ASHRAE Journal - December 2012 - 53

It is OK to have holes but only if there are not too many of them. How do you decide on what is too many? Easy. Experience. When the assembly doesn’t work that tells you that there are too many holes. Amazing. We will come back to this. Here is where it gets interesting. As walls get “tighter” and more heavily insulated, the number of holes that you can get away with goes down. So where are we at now? If you do not have hydrostatic pressure acting on your wall as a result of “perched” water (see ASHRAE Journal January 2012, “Hockey Pucks & Hydrostatic Pressure”) and you have back ventilation of your cladding (not a lot, just a little§) and you have a high drying potential (vapor open layers) either inwards or outwards—or even better to both sides—you can get away with nail holes and staple holes in building papers and building wraps and fully adhered membranes and liquid-applied coatings even with highly insulated airtight assemblies. But, you can’t get away with building wraps that are “perforated.” Not anymore. You used to. But not anymore. We have too much insulation, too many engineered wood products that are too water sensitive, and too many folks have gotten real good at airtightness. This efﬁciency and sustainability stuff is becoming a real pain. You can’t do what you used be able to do. A little bit of history is in order. Building paper was introduced early in the 20th century and its function was to reduce drafts in frame assemblies constructed with clapboards and board sheathing. It was not originally intended to be a water control layer (see ASHRAE Journal, June 2009, “The Evolution of Walls”). The water control function of building paper was introduced in the 1930s, and the codes were not changed to recognize this until after 2000.** Yup, they were originally “air inﬁltration” barriers. †† Building paper ultimately evolved into a key element of the water control system of the wall assembly. Lapped shingle fashion and integrated with punched openings became the norm. Not perfectly watertight, but good enough for the time. Not perfectly airtight, but good enough for the time. Being located on the outside of the frame assembly, with the insulation inside the frame assembly meant the building paper had to “breathe.” Boy, do I hate that term, but that is what we called it in many parts of North America: “breather paper.” How much did it have to breathe? Depends. Most of the folks say more than 5 perms less than 100. It is a Goldilocks thing. Not too vapor open and not too vapor closed. Traditional tar paper operated between 2 perms and 80 perms, depending on how wet it got (check out ASHRAE Journal, Feb 2008, “The Perfect Storm Over Stucco”). So what is wrong with tar paper? It has some pretty neat characteristics. It does not pass liquid water (except at nail holes and staple holes) and it does pass vapor water. Vapor permeable and hydrophobic. But it is tough to work with. It comes in narrow rolls, it blows off easily and it is really difﬁcult to tape
§What

it or seal it unless you mastic it. Your only good strategy is to lap it shingle fashion and give up on high levels of airtightness. Now you can achieve airtightness other ways. And many old folks love the stuff and believe me, I revere the old folks. The old folks say, suck it up and demand good workmanship and stick with the tar paper and put it up with cap nails and stop whining and make something else the air control layer. Yeah, but, it is getting really difﬁcult out there and these big wide roll products that let you tape them have a lot going for them. So we got the wide roll products made out of plastic stuff. You could tape them and print advertising on them. It was great. Except they had to “breathe.” Here is where it got really interesting. It is hard to make a “plastic” tar paper. Plastic by its nature when it comes in rolls does not have holes in it. So it is great at the water control thing. It lets no water through it. No water in any phase. Not liquid, not vapor, not solid, not adsorbed. None. Big problem. This layer has to breathe if you put it on the outside of the insulation layer, which most of the time is in the frame assembly. How do you solve this problem? Easy. Put holes in it. Perforate it. The holes need to be tiny. Very tiny. You want water in the vapor phase to pass through it, but not water in the liquid phase. Not so easy now. We don’t have the technology to make holes small enough to pass only water in the vapor form but stop water in the liquid form. A little bit about the physics of water is necessary now—the “kitchen physics” version. Sort of correct, but not completely correct. More like “engineering physics”—approximately correct. Water in the vapor form hangs out as individual molecules. Water in the liquid form clumps together in groups of 25 to 75 molecules. The “clumps” are bigger than the individual molecules. Think “golf balls” and “basketballs.” The golf balls are the vapor and the basketballs are the liquid. An ideal product would be a “screen” that has holes big enough to pass the water golf balls but small enough to stop the water basketballs. That product would be liquid water closed, but vapor water open. Now it gets a little bit more complicated. These miserable water molecule golf balls are really like tiny magnets; they have a positive and negative side, and they are polarized, much like the Republic. Now picture a screen that may or may not have an electrical charge to it. If the screen has an electrical charge to it and more of these crazy ass water golf balls show up, the electrical charge on the screen pulls them through the screen. The more of them that show up at the screen, the more of them that get pulled through. Bastards. This complicates things more than things need to be complicated. This means that some materials increase in vapor permeability as the relative humidity goes up. How crazy is that? It is tar paper crazy. Now it can get ugly. When the water vapor golf balls get together and form a gang of basketballs and become a liquid

does “little” mean? As small as 1/32 to 1/64 of an inch. Really. **You could actually build without any building paper in much of the United States even 10 years ago. Just siding and OSB and you were done. Wow, water must have been getting in. Yes. But things dried. Relax. Of course, retrofitting these making them more energy efficient is certainly going to be interesting. Ah, not my problem, the kids will have to figure this one out. ††“Air infiltration” sounds cooler than “draft.” That language change came about roughly when “swamps” became “wetlands.” George Carlin would be proud.

December 2012

ASHRAE Journal

ASHRAE Journal - December 2012

Table of Contents for the Digital Edition of ASHRAE Journal - December 2012