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Helpful? 1

Air Barrier or Vapor Barrier? - Building Science Podcast

Air leaks are a big source of moisture problems that lead to bugs, mold, and rot in homes

Posted on Mar 10 2010 by Joe Lstiburek, GBA Advisor

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This podcast series is excerpted from a two-day class called "Building Science Fundamentals" taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation.

For information on attending a live class, go to BuildingScienceSeminars.com

In our last episode, Dr. John Straube argued that building energy-efficient buildings takes just a little extra thought but makes economic sense. In this show, Dr. Joe Lstiburek offers a simple comparison between air barriers and vapor barriers, and warns that we need to worry more about airtightness if we want to keep our homes healthy and dry.


Smaller holes mean less moisture in your wall
You have to understand the difference between air and vapor. Vapor barriers can be ripped and torn and full of holes because the amount of water vapor that passes through due to diffusion is very small compared to the amount of water that can go through a hole or a crack due to an air pressure difference. I can move air, and if that air moves and there’s vapor in it the air will carry the vapor with it. For that to happen I need a hole and an air pressure difference. The likelihood of having a hole is very high. And the likelihood of having an air pressure difference is even higher.

So it behooves us to get rid of as many of the big holes as possible, and try to get as many of the small holes as well, but at the end of the day we’re still going to have some holes. It also means we ought to reduce the air pressures as much as we can, but at the end of the day we’re still going to have some air pressure differences. No matter how good we are, some vapor’s going to be carried by air as a result of a pathway and a pressure difference. Now let's put that aside.

Diffusion moves much less water than air leaks
If I have no holes, and I have no air pressure difference, but I have vapor on one side and I don’t have much vapor on the other side, I’m going to have a vapor pressure difference. And that material, depending on how easy it is for the water molecules to burrow through, will pass the water molecules. We call that vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. . Gypsum board is very vapor-open, so a lot of water will diffuse through it in the vapor form. But gypsum board is a fantastic air barrier. So if I installed gypsum board on the inside, and if I taped all of the joints together, and I had no windows — in other words a gypsum board box with five sides on a concrete slab, and I just caulked the bottom edge of the gypsum board to the slab — I would have a wonderful air barrier system. And I would have absolutely no moisture carried by air transport.

Now here’s the rub: the vapor transport is negligible compared to cutting a one-square-inch hole in that box and having just a modest air pressure difference between the inside and the outside. So what’s more important in controlling moisture transport? Air tightness. Now for the vapor tightness I could enclose maybe 90% of that enclosure with paint, which would be a vapor retarder. And the 10% I didn’t get — who cares? I’m reducing 90% of a small number. So I don’t really care if my vapor control layer is continuous because it doesn’t move that much moisture. But it’s real important that my air control layer is continuous. So air barrier continuity is much more significant that vapor barrier continuity.

Vapor barriers still work if they have holes in them
Now where it get’s real exciting, and interesting to me, is a concrete slab. So let’s say I’m putting 4 inches of concrete on top of the ground, and before I pour it I put down a plastic sheet — that sheet will be my vapor barrier. So let’s say before I pour my concrete I walk on the plastic sheet with golf shoes for about two hours. So what’s the total surface area of the punctures compared to the total surface area of the plastic? If I’m there for about two hours, maybe it’s 10%. So I basically have reduced the vapor control layer effectiveness of that plastic sheet by 10%.

Vapor flow by diffusion is a direct function — it’s linear. Airflow is not; it’s an exponential function of pressure. But let’s go back to the slab for a moment. What am I going to put on top of that ripped and torn and punctured plastic? Well, 4 inches of concrete. Concrete is a pretty good what? Air barrier — and it’s also a darn good vapor retarder.

So I haven’t increased, even from a measurable perspective, the amount of water vapor transmission from the ground into the floor with the ripped and torn plastic sheet. That’s why I always laugh at the people that say, “Well, you gotta tape the joints and you gotta be careful not to puncture it.” Give me a break! Now I don’t go out of my way to tell people to rip and tear it, and puncture it and leave gaps in it. And if they’re going to the trouble to tape the joints I’m not going to tell them, “Don’t go there.” It’s just not something I’m going to get bent out of shape about if they do a lousy job.

Know when your vapor barrier is also an air barrier
Now, what would happen if I took that concrete off of the plastic, and now I have a conditioned crawl space, and the only thing I have separating the ground from the inside of my house, which is the air in the crawl space, is a ripped and torn plastic sheet? Well, now I have a problem — because that sheet was supposed to also act as an air barrier. Now the amount of water vapor that goes through that plastic by diffusion is still very small, but the amount that will be carried as a result of air flowing across those rips and tears is huge. It’s typically 2 orders of magnitude — that would be a factor of 100. So that’s why we really care about air barriers, but we don’t care a hell of a lot about vapor barriers.


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Image Credits:

  1. Anna Robinson

1.
Thu, 03/11/2010 - 12:59

Excellent .......Dr Joe
by John Brooks

Helpful? 1

The Air Control Layer .....
My favorite subject.

The Simple and Continuous Red Pencil Line


2.
Fri, 03/12/2010 - 10:44

What about radon?
by Garth Sproule

Helpful? 0

I was always under the impression that a perfectly taped and sealed poly barrier under a slab was necessary to prevent radon gas from entering the conditioned space. In fact, up here in Sask., we are required to use "radon rock", a natural stone (no sharp edges) under slabs to help prevent the poly from being punctured. Does anyone know if radon gas behaves more like water vapor or more like air??


