Hi Bob,

An electrician drilled a one-inch diameter hole in my house's rim joist
while adding a new circuit. A half inch hole is sufficient to pull the
romex through but a larger hole is more convenient for him to insert a
conduit fitting into the sheet rock at the end of the hole.
Would this one inch hole significantly weaken the rim joist?

A one inch hole will not be a problem.

There are rules how big holes and notches can be in normal floor joists,
based on the size of the joist and where the hole is placed. This is to
ensure the joist is not weakened for carrying a load across a span.

However, a rim joist is generally supported along it's entire length,
either by the foundation/sill or the first floor walls below. Unless you
have a large point load, like a bearing post directly above, you can make
just about any size hole in the rim joist.

Of course, structure is good, so minimize holes and notches when possible.

Should I fill it with epoxy or something?

If there is more than 1/4" space in the hole, I would probably shoot in
some expanding foam, or push in a bit of foam backer rod (available at any
home center in many sizes). Then seal the exterior opening with a good
quality polyurethane caulk (exterior rated).

Of course, for a single small hole like this, caulk alone would probably be
all you need.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com