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Joe Lstiburek Discusses Basement Insulation and Vapor Retarders

An all-day seminar with Dr. Joe yields two news stories, a six-digit idea, and plenty of quotable opinions and conclusions

Posted on Jun 15 2012 by Martin Holladay, GBA Advisor

Dr. Joseph Lstiburek needs little introduction. The well-known Canadian engineer is a principal of the Building Science Corporation in Massachusetts. He’s also a regular GBA podcaster and Fine Homebuilding author.

On Wednesday, June 6th, I attended an all-day building science class presented by Dr. Joe in Westford, Massachusetts. As usual, his presentation combined salty language, corny jokes, light-hearted insults, and rock-solid building science information.

Although I’ve been listening to Joe’s presentations for at least 13 years, I learn something new each time I hear him speak. This time around, I harvested two news stories, one six-digit idea, and at least 16 interesting quotes.

News story #1: Don’t trust published perm ratings for vapor-retarder paint

Every now and then, someone posts a question about vapor retarders on GBA’s Q&A page. Over the years, I’ve often recommended the use vapor-retarder paint on interior drywall. Vapor-retarder paint is less problematic than interior polyethylene; moreover, the paint should satisfy a building inspector looking for a code-required 1-perm vapor retarder on a home's warm-in-winter surfaces.

I have also recommended the use of vapor-retarder paint on cured open-cell spray foam installed against the underside of roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. in cold climates. This method of vapor control was suggested to me several years ago by Joe Lstiburek, who told me that some type of vapor retarder is necessary in cold climates to prevent moisture accumulation in the sheathing.

Here’s the news: after delving into the nitty-gritty details of vapor-permeance testing of vapor-retarder paints, Lstiburek has concluded that the vapor permeance rating listed in the manufacturers' specs (usually 1 perm) only occurs in a lab, not in the real world.

The 1-perm value is an artifact of the laboratory testing method usually used by paint manufacturers — a type of test that doesn’t include a substrate. (In most lab tests, only the dried paint film is tested.) Once the paints are applied to a substrate, the permeance of the paint film rises.

Lstiburek explained, “There isn’t a vapor retarder coating that I know of that works on spray foam. Manufacturers claim there is a coating that works, but I don’t believe it. They do have a 1-perm paint; at least it’s 1 perm as tested according to the test procedures used for testing coatings. But when you put the coating on a substrate, it doesn’t behave that way. Is there a vapor-retarder paint that provides a 1-perm vapor retarder on drywall? It turns out, no — we’re not getting 1 perm. But we are getting close to 1 perm — close enough that I am a happy guy. On drywall, these coatings are giving us maybe 3 or 4 perms. That’s close enough. The 1-perm requirement in the code is arbitrary anyway.

“But when we apply the paint to low-density spray polyurethane foam, we don’t get 1 perm or even 4 perms. We’re not even close.”

So, if vapor-retarder paint won’t work on cured open-cell foam, what should we do if we want to use spray foam on the underside of roof sheathing in a cold climate? According to Lstiburek, we should just use closed-cell foam. “Open-cell foams work for roofs in climate zones 4 and hotter zones, but you need closed-cell foam in climate zones 5 and higher. Open-cell foam has most of the market in the hot dry and hot humid climates because it costs less and it works well. But I have a problem with it in the cold climates because it is too vapor-open and I don’t think the coatings work.”

Lstiburek mentioned one effective way to establish a vapor retarder when using open-cell foam: the foam can be shaved back flush with the bottoms of the rafters, and the insulated cavities can be protected by a layer of gypsum drywall painted with vapor-retarder paint. As long as the paint is applied to drywall rather than the foam itself, the paint will perform adequately. Of course, this method only works for roofs framed with dimensional lumber rather than roof trusses.

News story #2: Basement walls don’t need to dry to the interior

Joe Lstiburek’s Builder’s Guide — there are versions of the book for every climate in the U.S. — has been used by energy-conscious builders as a reference work for years. In his book, Lstiburek advised builders to choose only vapor-permeable types of rigid foam when insulating the interior of basement walls. Lstiburek wrote, “It is important that interior insulation assemblies and finishes [on basement walls] be constructed as airtight as possible (but vapor permeable). This will prevent interior moisture-laden air from accessing cold surfaces during both the winter and summer and still allow the assemblies to dry.”

