Insulation for Both Sides of the Sheathing
Insulation is a critical component to achieving net-zero energy.
The FHB Home uses a mix of fiberglass and mineral wool throughout the structure to achieve approximate R-values of R-60 (roof) and R-28 (walls). The wall cavities are filled with fiberglass batts, with rigid mineral-wool sheets on the exterior of the walls. The attic sports a hefty layer of blown fiberglass insulation, and the concrete walls of the basement were formed with insulated concrete forms (ICFs).
Part of the work to make the insulation successful was to place the air barrier at the outside face of the framing with Zip System sheathing and tape, using Titebond Greenchoice accoustical sealant and ProtectoWrap Triple Guard energy sill sealer. The attic will be a vented, unconditioned space, so the air barrier at the top of the house will be CertainTeed’s MemBrain sheeting at the second-floor ceiling. It will be attached to the bottom of the TJIs with 1×3 strapping installed perpendicular to the joists and seams sealed with a combination of sealant and tape. Builder Mike Guertin’s goal is to limit penetrations through the air barrier, and the 3/4-in. strapping plus 1/2-in. drywall creates enough space for low-profile junction boxes beneath the air barrier. The MemBrain will be installed before the partition walls are built. Then with the windows and doors installed, Mike will do a blower-door test and seal any leaks. Later, the ceiling joists will be buried with about 22 in. of blown fiberglass insulation. The MemBrain is similar in appearance to polyethylene, but it’s a polyamide film that is vapor variable.
When relative humidity is below 60%, it is about as permeable as polyethylene, but when relative humidity increases above 60%, the pore size of the material increases and enables vapor to pass through, allowing drying in both directions.
CertainTeed’s SmartBatts, rated R-21 at 5-1/2 in., insulate the 2×6 wall cavities and have a vapor-variable coating. The SmartBatt facing includes the kraft paper traditionally found on fiberglass batts. Typically, the vapor-retarding property of the facing comes from coating the back side of the paper with asphalt, which also adheres the paper to the batt. SmartBatts use about two-thirds less asphalt applied in stripes to adhere the paper.
A coating with the vapor-variable properties of MemBrain is applied to the outside of the kraft-paper facing. During winter, vapor drive is from the interior to the exterior, and the coating helps limit some of that moisture drive. When humidity levels rise above 60% in spring and summer, the pores in the facing open up, and vapor can migrate inward.
To boost the insulation level and prevent the framing members from forming a thermal bridge through the wall assembly, a layer of insulation will be installed on the outside of the sheathing. In this location, Mike will be using 2-in.-thick sheets of Roxul Comfortboard 80 IS, which will keep the sheathing above the dew point and should prevent condensation on the back side of the sheathing.
Roxul’s rigid mineral-wool insulation doesn’t have the global-warming potential of the spray agent in XPS, its performance doesn’t degrade as the temperature drops as polyisocyanurate’s does, and it is vapor open, so the wall can dry to the outside.
However, the semicompressible nature of mineral wool makes it a little more fickle to install than rigid foam. To support the siding, it needs to be installed with furring strips. That’s not a burden in this case because the FHB Home siding will be installed over a ventilated rain-screen assembly, which includes 1×3 strapping that is ideal for attaching the Roxul. Because the attachment point of the siding is cantilevered out from the sheathing by the thickness of the insulation, it’s important to verify that the fasteners and fastening schedule used for attaching the furring strips will support the siding. Because Roxul is compressible, driving the screws through the furring must be done with care to avoid overcompressing the sheets and introducing waves and dips in the siding.