A typical example here in Seattle would be a vapo(u)r barrier behind the drywall, when the exterior weather barrier is vapo(u)r impermeable. With an impermeable weather barrier, it is the vapo(u)r barrier and adding a second one can cause big problems.

Yes, I expected this to be a common example. We make it very clear that the drainage plane membrane is always vapour permeable but there are many who just label all the dashed lines as blueskip or twf without thinking twice.

Adding vapour barrieurs tou assemblieus with air barrieurs is a commoun case. But you alsou see this probleum with projects designed for warmeur (tropical) or coldeur (Canadieun) climates where the deuw points are differeunt.

^louve it

It's super great. Everyone in the field should follow it.

Great recommendation!

Just recently discovered BSFC. Great content!

Christine is awesome! Last night she moderated a wall contest I participated in. (I won.) https://www.instagram.com/buildingsciencefightclub/

YAY Wood Guy!!! COngratulations!!!

Fascinating WG.

Thanks Donna and Non! If you're interested, you can read about the contest here: https://www.instagram.com/bs_and_beer_kc/. I narrowly beat out my friend Ben and we decided to call it a draw, and give the award to a vocational school teacher--hopefully to encourage his students to continue on the high performance path.

That's lovely, Wood Guy!

WG, looking at your winning detail, It appears to me that you're suggesting a eng floor joist as a stud. Is there another reason other than increase cavity depth for R value? I know it's overkill for most uses but am curious on the wall height and struct load options (ie. mid-panel floor connection).

Non, I submitted the best system I've used, the panelized Passive House system developed by Ecocor, where I used to work, based on the Swedish platform framing system. One of my tasks when I was there was to get it certified by the International Passive House Association as a certified Passive House assembly--the first outside of Europe to do so.

The TJIs on the exterior were the best way we found to get a thick layer of exterior cellulose insulation. Same idea as Larson trusses from the 70s, but with much lower labor costs. Steve Baczek submitted a similar wall using custom open web trusses, but they aren't a good match for dense-packed cellulose because each one would have to be netted to retain the insulation. 

Ecocor has clever details for every connection. I helped develop a lot of them, among my other responsibilities as operations manager. (I quickly learned that running a factory was NOT a good fit for me, though.) You can see more about their systems here: https://www.ecocor.us/.

Here's a Fine Homebuilding article about Ecocor and their system: https://www.finehomebuilding.com/2017/09/12/making-passive-house-mainstream. 

Good info. thanks for the links. I think I've looked at ecocor before.

yes, but the exterior air membrane must be vapour permeable. If you have the junior labeling everything as blue-skin then you've got a problem.

Maybe. I just don't like absolute statements like that, because there are always exceptions and it shows a lack of understanding.

Also, roofs are done very similarly to double blue skin and they work just fine in our climate.

The only absolute statement I've seen above is that you should stop putting a VR in the same location every time. The OP's call to ask an expert would ferret out the exceptions, no?

(when I abbreviate vapor retarder no Canadians can complain about the spelling)

eugh, no to abbreviations.

lol ... or should it be loul?

“It is a damn poor mind that can think of only one way to spell a word.” Andrew Jackson

I've written this before, but one of building science's authors was my building science prof while in grad school.  Learned more in an 1hr in there than an entire undergrad semester.

NS, every time I get a chance to see a presentation he gives, I take it. I always come away having learned something.

Vapour. Vapor feels like it would be the name of a sports car

Vapor or GTFO.

You're leaps ahead of most architects if you can remember that. Of course, that doesn't work for all climates ... like hot humid climates where the problematic vapor drive is in the summer from the outside in.

That's the rule of thumb but tell me what I'm supposed to do - our weather swings from -40 in the winter to nearly 100f in the summer (ie, air conditioning season). It would seem that double VB would make sense in this instance.

To answer my own question, we take a different approach. I haven't used cavity insulation in my entire 10 year career.

Double VB never make sense unless you like to promote mold growth. High humidity areas should adopt a 1/3 : 2/3 rule. Vapour barrier is place 1/3 on the warm side of insulation and 2/3 of the insulation on the cold side.

The rule of thumb should be used as a preliminary tool to understand a basic concept, not as the final determining factor. If you use a rule of thumb to layout a preliminary structural grid, you don't insist that you don't need an expert (i.e. structural engineer) to validate it. I'd look at this the same way. Start with the rule of thumb to get the ball rolling, but validate the approach with someone who knows. FWIW, bowling_ball ... it seems like you know so you probably can skip the rule of thumb thing and just design it. That doesn't mean that others can't benefit from the rule of thumb though.

BB - I too have worked in a climate like yours (northern MN high humidity in the summer, cold in the winter). The vapor retarder needs to located based on several factors, most importantly where the dew point is occurring in the wall SYSTEM. The location of the dew point can occur in different locations depending on the season. When that happens I've found it beneficial to calculate which 'season' is the longest and install the vapor retarder based on that. When you're doing continuous insulation you need to calculate the insulation thickness so that the dew point is occurring before it reaches the building sheathing. I've never installed double vapor retarders or split the location of the vapor retarder.

You got it, Chad. There are many, many variables and I don't try to be a building scientist by any means. Re: dew point, our energy code mandates so much insulation that I don't envision a scenario where that could ever be a problem.

