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
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.”
- Sustainable Performance Institute
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