3.
Fri, 03/12/2010 - 11:21

Radon gas is transported by
by Joe Lstiburek, GBA Advisor

Helpful? 0

Radon gas is transported by air containing the radon gas. The movement of radon gas containing air is driven by an air pressure difference. Concrete is an excellent air barrier and therefore an excellent radon barrier when the concrete slab is sealed at the perimeter and at penetrations such as sump openings. Radon diffusion through concrete is negligible. Radon transfer through ripped and torn polyethylene covered with a concrete slab that is sealed at the perimeter and at sump locations is negligible. It is a great idea to ventilate the granular layer under the slab to the atmosphere to also deal with the air pressure driving force for radon transport. So construct your slab as an air barrier, vent the granular layer under the slab to the atmosphere and ignore the ripped and torn polyethylene. I love Sask but the requirement for radon control is not supported by the underlying physics.


4.
Fri, 03/12/2010 - 11:29

Thanks for clearing this up
by Garth Sproule

Helpful? -1

Thanks for clearing this up for me Joe.


5.
Wed, 03/17/2010 - 14:45

Concrete cracking
by Chris Ellis

Helpful? 0

A couple of further points - concrete slabs are likely to develop tension cracking, this is controlled by reinforcing but will occur at contraction joints and where the slab mesh is poorly detailed. The integrity of the underslab barrier is important in areas where the slab is cracked all the way through. It also seems to me to be important to have well detailed and finished underslab barrier in the event that the underslab drainage is damaged either during construction or in service and the underside of the slab is exposed to ponded water.


6.
Wed, 03/17/2010 - 15:11

Dry to the inside- EPDM roof over wd deck
by Joe DeScipio

Helpful? 7

If I learned anything from you Dr. Joe it is that continuity of the air barrier is paramount. Also, that two air barriers (not vapor barriers) is better than one, correct? Therefore, given the following roof section - (EPDM (060) membrane [perm of 0.05] over 1-1/2" ISO over 5/8" ply wd. sheathing over flat 2x10 joists @ 16" filled with R-30 fiberglass and a continuous layer of 5/8" drywall applied to the underside of the roof joists), I am correct in assuming that I should have a pretty darn good air barrier of the roof plane? Of course the intersection of where the roof meets the wall must be addressed. Also, with the very low perm of the EPDM membrane (i.e. 0.05) I want to be sure that the roof system can dry to the inside and therefore do not what to use any facing on the fiberglass nor, for that matter, use a paint on the drywall, correct?

Thanks and keep up the good stuff!

Joe D.


7.
Thu, 03/18/2010 - 09:16

Vapor Barriers in Crawl Space & Termites
by Marje

Helpful? -2

Great Pod cast, but it begs the question: If you have an existing attached townhome with water infiltration from existing graded gravel under your wood frame..Already added sand, regraded, sump--pumped and trenched, and plastic vapor-barriered the exposed wood floor joists and re-insulated with new dry fiberglass...and just tented for termites last spring...What remains to be done to prevent the whole swampy mess from happening again? Yeah, I ran a high-volume fan all last year to move the moisture out. Open to suggestions here.
Thanks for any help,
Marje


8.
Thu, 03/18/2010 - 09:46

What remains to be done
by Martin Holladay, GBA Advisor

Helpful? 0

Marje,
Here's what remains to be done:

1. Remove and discard the plastic vapor barrier you installed on the exposed wood floor joists. Instead, install a plastic vapor barrier on the re-graded sand floor.

2. Remove and discard the new fiberglass insulation. I don't know whether you installed it on the walls or between the overhead floor joists, but in either case you don't want fiberglass. Instead, insulate the crawl space walls with closed-cell spray polyurethane foam or rigid foam board.

3. Running a high-volume fan often adds moisture to a crawl space rather than removing it. (Warmer exterior air often contains moisture which condenses on cold crawl space surfaces.) Instead of running a fan, seal up your vents. If you're worried about the humidity, install a dehumidifier (at least temporarily) in your sealed crawlspace.


9.
Mon, 03/22/2010 - 13:13

basement remodel - dry to the inside
by Steve

Helpful? 0

OK, so we've remodeled our 1970 slab-floor basement with the new-ish idea that everything should be able to dry to the inside (based on reading a Fine Homebuilding article and Building Science Corporation articles). This includes: 2" unfaced EPS on walls and on the band/rim joist, 1" unfaced EPS on the slab floor, followed by 2 layers of 1/2" plywood on the floors and a standard 2x4 stud frame wall up against the 2" EPS to make up the walls. Standard 1/2" Gypsum over that. We are planning to use a high-breathability low-VOC primer like the one Mythic makes followed by breathable wallpaper. The floors though is where it gets tricky. In some areas we want to do tile - is there any way to make that breathable? If not, is it acceptable to rely on the untiled (carpet or bare slab) areas to dry any floor moisture? Any suggestions on the wallpaper also? Thanks, Steve


10.
Mon, 03/22/2010 - 18:28

Air Barriers v Vapor Barriers
by Bob Guthrie

Helpful? -1

This was an excellent piece and I learnt a lot from it.
However, another important function of the vapor barrier under a concrete slab is to _retain_ moisture in the slab to increase the curing time / strength of the concrete.
For this reason, I would suggest repairing of holes is still very important.


