It just drives me crazy! When you see news stories in the media about improving your home's energy efficiency, they invariably tell you to caulk your windows and weatherstrip your doors. You know, there are enough home energy raters and auditors now that we oughta be able to cure them of foisting this uneducated advice on homeowners, but it seems to be one of those self-perpetuating beliefs that resurrects itself every year and won't go away (kinda like Brett Favre).
See that photo above? That's just one small section of a gap that allows attic air to move right down into the walls of this house (or from the house into the attic). The whole thing is probably 20' long and adds up to many times more infiltration than this house (or just about any other) gets through all the windows or doors. The discolored insulation shows that air has been moving through that gap.
That's probably the biggest source of air leakage in this house, but there are plenty of others. The photo at right shows a gap between the top plate and ceiling drywall, and light comes straight through from the room below because that corner was never taped and mudded. Why? Because there's crown molding there, of course. Why waste labor and materials in a place where it's unnecessary?
Actually, even without crown molding, unsealed gaps at the top plates can produce a lot of air leakage in a house. When a HERS rater does a Blower Door test to determine the infiltration rate of a house, you can walk around and feel the air blowing through receptacles and switches - even on interior walls - because of those unsealed top plates.
So, before you go around caulking your windows and weatherstripping your doors, get up in your attic and seal the real leaks! And you definitely want to air seal the attic before you add insulation. Otherwise, you're covering the leaks with material that will simply filter the dirt but not stop the air movement.
What this homeowner did instead was to make those leaks irrelevant by using spray foam insulation on the roofline, as shown here. That made the ceiling an interior partition and brought the attic inside the building envelope so it's part of the conditioned space (although it's only conditioned indirectly).
The homeowner in this case was Jason Payne of Structured Energies, and he paid for the work by refinancing his home with an Energy Efficient Mortgage. He now has a much tighter, better insulated home and pays less than he did before in monthly mortgage payments and energy bills.