Don't become overly concerned over this issue. The cost vs effect isn't there. What it takes to seal it properly cost more than the energy save to a point that your better off to put the money in the bank & collect interest). Also, there isn't a structural concern. Your sill isn't going to rot out from the condensation (Even frost) produced. When it warms up, it dissipates. If it was a problem, Virtually every house built with a sill would be suffering collapse.
If you neatly install faced fiberglass, although you wil see some condensation, it's nothing to worry over.