11.
Mon, 03/22/2010 - 19:10

No vapor barrier - seal edges?
by Carmela

Helpful? -2

My 9x12 mudroom is on a slab, pretty sure no vapor barrier beneath since it was built about 50 years ago. Humidity in this room, even tho open to rest of the house, is consistently 10% higher. If I remove the baseboard and inside trim around the exterior door and spray these areas with a slow expanding foam, will it create an air barrier, at least partially, and help reduce moisture? Seal the concrete? Both? Thank you.


12.
Mon, 03/22/2010 - 19:49

Finding Knowledgeable Builders
by Anonymous

Helpful? 1

All of the foregoing is very helpful. However, my question is broader: How does one find a builder or contractor who knows (and cares about) the correct principles of sound building practices? I have the feeling that most builders are going to do things the way they always have done, and don't much care to learn or change. Thanks for your help.


13.
Mon, 03/22/2010 - 20:27

Vapor barrier or retarder
by David Brooke Rush

Helpful? 1

Dear Joe,

Another very interesting article about the science of building. Another way to look at the difference between an air barrier and a vapor retarder is to think of a helium balloon. Fill up your balloon and let it will float around for a couple of days and then it will be laying on the floor. No hole in the balloon, the air barrier is still in tack, but the helium has “diffused” through the walls of the balloon.

Now put a small pin hole in the balloon and then fill it with helium, the deflated balloon will be on the floor in a couple of minutes, not days. Clearly the hole in the air barrier is the issue. Why worry about diffusion. Like you have said in your presentations, there is no such thing as a vapor barrier only a vapor retarder. Nothing stops diffusion, it just slows it down. “Look for the hole”


14.
Mon, 03/22/2010 - 21:03

and ADA?
by John

Helpful? -1

I was at a presentation on air barriers and vapor barriers in which they were pushing air tight drywall approach. They made the same arguement about the difference between air moving through the wall and diffusion. Here in "leaky condo" country (Vancouver) where I still see townhouses being ripped apart from the outside it seems to be the current thinking to have the wall cavity capable of drying towards the inside. They wanted the house wrap sealed and pressure tested with a reversed door blower test. I was always a "sealed poly" kind of guy but after the presentation I am almost completely converted to ADA at least for our specific region.

Can you provide any commentary on this concept of drying to the inside?

Thanks

John


15.
Tue, 03/23/2010 - 05:40

Sealing drywall at baseboard
by jayne

Helpful? -1

We have a heated slab with tile floors & the baseboards were never installed. Before installing baseboard, we had planned on using expanding foam to seal large gaps, but other areas where the gap between drywall and tile was small, it is filled with grout from the tile. Should I seal the grout with something?


16.
Wed, 03/24/2010 - 16:24

Answers to the previous questions
by Joe Lstiburek, GBA Advisor

Helpful? -1

Chris Elliis,
Actually the integrity of the underslab barrier is not important as
it is rare that a crack that extends all the way through the concrete
slab also aligns with a puncture in the membrane.

Joe DeScipio,
Yes, you are correct.

Marje,
Go to
http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-c...

Steve,
Paint the top of the slab. Go to
http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor...

Bob Guthrie,

The cure of the slab is not affected with holes in the vapor barrier.
The ground under the vapor barrier is at 100 percent relative
humidity so downward desiccation of the slab is not going to happen.

Carmela,
Paint the top of the slab as well.

John,
I developed ADA in the early 80's when I still drank beer and lived
in Canada. The approach allows inward drying in the summer. Had it
been widely used in Vancouver the condo crisis would have been much
smaller.

Jayne
No.


17.
Wed, 04/07/2010 - 09:05

Protecting brick foundation while upgrading basement
by Brian Milani

Helpful? 2

We're renovating a basement with a brick foundation that has been drying to the inside for over 100 years, including lowering/underpinning and a new well-insulated radiant slab, interior drain tile with sump, etc. The tight city site in Toronto doesn't allow any external foundation work, and various people have told us to be careful to not make the basement so tight inside that it traps too much moisture in the wall. We have interior drain tile and a way for water on the foundation wall to drain down and be carried away. It's been suggested we leave out the bottom 16 in. of insulation in the newly framed wall, and will have a moisture barrier up to grade level. I suspect we should simply leave out the poly a/v barrier inside, but we've got different opinions about the insulation: roxul batts? closed-cell foam? open-cell foam? What will allow continued drying to the inside while giving us better thermal efficiency without big problems with condensation?


18.
Wed, 04/21/2010 - 09:41

Build Tight Ventilate Right Conundrum
by Marc Manley

Helpful? 7

The one thing I don't see addressed in the tight envelope discussions is what appears to be an underlying but unspoken assumption, being that we are to live in homes that are sealed shut at all times except under the most perfect climatic conditions (68 to 75 degrees F, 45-60% rel humidity. Please don't pick apart the numbers. That's not the point of my question!). I mean, isn't an open window a pretty big hole in the envelope?

