Seal the following areas:

• Around the plumbing stack and any other pipes. For plastic plumbing stacks that move up and down due to thermal expansion, use a flexible gasket or a polyethylene sleeve sealed to a plywood collar (see Figure 5-6).
• Around wires or ceiling light fixtures that penetrate the attic floor and partition and exterior wall top plates (see Figure 5-7 and Figure 5-8.) See Fire and other hazards in attics, on how to deal with recessed lights.
• Around ducting that enters the attic from inside the house. Seal duct joints with aluminum duct tape or paint or mastic. Seal the gaps where ducts penetrate the ceiling. The ducts should stay below the insulation or should be wrapped with insulation and be protected from being crushed. Exhaust fans should always discharge to the outside but not directly below the eave vents.
• At the junction of the ceiling and interior wall partitions, pull back the insulation to locate and caulk cracks along interior walls (see Figure 4-4).
• At the top of interior and exterior walls, check to see if all wall cavities are blocked from the attic (usually by a top plate). Block any open spaces in the exposed cavity with a piece of rigid board insulation and caulking. If the top plate is cracked or poorly fitted, use caulking and polyethylene to create a tight seal (see Figure 5-7).
• Around attic hatches (see Figure 4-7).
• Around the chimney. Building codes require that air spaces between chimneys and floor or ceiling assemblies through which they pass be sealed with a non-combustible fire stop (see Figure 5-3 and Figure 5-4).
• Along the edge of shared walls. There is often a gap between the party wall (i.e. the shared wall between units in duplexes, triplexes and row houses) and the edge of the attic floor. Ensure that this gap is well sealed. If you suspect that air is leaking up and out of a party wall made of concrete blocks, hire a contractor to seal the blocks.