Remodelers face other challenges in installing foam-backed siding due to the thickness of the product. For one, Lutes says, it produces bigger J-channels at window penetrations, as wide as 11/2 inches compared with a standard 5/8-inch or 3/4-inch channel. “You may have some more aluminum to bend around windows or other areas, to have a big enough flashing or to have a trim detail come out away from the house farther to cover the profile of that siding,” Lutes says.
Remodelers who use backed siding say that the decision to use the product rather than to apply rigid sheathing has to do, in part, with the needs of the project as well as client preference. Brick says the surface of the home’s exterior plays into the decision: Walls that show too much wave require a thick application of sheathing. “If you nail your siding directly to something that has a lot of bellies in it, you get that roller-coaster look, and you don’t want that,” he says. Cost is a factor, too, as backed siding can cost up to 50% more than regular vinyl. Clients whose priorities are budget and aesthetics tend to prefer a thin layer of sheathing and more-affordable vinyl, Lutes says. If energy efficiency is a priority, as is increasingly the case, Lutes usually recommends foam-backed siding.
Foam and Energy Performance
Of course, the foam-backed product is only an option when the client is interested in vinyl. But even when considering vinyl only, rigid foam appears to provide better energy performance. CraneBoard, the leader in the foam-backed market, offers a value of R-4. Alside Prodigy, which Lutes uses, offers an R-value of 5. By comparison, rigid-foam sheathing generally offers a value of R-5 per inch, and the sheathing can be installed to whatever thickness is desired. Backed siding will also necessarily have gaps because the panels have seams where they join, which means it doesn’t serve to tighten the house the way foam sheathing does.
“With foam sheathing, you get a continuous layer of insulation, which will serve to tighten up the house in case of any air leakage,” says Michael Anschel, president of Minneapolis remodeling firm Otogawa Anschel. “And you also move the dewpoint outside of the walls into the middle of the foam so that any condensation that occurs occurs in the middle of the foam where there’s no opportunity for mold to grow.”
One to 11/2 inches is a standard thickness for rigid foam, but Anschel says that’s not necessarily enough. He often specs 3 to 4 inches of rigid foam, which adds R-value and ensures that the foam will push the dewpoint of the house outside the building envelope.
Building the sheathing up to that thickness, however, can lead to trouble when it comes to flashing and trimwork. “It can be tricky,” Anschel says. “We’ve ordered custom millwork that’s thicker to make up for the difference that foam leaves in there. In other applications we’ve actually put a piece of plywood behind the trim. We’ve also pulled the trim off. If the trim is still in good condition or if it’s a complicated profile, we carefully pull it off and put a rigid backer behind it so we can reinstall it.”
To help mitigate the challenges of siding over thick applications of sheathing, Lutes, the Michigan remodeler, recently started installing IsoBoard, an ultra-thin sheathing. “It’s about a 1/4-inch to 1/2-inch thick and it has an R-value that surpasses that of both the rigid-foam board and the siding with the insulation adhered to the back of the profile,” he says. “We’ve found it to be a little easier to work with, and it provides a higher insulation value, although the cost is higher than the cost of the standard rigid-foam insulation.”
Give Me Air
Another essential consideration in jobs that call for foam sheathing is the danger in restricting airflow. If the house has atmospherically vented appliances, too much tightening can create a backdraft.
“If you tear the siding off a home that previously had strong or good air exchange and you put your rigid insulation all over it and re-side it or re-stucco it, suddenly that house is no longer breathing,” Anschel says. “The air inside the house is also no longer being exchanged. You can create a really hazardous situation.”
After you sheath a house with rigid foam, conduct a worst-case depressurization test to determine whether the increased tightness has created a backdraft. If the tests find positive pressure — meaning gases can’t escape the house envelope — you have a backdraft issue that has to be addressed, either by replacing the appliances with power- and direct-vent sealed combustion models or atmospherically venting appliances with smaller exhausts.
“You can have atmospherically re-venting appliances in a house if the venting off the appliance is done properly and you don’t have a lot of large-exhaust devices in the home,” Anschel says. “So if you have a 600 cfm hood on a range, that’s pulling a lot of air out of the house. If you have a little 200 or 300 cfm fan, then that’s not as big a deal.”
Obviously, finding out at the end of a project that it’s necessary to install new mechanicals adds to the bill, so best practice calls for conducting a test before construction begins to attempt to predict the results of the post-construction test and build any necessary mechanical work into the budget.
—David Zuckerman is a freelance writer in New York.