Image Credit: Energy Vanguard
Image Credit: Energy Vanguard Fig. 2. The War Against Water was a booklet published in the early 1950s by the National Paint and Varnish Association. It reads like an anticommunist manual.
Image Credit: From Bill Rose's book, Water in Buildings
More Building Science
Back in the 1930s, a rash of paint-peeling showed up across North America. One thing that most of these homes had in common was insulation in the walls. Painters put two and two together and decided that the problem was the insulation. According to building scientist Bill Rose, the painters surmised that the problem was happening because insulation “draws water,” and some refused to paint insulated houses.
Now, I know what you’re thinking. Those painters didn’t want to paint insulated buildings because building science hadn’t been invented yet, and they thought the insulators were jumping the gun. Or was it that painters thought that stuffing the cavities with insulation was silly when all they needed was some good insulating paint? Then again, maybe I’m just jumping to conclusions here, as, it turns out, the proponents of insulated buildings did in their response to the painters’ revolt.
Insulation and the early building science researchers
I’ve mentioned Bill Rose’s excellent book, Water in Buildings, in this space before, and it’s a wonderful resource. Chapter 3, “Water and Building Materials,” lays out the U.S. history of building science research spurred by the paint-peeling episode of those early adopters of insulation.
On the first page of that chapter, Rose outlines how a set of moisture management practices developed in the period from 1937 to 1942, and that’s pretty much how we’ve treated buildings ever since. I’ll abbreviate his six bullet points to three (since I’m not going to delve into profile analysis in this article):
It’s a fascinating history, and Rose goes into the details of the different people who advanced the theory of diffusion and vapor barriers, the papers they wrote, and nearly two full pages on the 1952 condensation conference. The big names of the early building science research were F.L. Browne, Larry V. Teesdale, T.S. Rogers, and Frank Rowley. (For more information on Teesdale, Rogers, and Rowley, see Do I Need a Vapor Retarder?)
One of the most amusing parts of the story is how the National Paint and Varnish Association got involved and declared “War Against Water.” Figure 2 below shows thecover of one of the booklets they published in the early 1950s. Written near the beginning of the Cold War, the booklet villainizes moisture much the same as McCarthy maligned communists. For example:
They seem innocent enough, these three pools of moisture: the milk from the bottle, the steam from the shower, the vapor rising from the whistling tea kettle. But are they? Oh, no… they’re up to no good. Where do they go from here? Believe it or not, they have an engagement. At the “dewpoint” — if you please.
Yeah, we can laugh now, but back then building professionals and homeowners alike were practicing their duck-and-cover drills at the slightest hint of water vapor!
Will the real culprit please stand up?
Those early building scientists did some good research and advanced our knowledge of vapor diffusion and other building science topics. For example, Teesdale found that a material’s wetness is related to its temperature in what Rose calls the Fundamental Rule of Material Wetness: Cold materials tend to be wet and warm materials tend to be dry.
They misfired, however, on the cause of the peeling paint. The industry, led by Teesdale, Rogers, and Rowley, focused almost entirely on moisture diffusion and the need for vapor barriers. (These are also the guys who gave us vented crawl spaces, but that’s another story.) Browne is the one who got it right, way back in 1933. Yes, he mentioned diffusion as one mechanism for the wetting of walls and peeling of paint, but he also called out “poor carpenter work or faulty design,” as Rose quotes him.
That is, the bigger problem was bad flashing details, which allowed rainwater to get into the building assemblies — and then stay there. Before insulation, it didn’t matter so much because of the Fundamental Rule of Material Wetness. Uninsulated walls stayed warmer and thus dryer. With insulation in the walls, the cladding was colder and that meant it had less tolerance for bad flashing.
Another factor more important than vapor diffusion is air leakage. Air moving through leaks in a wall can carry far more water vapor than diffusion allows. Dr. Joseph Lstiburek just wrote about this in his latest article at the Building Science Corp. website: “Air leakage was and is more important than vapor diffusion. Things have not changed.” It’s a great article about MacBeth and vapor barriers, and even though Joe is full of sound and fury, he’s not an idiot. Go read it.
The moral of the story is not to jump to conclusions. We learned a lot about vapor diffusion, but our decades-long obsession with vapor barriers was counterproductive and hindered us from learning the more important lesson: It’s generally more important for building assemblies to be able to dry out than it is to prevent wetting by vapor diffusion.
