Envelope consultants drive me crazy. For half of them, I feel like I need to mansplain the concepts to them. The other half keep saying the same things over and over again even when they know they're not exactly right ...

Me: Why do we need a vapor retarder on the metal roof deck? Isn't metal a vapor retarder?

Them: Yes, but we need a vapor retarder anyway to keep the warm interior air from condensing in the roof assembly. 

Me: But we already have a vapor retarder ... the metal deck. I think what you mean to say is we need an air barrier, but we already have an air barrier ... the roof membrane.

Them: Yes, but we need a vapor retarder on the metal deck to keep the air out of the roof assembly.

Me: So we need an air barrier on the metal deck?

Them: Yes.

Me: What force is driving air into the roof assembly if we are using an adhered membrane (i.e. no billowing)?

Them: Well, we just need a vapor retarder on the metal deck.

Me: No, the metal deck is a vapor retarder. You are saying we need an air barrier in addition to the air barrier we already have to prevent the flow of air into the assembly that will magically happen without any external force to drive it in there.

Them: Yes, we need a vapor retarder.

EI, I usually spec one to cover the joints in the steel deck sheets but not always.

EI: The "force" driving humidity into contact with the metal decking (which is vapor impermeable, but also a very good heat conductor) is the Maxwell-Boltzmann distribution of water vapor particles as applied via the Ideal Gas Law. If the metal deck is cold and humid air comes into contact with it, you'll get condensation on the humid air-contact surface unless something prevents (or retards) the distribution of humidity in the air from its source within the building into the area adjacent to the metal deck.

This is only an issue if the deck is on the "cold side" of the insulation. If the insulation is above the deck (so that it is on the "warm side", then you're right, except that the metal deck is usually full of holes. Even with an air barrier above it, you'll still get a Maxwell-Boltzmann distribution of water vapor throughout the communicating air volumes. That's just physics.

I can confirm the above. We've done this with a steel deck on cold side of insulation.

Just adopt the Perfect Wall approach, combine the vapor and air barrier, and you'll be fine.

https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall

I would need a wall cavity so big that it would be ridiculous to successfully use the perfect wall with the combined air/vapour membranes as a typical detail. Code reqs R values are too great in my region. 

Gwharton, one of the references at the bottom of that article you linked is from my building envelop professor during my M.arch.

I'm assuming by "wall cavity" you mean overall assembly thickness, not an actual airspace, since the thermal layer is outboard of the air/vapor barrier in a perfect wall assembly.

That's true, but only if you're using conventional light framing, which is optimized for internal insulation within the wall cavity. If you take the insulation out, you still have the cavity. And the structure itself is of poor visual quality so you have to put a separate finish layer on it.

But if you switch to a different structural system, or do something to clean up the framing for appearance, you don't have that problem.

Example:

huh, there's a lot to like about that house

Yes shellumi, you shouldn't be a dick to non-architects. Nor should you really be a dick to architects, which leads me to one obvious, simple conclusion: Don't be a dick. Unless you actually are just one big dick, in which case you shouldn't pretend that you are an architect. If you are a dick, you should find your favorite bathhouse, and move in, so you can get serviced by anonymous strangers all of the time -- at least until you contract HIV, hepatitis, and HPV.

To be clear, you don't have to identify as anything in particular to do this; the medical community stopped using offensive terms like the ones JimBob used several years ago because they learned that the real problem with the spread of the AIDS epidemic was not orderly and respectful queers, who use condoms, see their doctor regularly, discuss safe sex as a form of foreplay, maintain healthy boundaries with partners who cannot manage boundaries themselves, form activist groups, finance community organizers, get elected, support the rule of law even when it is inconvenient, remain civil, and patiently explain. On the other hand, I'm told that the epidemic largely continues to grow because of men who have sex with men and self-identify as heterosexuals.

I suppose love is like water; it gets in even where we pretend we don't want it. So no, don't be a dick, or a bro. Don't join a frat, and for gods sake, don't eat the ooky cookie, because you have no idea what those guys have done with their junk. Use effective membranes. Figure out how to be a decent person, or barring that, find a place to live where you are the least dickish person around, and lead a very private life, and hope that cock and caulk aren’t actually spelled the same. If you cannot avoid being harmful, at least minimize the harm you cause. But don't pretend to be a functional professional if you are so full of hate you cannot avoid posting your bile in a professional forum.

In a sort of round-about way, this set of arguments about membrane location is an interesting example of the real problem we have. A homeowner, or student, should be able to come on here and post their questions about membrane placement, without being trolled. One or two of us could share with the homeowners a story about the real consequences of the mistake they are making, and earn a referral fee from a competent local architect in our professional network who can actually make their home safe. A student can get a reference to a textbook that will help them think the problem through, or if not, then one of us can realize that we need a real textbook about building envelope design, and someone can make a real career out of that. Also, practice safe sex.

Thanks for posting that article that GWarton. It is rife with grammatical errors, and helpfully offensive -- in the middle of it the author calls out a person by name for not having children with them. I hope they work out their family problems, but it is also a problem for us because the best we are willing to share about what, from my perspective, is a real, unresolved, problem in architecture, is an AutoCAD schematic that would get you flunked out of interior design in the fifth, or possibly sixth, semester.

It disturbs me that someone needs to say this, and I'm not sure that many of you will listen to a reasonable argument on the subject, but your sexism, heteronormitivity, and hate speech will, ultimately, hurt you. Seven of the people you know well enough to be your facebook friends have been struggling with intersexuality all of their lives; another thirteen are just beginning to. Only half of Homonormitives (gay men and lesbian women) are actually out in the first world, what do you think it looks like elsewhere? How many of your past and future clients are?

Yes, this sample case is heavy, thank you. And I didn't mean to imply that shellumi was, specifically, a bro.

/\ whut? did i just get mainsplained?

Gwarton, that's a nice example and we've got one on & off project at the moment where we're hoping to removing the insulation in the stud cavity to expose the steel.  It's just a little hard to design when you need R27 and all of it is hanging on 8" deep girts.

Related to the "perfect wall," it looks like a couple of people are getting upset over what they consider an incorrect placement of the vapor barrier in one of the NCARB examples for the new ARE 5.0 test question types. I would say, "if only more people would read Lstiburek," but the reality is they just didn't pay close enough attention to the problem and are assuming things. That's just a failure of Test Taking 101. 

https://youtu.be/mv3jWnV8x1Y