We live in a Durisol ICF/open cell spray polyurethane roof home in a mixed climate and can have really low utility bills for 6 to 8 months of the year, IF we open the windows and allow the house to cool off overnight. Of course doing so means a higher-than-ideal humidity level in the home (75-80%), equalized, more or less to the outdoor environment. If we don't want the humidity that high than obviously we have to seal off the house, lose the enjoyment of the breezes blowing through, AND pay for it by using lots more fossil fuels (coal-based electric grid), thus supposedly being LESS environmentally friendly.

So am I missing something here? Or is my assumption about your assumption correct? If so, how would you respond to this? And if I am missing something, I would greatly appreciate any suggestions as to a better strategy.

Thanks!


19.
Wed, 04/21/2010 - 10:52

Answers for Brian and marc
by Joe Lstiburek, GBA Advisor

Helpful? -4

Brian Milani

The following links and papers will help you answer your own
question. Read them in the order they are listed as I think they
will make more sense that way. To answer two of your specific
questions leave out the poly, but do insulate full height.

http://www.buildingscience.com/documents/digests/bsd-103-understanding-b...
http://www.buildingscience.com/doctypes/researchreport - download
RR-0509c: Renovating Existing Basements
http://www.buildingscience.com/documents/insights/bsi-011-capillarity201...

Marc Manley

When you build tight and have operable windows and a controlled
ventilation system you can control your interior environment to suit
your particular requirements. In a leaky building that is not
possible. In a tight building you can ventilate passively with
operable windows - as apparently you are doing - and be subject to
the limitations of the exterior conditions - which is ok in my view.
Or you can mechanically exchange air and condition the interior using
energy for heating, cooling, dehumidification - which is also ok in
my view (if you do it efficiently).

In doing all of this you have to understand the physics of your
environment. When the outside air is humid and hot you are going to
have to spend energy to make it colder and drier. The tighter the
house and the better insulated, the less energy you are going to have
to spend to make it colder and drier. You get to pick how cold or
dry you want to make the air and you get to pick how much air you
want to bring in. Those choices have energy and other implications.
You can't do all of that in a leaky house.


20.
Fri, 05/21/2010 - 11:11

Vapor Barriers under slabs.
by Hank

Helpful? 6

As an architect with 32 years of practice I find your comments about vapor barriers under concrete slabs wrong and misleading. Concrete is a very poor vapor barrier. Over the years I have seen glue down wood floors on 4" of concrete with a typical 6 mil visqueen buckle 6" high. Just tape a 16" square sheet of visqueen to the slab and wait a day or so and watch how much moisture accumalates under the square sheet. Virtually every floor manufacture we specify specifically notes in their warranty the allowable moisture content in the slab and in every case I have seen of a floor material, VCT, wood, etc. which failed was due to vapor transmission thru the light weight visqueen and the 4" concrete slab. We do a calcium cholride test on every job with a slab. Most floor adhesives used today are water based, the moisture vapor transmits through the slab and becomes trapped under the impervious flooring material and turns the glue to a nice green moldy substance.

Re: ASTM 1643-09 sections 5..32.1 & 5.3.2.3 ACI 302.2R-06 section 9.3

Typically this occurs humid climates where the indoor RH is much lower than outside air, moisture vapor migrate from a moist conditon to a dry condition right thru the slab. If you are installing a glue down flooring material in a humid region, beware.


21.
Fri, 05/21/2010 - 13:41

Ripped plastic isn't the issue
by Joe Lstiburek, GBA Advisor

Helpful? -1

Hank,

The vapor transmission through a 4 inch concrete slab placed on top of a ripped and torn 6 mil poly sheet over stones is trivial. I challenge you to show me any analysis method that shows otherwise.

There is an enormous amount of water in the concrete when it is first placed. Depending on floor covering and climate it may take months to years to equilibrate. When you place that sheet of poly on a slab it matters what the age of the slab is.

Calcium chloride tests are a joke. That flooring manufacturers still refer to them is a travesty and their warranties are written to protect them not the user. In terms of the re-emulsification of
adhesives and a more detailed response to you comments check out the following link.

http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor...


22.
Thu, 06/10/2010 - 23:22

Uplifting Moments & Air Barriers
by Ronald D. Crouch, AIA

Helpful? -1

Hi Dr. Joe from Richmond, Virginia. Hope you are well. I will attend your Summer Camp one day.

Did you know it takes 50 Richmonders to change a light bulb? One to change the bulb. 49 to stand around and talk about how they liked the old bulb better.

The same scenario is true with our firm’s “current” approach to a better “compact” low slope roof design (we believe) versus the sincere but conservative nature of some of our loyal local customers. I’ll explain.

In the vein of your “classic” BSI-019 Uplifting Moments paper, we believe we must get that horribly leaky steel deck airtight. We indicate a ½” gyp sheathing "substrate board" fastened to the steel deck, then a self-adhering air barrier membrane, then the double staggered insulation layers, then cover board, then a fully adhered white TPO roof membrane or sometimes modified bitumen. We get calls and letters of concern. Why are we building a "double roof" we are asked.

Our veteran mechanical engineering leader asks “Please stop the plenum air from being able to be driven up (via positive building pressure) thru the leaky steel deck where it can condense in the roof insulation joints.” That is what we are trying to do but it is not understood. Trapped moisture vapor questions then abound.