This advice — to allow basement walls to dry to the interior — has always caused some head-scratching, and the topic has come up several times on GBA’s Q&A pages. For example, in October 2011, Richard Baumgarten wrote, “I’ve always read that basement wall insulation should be semi-permeable to water vapor, allowing the wall assembly to dry to the interior, but I don't recall ever reading why.”

Here’s what I wrote in response to Baumgarten’s question: “I have puzzled through the same question, and I have concluded that there is no reason for a foundation wall to dry to the interior, in spite of what you sometimes read. Walls insulated on the interior with closed-cell spray foam perform very well — and they certainly don’t dry to the interior.”

A similar question was asked almost a year earlier. In November 2010, I wrote, "It is illogical and unnecessary to encourage a concrete wall in contact with the damp soil to dry inward." Similarly, in December 2010, I wrote, “The experts at the Building Science Corp. believe that a concrete basement wall should be able to dry to the interior, but I think that the importance of this detail is exaggerated. After all, the soil on the outside of your basement wall is usually damp. If you encourage a basement wall to dry to the interior, you are inviting a lot of moisture into your home.”

The question popped up again in March 2011, when I wrote, “Some experts come up with a logical explanation for why it’s useful for basement walls to be able to dry to the interior, but I don’t buy it. … Masonry and concrete walls aren’t hurt by a little moisture.”

Here's the news story: Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”

Lstiburek's former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall.

Joe’s six-digit idea: Siding and OSB manufacturers should develop products with bumps

I’d like to tip my hat to Chris Briley and Phil Kaplan, the Maine designers responsible for the Green Architects’ Lounge, for coining the term “six-digit idea.” A six-digit idea is an invention that has the potential to earn big bucks.

At his recent workshop, Joe gave away a great six-digit idea. He suggested that it was time for manufacturers of fiber-cement siding to create siding which is bumpy on the back side. After all, if it’s bumpy, it will allow for drainage and ventilation. Great idea!

He also said, “Why isn’t anyone making bumpy OSB?” That’s an excellent question. If the back of the siding can’t be bumpy, why not make one side of the OSB bumpy?

Now that this six-digit idea is out there, it’s time for manufacturers to get cracking. Don’t forget to send Joe a nice bottle of wine as a thank-you present.

A collection of 16 quotes

Joe is very quotable. If any of his quotes strike you as outrageous, blame the journalist (me) who quoted Joe out of context.

“They call me a building whisperer. I can hear the building talking to me.”

“A steel stud is a thermodynamic obscenity. All-steel buildings insulated with fluffy stuff are an embarrassment.”

“The best place to insulate is on the outside.”

“Reverse lapping of housewrap happens at almost every job site.”

“There are only two kinds of windows: windows that leak and windows that will leak.”

“An innie or an outie? I prefer innie windows because they look better from the inside. The water control details are more complicated with an innie than an outie, but thermally it performs better and it will last longer. However, the reason I like innies is not because they perform better thermally; it’s because they look good.”

“I hate the windows with suspended films. I don’t believe that they work. We have some of the Hurd windows with Heat Mirror glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. in our barn, and every one of them is… [Joe’s voice trails off]. Forget the suspended films. Choose windows with three layers of glass. None of this film stuff for Joe.”

“A leak is not a leak if the client never sees it.”

“Insulating the slab and basement walls controls mustiness and mold in the summertime. Now moisture doesn’t condense on cold surfaces in the basement. We insulate less for energy efficiency than for comfort and odor control.”

“If the outcome of a test isn’t going to change your course of action, why would you want to do the test? A classic example is mold testing. Why would you want to test for mold? Just find out where the wet area is and make it dry and make sure it doesn’t get wet again.”

“When paint peels, it’s the paint telling you that you did something stupid here. ‘Danger, danger, Will Robinson.’”

“Vinyl siding is foolproof. There’s a lot of air circulation. It doesn’t store water, so there is no solar-driven moisture. But if you install stucco over OSB, you are doomed unless you put in an air gap.”

“We stripped the roof shingles and put 6 inches of rigid insulation on top of the sheathing — three 2-inch layers of polyisocyanurate. So what’s the payback? Never. I save maybe $45 a year on my heating and air conditioning bill. This isn’t about payback; it’s about ‘My R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. is bigger than your R-value.’”

“There is a big difference between OSB and plywood. Both have variable permeance, but the vapor permeance of plywood is more variable at the high end of the relative humidity scale. In the same microclimate, plywood works significantly better than OSB. Plywood dries much faster, while OSB needs to be more protected than plywood. OSB can’t get as wet and it needs to be more deliberately dried. That’s why it is critical to have an air gap between OSB and your claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. . Fiber-cement siding can go directly over plywood, but with OSB you need an air gap.”

