Here are some important air sealing locations for building a tight energy efficient home, if you choose to do all of these, you should seriously contemplate a Heat Recovery Ventilator  (HRV) for comprehensive Mechanical Ventilation, superior indoor air quality, and moisture control.

During framing:

• Use sill seal between foundation and mud-sill; caulk framing to concrete where sill seal is not tight to both the concrete and the stud wall (such as a step up in the foundation wall.)
• Seal rim-band to mud sill with caulk or minimally expanding foam.
• Seal all plumbing and electrical penetrations in rim bands with minimally expanding foam or fire-rated caulk.
• Seal rim-bands to subfloor and subfloor to bottom plate with caulk or minimally expanding foam.
• Foam rough openings for windows and doors.  A small bead of minimally-expanding foam will fill the gap without distorting the frame.
• Use rigid draftstop over soffits and behind soffit footprint on outside wall.
• Walls balloon framed into the attic require sealed draftstop at ceiling plane.
• Tongue and groove ceilings require sealed rigid air barrier

During drywalling:

• Seal bottom of drywall to subfloor.
• Foam or caulk electric boxes to drywall.
• Install rigid air barrier (sheetmetal, plywood, drywall or other) behind tub and fireplace enclosures.

Before attic insulation:

• Foam all penetrations in the drywall including: electric boxes, smoke alarms, bath fans, duct boots, and recessed lights.  For recessed lights use fixtures that comply with the WashingtonState air leakage standard.
• Cover service chases with sheetmetal, rigid foam insulation, drywall or plywood.  Use foam or caulk to join everything into an airtight lid on the chase.
• Seal holes in top plates (such as those for wires or plumbing vents).
• Foam top plates to ceiling sheetrock.
• Seal the attic hatch with caulk on inside and outside of trim, or with gasket on backside of trim.