diffusion barrier can be omitted, an air barrier is essential.

Not only is a diffusion barrier probably not needed, but also if there is significant air conditioning in the summertime, a diffusion barrier at the interior wall may condense vapor in warmer exterior air entering from the breathable exterior. So by using only an air barrier in the form of airtight drywall, exterior vapor condensing on the cooler back of the drywall during air conditioning will be able to dry to the interior because the airtight drywall is not a diffusion barrier.

Here is my question:

If diffusion is so slow through an air barrier of airtight drywall that it is inconsequential, how can it be relied upon to dry condensed moisture out of the wall cavity through the interior wall? Because the interior wall is an air barrier, there can be no air movement to carry vapor into the interior to aid drying. All such drying would have to rely on diffusion, which is said to be a relatively poor mechanism for vapor transmission.

It seems to me that if you were to cool the interior space on a daily basis for a few weeks or more, the inward drying would not be able to keep up with the accumulation of condensing moisture inside of the wall cavity.

So how can airtight drywall be a suitable diffusion barrier to prevent outward movement of vapor during wintertime heating, but not be a diffusion barrier that would impede inward movement of vapor during summertime cooling?


45.
Tue, 06/26/2012 - 23:46

Re: Martin Holladay
by c talwalkar

Helpful? 0

Thanks, Martin. So let's say you're using 2" Roxul (boards, right? The batts don't seem appropriate to use as outsulation).

Does the window have to project past the Roxul, the furring, and the cladding? If not, what would be the preferred way to carry that penetration past the cladding layer? How would this work with the 10" thick Roxul product Albert mentioned?

Sorry to be so dense. The earlier posts refering to Roxul's compressibility and low adhesion to tape made me wonder how to seal and trim penetrations through a squishy, potentially very thick substrate.

Thanks for your patience!


46.
Wed, 06/27/2012 - 05:21

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Q. "How can airtight drywall be a suitable diffusion barrier to prevent outward movement of vapor during wintertime heating, but not be a diffusion barrier that would impede inward movement of vapor during summertime cooling?"

A. Airtight drywall is not a diffusion barrier; it is an air barrier. It does not prevent outward movement of vapor during the wintertime. Luckily, outward diffusion of vapor during the wintertime is not really a significant problem.

In the wall you describe, the biggest risk is connected to the cold wall sheathing on the exterior. The best way to lower this risk is with a ventilated air gap between the siding and the sheathing. For more on this risk, see How Risky Is Cold OSB Wall Sheathing?


47.
Wed, 06/27/2012 - 05:27

Response to C Talwalkar
by Martin Holladay, GBA Advisor

Helpful? 0

C Talwalkar,
Q. "Does the window have to project past the Roxul, the furring, and the cladding? If not, what would be the preferred way to carry that penetration past the cladding layer?"

A. It's possible to do it both ways. For more information, see ‘Innie’ Windows or ‘Outie’ Windows?

Our GBA detail library includes details for both innie windows and outie windows. Links to these details can be found here: Building Plans for a Deep-Energy Retrofit.


48.
Wed, 06/27/2012 - 11:56

Why Is Diffusion Not a Problem?
by Ron Keagle

Helpful? 0

Here is the point that I am not understanding: Why is outward diffusion in the wintertime not a problem?

Is it because there is not enough vapor available ahead of the membrane to cause a problem once it goes through the membrane? Or is it because the membrane slows the passage of vapor enough to minimize the problem once it gets through?

Certainly there is a difference in vapor pressure from interior to exterior, so this would result in diffusion if a barrier were sufficiently permeable.

My impression was that drywall is a good air barrier, and a good enough barrier to slow diffusion to the point where it is not a problem. My understanding was that poly would be a much better diffusion barrier, but airtight drywall is good enough. Moreover poly has the drawback of stopping inward drying.

I understand your point about the risk of outward vapor condensing on cold sheathing on the exterior. But it can also condense in the insulation before it gets to the cold sheathing.


49.
Wed, 06/27/2012 - 12:47

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Ron,
Q. "Why is outward diffusion in the wintertime not a problem?"

A. Because the amount of water vapor that diffuses through materials used to finish walls is too small to cause any problems.

Q. "I understand your point about the risk of outward vapor condensing on cold sheathing on the exterior. But it can also condense in the insulation before it gets to the cold sheathing."

A. No it can't. As William Rose has explained, "The language ‘reaching dew point’ seems to indicate that one could plot a temperature profile through a wall, find the point where that profile intersects a horizontal line indicating indoor dew point temperature, and expect burgeoning water at that location. This impression is decidedly incorrect. If water accumulates, it does so on the surfaces of materials, not within the thickness of materials.”

Anton Tenwolde agrees with Rose: "The perceived importance of condensation has been bolstered by the wide misuse of the dew-point calculation. … Many of you are familiar with a chart like this: you project the temperature profile through the wall to calculate saturation vapor pressures. Then you calculate vapor pressures based on the permeance of the materials, and you come up with a profile like this. I have seen hundreds of these profiles, and many seem to show condensation occurring in the insulation. This has encouraged a lot of research into the performance of wet insulation. But the picture is wrong, because the vapor pressure has to be below the saturation pressure. You need to make a correction, and if you do that, if you redraw it, the condensation does not occur in the insulation. We thought there would be a problem with condensation in the insulation, but all the action happens on the sheathing and the interior vapor barrier. We’ve confirmed this by opening up walls. The action is never in the insulation."

For more information, see Are Dew-Point Calculations Really Necessary?


50.
Wed, 06/27/2012 - 13:13

Clarification Please
by Ron Keagle

Helpful? 0

Martin, regarding your answer to my question:

"Ron,
Q. "Why is outward diffusion in the wintertime not a problem?"

A. Because the amount of water vapor that diffuses through materials used to finish walls is too small to cause any problems."

But why is it too small? Is it because there is not enough vapor available to pass through the drywall; or is because the drywall slows the vapor transmission to the point where it does not accumulate in the wall cavity?


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