“The humble vented attic has the best performance for the least amount of money. Build an airtight ceiling, install continuous soffit and ridge ventilation, and pile on lots of fluffy stuff. This type of attic works in all climates. But for this to work, the only thing above the ceiling should be fluffy stuff. There should be no storage and no mechanicals.”

“I have a problem with the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. people, because they are building ugly freaking boxes. The only way to get a building to last a long time is if the building is maintained, and people have to want to take care of it. People do not take care of ugly things.”

Last week’s blog: “Broken Ventilation Equipment Goes Unnoticed for Years.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Sustainable Performance Institute
Fri, 06/15/2012 - 06:42

Edited Fri, 06/15/2012 - 14:50.

There is only one Joe L.
by aj builder, Upstate NY Zone 6a

Helpful? -2

There is only one Joe L. Great, fun, quotes. But tell Joe I have two homes doing perfectly fine and moisture free with open cell spray foam done how he now says won't work.

I do see he agrees with my spray foam guy that overinsulating is a waste as little is to be saved after the first few inches if done well. Good sighting, good windows and design.... for me has resulted in PGHs (pretty good homes). And have to mention Panasonic bath fans. Panasonic fans have turned out to be singularly good, great, choice to include in homes for too many simple reasons. They do a good job dependably and quietly. And require no maintenance, sensors or warning lights, or maintenance contracts, or changes in real estate sales laws. A six digit idea that already exists.

One more smile. Glad to see Joe likes OSB as much as I do. Choose plywood people. The cost difference is tiny compared to the project cost.

Plywood, Panasonic and Joe L, all the best parts of a PGH.

Fri, 06/15/2012 - 06:53

Edited Fri, 06/15/2012 - 06:56.

How about house wrap with bumps?
by Carl Seville, GBA Advisor

Helpful? 0

Don't have any experience with this stuff, and I am not sure if the bumps are big enough to create a large enough gap to actually function as a rain screen, but this stuff from Obdyke looks like an interesting option:


Fri, 06/15/2012 - 07:52

Response to Carl Seville
by Martin Holladay, GBA Advisor

Helpful? 1

There are lots of brands of bumpy (or crinkled) housewraps out there, and they work.

The one that Joe Lstiburek usually uses is Tyvek DrainWrap.

Fri, 06/15/2012 - 10:57

Edited Fri, 06/15/2012 - 11:00.

A softening position?
by albert rooks

Helpful? 2

“I have a problem with the Passivhaus people, because they are building ugly freaking boxes. The only way to get a building to last a long time is if the building is maintained, and people have to want to take care of it. People do not take care of ugly things.”

Well... I guess his complaints about Passivhaus people are no longer that we employ:
-Excessive insulation.
-Excessive airtightness.
-Excessive personalities. Well, perhaps he's still right on that one :).

His holdout position is down to: --that we have no taste--

I can live with that because I know that his expertise is in engineering, not design :)

Fri, 06/15/2012 - 11:40

New (to Me) Video
by John Brooks

Helpful? 0

I was looking at the related articles mentioned above.
I was not aware of the "Roofs Video, Part 2" until today

Very Nice

Fri, 06/15/2012 - 12:32

Edited Fri, 06/15/2012 - 12:38.

Thanks for the recap...
by Armando Cobo

Helpful? 0

I’ve follow BSC detailing on basement insulation for many years with good results for CZ2-5. The worry I always have is how long to let the wall dry as much as possible before we finish it, as concrete can take a long time to dry.
BSC wrote a few good papers back in 2002 and 2006 about basement walls and insulation, but I wish they update them if Joe has change his mind about some of those issues. “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.” Really puts a twist on few designs I’m doing now.
Thanks for showing some Joe'isms, they are always fun to read/hear.

Fri, 06/15/2012 - 17:04

Martin - Wrinkled house wraps - do they really work?
by Carl Seville, GBA Advisor

Helpful? 0

I read this in a post about wrinkled house wrap by some nerdy guy who seems pretty smart:

"In order to evaluate these products, one question arises: How thick does a corrugation need to be in order for water to drain? Unfortunately, building experts disagree on this issue, which awaits further research. “An air gap does not have be 3/4 inch,” says Mark Bomberg, editor of Journal of Thermal Envelope and Building Science. “I am quite happy with an air gap that is less than 1/8 inch. It does not ventilate, but it allows local drainage.”