In Virginia it can be a real exercise determining whether the above subject roof air barrier membrane should be vapor permeable or impermeable. Chapter 3 of the NRCA 5th Edition manual on low slope roofing explains some analytical methods including Wayne Tobiasson’s time honored 1986 “CRREL” method which uses the expected winter time indoor RH as a main calculation guide. I know you have a great deal of respect for Mr. Tobiasson.

Whatever we determine on vapor permeability, this on thing we believe; The Owner should want to stop the air movement thru his leaky steel deck. If for no other reason, chose hurricanes. We do projects near the eastern seaboard coast. We will get our “Katrina” or “Andrew” here one day in our life times. Pray not.

I want our VP’s getting elated calls from our customers the day after the hurricane, saying “Our roof is all still on! We’re dry! It was worth the money. Thank You!”

One Owner eliminated the subject roof air barrier so he would know if their roof was leaking. He wanted the water to show thru the leaky steel deck as a fast “tell tail” sign (nautical term you know) but honestly the leak could be 100's of feet away and not show up fast at all - if ever.

This good customer also worried that the subject air barrier will trap the same leaking water, which honestly, I guess it does. They have got me on that one. I suppose this is why thermal cameras are made and can be rented for $100/day or he could buy one and check his roofs as often as he likes.

Q: So, which is the lesser of two evils? 1) Possible trapped water or 2) the possibility of condensing water and eventually a roof membrane that might try to fly to the next county? I am looking for the most sincere and heart-felt response to these trusted freinds of ours that might turn on a "light buld" that our approach truly is a better way.

We do appreciate their concerns and often customers are far smarter than their architects. We always listen very closely.

Thanks for your constant generosity of sharing of your knowledge.
Ron


23.
Thu, 06/17/2010 - 16:09

metal buildings and use of SPF
by Teri England

Helpful? 1

Hello Joe,
We attended your workshop in Portland, Oregon a couple months ago. Very informative. Love your style.

Anyways, we have a metal building we are spraying with SPF as insulation both on the walls and on the ceiling. I am having a difficult time locating any perms or calcs on shear strentgh, snow load, moisture, thermal or air barriers.

The building code officials here, as well as the local architects and engineers don't know much if anything about SPF and don't want to spend the time or resources learning or researching any "new" product. It is difficult to get them to open up to the benefits of SPF.

So the two things I am in search of:
1) perms and calcs on metal buildings in one place
2) a way to educate the decision makers when it comes to SPF and it's many uses.

Thank you!!

Teri


24.
Fri, 06/18/2010 - 20:16

Response to Ronald Crouch and Teri England
by Joe Lstiburek, GBA Advisor

Helpful? 8

Ronald Crouch,

It is absolutely critical that there be an air barrier at the deck.
Whether this air barrier is vapor open or vapor closed is irrelevant
because the deck is typically metal. Hence, the deck is the vapor
barrier, and the air barrier on top of it is - yes you guessed it -
the air barrier. So, you always end up with a double vapor barrier
when you have a metal deck and a roof membrane with insulation in
between. What you also want and need is a double air barrier.

It is just dumb to not install an air barrier in order to to see roof
leaks when the roof leaks. Why? Because if you don't have the air
barrier at the deck you get uplift problems and you get condensation
problems that are far worse than the roof leaks. Besides, not
installing an air barrier is not the way to find roof leaks. Roof
leaks are why God invented the infrared camera.

Finally, the reason that double vapor barriers are not a problem in
roofs but they are a problem in walls is that in roofs it is possible
to get two almost perfect air barriers and two almost perfect vapor
barriers and so the assembly works. That is almost impossible to do
in walls.

So, double vapor barriers work in roofs if you also have double air
barriers. The one that most folks screw up is the air barrier at the
deck. If you have an air barrier at the deck the other air barrier
is the upper roof membrane itself. Whether or not the lower air
barrier is also a vapor barrier or not does not matter because for
all practical purposes you have a vapor barrier anyway - the deck
itself.

As a consultant you can have a lot of fun making money doing
meaningless vapor transmission and dewpoint calculations. As long as
that lower air barrier is in place any calculation you do will always
be correct because the roof works regardless of the calculation and
regardless of whether you specify the perm value of the lower
membrane air barrier. Neat eh?

Teri England,

The perm values for foams are listed in ASHRAE Fundamentials and can
also be obtained from the manufacturer's. In general low density
foam is around 50 perms per inch of thickness and high density foam
is around 3 perms per inch of thickness.

Good luck with getting shear values with spray foam on metal panels
attached to metal building frames. The current best approach is to
assume the foam provides no shear and you go with the metal building
standard structural calcs for wind and ground snow loads.

The manufacturers of spray foam can provide you with information
regarding thermal barriers as their code acceptance requires them to
provide that information and the limitations of use for their
products.

In terms of air barrier information check out the following link:
http://www.buildingscience.com/documents/digests/bsd-104-understanding-air- barriers/?topic=/doctypes/digest

Joseph Lstiburek


25.
Wed, 06/23/2010 - 10:34

Answers for Ronald and Teri
by Joe Lstiburek, GBA Advisor

Helpful? -10

Ronald Crouch,

It is absolutely critical that there be an air barrier at the deck.
Whether this air barrier is vapor open or vapor closed is irrelevant because the deck is typically metal. Hence, the deck is the vapor barrier, and the air barrier on top of it is - yes you guessed it - the air barrier. So, you always end up with a double vapor barrier when you have a metal deck and a roof membrane with insulation in between. What you also want and need is a double air barrier.

