ASHRAE Journal - January 2010 - 60

So, how big should this air gap be? Easy, 3/8 in. (10 mm). Can it be smaller? Yes, 3/8 in. (10 mm) is once again very, very conservative.# Let us go back to the cavity insulation. Things get risky if you use a high-density spray foam on the inside of OSB sheathing (Photo 5). There is no way that any appreciable moisture in the OSB can dry inwards. The only possible drying is outwards. We need the gap. Same thing with structural insulated panels (SIPs). These are foam panels lined on both sides with OSB. There is no way that an exterior OSB skin that gets wet on a SIP can dry inwards. We really need the gap with a SIP. How much of a gap? Oh, how about 3/8 in. (10 mm)? Remember the discussion about being conservative. It can be smaller with small loads. We’re not done yet. The housewrap actually has to work. It can’t be perforated, and it can’t lose its water repellency due to surfactants such as tannins, wood sugars and other water-soluble extractives found in OSB and wood-based sidings, or dispersion agents found in paints and workability additives in stuccos and mortars. Most housewraps don’t work. That will be a discussion for another day. And, then they actually have to be installed so they don’t blow off. And, they have to be lapped properly and taped or sealed to be effective water control layers and air control layers. So, can we make OSB work? Yes, get a good water resistant OSB, a non-perforated surfactant resistant housewrap and make sure you have a gap between the cladding and the housewrap/OSB interface. You now have 1950s durability coupled with awesome energy efficiency. And, a whole bunch of structural strength that you may or may not need. But it is nice to know that if you need the structural strength the wall won’t rot because you have that gap on the outside of the OSB. Knock yourself out with whatever cavity insulation you want. The gap makes them all work, including SIPs. ’Nuff said. Photo 5: High-Density Spray Foam Cavity Insulation. High tech and low perm. Great air sealing, great thermal performance, great structural strength. It does not allow for any appreciable inward drying of the exterior sheathing. Photo 6: Shear Resistance. Premanufactured metal inset shear panels are quite common in high seismic risk zones. Foam Sheathing Foam sheathings are definitely not plywood. They are also not OSB. Foam sheathings come in various flavors: extruded polystyrene (XPS) and foil-faced isocyanurate are the most common. For purposes of this discussion they will all be treated pretty much the same way, as impermeable,** and without any shear or racking structural resistance. It will be assumed that none of them store nor absorb any significant water and that none of them rot. If you tape or seal the joints of foam sheathings, they can also act as water control layers. Let me translate. If you tape or seal the joints, foam sheathings do not need to be covered with a housewrap or building paper. Foam sheathing with joint treatment can replace OSB and a housewrap and you get all of that insulating value from the foam sheathing to boot and nothing to rot. Easy. We’re done. Well, not quite. Three details need to be addressed. Foam sheathing provides no structural strength, foam sheathing does not breathe, and we have to control hydrostatic pressure. These are not so minor problems. First, the structural problems. When you use foam sheathing, some means of shear resistance must be provided and the necessary shear resistance will depend on where you are. The more wind, the more shear needed. The greater the seismic risk, the more shear needed. The greater the snow load, the more shear needed. You can get all the shear you need without OSB sheathing. There are alternatives to the shear properties provided by OSB sheathing (Photo 6). What about attaching cladding? What about it? You do not attach the cladding to foam sheathing. You attach the cladding through the foam sheathing to the structure under the foam sheathing. It is pretty straightforward. You can attach vinyl and wood siding and hardcoat stucco lath and manufactured stone veneer directly through all flavors of foam sheathing up to 2 in. (51 mm) thick. With fiber-cement siding this is limited to 1.5 in. (38 mm) of foil-faced polyisocyanurate. For thicker foam sheathings, 1×4 wood furring is necessary. The 1×4 wood furring is installed over the foam sheathing (up to 6 in. [152 mm]), and then the #There are “draining housewraps” and “draining building papers” that are about 1/16 in. and even smaller that work very well. Even two layers of building paper work pretty darn well. So why not just specify 1/16 in. (2 mm) or two layers of building paper? Ah, it depends on the “load,” a fancy word that means sometimes we get lots and lots of rain, and when things get very ugly, 1/16 in. (2 mm) or two layers of building paper does not always cut it. Low-rise houses without much of an exposure easily work with 1/16 in. (2 mm) gap or two layers of building paper. But taller buildings with big exposures and no overhangs and lots of rain and wind with the 3/8 in. (10 mm) gap makes me sleep at night. Sometimes someone somewhere has to throw some judgment at the problem. ** OK, everyone take a Valium. I know that this is absolutely not true. All kinds of foam sheathings do breathe to varying degrees. Yes, I know about expanded polystyrene (EPS)—3 perms for a 1 in. (25 mm) thickness. Yes, I know about fiber-faced isocyanurate—also 3 perms for a 1 in. (25 mm) thickness. Yes, I even know that most 1 in. (25 mm) unskinned extruded polystyrene (XPS) is between 1.0 to 1.5 perms. But much of the stuff used for sheathings is faced with foil or polypropylene skins and so the limiting case is a vapor impermeable foam sheathing. If things work when they are completely vapor closed, they will also work when they are semivapor permeable. And, in many assemblies, they work much better when they are more vapor permeable. But that is a discussion for another time. ASHRAE Journal ashrae.org January 2010

ASHRAE Journal - January 2010

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