This seems to contradict the idea about that bumps would provide ventilation and drainage. Seems to me that bumps on OSB or siding wouldn't be any bigger than the wrinkled or bumpy wraps.

Has there been further research that I missed a report on?

Sat, 06/16/2012 - 06:20

Edited Sat, 06/16/2012 - 06:54.

Response to Carl Seville
by Martin Holladay, GBA Advisor

Helpful? 0

You're right that crinkly housewraps don't provide very much ventilation drying -- although, believe it or not, they may provide some useful ventilation drying. These crinkly housewraps can definitely provide what Joe Lstiburek calls "hygric redistribution." In some applications -- notably when used between exterior rigid foam and OSB sheathing -- this hygric redistribution can be extremely useful.

Here's why Joe wrote in his article Mind the Gap:

"Yes, closed cell high-density foam cavity insulation applied to the inside of OSB sheathings that are in turn covered on the exterior with impermeable foam sheathings is risky. Unless you provide a small gap between the exterior face of the OSB and the back surface of the foam sheathing to provide for some hygric redistribution. Or if you are “perfect” with your rainwater control such as when you use a fully adhered membrane – think roof membrane standing up applied to a wall. Otherwise, go with a gap. What works? Grooved foam, “bumpy” OSB, “crinkled” building wrap, 1/8-inch polypropylene mesh, dimpled polypropylene sheets. "

If you want to design an air gap between your siding and sheathing to help dry a wall through the ventilation effect, deeper gaps work better than narrow gaps. It's also beneficial to have openings at the bottom and top of the wall. The bigger the gap (up to a reasonable depth, usually maxing out at about 3/4 inch), the better the ventilation effect.

However, not all walls need ventilation drying to stay out of trouble.

In one of his papers, John Straube discusses the findings of a 2001 research study. The study found:

"Walls with cavities (vented and ventilated) dried faster than comparable panels without cavities (face-sealed). There was a substantial range in the drying rates: as much as three times higher drying rate for comparable walls with a ventilated cavity than for those without.

"Ventilation (top and bottom vents) resulted in marginally faster drying than vented (bottom vents) walls. The width of cavity was also important, and those walls with cavities of 19 mm dried faster than 10 mm.

"Walls with plywood dried faster than comparable walls with OSB sheathing. OSB has a lower vapour permeance than plywood and may have restricted the drying through the sheathing to the exterior."

However, further research has shown that even very small gaps are quite useful. In The Role of Small Gaps Behind Wall Claddings on
Drainage and Drying
, John Straube wrote,

"The experiments to date have conclusively shown that even small gaps (less than 1 mm) can drain more water than would normally be found in a drainage gap. It was also found that in some cases small gaps will store less water than a large drainage gap. It was also found that ventilation drying can play a role in very small gaps of approximately 1 mm, at a pressure difference of only 1 Pa. More research is required to further analyze optimal ventilation gap sizes and compare the laboratory results to hygrothermal modeling."

Sat, 06/16/2012 - 07:26

Basement wall
by shane claflin

Helpful? 0

If the basement wall doesn't dry to the inside, where does it dry to? Or is it just always permeated with moisture?

Sat, 06/16/2012 - 11:05

Response to Shane Claflin
by Martin Holladay, GBA Advisor

Helpful? 0

For strong concrete, you want to make the mix as dry as possible before it is placed, with a minimum amount of water -- just barely enough water to allow the concrete to be placed.

Once the concrete has been placed and has begun to cure, you want to keep the cured concrete as damp as possible. The concrete will continue to strengthen for months, even years, as long as you keep it damp.

In other words, damp concrete is happy concrete -- and strong concrete.

However, you don't want a damp basement, and you don't want damp concrete to be drying to the interior or adding moisture to your home. That's why a layer of closed-cell spray polyurethane foam or rigid concrete between the concrete wall and the interior is such a good idea. It prevents evaporation.

Sat, 06/16/2012 - 15:16

Edited Sat, 06/16/2012 - 15:18.

Agree, but...
by Armando Cobo

Helpful? 0

I understand and agree with your statements Martin, however, there is the issue that we'll trap moisture in the concrete wall between the asphalt waterproofing and the close cell foam...If that is good for the concrete, at what point should we install the rigid foam, right after we waterproof the outside or wait till we finish a basement?
Now I’m designing a lake house, east of Dallas, with a walkout basement where all the social areas (kitchen, living, dining, and office) are located, and we’ll be finishing the basement as we finish the house; so my concern is trapping too much moisture on the foundation wall too quickly. I’m specifying 2” of rigid foam against the concrete, flat 2x4 wall with no insulation, and sheetrock. Would that create problems?

