ASHRAE Journal - December 2012 - 54

BUILDING SCIENCES
then desorbs water. Think of something that likes to slow dance with the water golf ball, but ultimately turns down the golf ball, and the golf ball then leaves the ballroom. Typical promiscuous water golf ball. Tries to get lucky but doesn’t. Comes in one side of the dance hall, slow dances and then leaves out the other side. The botanists call this “transpiration” and amazingly enough the chemical engineers like it as well. I call it being turned down. The formal name for this type of product is a non-porous permeable ﬁlm. This product rolls. You can also do it the way the second to last footnote says. But you can’t do it by punching holes in something. So how do you tell if you have got something that actually works? Something that rocks and rolls? A plastic that works like tar paper but does not blow off that you can tape and that handles surfactants? Well you need to run the “Joe Tests.” Specially developed just for this purpose. They can be run in a kitchen since they use “kitchen physics.” You might also need to know when a product does not work. Remember, I said I would come back to this? One of the ﬁrst signs that a product does not work is that your house smells musty or the paint is beginning to fall off your siding or both. Check out Photo 1 on Page 52—nice house. Now look at Photo 2—not so nice. The sheathing is almost gone. Photo 3 in the sidebar shows the back of the siding. Wet. I think we can pretty much conclude that this assembly is not working. Joe Test 1 is shown in Photo 4. It is run on tar paper in this example. A teaspoon of water is placed on tar paper placed over a paper towel. If the water beads up and does not wet the paper towel, it passes Joe Test 1 (Photo 5). In the case of the house we were looking at, the sample product removed from the wall failed Joe Test 1 (Photo 6). Most products pass Joe Test 1 before they are installed. But all products that fail in the ﬁeld seem to fail Joe Test 1. I ﬁrst started doing Joe Test 1 in 1995 (Photo 7 and Photo 8). It was not very

Photo 2 (left): Not So Nice. Photo 3 (right): Wet Siding. Water that used to drain and not be absorbed now passes through the field of the building paper and is absorbed. More water is now retained for a longer period of time. they tend to behave badly as gangs typically do. The basketball gang sometimes goes and hangs out with other gangs. One of the most miserable gangs out there is the surfactant gang and the most vicious members of the surfactant gang are the soaps. These soaps not only get your whites whiter than white, but they have tattoos, date your daughters and drive big Harley hogs. Worse, they make the screen wires slippery. And slippery is worse than charged. They grease the skids on the screen and basketballs just pour through. So we need a plastic sheet product that passes vapor golf balls but not liquid basketballs and handles gangs with surfactant members. There are only a couple of ways to do this. None of them involve punching holes in sheets. The most common is to start with big holes that get ﬁlled in. You can’t start with no holes and add holes because the holes that need to be added are so small no mechanical perforation process can make them small enough.‡‡ A water golf ball is 3 × 10–10 m (one billionth of a foot). The smallest holes that we know how to make using mechanical perforation are about 100 microns—the thickness of a dollar bill—the limit that an eye can see without magnification, which is about 1 × 10–4 m, or 100,000 times bigger than a water golf ball, and 1,000 times bigger than it should be. If you can see the hole, it is too big. Trouble is, some of us can’t see even big holes. Tar paper starts with a bunch of cellulose ﬁbers that are smushed together so that the gaps between them are about the size of water basketballs. The ﬁbers are then coated with bitumen that makes the gaps smaller than water basketballs—in essence “clogging” up the screen. The bitumen also makes the surface hydrophobic; it repels the little water bastard basketballs while protecting the cellulose from decay. The old guys love this product for good reason. You can take tiny plastic ﬁbers rather than cellulose ﬁbers, ﬁbers that are even smaller than cellulose ﬁbers, and do pretty much the same thing. And if you choose the right plastic, with a little chemical engineering magic you can make the surface hydrophobic without coating them with anything. This type of product is called polymeric ﬁbrous. In essence, a plastic tar paper. This product rocks. You can also take much bigger plastic ﬁbers that create a structural screen mat that supports a thin plastic ﬁlm that loves water. Something similar to nylon that adsorbs water, then absorbs water and

‡‡You can start with no holes and stretch a film that has particles imbedded in it until it begins to tear where the particles are. Clever, eh? The particles initiate the tears. If you stop stretching it at the right point the tears and gaps that are left are small enough to pass only the water golf balls. These materials are more formally called micro-porous films. §§That would be my then 12-year-old son who apparently understood intuitively that breaking the surface tension of water results in wetting of the surface and subsequently a leak.

ASHRAE Journal

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December 2012

ASHRAE Journal - December 2012

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