Air Barrier or Vapor Barrier? - Building Science Podcast

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

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

The Simple and Continuous Red Pencil Line

What about radon?
by Garth Sproule

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??

Radon gas is transported by
by Joe Lstiburek, GBA Advisor

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.

Thanks for clearing this up
by Garth Sproule

Thanks for clearing this up for me Joe.

Concrete cracking
by Chris Ellis

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.

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

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.

Vapor Barriers in Crawl Space & Termites
by Marje

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

What remains to be done
by Martin Holladay, GBA Advisor

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.

basement remodel - dry to the inside
by Steve

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

Air Barriers v Vapor Barriers
by Bob Guthrie

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.

No vapor barrier - seal edges?
by Carmela

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.

Finding Knowledgeable Builders
by Anonymous

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.

Vapor barrier or retarder
by David Brooke Rush

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”

and ADA?
by John

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

Sealing drywall at baseboard
by jayne

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?

Answers to the previous questions
by Joe Lstiburek, GBA Advisor

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.

Protecting brick foundation while upgrading basement
by Brian Milani

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?

Build Tight Ventilate Right Conundrum
by Marc Manley

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!

Answers for Brian and marc
by Joe Lstiburek, GBA Advisor

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.

Vapor Barriers under slabs.
by Hank

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.

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

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...

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

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

metal buildings and use of SPF
by Teri England

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

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

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

Answers for Ronald and Teri
by Joe Lstiburek, GBA Advisor

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...

Vapor/Air Barrier
by Matt R.

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?

Re- Matt R
by Joe Lstiburek, GBA Advisor

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.