Sat, 06/16/2012 - 15:29

Edited Sat, 06/16/2012 - 15:30.

Response to Armando Cobo
by Martin Holladay, GBA Advisor

Helpful? 0

I'm happy to defer to other opinions here, but my guess is that, as long as a portion of the basement wall extends above grade on the exterior, as it should, the exterior air and sunshine will ensure that the wall eventually dries out enough for you not to have to worry about it.

Here's another reason you shouldn't worry: ICF basement walls seem to perform well, and they have foam on both sides.

Sat, 06/16/2012 - 17:27

by Armando Cobo

Helpful? 0

Good thinking on the ICF, I've done a few, and never worried about that before.

Sat, 06/16/2012 - 20:25

Venting rigid
by Dan Kolbert

Helpful? 0

Leaving a drainage/vent plane behind the exterior rigid seems counterintuitive to me - why isn't that short-circuiting the rigid as insulation?

Sat, 06/16/2012 - 21:39

Venting Rigid
by John Brooks

Helpful? 0

Joe addresses this on page 7 of "Mind the Gap, Eh"

Sat, 06/16/2012 - 21:59

Edited Sat, 06/16/2012 - 22:01.

by Dan Kolbert

Helpful? 0

Here's the quote:

"What works? Grooved foam, “bumpy” OSB, “crinkled” building wrap, 1/8-inch polypropylene mesh, dimpled polypropylene sheets. Lots of stuff. But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap. With “crinkly” stuff you loose next to nothing.7 Is it worth it? Yes, in my opinion, the loss in thermal performance is trivial compared to the reduced risk and improved durability."

He's talking about pretty tiny gaps. I would guess the thermal loss increases more than linearly as the space increases but of course am speaking out of my ass.

Sun, 06/17/2012 - 06:24

Response to Dan Kolbert
by Martin Holladay, GBA Advisor

Helpful? 0

You're right: if you're choosing a product to provide a gap between OSB and rigid foam, you want to choose a product with a tiny gap, so that the gap doesn't lower the thermal performance of your wall very much.

The large ventilated rainscreen gaps -- especially those achieved with vertical furring strips -- always go between the siding and the sheathing, not the rigid foam and the OSB.

Sun, 06/17/2012 - 15:16

Edited Sun, 06/17/2012 - 15:18.

Using spay foam on basement exterior
by Ron Keagle

Helpful? 0

Is it acceptable practice to use closed cell spray foam on the exterior of a concrete basement below grade, and then backfill against it?

Sun, 06/17/2012 - 23:51

windows with suspended films
by Rachel Wagner

Helpful? 0

I'd like to know more about why Joe "hates" this technology. What, specifically, does he believe doesn't work about them? I live in the cold north and have used triple glazed windows for more than a decade, but have recently specified windows with suspended films on a few projects.

Mon, 06/18/2012 - 02:38

ICF analogy
by Katy Hollbacher

Helpful? 0

Martin, I'm not sure about the statement "ICF basement walls seem to perform well, and they have foam on both sides." But that's with (semi-permeable) EPS. In my plan reviews I've stated concern with proposed assemblies that include, for example, a few inches of XPS on the outside of a basement wall, and high-density spray foam to fill the furred-out cavity on the inside. Joe would call this a "vapor barrier sandwich"... isn't he only saying that, if the outside is vapor open, it's then OK to have a vapor barrier on the inside? He hasn't said (that I know of) that it would not cause problems to have a vapor barrier on both sides of a basement wall.

Mon, 06/18/2012 - 05:19

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Q. "Is it acceptable practice to use closed cell spray foam on the exterior of a concrete basement below grade, and then backfill against it?"

A. Yes. Researchers have found that spray foam performs well in this application. For more information, see these documents:

In situ performance evaluation of spray polyurethane foam in the exterior insulation basement system.

Spray Polyurethane Foam for Exterior Subgrade Thermal and Moisture Protection.

Mon, 06/18/2012 - 05:22

Response to Rachel Wagner
by Martin Holladay, GBA Advisor

Helpful? 0

Q. "I'd like to know more about why Joe 'hates' this technology [windows with suspended films]."

A. I'll send Joe an e-mail today and ask him to comment on your question. I know that some of the Heat Mirror units from the 1980s had problems with transparency and film durability.

