Many of you may have  heard Joe Lstiburek say

"Build tight, ventilate right and don't eat your sweater.."

or you may heave read where John Straube called walls without outside insulation "Turds"

I would say that BSC might have a bias against double stud walls, many Euoropean style walls and even the "Riversong Wall"

I think double stud walls would be more "buildable" and "affordable" and maybe even more "forgiving"  than the BSC "perfect wall" and or many of the other outside insulation walls promoted by BSC.

Are double stud walls and/or "inside insulation walls" really "Turds"... ?

Does anyone here know of any failures that were related to double stud walls with good air barrier systems?


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Here's a link to a powerpoint from "Summer Camp" 2012

At least BSC is doing some research

It's hard to learn much just by looking at the slides

If I am reading correctly it looks like sheathing was indeed colder and wetter (as many would expect)

but is the double stud wall a "failure"?

and here is a link to a long discussion about "Apples & Turds"

BSC - the perfect wall:

If Robert wishes to jump in, he can add a link to the best description of his wall

In answer to your question on air they "turds" - yes, no, & sure depending on your point of view & what you maybe comparing them against. For example how many of us have referred to FG as nothing more than an air filter?

Failures, not only do you need good air barriers, but also water management features - if it fails or is done wrong it doesn't matter which wall you have, it will fail.

As a builder a double stud wall is not more build-able than a normal framed wall nor affordable. The control layers are required on all walls so that doesn't that to the cost & neither do the bucks or additional trim as both systems require it. The only cost difference might be between the foam sheathing & the cellulose but I dare say it would almost be a tie. Now assuming you don't like foam - check out Bensonwoods system which he has used for Passiv Haus built in the US

As a remodeler - hands down the "perfect" wall comes out on top as I don't have to worry about a ton of cellulose falling in the space, trying to figure out how to replace it, etc... I can simply open up the walls, make the needed utility changes & seal it back up - need to run new AV cables or something similar, I don't have to worry about fishing through everything, or hoping that they installed the eXapath homepath system, as the whole assembly is open.

Are they anti or biased against other systems - no, they have their optimal solution but they also talk about how to do all the other systems properly, best practices, etc... This applies to almost all of us, I have preferred solutions but if someone wants to go with FG, I will make sure it is installed properly, etc...

A good description of the Riversong Truss wall system is at with another link there to the story of its evolution. There are also cross-sections and details of both my truss wall and several variations of double-wall envelopes that I've built or designed at

I'll have to differ with you on the buildability and cost-effectiveness of my own or any double-wall system, as both my own 30 years of super-insulation experience and published studies have shown that it is one of the least costly superinsulated wall systems, and certainly less costly than any application of foam for similar R-values on new construction.

I can't tell by looking at the BSC summercamp slides how the air barrier system is detailed.

Is the ZIP system "the" air barrier ?

I would think a good air barrier system should be on the warm /wet side of the wall?

(and probably not include OSB)

Robert Riversong, Thorsten Chlupp, the Euros and some other (not-so-foamy) cold climate High-R wall builders provide an air barrier system on the conditioned side of the wall....that makes sense to me for a Cold climate.

I think a mixed climate wall should have an air barrier system on both sides of the wall.

Sorry John, I am stuck in a hotel with slow internet so I can't really at the docs, but I would say all climates should have an air barrier system on both sides of the walls - normally this would be the drywall (or better yet plaster) on interiors followed up by OSB or plywood sheathing on the exterior properly detailed, etc...

Nice explanation Robert, can't wait till I get back in town to look at yours (sorry it has been a while so some parts are fuzzy)


I would say that air barriers on both sides of a wall system are necessary only with highly permeable fibrous insulation like fiberglass and mineral wool. This was the original purpose for exterior housewraps - as a wind barrier as well as secondary drainage plane.

As long as interstitial air movement is controlled, by sealing utility penetrations for example, if the primary purpose of the air barrier is to prevent heat loss and moisture-laden air from entering the envelope, then the air barrier layer is best placed on the same side as the vapor control layer - inside in a heating-dominated climate, outside in a cooling-dominated climate and perhaps on both sides in a mixed climate.

