I assume when you say R25 you are referring to fiberglass batts. The sill/foundation area typically allows alot of air flow. In this situation I would consider an application of spray foam. Expanding foam does alot better job sealing cracks and crevices than fiberglass does. Keep in mind that a well sealed basement or crawlspace may allow previously unnoticed mosture problems to come to light and some type of controlled ventilation may become necessary which is not a bad thing.