Mon, 06/18/2012 - 05:28

Response to Katy Hollbacher
by Martin Holladay, GBA Advisor

Helpful? 0

You're right that the EPS foam used for ICFs is somewhat more permeable than XPS, but once you increase the thickness of the EPS to 4 inches, the permeance is below 1 perm -- so you aren't getting much drying through your foam.

You wrote, "In my plan reviews I've stated concern with proposed assemblies that include, for example, a few inches of XPS on the outside of a basement wall, and high-density spray foam to fill the furred-out cavity on the inside." But what is your concern? It's one thing to be concerned when OSB is sandwiched between impermeable layers, because OSB is a wood-fiber product that is susceptible to rot. But concrete doesn't rot. In fact, it likes to be damp.

I will send Joe an e-mail and ask him whether he is concerned about the creation of a "vapor barrier sandwich" with concrete in the middle of two layers of foam. But here's my answer: I'm not concerned.

Mon, 06/18/2012 - 06:48

Response to Katy Hollbacher
by Joe Lstiburek, GBA Advisor

Helpful? 0

When I say I made a mistake about basements, the context is important. The assemblies I recommended back then all work. Even today.

The mistake was that I thought that some drying of the concrete was necessary or important. I do not believe that any longer. That is what i consider my "mistake." Additionally, we have found that most foundation walls dry upwards and outwards at grade in any event.

So to answer your question directly, I do not believe there is a problem if there is a vapor barrier on both sides of a concrete wall and the concrete never dries. The concrete is perfectly happy. The key is to make sure that the interior surfaces of the foundation are protected from the wet concrete and that interior air can not access cold surfaces.

Mon, 06/18/2012 - 06:50

Response to Rachel Wagner
by Joe Lstiburek, GBA Advisor

Helpful? 0

I have only had bad experience with windows with suspended films as they experience creep and the tensioning methods are not able to compensate. The creep phenomenon is not uniform and leads to the films becoming visible overtime and unsightly. There is no apparent effect on thermal performance, only aesthetics.

Tue, 06/19/2012 - 02:21

Concrete, ICFs & suspended films
by Katy Hollbacher

Helpful? 0

Joe & Martin,

Thank you for clarifying--this is very helpful & I'm pleased with the revised view on this. I do still feel a bit uneasy about the idea of trapping water very deep below grade; if the wall is fully insulated on both sides all the way up to the sill, and there's a capillary break between the top of wall & mudsill, the wall won't be able to dry out above grade either, right? But your point is, who cares. Hmm... could concrete efflorescence degrade insulation over time? I suppose that's a question for any situation.

On the other topic of window suspended films: Joe, can you share more about your observations: are these newer or older products? large, average or small openings? squarish or rectangular? I've heard about issues with very large (say 5x6 or larger) openings, or those with very different aspect ratios, showing distortion at corners--but haven't heard of this with average opening sizes made with current generation products.

Tue, 06/19/2012 - 03:45

Response to Katy Hollbacher
by Martin Holladay, GBA Advisor

Helpful? 0

Q. "Could concrete efflorescence degrade insulation over time?"

A. Efflorescence requires several conditions: the concrete has to be taking up water with dissolved minerals; this often occurs when the water rises through a footing that lacks a capillary break. So one way to interrupt or limit efflorescence is to include a capillary break between the footing and the wall.

Another requirement for efflorescence: the concrete wall has to be exposed to an environment that is dry enough to permit evaporation of the moisture from the concrete. This doesn't happen when the concrete is covered with rigid foam. So you aren't going to have "hidden" efflorescence magically occurring under the foam. Water will not be evaporating from the concrete, so the buildup of minerals won't occur.

Tue, 06/19/2012 - 23:45

Edited Tue, 06/19/2012 - 23:47.

Great comments, concerns and questions.
by albert rooks

Helpful? 0

Valueable insights by all. Thanks Dr Joe, Martin, and (mama?) Katy.

Hmmm... "Martin, Dr Joe and Mama Katy"... Kinda sounds like the name of a 60's rock band...