The primary advantage of an exterior air barrier, such as air-tight sheathing, is that it's easier to make it continuous. But, since the Air Barrier Association of America uses 1/2" drywall as the gold standard for an air barrier material (≤ 0.02 l/sec-m² @75 Pa or 0.004 cfm/sf @ 1.57 psf – 25 mph wind equivalent), and since cold climate air barriers are better located on the interior, I've always preferred the Air-Tight Drywall Approach (which, ironically, was developed in Canada by Joe Lstiburek even though he no longer favors it), but which requires some air sealing between assemblies during framing.

While Building Science, Inc. is one of the best sources of information on moisture issues, they do seem to have a strong bias toward rigid foam, particularly exterior foam and the "warm sheathing" approach.

Ironically, while they popularized the "moisture balance" equation (below), they tend to ignore "safe storage capacity" in their approach to the "perfect" wall system.

I was fortunate to have received my first formal training in energy-efficient design and construction from engineer Charlie Wing, whose book The Visual Handbook of Building and Remodeling is one of the best reference manuals available. I still recommend his out-of-print books, From the Ground Up (available in full on-line at and From the Walls In, as the best introductory texts for energy-efficient building and remodeling respectively.

One of the "rules of thumb" that he taught me 30 years ago was that, in a cold climate, the outside "skin" of a house needs to be five times as vapor permeable as the inside "skin" to avoid moisture accumulation and damage. I've lived by this rule for 30 years and found it to be valid. Interestingly, the latest International Residential Code requires a 1 perm inside vapor retarder and an exterior weather-resistant barrier material (it names #15 felt as the gold standard) that's at least 5 perms. It's heartening to know that the latest accepted building science, as embodied in our new and best code, is based on this same 5:1 rule.

Another fundamental rule that it took me years to fully appreciate, is the Law of Unintended Consequences. The more complex our buildings become, the more probable it is that almost any design decision or material choice will result in consequences that weren't anticipated. It is also commonplace, as CBS anchor Eric Sevareid famously said, that "the chief cause of problems is solutions".

OSB, for instance, was a "solution" to the over-harvesting of the virgin trees that are required for plywood veneers and of the increasing cost of sheathing. But it was a "solution" that came with the cost of increased vulnerability to moisture. Spray foam insulation was another "solution" to getting higher R-values in thin cavities, but it caused the problems of reduced drying potential, potentially trapped moisture and no moisture buffering capacity. Similarly, polymeric housewraps were a "solution" to wind-washing of highly air-permeable insulation like the all-too-common fiberglass, but it also tended to trap condensation behind the cold barrier material and keep sheathing wet (until the water could evaporate and pass through).

The "solution" that BSC prefers (and has the audacity to call the "perfect wall") is exterior rigid foam sufficient to maintain warm sheathing above the average winter dewpoint. As I teach in my HygroThermal Engineering class (, the unintended consequence of this system is the problem of exterior water leakage, through the difficult-to-design flashing details, and - ironically - the warm sheathing itself.

I'm aware of only two studies, one an on-the-ground monitoring and the other a detailed computer simulation, which deliberately introduced a typical exterior leak to a wall and a roof, respectively. What the monitoring study found was that, with walls sheathed on the exterior with foam board, not only did the wall framing (particularly the bottom plate) never dry out in the two years of the study, but the insulation maintained the wall sheathing within the temperature range necessary for mold growth and possible rot. The results of the computer simulation for spray foamed unventilated roofs were similar, requiring as much as six months in a cold climate for drying.

So, because OSB sheathing, foam insulations and polymeric housewrap have become today's conventional building approach for energy-efficiency, the combined liabilities require an additional, problematic and cost-added rainscreen cladding system to compensate for the problems created by the other "solutions". But rainscreens not only add cost, they also make weather barrier to flashing integration more problematic and hence make the biggest source of moisture - environmental rain - more likely to inject itself into our envelopes (even Dr. Joe admits he's made some serious flashing mistakes in his own attempts at the "perfect wall").

Where my Riversong Truss wall differs from most double-frame systems, is that there is no sheathing (let-in metal T-bracing on the inner wall for shear) and the 3/4" shiplap wood siding is primed with solid-color latex stain on all six sides before installation to minimize sorption of water but still keep the exterior "skin" highly vapor-open. In my KD double-wall designs (which eliminate the troublesome thermal bridging of the floor assembly), I specify 1/2" CDX sheathing with #15 felt WRB and wooden siding of the customer's choice (avoiding any kind of vertical siding, however).