It is just dumb to not install an air barrier in order to to see roof leaks when the roof leaks. Why? Because if you don't have the air barrier at the deck you get uplift problems and you get condensation problems that are far worse than the roof leaks. Besides, not installing an air barrier is not the way to find roof leaks. Roof leaks are why God invented the infrared camera.

Finally, the reason that double vapor barriers are not a problem in roofs but they are a problem in walls is that in roofs it is possible to get two almost perfect air barriers and two almost perfect vapor barriers and so the assembly works. That is almost impossible to do in walls.

So, double vapor barriers work in roofs if you also have double air barriers. The one that most folks screw up is the air barrier at the deck. If you have an air barrier at the deck the other air barrier is the upper roof membrane itself. Whether or not the lower air barrier is also a vapor barrier or not does not matter because for all practical purposes you have a vapor barrier anyway - the deck itself.

As a consultant you can have a lot of fun making money doing meaningless vapor transmission and dewpoint calculations. As long as that lower air barrier is in place any calculation you do will always be correct because the roof works regardless of the calculation and regardless of whether you specify the perm value of the lower membrane air barrier. Neat eh?

Teri England

The perm values for foams are listed in ASHRAE Fundamentials and can also be obtained from the manufacturer's. In general low density foam is around 50 perms per inch of thickness and high density foam is around 3 perms per inch of thickness.

Good luck with getting shear values with spray foam on metal panels attached to metal building frames. The current best approach is to assume the foam provides no shear and you go with the metal building standard structural calcs for wind and ground snow loads.

The manufacturers of spray foam can provide you with information regarding thermal barriers as their code acceptance requires them to provide that information and the limitations of use for their products.

In terms of air barrier information check out the following link:
http://www.buildingscience.com/documents/digests/bsd-104-understanding-a...


26.
Fri, 07/23/2010 - 12:55

Vapor/Air Barrier
by Matt R.

Helpful? 1

I was told that the vapor barrier in a refrigerated warehouse roof cannot be repaired. It can only be replaced. I think there may be a greater problem with the air barrier than the vapor barrier. What can you tell me about this? Is there a way to patch and repair?


27.
Fri, 07/23/2010 - 13:50

Re- Matt R
by Joe Lstiburek, GBA Advisor

Helpful? -8

Re- Matt R
You are not being helpful enough in you description of the problem. A cross-section of the roof is necessary. Having said that, in general, with respect to refrigerated warehouse roof assemblies, the drying direction is downward and the roof membrane is typically both the air barrier and the vapor barrier and no additional layer is required. If your roof is not leaking rainwater and if you are not leaking air at the roof perimeters where the roof meets the wall you should not have a problem. Of course all of this changes if it is a metal roof and a metal industrial building.


28.
Wed, 09/01/2010 - 15:04

Air barrier AND vapor barrier
by Pam W.

Helpful? -8

I am an architect in WV. On a wood framed building with 5/8" gyp board on the interior, plywood sheathing, 1" airspace and brick veneer on the exterior, we commonly use plastic vapor retarder on the inside and building wrap (air barrier) on the outside. Is this practice overkill, adequate, or inadequate for our climate?


29.
Thu, 09/02/2010 - 04:23

Response to Pam W.
by Martin Holladay, GBA Advisor

Helpful? 5

Pam W.,
Interior polyethylene is a mistake in your climate. If anyone ever installs air conditioning, the poly can be come a condensing surface during the summer.

After your brick veneer gets wet from a rainstorm during the summer, your wall is in danger of getting wet when sunshine causes inward solar vapor drive. It's a good idea to install rigid foam sheathing behind brick veneer, in order to limit inward solar vapor drive.


30.
Wed, 09/29/2010 - 13:41

vapor barrier needed between plywood subfloor & 1.5" concrete??
by Anonymous

Helpful? 1

Our crew is about to pour 1 1/2 inches of concrete (straight concrete, no additives) on the main floor of our house (2 story house -- main level with a walkout basement) -- radiant will go in it and we'll have a type of flooring installed over it (not sure what yet). They need to know what we want to use as a barrier between the concrete and the exterior grade CDX plywood subfloor. My wife is chemically sensitive, so we need to be careful with the material selection.

Any ideas for what material is typically used for this application? Vapor barrier or air barrier or??
Thanks!


31.
Fri, 10/01/2010 - 11:22

weather resistant barriers
by Eddy E.

Helpful? 0

Joe,
Regarding inward solar vapor drive. I understand this to be a problem when typical housewraps or felt papers are utilized and the moisture condenses behind the housewrap and gets absorbed into the sheathing. I have seen new weather resistant barriers that are liquid applied with meshed seams that are coated with the liquid membrane. Will this application reduce (or eliminate) the amount of vapor drive into the sheathing behind?
thanks,
Eddy


32.
Fri, 10/01/2010 - 11:39

Response to Eddy
by Martin Holladay, GBA Advisor

Helpful? 1

Eddy,
As I understand it, liquid-applied water-resistant barriers are designed to be vapor-permeable, just like housewraps, so they won't solve the inward solar vapor drive problem.