I’ll give the last word to Bill Rose on this topic: “Given the fact that a very small percentage of building problems (1 to 5% at most in the author’s experience) are associated with wetting by water vapor diffusion, the argument for enhanced drying potential becomes much stronger.”
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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16 Comments
Vapor loss and heat loss
As warm interior vapor travels outward through the wall, doesn't this result in some heat loss? Has this been quantified? Since I never see it mentioned, I assume (or just hope) that it is insignificant... But I'm still curious.
Heat loss as water vapor, sure...
The heat of vaporization is about 940BTU per lb of water than ends up as adsorb in the sheathing. Give how truly miniscule the rates of water transport via vapor diffusion is (even through UNPAINTED wallboard or high-perm housewrap), that's a very tiny heat loss too small to be measured by conventional methods.
You'll be moving several orders of magnitude more moisture/heat out of the house with the ventilation air.
Responding to the blog article:
"That is, the bigger problem was bad flashing details, which allowed rainwater to get into the building assemblies — and then stay there. Before insulation, it didn't matter so much because of the Fundamental Rule of Material Wetness. Uninsulated walls stayed warmer and thus dryer. With insulation in the walls, the cladding was colder and that meant it had less tolerance for bad flashing."
It's pretty common to find early 20th century or early wood-framed homes that have NO window flashing of any type (!), since it really wasn't necessary, except on homes with very limited eave/rake overhangs. Before insulating an older house it's important to figure that all out. On a craftsman bungalow with 18-24" overhangs everywhere it won't matter much, but on a cape style or Greek revival houses that can sometimes come with mere 6" overhangs, having the flashing in place really counts.
Response to Dana Dorsett
Indeed. Not only do some old houses have no flashing, some have no sheathing either. Insulating old houses can be a tricky business, and it needs to start with determining the state of its moisture management systems.
Questions About the Early Research
What indicates that the early researchers were only concerned about stopping diffusion? Is it possible that they were simply concerned with stopping outward vapor movement caused by both diffusion and vapor transport by air leaks? How do we know that the intent of their “vapor barrier” was not to stop both mechanisms of vapor transport? With both mechanisms, the ultimate intent was to stop vapor from entering the walls, so the term, “vapor barrier” could easily refer to both mechanisms of vapor transport.
The early researchers may have reasoned that a properly installed vapor barrier would prevent moisture from gathering in the walls, and thus the fact that the vapor barrier did not permit inward drying was irrelevant. If you prevent vapor from entering the walls in the first place, there is no need for the walls to dry. And besides, they would have been able to dry to the outside as a backup reassurance.
Perhaps the early researchers simply assumed proper workmanship was a given, and that it would result in a “vapor barrier” that would prevent both diffusion and air leaks. Perhaps they also assumed proper workmanship would prevent rain from entering the wall from the outside. They might have reasoned that if rain did enter a wall, it would be a separate problem that should be addressed directly; as opposed to a problem that required the wall to have extra ability to dry in case rain got in.
Response to Ron Keagle
Read Bill Rose's book, Water in Buildings, and you'll see that it's pretty clear they focused on diffusion and not vapor transport by air leakage. They went down a wrong path and that set up the vapor barrier problems we've been experiencing ever since.
Actually it's gotten worse since their time because we air condition a lot more houses now. That's the reason why plastic on the inside of a wall doesn't work. When you air condition a home, you have to be able to dry to the inside. See Joe Lstiburek's latest article, Macbeth Does Vapor Barriers (Double, Double Toil and Trouble).
Unintentional Interior vapor barrier?
OK, I get it that you don't want to put plastic on the inside of a wall in an air-conditioned home because it's vapor impermeable. So, how do you finish out the interior surface of a bathroom on an exterior wall in an air-conditioned home? Current wisdom indicates that conventional gyp board (or even green board) is no good because of moisture generated in the room from showers, etc. There are several options of "paperless" drywall, but they all appear to be vapor impermeable (just like the plastic sheeting is). So, what should I hang on the bathroom wall?
Specifically, I'm in Zone 2B.
My wall is brick veneer, 1" air gap, 30# felt paper, 1/2" plywood, 5 1/2" open-cell foam in 2x6 wood framing, and then 1/2" drywall.
Response to John Walls
John,
As long as you're not talking about a tiled shower or a tiled bathtub surround, then ordinary moisture-resistant (green) drywall is fine in this location.
If you want something more water-resistant -- and I don't think you need it, unless your kids splash a lot -- you could install cement backerboard on your walls, and finish the walls with a skim coat of drywall mud or plaster.
