Comments on: You Don’t Need a Vapor Barrier (Probably)

By: Jim

Jim — Tue, 02 Sep 2014 11:40:04 +0000

Building codes must be based
Building codes must be based on personal opinions.One or two years do it this way then no no no we "learned" THIS is the way you really really do it.

By: Allison Bailes

Allison Bailes — Fri, 29 Aug 2014 00:03:31 +0000

Terry: It
Terry: It depends. If you're in a really cold climate, maybe not. It also depends on the other components of your wall assembly. Many older homes have had cellulose insulation put in the walls without a problem, though.

By: Terry

Terry — Thu, 28 Aug 2014 18:32:16 +0000

So can I insulate the walls
So can I insulate the walls of my
existing home (built in 1956) with cellulose and not be concerned about whether or not there's a vapor barrier?

By: Richard Mozeleski

Richard Mozeleski — Wed, 26 Feb 2014 19:01:35 +0000

Is this your opinion or did
Is this your opinion or did you perform tests to prove your statements. I would like to see some evidence. Please
Thanks for the article

By: Allison Bailes

Allison Bailes — Fri, 06 Jul 2012 23:01:27 +0000

G. Curmudgeon
G. Curmudgeon: Hooray for the statute of limitations, right? I'm sure there were some nice (aka nasty) photos to be had when they opened those walls.

Steve K.: I'd love to come back and participate in one of your regular Building Science Discussion Group meetings! I'll be heading your way at the end of this month for Building Science Summer Camp but won't catch your meeting then.

Thomas P.: Good point about the integrity of the airtight drywall approach. A double-wall construction would solve that problem and reduce thermal bridging, too. In my more general article on vapor retarders, I linked to a Joe Lstiburek article that gave a lot of different ways of dealing with moisture management, some of which had the vapor retarder in the middle.

By: Thomas A. Peterson

Thomas A. Peterson — Tue, 26 Jun 2012 00:08:28 +0000
Sorry that I missed you in
Sorry that I missed you in Maine, Allison.

My concerns with the air-tight drywall approach is that drying framing members and potential intentional holes made by the occupants, a carpenter ant or mouse, or an electrician adding an outlet; could severely compromise the vapor retarding ability of the system with no easy way that I can think of to correct such compromises. After a house has aged for a few decades, I am sure that some compromising of the air-tight drywall system will have occurred.

As I have been promoting and building "double-wall" construction for 30 years and building only double-walls for the last 15 years, I believe that I have developed a unique approach that helps to eliminate all of the negatives. I install my vapor barrier in the middle of the wall system between the two walls. Such a location works both as a vapor barrier as well as an air barrier and eliminates the need for an external air barrier (not a easy application on a multi-storried building) as well as any special electrical pans and their sealing. The few holes in this barrier required for exterior lights/switches can be easily sealed with a can of foam or silicone mushroomed on both sides of the vapor barrier.

To thwart the comments of the expense of the double-wall headed my way, I would like to add that the second wall that I add to the interior is totally non-structural (24" O.C.) and has less integrity than an interior partition wall. Its only job is to provide a chase for electrical, support window and door trim, another 3.5" of insulation, and 5/8" of sheetrock (part of the building's thermal mass). This is off-set by no labor or materials for an external air barrier; no electrical pans, sealing, and their labor; no airtight drywall sealers and their labor; no special vapor barrier paint; a much less costly heating/cooling system; and much lower heating cooling bills. Yes, your window & door jambs are deeper (wider), but this only really adds to materials costs, and the application to install the vapor/air barrier is a bit more involved. You do need an air-to-air heat exchanger, but any tight building needs this regardless of the wall's thickness. Fire away, I am sure that there are some additional concerns that I have not addressed.

By: Steve Konstantino

Steve Konstantino — Mon, 25 Jun 2012 20:29:33 +0000
Allison, thank you for your
Allison, thank you for your usual direct, clarity on this topic. We still find so many people insisting on using plastic layers in their wall assemblies but I really think most of these layers end up with penetrations over time and negate their purpose. The plastic then becomes more of a problem than a solution.

Come back and visit us again at the BSDG!

By: Green Curmudgeon

Green Curmudgeon — Mon, 25 Jun 2012 19:13:28 +0000
Good points as always. Vapor
Good points as always. Vapor retarders seem to me to be a backup in cold climates for poor air sealing, if they even work at all. It is amazing how things like vapor retarders, along with similar things like roof and crawlspace ventilation, get put in codes and never get reconsidered or revised. Hopefully increased requirements and testing for air sealing will lead to tighter homes and less vapor movement through convection in and out of buildings.

I remember in the 1980's there was a big push to install polyethylene barriers on framed walls before the drywall. Once company would charge you to do this and guarantee the energy savings. I never went that route as a contractor I did plenty of what I now know to be stupid things that did plenty of damage to homes I was building or renovating.

Never personally saw one of the those poly sealed buildings after they tore open the walls to fix all the rot but I sure would like to.

By: Allison Bailes

Allison Bailes — Mon, 25 Jun 2012 18:48:39 +0000
Charles:
Charles: Good point about the retarder vs. barrier confusion. I was relying on people to remember what I wrote in the article on vapor retarders, but I'll go back and edit the article to make it clear. Thanks!

Yes, there's a lot of confusion about the difference between permeability and permeance. I discussed that in the other article, too, and pointed out that permeability is like density and permeance is like weight.

By: Allison Bailes

Allison Bailes — Mon, 25 Jun 2012 18:35:22 +0000
Debra M.:
Debra M.: Thanks! I didn't get into the airtight drywall approach to air sealing here, but in a cold climate, that would be the way to go.

Bob: Spray foam is one way to deal with the air sealing.

Franklin M.: Ah, yes, building inspectors. I didn't mention their role in all this in the article, but building codes are coming along gradually and getting the message. Until they all get it, you just have to do what they say and try to do it ways that won't cause problems. Putting plastic on the inside of a wall in a cold climate isn't a bad thing. It's just not a guarantee that you're going to solve moisture problems. Putting plastic over closed cell spray foam is redundant and superfluous as well. Putting plastic on the interior side of a wall down here in Georgia or North Carolina is a recipe for failure.