The thing that's always bothered me is that with POLYISO and XPS insulation is that they loose around 50% of their R value when wet and take a long time to dry. If the dew point is occurring within the insulation then what? How dose the water get out? Are we setting up a bunch of exterior envelopes to fail in 15 years when the insulation deteriorates?

This is why we pay our envelope consultants handsomely

Chad, as long as that deterioration period is longer than the Statute of limitations, think of all the extra fees!

double vapor barrier is bad. the newer codes will tell you that you can't use visqueen at all in temperate climates anymore.

NS - I try to not think about that, Colorado has some rather generous standard of care and liable laws when it comes to construction system failures.

remember you have to temper the chocolate for it to set right

That's awesome, I'll be tuning in. Thanks for the heads up!

Christine happens to be Joe L's daughter, which both makes sense and blows my mind. Two ridiculously smart-but-practical experts in the same family.

I honestly gave up correcting the retarder/barrier terms. I should probably fix that habit.

BB, great, I hope to see you there! This is a good interview with Christine: https://entrearchitect.com/podcast/entrearch/building-science-fight-club/. She talks about her relationship with her dad and working at BSC. Another big influence for her was working with Chris Bennedict, an amazing NYC architect who is a leader in building energy performance, among other things.

Non, that's probably wise. I try to pick my battles but can be stubborn on some hopeless issues. 

Wood guy, I hear ya. I'm working with ownership on establish naming conventions and eventhough using retarder/barrier is far on the to-do list, I'll make note of it. Hard to impose standards when the senior staff can't get the basics organized.

WG - very nice, I will be listening in! Is there a link for the talk? Not sure I saw one.

One other thing, in the above reply you mentioned 4 new terms, which I find insanely useful. But you said you need 3 in any buildup - where's the odd one out ?

Bench, I try to be careful about spamming--I can be prone to too much self-promotion. But since you asked, you can join the mailing list here https://www.thebsandbeershow.com/ or view the invitation for today's talk here https://mailchi.mp/e6679ed9e87a/the-bs-beer-show-april-30-4506448

As for the new terms, I'm glad you like them. I only wish I could do basic arithmetic. There are four control layers and they're all important. (Apparently four semesters of calculus and one of differential equations were not enough.) 

Thought that might have been a typo ... !

For a young, soon-to-be licensed architect (hopefully), what do you recommend for getting a better understanding of everything discussed here? I've done my own wall details previously and seemed to have a decent handle on it, but none of the comments here seem to be anything i've given too much thought to in the past. I'd like to have a better understanding but its very complex.

That's cool wood. I might try to listen in. @Bench, it is one of those things you'll have to search around to find good information. For myself, I am admittedly weak on this vapor drive stuff and psychrometric charts. What I do know is we are seeing a lot more issues in recent years due to spray foams and reflective roofs for energy conservation. Residential is a huge issue because unlike commercial, the mechanical code doesn't have a outdoor air requirement.... so humidity levels tend to escalate until eventually you get one of those days where condensation forms; added issue is architects don't have much training, and mechanical engineers don't do building shell... so there is a gap in areas of expertise and knowledge.

Naming conventions; WRB. Our office just changed this one to "water resistant barrier" in our legal reports instead of the old "weather resistive barrier". Basically, the IRC (R703.1.1 & R703.2) talk about weather resistance, but now call it "water resistant barrier"... Sometime it just feels like they change things for the hell of it.

I roll with whatever people want to call a WRB. I've yet to find a clear consensus. Even the IBC can't agree with itself (references from 2018 IBC): Section 1403.2 requires a "water-resistive barrier," but Section 1403.3.4 mentions a "weather-resistive barrier as specified in this chapter." That term "weather-resistive barrier" only occurs the one time in Chapter 14. Chapter 2 has "water-resistive barrier" defined, but it also includes multiple references to "weather-resisting barrier" and "weather-resistant barrier" and also simply "weather barrier." In the end, I accept them all as interchangeable and don't argue with the pedants (not used as a pejorative) that insist on trying to argue about which is correct. In my specs though ... I follow CSI MasterFormat and use "weather barrier." And since I make sure the terminology in my drawings and specs match ... call it a weather barrier or feel the wrath of my red pen.

Bench, I think this article is an excellent overview of the four control layers: https://www.greenbuildingadvisor.com/article/an-intro-to-the-control-layers-of-a-wall. If you don't have a subscription, I'd consider getting one--it's a good value for the content. If that's not in the cards, an abbreviated version is here: https://www.finehomebuilding.com/2019/01/03/crash-course-in-control-layers. (The author of both, Brian, is the editor of Green Building Advisor and one of my co-hosts for BS + Beer. I tried to get him to go with my terms above but he wasn't ready for that.)

This is another one, classic but relevant:  https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

LOL. Did you see SNL last weekend? Your guy was good: https://www.youtube.com/watch?v=uW56CL0pk0g

Several of my friends sent me that clip moments after it aired. My guy is *always* good.

That man ages well!

Honestly I feel like this is one of the best Twitter jokes I’ve made.

it's very necessary and no, it's part of the hold-up your pinky while you drink your coffee system. pesant.

Non is that "peasant" or "pissant"?

Donna, I don’t know. I had to lay off my dictation team and I can’t work the spell check without them.

puissant

I think the word 'yal are lookin' for is Canadian.

Non, you joke about your dictation team but honestly I bet people like Bjarke and Rem have folks to just follow them around and write down or record the stuff they say.