Reply to John Walls
John,
I see two separate issues in your question. One is the ability of the interior facing surface to withstand moisture, and the other issue is the ability of the interior surface to prevent vapor from diffusing through the wall and condensing when it encounters a temperature below the dew point at the sheathing or somewhere within the wall cavity.
When you speak of the type of drywall, the issue is the direct wetting from the bath water and high humidity. When you speak of the plastic film vapor barrier, the issue is vapor diffusing through the wall and condensing at the dew point.
I think you are asking how to prevent the condensation from the relatively high vapor pressure diffusing outward from the bathroom; if you omit an impermeable vapor barrier because it can lead to condensation of reverse vapor drive during summer air conditioning.
Response to Ron Keagle
Ron,
Anyone who wants to reduce outward wintertime vapor diffusion in a home that is air-conditioned during the summer should choose a smart vapor retarder like MemBrain instead of polyethylene.
Bathroom walls and ceilings
Martin & Ron, thanks for your responses. Yes, my concern is two-fold; but my main concern is condensation on the back side of the drywall during the summer time. So, it sounds like green-board is a good solution (vapor permeable with some mold resistance). My secondary concern is water splashing by occupants that would lead to drywall deterioration . So, I suppose I could use cement board up beyond "splash" height, and then go to green board from there up. Is that a difficult joint to finish (cement board-to-green board--different thickness, no taper, different joint compound, etc.)?
Some related questions I have are:
1) Is green board necessary/recommended for the rest of the bathroom, or would regular gyp board serve well? Walls & Ceilings? (Semi-conditioned attic---open cell spray foam in roof rafters).
2)What about the ceiling in the shower. Green board? (note: had planned on using 5/8" gyp on other ceilings).
3) Is there any need to use cement board at sinks? How about at washing machines?---because of splashing or potential plumbing leaks. Or is that overkill?
4) Is drywall "primer/sealer" vapor permeable? I've asked a couple of paint reps. They don't test for it and won't venture a guess.
Response to John Walls
John,
It isn't hard to finish a tape joint between concrete backerboard and drywall, as long as the two boards are the same thickness.
The answers to the rest of your questions will never be definitive. These are judgment calls. It's your bathroom, and you can finish it any way you want. Many designers and homeowners like all surfaces in a bathroom to be covered with a durable tile. Others are happy with drywall. If you decide to install tile, I would stick will greenboard, even for the ceiling (as long as your framing is 16 inches on center or closer).
Most paint primers are vapor-permeable, unless they are specifically sold and labeled as a vapor-retarder paint.
No Tile
Martin, thanks for the advice.
One clarification, please----I will not be installing any tile (except stone slabs on the shower walls---I'll be using cement board on those walls). The tub and sinks will have a stone top and a short stone back splash (so greenboard should work ok there.).
So, in that case, are you still preferring green-board throughout the rest of the room, or would conventional gyp board serve ok? I realize its a judgement call, but I value your opinion. What would you do if it were your's?
When you say "even for the ceiling", are you including the shower ceiling (I had planned for no tile on the ceiling)?
Reponse to John Walls
John,
I know that you aren't installing tile. I just pointed out that this is a judgment call. You seem to want to use drywall or cement backerboard. But other homeowners prefer tile. The point is, there is a continuum of expectations for how waterproof to make bathroom walls. Only you can decide what you want to specify.
My bathrooms have greenboard, and I'm happy with them. But you might not be happy.
And, yes, I chose to install greenboard on my bathroom ceiling.
Green board in the bathroom
John, the bathroom can be a high humidity location, even with a fan so some people will install green board throughout the entire bathroom.
That is what I did in my son's bathroom and the cost differential is not large. It cost an additional $60 to apply green board to the walls and ceiling in a 5x9 bathroom with the exception of the tub area. We put cement board (on the walls only) as it is going to be tiled. However, the ceiling over the tub enclosure is still green board.
FYI: We had a new house with the tub along an exterior wall and the builder used only green board. Within 5 years, I had to rip it out and redo with cement board. And we live in a very dry and cold climate (Edmonton, Alberta, zone 7) so there is little humidity and we had no air conditioning. However, at that time, we did have a teenage son and daughter that liked their showers.
Purple Board?
Thanks Martin , et al for your comments.
Any experience with, or comments on Purple Board?
AG makes an "improved" board (colored purple) that is marketed as an upgrade of green board.
The literature suggests that its similar to green board, but with added chemicals to retard mold growth.
Response to John Walls
John,
I have no experience with purple board.
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