ASHRAE Journal - January 2010 - 61

cladding is attached to the 1×4 wood furring (Photo 7). Second, the water vapor impermeability of the foam sheathing must be dealt with. Condensation from interior moisture-laden air on the interior (cavity side) surface of exterior impermeable insulating sheathing must be Photo 8: Nightmare. Exterior insulated finish controlled. The only way to do this is to Photo 7: Furring: When foam sheathings system (EIFS) directly adhered to water-senincrease the thermal resistance of the exceed 2 in. (51 mm) in thickness 1×4 fursitive OSB sheathing. Water passes through foam sheathing high enough to limit ring becomes necessary. This wall has 4 in. and around stucco cladding and is trapped condensation. This, of course, depends (102 mm) of foil-faced polyisocyanurate between the foam sheathing and the OSB. on where you are (i.e., climate locainsulating sheathing installed over a 2×6 tion), what is going on inside (i.e., the wood frame wall. The wood furring acts as Water managed EIFS does not have this interior moisture load), and how much the nail base for the fiber-cement cladding. problem and is an awesome system. insulation is already in the cavity you Wow, cows and sheep sharing the pasture. Who would are insulating on the exterior with insulating sheathing. The have ever believed that it could happen? Well, it is. Not evgood news is we have figured all of this out. It is already erywhere, and not in huge numbers. But in numbers that are memorialized in the codes. Most homes in cold climates need between 1 in. and 2 in. (25 mm to 51 mm) of insulating becoming impressive. It has not been easy, and like the range wars of the 19th century, most of the problems have been due sheathing on the outside to control this condensation risk. to misunderstandings and entrenched interests. With foam sheathing (of the appropriate thickness, an The foam sheathing will be thinner where it is installed over R-value based on condensation control as noted earlier) the the OSB at building corners so that the exterior face of the type of cavity insulation becomes irrelevant. Fiberglass batts, cellulose, low density foam, high density foam, ground up “blue foam sheathing at the corners lines up “flush” with the exterior face of the foam sheathing in the “field” of the wall. The jeans,” whatever, it no longer matters, they all work. Just don’t install an interior side plastic or foil or vinyl vapor barrier. A wall exterior face of the foam sheathing will be the water control assembly has to dry in at least one direction and with that foam layer and there will be a gap (there must be a gap) between the cladding and the exterior face of the foam sheathing. Yes, on the exterior the only practical drying direction is typically a 3/8 in. (10 mm) gap is a pretty good gap to start with. inwards. Remember that the assumption here is that the foam There will be a few houses where entire walls will be on the exterior is impermeable, which is not always the case. sheathed with OSB (not just at the corners) and again these However, once again we are being conservative. OSB sheathed walls will be covered with foam sheathing. Third, we have to control hydrostatic pressure. Remember the And, finally, there will be a few houses where the entire stucco discussion earlier with the OSB? Same problem here, and building will be sheathed with OSB, and, in turn, this OSB the same solution applies. The exterior face of the foam sheathcompletely over-sheathed with foam sheathing. ing is typically the water control layer for the assembly, and we This installation of foam sheathing over OSB sheathing need an air gap between it and the stucco cladding to drain the leads to some interesting questions: some with easy anrainwater penetrating the stucco downwards and outwards swers, some with not so easy answers. preventing the buildup of hydrostatic pressure. How much of an First the easy stuff. Let’s look at stucco cladding over foam air gap do you need? Same as with OSB: 3/8 in. (10 mm). sheathing over OSB sheathing. Providing an air gap between But we are not done with the water risk. The foam sheaththe stucco cladding and the exterior face of the foam sheathings do not absorb or store any water. And, that is both good and bad. The bad thing is, if you have wood or wood-based sid- ing prevents the debacle we got with “face-sealed” exteriorinsulated finish systems (aka “EIFS,” aka “synthetic stucco” ing. The wood rots, and the paint falls off because the wood or [Photo 8]). With the air gap we get drainage and the control of wood-based siding cannot dry into the foam sheathing. Guess what fixes this? Yes, you guessed it, a small air gap between the hydrostatic pressure. The gap can go behind the foam sheathing as well (Photo 9). If water does get into the OSB from the wood cladding and the face of the foam sheathing. How much outside, it will be minor in nature, and the OSB can dry to the of a gap? Yes, right again, about 3/8 in. (10 mm). interior as long as we have cavity insulation that is vapor open and no interior vapor barrier. This works for all claddings, not Foam Sheathing Used With OSB Sheathing just hardcoat stucco. But, you do need the gap. I don’t think I am going out on a limb saying most builders What if I use a vapor closed cavity insulation? Well, we now construct houses with wood framing and OSB sheathing at corhave a problem. Ah-hah. A problem! Yes, closed cell highners for shear strength. Everything will be covered with foam density foam cavity insulation applied to the inside of OSB sheathing, including the OSB sheathing at the building corners. sheathings that are covered on the exterior with impermeable Foam sheathing will be used with OSB, not in place of OSB. January 2010 ASHRAE Journal 61

ASHRAE Journal - January 2010

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