Most wall cavities are composed of a horizontal top plate, a horizontal bottom plate, and the vertical studs. When gypsum board and wall sheathing are caulked and fastened to the inside and exterior faces of these components, they create an air-tight six-sided wall cavity. If one element is missing from this assembly or is not adequately air sealed to the other elements, air will flow through the cavity, robbing any insulation present of its insulating value. Some wall designs have no top plate so the wall cavity is open to the area above, which may be an unconditioned attic. This opening can become a pathway for allowing unconditioned air from the attic to flow down into the wall cavity and conditioned air from the wall to flow up into the attic. The result is unwanted heat loss or heat gain, cold spots in walls, and an increased potential for moisture problems in the wall or attic.
Missing top plates can sometimes occur when a room of one ceiling height abuts a room of a taller ceiling height. They can also occur when buildings are designed with balloon framing. Designers should not specify balloon framing. If the house design includes varying ceiling heights, blocking should be specified where the top of the lower wall meets the side of the higher wall if no top plate is present. This blocking material could be rigid foam, plywood, OSB, or lumber that is cut to fit. Alternately, the open stud cavities can be filled with fiberglass batting that is rolled and tucked into the cavity opening then covered with spray foam.
How to Air Seal a Wall with a Missing Top Plate
- dentify missing top plates in adjoining walls with different ceiling heights.
- Select a rigid air-blocking material (rigid foam insulation, plywood, OSB, lumber). Cut into pieces to fit each stud bay. Wrap a thin piece of strapping around the board to hold it in position while you glue each end with caulk or spray foam. Pull the strap out and glue the remaining two sides.
- 3. Or, roll a piece of unfaced fiberglass batt insulation for each cavity. Pressure fit the fiberglass batt roll into the top of the stud cavity. Cover the top with spray foam to air seal the roll and hold it in place.
Figure 1 - Ceiling heights may vary within a house design, for example, the ceiling in a hallway or bathroom may be lower than the ceiling in an adjoining dining room or bedroom. Where a lower ceiling meets the wall of a room with a higher ceiling, the lower wall may be missing a top plate, creating an open air pathway from the stud cavities to the attic space.
Figure 2 - Top plates may be missing where a lower ceiling meets the wall of a room with a higher ceiling. The open wall cavities should be closed off with an air-blocking material like rigid foam, plywood, or dimensional lumber that is cut to fit the stud cavity and sealed in place with caulk or spray foam or fiberglass batt insulation that is rolled and friction fit into place and air sealed with spray foam.
How to Air Seal Open Wall Cavities in Balloon-Framed Walls
- Identify open wall cavities in balloon-framed walls. Note, balloon framed walls are walls that have no top plates so wall cavities are open from the bottom plate to the attic. This style of construction is not recommended.
- Roll a piece of fiberglass batt and stuff it into place at the top of the wall where the top plate is missing.
- Cover the roll of fiberglass batt with spray foam to air seal it in place.
- Fill the attic with additional insulation.
Figure 3 - Balloon framing at a gable end wall allows air to flow from the attic down into the wall cavity
Figure 4 - The space at the top of the wall can be filled with a piece of fiberglass batt that is rolled up and stuffed in place
Figure 5 - The fiberglass roll is covered with spray foam to air seal the top of the wall. The top plate of the wall under the rafter can also be spray foamed between the ceiling gypsum and the bottom side of the rafter insulation baffle. Then the area can be covered with blown insulation.