Pro Insulation Tips From the Jamestown Net-Zero House
TOH host Kevin O’Connor gets expert Tom Kelly to explain the decision behind the Jamestown Net-Zero house insulation
At the TOH Jamestown Net-Zero House, closed-cell foam was installed against the underside of the roof sheathing, followed by a thick layer of open-cell foam. According to the installer, closed-cell foam forms a vapor barrier. I thought the usual practice was to install a vapor barrier on the warm side of the building. Why wasn’t the open cell applied to the sheathing first?
—Andy Perlik, via e-mail
I’ll let Tom Kelly explain the science behind this approach. He’s the owner of Ecologic Insulation, the Rhode Island–based spray-foam company that insulated the Net-Zero House.
“You’ve hit on one of the most important and most confusing issues in residential construction these days: the behavior of water vapor as it moves through a house. The big concern in cold climates like ours is how to prevent vapor warmed in the interior of the house from reaching a cold surface and condensing. When that happens, you’re likely to have mold, rot, and poor air quality.
“Plastic-sheet vapor barriers are often installed near the warm side of the building, as you suggest, to keep water vapor from diffusing through air-permeable insulation and contacting the cold surfaces closest to the outside of the house envelope. Fiberglass, cellulose, and mineral wool are examples of air-permeable insulations.
“Plastic wrap is not the only way to deter condensation, however. Thanks to careful scientific studies of different insulation strategies, the International Residential Code (IRC) provides architects, builders, and installers like me with other options, all of which result in dry, healthy interiors, if done properly. The options, which vary among the eight climate zones in the United States, are spelled out in the IRC’s Table R806.5, in case you’re interested.
“For the Net-Zero House, in climate zone 5, we chose to lay 5-inch-thick panels of mineral wool on top of the roof sheathing, for a total R-value of 20. According to the code, that’s enough to keep the roof sheathing warmer than the dew point—the temperature where water vapor condenses into liquid water. Then we applied a 2-inch-thick layer of closed-cell foam to the underside of the sheathing, followed by 7 ½ inches of open-cell foam. Both types of foam are air impermeable, as are rigid foam panels. So yes, the closed-cell foam does form a vapor barrier, but its main purpose, in combination with the open-cell foam, is to pack enough insulation in the rafter bays to meet the code’s R-49 minimum requirement for climate zone 5 ceilings.
“Even if we hadn’t used rigid mineral-wool panels, and all the insulation had been under the sheathing—the typical scenario in retrofit situations—I still would have applied the closed-cell foam first, directly to the sheathing and rafters, followed by the open cell. That closed-cell layer would have to be roughly 3 inches thick—about R-20—to minimize the risk of condensation forming in the open-cell insulation. In any case, closed cell needs a firm substrate to bond to, and open-cell foam’s spongy, angel-food-cake consistency doesn’t offer that.
“By the way, condensation is also an issue in warm, wet climates where air-conditioning is heavily used, but there the cold surfaces are inside the house. That’s a discussion for another time.
Thanks to: Tom Kelly, Ecologic Insulation, Tiverton, RI