ASHRAE Journal - January 2010 - 57

Mind the Gap, Eh? By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE heathing does more than deal with wind. And, sometimes it doesn’t even deal with that. It wasn’t always that way. We didn’t always even have sheathing. We had a timber structural frame (or not, sometimes we just had rocks, or bricks, but you get the idea) and a cladding. The cladding kept the outside out (kind of), and the structure kept the building from falling down and blowing over (kind of). A century or two ago we began to get boards as a sheathing whose function was more to provide intermediate support for a cladding than anything else, and then less than a century ago, we got building papers that kept the wind out. The board sheathing evolved into plywood, and we got the modern residential wall: a wood structural frame sheathed with plywood sheets, wrapped with building paper covered with a cladding. This configuration took us through almost half a century. The last half of the 20th for those who don’t quite know how to count them. The key to the modern residential wall was the magic of plywood (see sidebar). Plywood was and still is an amazing product, so we continue to build furniture, houses, boats and planes* out of it. We no longer build as S ing we see two alternatives:† oriented strand board (OSB) (Photo 1) and foam boards (Photo 2). One of these two alternatives is structural (the OSB) and the other (the foam board) is most definitely not. One of these two alternatives is insulating (the foam board), and the other (the OSB) is most definitely not. One of these two alternatives rots (the OSB) and the other (the foam board) most definitely does not. One of these two alternatives “breathes” somewhat (the OSB) and the other (the foam board) sometimes does and sometimes doesn’t. What is a builder or designer to do? Use both. Really. Pick your spots, and use both. Both are great alternatives that can be complementary. Photo 1 (top): Ubiquitous Sheathing. A pretty familiar sight is a home completely sheathed with oriented strand board (OSB). “It’s ruling the range.” Photo 2 (bottom): Upstart? Fifty years as a sheathing but still considered an upstart. Extruded polystyrene (XPS) completely sheathing the structure, providing a continuous thermal break. “Beginning to show up on the range and confusing the cowboy builder and architect.” many of these things out of plywood as we once did because plywood is being replaced by alternative materials. This is most evident in housing where plywood is pretty much gone. In hous- OSB Sheathing It took us about 20 years to get OSB right. The early stuff was unbelievably water sensitive. The edges swelled and the only thing you could do was invest in a belt sander. Some of the cheap stuff today is still pretty bad. But if you shop well you can now get OSB with nail-holding characteristics as good as plywood’s, and OSB shear strength that just rocks (actually it doesn’t rock, which is the point). And, it can be rained on and not swell. We don’t need big trees, and it is much less expensive than plywood. Pretty awesome, all things considered. * Arguably, the best high performance aircraft of the Second World War—next to the Messerschmitt 262 twin-engine jet—was the de Havilland Mosquito fighterbomber. The Mosquito was nicknamed the “Plywood Wonder.” Its fuselage and wings were a plywood monocoque. It was such an impressive aircraft that wreaked so much havoc and destruction that the Germans awarded two victories to anyone shooting a Mosquito down. “The Mosquito famously annoyed the Commander in Chief of the Luftwaffe, Reichmarschall Herman Göring, when on 20 January 1943, the tenth anniversary since the Nazis seizure of power, a Mosquito attack knocked out the main Berlin broadcasting station putting his speech off air” (http://en.wikipedia.org/wiki/de_Havilland_Mosquito). It gets better. Being built out of wood instead of metal it had a much lower radar signature. It was an early use of “RAM” or “radar-absorbing material.” It was the original stealth fighter. I grew up with the legend of the Mosquito. More than 1,000 were built in Downsview, Ontario, by de Havilland Canada. My dad started as a tool and diemaker working for de Havilland at the Downsview plant and left as a controls engineer. I went to Downsview Secondary School, and we could see the de Havilland runway from our classroom. The Mosquito had long since left service by the time we youngsters saw them, but a few were still around. My dad would mist up whenever he saw one. Me, too. † In commercial wall assemblies, we typically see a third alternative: gypsum sheathing. The most common gypsum sheathing used is glass-fiber faced and is semimoisture tolerant with a very high vapor permeance. Think of it as a more vapor permeable plywood sheathing with minimal structural properties that does not burn. Most of the discussion that follows is relevant to this wall as well. The gaps are as important to this wall, as to all the residential walls, in that the performance is significantly enhanced. Having said that, if the gaps are missing, this commercial wall does not have as many of the moisture problems associated with “gapless” OSB sheathing or even “gapless” plywood. Why? It does not rot and stores and redistributes moisture better. January 2010 ASHRAE Journal

ASHRAE Journal - January 2010

Table of Contents for the Digital Edition of ASHRAE Journal - January 2010