I've been using these systems for 20 years in cold climates (7,000 - 8500 HDD) and have had no moisture problems. This is largely due to the moisture buffering ability of dense-pack cellulose, which is so highly hygroscopic that it protects wood framing from moisture accumulation in the event of interstitial condensation or minor exterior leakage (and, of course, interior moisture control and ventilation as well as ventilated "cold" roofs).

Compact Asphalt Shingle Roof Systems: Should They be Vented? by Peter E. Nelson and Jason S. Der Ananian, Journal of ASTM International, Vol. 6, No. 4, Paper ID JAI102057, 2009


Selected findings of an IRC/NRC study of the wetting and drying potentials of wood-frame walls exposed to different climates, Rousseau, M.Z.; Dalgliesh, W.A., NRCC-47066, also called the Moisture Management in Exterior walls study (MEWS)

Speaking of "compact asphalt shingle roof systems"

Up until the summer of 2010 ... I believed compact unvented roofs were perhaps "not-so-bad".

Then in the Summer of 2010 I saw an article in JLC magazine "Repairing a Rotting Roof" with some startling images.

one of the photos showed the OSB after the shingles were peeled off ... the OSB over the compact roof looked to be sort of carmelized...while right next to the compact roof was a vented portion of roof where the OSB looked to be "brand new"....hmmm...

and Then August 2010 at  Building Science "Summer Camp" I saw the toasted shingles on Joe Lstiburek's not-so-old roof.....Someone asked Joe what was going on....Joe said "cheap shingles" and that they would be replaced soon along with a roof upgrade.

Maybe if Joe's shingles were not-so-dark and maybe if the shingles were not-so-cheap...they would of lasted longer....maybe....hmmm....

To me the toasted shingles looked like a clue...a clue that unvented compact roofs might not be such a good idea.

When I first read the Title of BSI-063 ... I was thinking, OK.... maybe he is going to replace his roof with a VENTED over-roof... however, it looks like he beefed up the R-value and then re-built a compact roof with not-so-dark and not-so-cheap shingles.

Cheap shingles my arse!. Dr. Joe just doesn't want to admit that his "perfect roof" system is less than perfect.

I would never build an unvented roof in a cold climate and I emphasize the importance of roof venting in all my classes. Most building scientists tend to agree (even Dr. Joe):

Bill Rose, ASHRAE, Illinois Building Research Council:

Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.


Don Fugler, research director, Canada Mortgage and Housing Corporation:

Houses with ceilings tight enough to meet Canada’s strict R-2000 standard “probably could get by without roof venting.” But most Canadian houses, even new ones, don’t have such perfect ceilings.


Ned Nisson, author, The Superinsulated Home Book, editor, Energy Design Update:

"I hesitate recommending [unvented roofs] to clients unless I’m absolutely assured of impeccable quality control. In my opinion, roof ventilation is cheap insurance against expensive callback problems. Why gamble?"


Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center:

“Because of the monumental problem of ice damming, there is no question in my mind that the ventilated roof is an order of magnitude better in cold regions.”


Anton TenWolde & William B. Rose, members, ASHRAE:

We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.


Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst:

"There are many ways to treat the symptoms [of ice damming], but proper air sealing, insulation, and attic venting are the best ways to eliminate the problem."


Joe Lstiburek, Building Science Corporation:

"Vented attic/roof designs have the advantage of a long, proven historical track-record.  However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs."

"In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to avoid ice damming. The vented air space is needed to flush heat away trapped by the insulating value of relatively thick snow."

  • in a cold climate, the outside "skin" of a house needs to be five times as vapor permeable as the inside "skin" to avoid moisture accumulation and damage.
  • Law of Unintended Consequences.
  • Eric Sevareid famously said, that "the chief cause of problems is solutions".
  • moisture buffering ability of dense-pack cellulose, which is so highly hygroscopic that it protects wood framing from moisture accumulation in the event of interstitial condensation or minor exterior leakage

awesome post.


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