To learn more about inward solar vapor drive, see
When Sunshine Drives Moisture Into Walls

To learn more about liquid-applied WRBs, see
Housewrap in a Can: Liquid-Applied WRBs


33.
Wed, 10/06/2010 - 15:24

Air barrier
by Nick Coulson

Helpful? 0

Our climate is mixed humid (deep south)
Is there a brush applied air barrier that can be "painted" on to
the inside sheetrock of wood framed houses that will help air "seal" the house,
but still be permeable (enough) to moisture vapor to avoid condensation risk ?
Or is there another way or am I dreaming ?


34.
Wed, 10/06/2010 - 20:28

Response to Nick Coulson
by Martin Holladay, GBA Advisor

Helpful? 0

Nick Coulson,
Q. "Is there a brush-applied air barrier that can be "painted" on to the inside sheetrock of wood framed houses that will help air "seal" the house, but still be permeable (enough) to moisture vapor to avoid condensation risk ?"

A. No.

Q. "Is there another way?"

A. Yes. For an existing house, the "other way" is called blower-door directed air sealing. You need to hire a home-performance contractor to find the air leaks in your house, and to seal them with spray foam, caulk, and other materials, while a blower door is depressurizing your house. To learn more, see:
Blower Door Basics


35.
Fri, 11/12/2010 - 14:25

Uplifting Moments & Air Barriers Revisited
by Ronald D. Crouch, AIA

Helpful? 1

Hi Dr. Joe,
Thank you for your thorough response on the “Uplifting Moments & Air Barriers” blog query above. Very helpful.
Please take the scenario of that SAME subject low-slope compact roof but with a twist. Bear with me. Tilt that roof on a pitch. Cover it with a structural lock-seamed metal roof. This top membrane (over the insulation) is being forced upon us by “some” metal roof manufactures even if the good ones (the ones we actually spec) don’t require such a continuous membrane. That’s a whole ‘nother discussion. Ughhh.
Anyway, these roof manufactures that are requiring a water shedding membrane under their roof panels are also saying it has to be an asphaltic “vapor-closed” type membrane. Impermeable. No 20 year watertight warranty is their leverage. Set that aside for the moment.
In this same new roof assembly we still want your “critical” air barrier at the structural steel deck region – hopefully placed over a faced gypsum substrate board or maybe a thin starting layer of my double layer of insulation (to save another layer of stuff). We know to keep the air barrier membrane above the dew point temperature. (NRCA and Wayne Tobiasson have some ways to figure that.)
MY QUESTION: I know you said the low slope compact roof could have a vapor open or vapor closed air barrier at the structural steel deck in the original blog answer. Didn’t matter. You were only after stopping the air from coming thru the steel deck. We agree. But, does that opinion change in the above “new” roof scenario where we are poking holes in our top vapor-closed membrane for metal roof panels? I know these ice and water membranes seal very well around fasteners and we can get high temp types.
You see, we want to take your “Perfect Wall” (BSI-001) and tilt it on an angle to make the “Perfect Roof” but these roofs manufactures who “must” have plastic underpants - that will still be riddled with clip anchor holes – sure throw a monkey wrench into the simplicity of things.
I actually hope you think “vapor-closed” membranes top AND bottom does work in this new scenario. They are sure stickier to apply in the field and fasteners do seal well around them but some have suggested to us that the bottom membrane be vapor open on this case.
Thoughts? We are home based in Virginia but do work in the Carolinas as well.
Thanks,
Ron


36.
Sun, 11/14/2010 - 22:27

Air Barrier
by Del

Helpful? -1

Is it okay to place foil-backed white styro-foam floor underlayment over the dirt (purpose is as a cushion to prevent punctures in 6 ml poly overlay - has been only tack-taped and does not fully cover the floor)? Also, how can I effectively seal the poly to the concrete walls of the crawlspace? Thank you for your good works.


37.
Mon, 11/15/2010 - 05:39

Response to Del
by Martin Holladay, GBA Advisor

Helpful? -2

Del,
Q. "Is it okay to place foil-backed white styrofoam floor underlayment over the dirt (purpose is as a cushion to prevent punctures in 6 ml poly overlay - has been only tack-taped and does not fully cover the floor)?"

A. Yes.

Q. "How can I effectively seal the poly to the concrete walls of the crawlspace?"

A. The best way is to use a bead of Tremco acoustical sealant between the poly and the concrete, and then to compress the Tremco by placing a horizontal 1x3 or 1x4 batten on top of seam and fastening the batten to the concrete wall with Tapcons or masonry fasteners.


38.
Mon, 11/15/2010 - 12:20

To Ron Crouch
by Joe Lstiburek, GBA Advisor

Helpful? -2

The roof assembly works with vapor closed membranes top and bottom.


39.
Mon, 01/31/2011 - 11:20

Close cell foam under roof deck
by John Duggan

Helpful? 0

Hi Dr Joe.
I live in south Louisiana and want to spray foam under my roof deck. I have asphalt shingles and tar paper underlayment. Which foam should I use, open or close, and why?


40.
Mon, 01/31/2011 - 11:59

Response to John Duggan
by Martin Holladay, GBA Advisor

Helpful? 0

John,
You can use either one -- it's up to you. Just be sure that your insulation contractor installs AT LEAST the minimum thickness of insulation required to meet minimum R-value required under your local building code. Most spray foam contractors don't -- they skimp.


