Sealing Side Attics
To insulate a side attic, most builders would wait until framing was complete, and then hang insulation in the kneewall just as if it were on the outside wall of the house. That makes the kneewall the building's thermal barrier and the place to stop heat loss and air leakage. However, most builders don't realize that the kneewall sits above a king-size air leak. Indoor air can freely travel through the space between floor joists to the side attic where it escapes through attic vents.
The drawings on this page show four ways to insulate and air seal a side attic.
Blocking Between Joists
Placing a barrier in the joist cavity directly below the kneewall will block air flow to the side attic. This barrier can be solid wood blocking, a piece of rigid insulation or a sheet of coated cardboard, such as an attic vent baffle. For all materials, you must run a bead of caulk around the barrier material to adequately seal it against air leakage. Additional caulking and gaskets continue the air barrier down along the interior of the building.
Extended Floor Sheathing
In a slight variation of the typical side attic, floor sheathing on the second story runs all the way to the perimeter wall, instead of stopping at the kneewall. Caulk the gypsum board on the kneewall to the floor sheathing and the floor sheathing to the rim joist. This carries the air barrier to the outside of wall of the building. Insulation can be installed on top of the floor sheathing if you need to achieve a greater R-value.
Rafter Insulation with Interior Wall Air Barrier
Insulation and air sealing can be simplified by moving the thermal boundary to the rafters. Place cavity insulation between the rafters and attach rigid insulation to the face of the rafters. The rigid insulation runs all the way to the outside walls. All joints are carefully sealed with contractors tape that is compatible with the foam. A bead of sealant connects the air barrier of the roof to the wall to maintain continuity. This approach is simpler to build and provides conditioned storage behind the kneewalls.
Rafter Insulation with Exterior Wall Air Barrier
An exterior air barrier adds one more simplifying feature. Rigid insulation on the outside of the first floor wall is sealed with tape to form the wall's air barrier. The insulated sheathing is sealed to wall framing at every stud and plate with a continuous bead of construction adhesive or caulk. Although rigid insulation is impermeable to moisture, this is not a "cold side vapor retarder." The insulating value of the foam board protects against condensation by raising the temperature of the wall cavity.
This article appeared in Energy Source Builder #51 June 1997,