Thu, 06/21/2012 - 01:13

Latex Paint - Code - Air gap
by Robby Schwarz

Helpful? 0

Thanks for the great article. Especially is dry climates I have never understood the rational in the code to only allow latex paint as a class 3 vapor barrier if there is a gap between the siding and the drainage plane. The code language give examples of this gap but does not go far enough on the rational behind it to allow code officials to feel comfortable allowing other assemblies to utilize a latex paint VB. Plastic in our walls seems like a bad idea in almost every climate because it is installed so badly and has such a potential to trap moisture in the wall that is carried in via air leakage. I believe Joe got this language into the IECC so I was wondering if he commented on it more that to say that paint works as a VB when installed on drywall.


Thu, 06/21/2012 - 05:53

Edited Thu, 06/21/2012 - 05:55.

Response to Robby Schwarz
by Martin Holladay, GBA Advisor

Helpful? 0

Joe Lstiburek didn't address your questions in his recent all-day seminar. However, I'll do my best to answer them.

Q. "Especially is dry climates, I have never understood the rationale in the code to only allow latex paint as a class 3 vapor barrier if there is a gap between the siding and the drainage plane."

A. The need for interior vapor diffusion retarders has been greatly exaggerated for decades. Vapor barrier requirements crept into our building codes based on a misunderstanding rather than science or research.

Wet-wall problems are almost never caused by outward diffusion during the wintertime. The most common cause of wet-wall problems is wind-driven rain that gets past windows or flashing. The second most common cause is condensed moisture that entered the wall cavity with exfiltrating air.

The code changes you are referring to (the provisions shown in Table N1102.5.1) were an attempt to reduce the stringency of illogical requirements. These changes were introduced by building scientists (including Lstiburek) who knew that interior poly was doing more harm than good in many climates. The table indicates the types of walls that are so safe that interior poly is, by universal agreement, unnecessary. These walls include those with adequate insulated sheathing (usually rigid foam, although mineral wool would also work) and certain walls with a ventilated air gap between the siding and the sheathing.

Q. "The code language give examples of this gap but does not go far enough on the rationale behind it to allow code officials to feel comfortable allowing other assemblies to utilize a latex paint VB."

A. You're right. But then again, the code has never been very good at providing rationales or explaining who certain provisions exist.

Q. "Plastic in our walls seems like a bad idea in almost every climate because it is installed so badly and has such a potential to trap moisture in the wall that is carried in via air leakage."

A. I agree.

Fri, 06/22/2012 - 13:12

Builders Guides
by Bill Burke

Helpful? 0

If only there WERE Builders Guides for every climate zone! Those of us living in the Marine Climate long for the day Joe and Building Science Corp produce a guide for our climate. In the interim, we make due with the Building America Marine Climate Best Practice Guide available at This guide was produced by Building Science Corp for US Dept of Energy, but the section on building is not nearly as large as what you'll find in the Builders Guides. We really do need a full guide for the Marine Climate!

Fri, 06/22/2012 - 13:34

Response to Bill Burke
by Martin Holladay, GBA Advisor

Helpful? 0

You make a good point.

So Joe -- if you're reading this -- you've got at least one eager customer ready to buy the next edition of your book, as soon as it's written ....

Fri, 06/22/2012 - 21:07

Houses Without Basements
by Ron Keagle

Helpful? 0

For houses, how popular is the idea of simply eliminating basements and all direct ground contact (including slab foundations) in order to eliminate the affect of moisture and mold?

Basement-less houses still need foundations, but foundations could be provided as piers. If you need the space provided by a basement, you can just make the aboveground house larger.

With super tight, superinsulated houses, the basement can really drag down the quality of the house space.

Sat, 06/23/2012 - 05:01

Response to Ron Keagle
by Martin Holladay, GBA Advisor

Helpful? 0

Most builders who are frustrated by the disadvantages of basements and crawl spaces are happy with slabs on grade.

Pier foundations work in warm climates, but they are more difficult in cold climates, where plumbing pipes and drain pipes can freeze.

Sat, 06/23/2012 - 16:06

very good
by John Klingel

Helpful? 0

Very good info here, and I am glad to see that Dr Joe, etc, can admit to being human and accepting a change of opinion as BS evolves. One comment is disappointing, though. To hear a person who is well respected in the BS community say that PH is building ugly boxes is at best unfortunate, and at worst ammunition for the crowd who does not buy into the concepts of conserving energy. Beauty is in the eye of the beholder, and there is nothing, IMO, but visceral excitation in seeing low energy bills. I'd suggest nothing negative be said about those who chose function over form; an age-old battle.

