Where Not to Caulk
Don't caulk joints that aren't affected by water and won't mar the
appearance of the final paint job. Two examples are horizontal joints
under lap siding (Photo 5) and the tops of drip caps over windows and
doors where flashing carries water to the exterior. Joints protected
by roof overhangs don't need caulk either.
In general, don't use exterior caulk as a way to make your home
more airtight for better energy efficiency. Moisture generated inside
must have ways to get out. If you seal your home in an airtight skin,
you'll trap that moisture in the walls, especially in cold weather, and
it'll soak the wood, cause rot and lift the exterior paint.
Still, a good caulking job will make your home more airtight. So
afterward, check your gas- or oil-burning appliances to make sure
they continue to vent waste gases properly.