Conventional platform framing with insulation bats filling the open spaces between the 2 × 4 stud framing leaves multiple thermal bridges through the wood framing to the outside (Figure 17.1). Each stud reaches all the way through the wall. The framing around windows and doors reaches all the way through the wall. The sill plates reach all the way through the walls and often insulation is not placed against the band joists. Ceilings are usually insulated at a higher level than floors and walls. The heat losses from a typical house are as follows: infiltration, 26 percent, floors, 28 percent, windows, 20 percent, walls, 14 percent, roof, 11 percent, and doors, 1 percent (Nisson and Gautam 1985, 38).
The framing around doors and windows provide air gaps between the inside of the house and the outside. These gaps, especially the gaps between the rough framing and window and door frames, need to be filled with sealant to minimize air leakage. It is also important to put a bead of sealant on sill plates to limit air leakage. The most overlooked air leakage problems are openings through the ceiling into the attic. Recessed light fixtures, attic access hatches, pluming pipe and HVAC vents need to be sealed. The ceiling surface should be extended from plate to plate with any dropped soffits added later to avoid a large heat loss path to the attic.
Air infiltration heat loss in a well-insulated and sealed house causes about one quarter of the heating needs. Infiltration heat losses come from the following places (Figure 17.2): sills at the ceiling and floor, 31 percent, HVAC, 15 percent, fireplace, 14 percent, pipes, 13 percent, doors, 11 percent, windows, 10 percent, vents, 4 percent, and electrical outlets, 2 percent (Nisson and Gautam 1985, 42). Note how much air infiltration happens at the sill plates where the wall meets the floor and the ceiling. Also note how much air leakage happens through a fireplace even with the flue closed. If one leaves the flue open the air leakage will increase. The air leakage through a fireplace dramatically increases when there is a fire in an open fireplace. The fire creates a thermal draft drawing heating system warmed air out of the house which will suck outside cold air in through cracks around windows, doors and through sill plates. A fireplace with glass doors, access to outside combustion air, and a path for room air to circulate behind the steel fire box and back into the room will add heat to the room.