Tue, 06/26/2012 - 17:40

Choose function and form. As
by aj builder, Upstate NY Zone 6a

Helpful? 0

Choose function and form. As to Joe, if you go to his seminars, every word is worth the price of admission. And you will also find out he has opinions and is great at expressing them! Much more good than harm ... ya just gotta know, Joe.

Wed, 06/27/2012 - 09:18

More on windows with suspended films
by Martin Holladay, GBA Advisor

Helpful? 0

Joe Lstiburek has been responding to several e-mailed questions on insulated glazing units with suspended plastic films, and pointed out that he installed 18 such windows and "all have gone bad." While it's true that many of these windows are performing well, even after 17 years or more, some (like Joe) have had less satisfactory experiences.

Joe wrote, “I am done with films. I am replacing everything with triple-glazed units. Tired of being someone’s guinea pig. In this case -- been there, got burned, got the T-shirt, and have moved on. I looked at the engineering solution to the physics problem and have said no.”

Thu, 06/28/2012 - 00:15

AJ: Small
by John Klingel

Helpful? 0

AJ: First, this is NOT a large issue, but I think worth mentioning to keep "the team spirit" intact. Joe is knowledgeable, etc, etc, but he ain't perfect and he does not levitate. A little tact can go a long way. Too, IMO, a good minister is very careful when preaching to the choir because the words go beyond the choir. That's my point; other folks, esp PH folks, may take offense. It's just a small thing, but something to be aware of. I've shot myself in the foot being "humorous" more than once.

Thu, 06/28/2012 - 19:50

Edited Thu, 06/28/2012 - 20:27.

John, Joe deals with the
by aj builder, Upstate NY Zone 6a

Helpful? 0

John, Joe deals with the likes of me and builders and tradesmen, etc and his preaching is perfect for us roughians. You want Jesus, go to church my man.

Oh, and am sure Joe levitates, deviates, hesitates, necessitates, eradicates, delineates, desiccates, and especially pontificates ..

Fri, 06/29/2012 - 07:30

Mike Eliason responds to Joe Lstiburek
by Martin Holladay, GBA Advisor

Helpful? 0

Evidently Mike Eliason didn't agree with Joe's observation that "the Passivhaus people ... are building ugly freaking boxes." So he wrote a blog in response: Dr. Joe should stick to building science."

Fri, 06/29/2012 - 14:44

Response to Joe Lstiburek
by Don J.

Helpful? 0

Great guy. I followed the advice on BSC's website and wasted several months of work and $6,000.
Perhaps our installers mess up and it's not BSC's fault but it seems that the BSC docs are too theoretical and make assumptions about the environment that simply don't apply in reality.

Sun, 07/01/2012 - 18:42

Edited Sun, 07/01/2012 - 18:47.

Don, your last post is
by aj builder, Upstate NY Zone 6a

Helpful? 0

Don, your last post is premature IMO. You should ask them to inspect your home. Both of you and us would learn from getting your home properly functioning. At this point they should if you all could agree to stay out of court help you pro bono IMO.

Mon, 12/10/2012 - 18:29

Insulating limestone "rubble" foundation walls
by Steve Wilke-Shapiro

Helpful? 0

Most of the closed-cell foam insulation conversation above refers to concrete foundation walls. I'm wondering about how this information applies to retrofitting insulation into historic home basements with limestone rubble foundation walls that do not have any exterior waterproof coatings.

These basements almost always have moisture migrating in through the wall (sometimes in bulk water form, other times in vapor form). Historic mortars behave in a substantially different manner than concrete, though clearly a masonry wall could dry up and out rather than in to the basement just like a concrete wall could.

I'd be interested in anyone's thoughts specific to this situation.

Tue, 12/11/2012 - 08:03

Response to Steve Wilke-Shapiro
by Martin Holladay, GBA Advisor

Helpful? 0

The details of your insulation retrofit will depend on how wet your basement gets. Here is a link to the oft-cited report by Joe Lstiburek on his own rubble-foundation interior insulation retrofit: Rubble Foundations.

If you anticipate a lot of water coming through the wall, it makes sense to include a three-dimensional drainage mat product plus geo-textile fabric between the stone and the spray foam. However, many such foundations are insulated by spraying the insulation direction on the stone. After all, there are enough cracks and crevices in such walls for any liquid water to find its way to the footing. It's usually a good idea to have an interior French drain (connected to a sump) to handle any water that gets through the wall.

Closed-cell spray foam is tenacious, and helps bind loose stones to the wall. You end up with a stronger foundation.

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