41.
Mon, 01/31/2011 - 16:35

Close cell foam
by John Duggan

Helpful? 0

Hi Martin,
If I read Dr Joe's articles correctly If I put an impermeable product like close cell foam I will be trapping moisture in the roof deck.


42.
Mon, 01/31/2011 - 16:44

Second response to John Duggan
by Martin Holladay, GBA Advisor

Helpful? 0

John,
I don't follow your logic. Are you worried about roofing leaks? If so, you are not alone. It's true that many people worry that when their roof eventually leaks, water may rot their roof sheathing before they notice the leak.

Is there another concern?


43.
Mon, 01/31/2011 - 17:22

Answer to John Duggan
by Joe Lstiburek, GBA Advisor

Helpful? 1

Closed cell foam is not impermeable. You can use it on the underside of the roof deck. It is a vapor retarder not a vapor barrier.


44.
Mon, 01/31/2011 - 17:47

close cell foam
by John Duggan

Helpful? 0

Martin
I am worried about roof leaks, but I was also worried about traping moisture in the deck between the tar paper and the foam. Dr Joe just answered my moisture question. I obtained some open cell foam and have been testing it. I have found that the open cell will hold water a long time. I simulated a leak into the foam and it soaked into the foam and it has been 3 days now and the foam is still saturated. This has me worried about the open cell foam. The closed cell will not saturate but a leak won't go through it.


45.
Fri, 05/13/2011 - 13:08

Need to use BIN shellac on entire apartment in NJ. Mold danger?
by K. G.

Helpful? 0

On the verge of moving into the perfect mold-free apartment, perfect six weeks ago that is, my chemically sensitive husband and myself have been flummoxed by the paint applied a month ago to the apartment, which turns out to contain vinyl acetate and isn't curing fast enough. It stinks and is making us ill, so we can't move in. Discovered this after signing lease. Desire to fix apartment so we can move in!

We've heard that shellac primer, "BIN classic" I suppose you could say, would reek on first application but thereafter seal all the outgassing and be inert. Perfect--but now here's why I'm here: we've also been told this may not be safe to use in NJ on the inside surface of outside walls, the BIN providing too good a vapor barrier for that location.

Is this true? Does it depend on the type of wall construction? The walls may be steel-framed, and there is an attic above the apartment, but we don't know about wall venting. It would be great to be able to use the BIN without a mold worry because no other products we've heard of sound as if they offer 100% chemical odor blocking, and for vinyl acetate, we need 100%.

Any ideas? And what should we learn about the wall system in order to be sure? Thank you very much for your comments and help.

P.S. The whole apartment was painted, including ceilings, which means ceilings are in view in my question as well as exterior walls.


46.
Fri, 05/13/2011 - 13:37

Edited Fri, 05/13/2011 - 13:39.

Response to K.G.
by Martin Holladay, GBA Advisor

Helpful? 0

[Joe Lstiburek just sent me this answer by e-mail and asked me to post it:]

In general shellac is not a good idea to apply on interior surfaces in air conditioned assemblies - just as it is not a good idea to install vinyl wall coverings on interior surfaces in air conditioned assemblies.

You might want to consider covering the paint with an acrylic latex paint that is vapor permeable. Check on the chemical compatibility of this paint with the vinyl acetate.
-- Joseph Lstiburek


47.
Fri, 05/13/2011 - 14:56

If the new acrylic latex is vapor permeable . . .
by K. G.

Helpful? 0

Thank you very much for the response. I have greatly appreciated discovering Mr. Lstiburek's books and some day I hope we can build a house and do it right from the ground up.

I am puzzled, though. If we coat over the toxic paint with something that IS vapor permeable, then won't the outgassing from the toxic paint seep through the new paint and eventually reenter our living space once again after a honeymoon period of apparent camouflage?

Or does it depend on the new paint used?

Also--I'm intrigued by the suggestion to check the compatibility of the second paint with vinyl acetate. I wouldn't know how to go about that.

Thank you, and any more light you all can shed on our difficult situation is greatly appreciated.


48.
Tue, 12/06/2011 - 00:38

Edited Tue, 12/06/2011 - 00:43.

Why are plastic vapor barriers recommended over concrete slabs?
by Scott Morgan

Helpful? 0

Thanks to Dr Lstiburek I think I have a good understanding of the limitations and precautions of vapor barriers on walls,.but I am still unclear about vapor barriers on concrete slabs. In a retrofit situation on a dry slab over 50 years old, will a poly vapor barrier cause excessive moisture to accumulate under the plastic due to vapor diffusion through the slab? Why do wood and laminate floor manufacturers recommend poly vapor barriers if so much moisture would accumulate under them?


49.
Tue, 12/06/2011 - 04:46

Response to Scott Morgan
by Martin Holladay, GBA Advisor

Helpful? 1

Scott,
Assuming that the soil under your slab can be damp, the idea of the vapor barrier is to separate the damp soil from the dry flooring above. Wouldn't you rather have dampness under the poly rather than dampness in your flooring?

Even better than installing a layer of poly on top of an old slab is installing a layer of rigid foam on top of an old slab. That way, you reduce the moisture transmission -- and you also get some R-value.


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