IRS interest rate question

[five2one]

We are finally coming down to the wire for our refi/foreclosure/IRS mess. Next week, the IRS consumer advocate is supposed to have a response for me regarding our tax lien (long story short: timely mailed is timely filed, have proof, IRS still charging HUGE penalty for filing late, despite copy of metered envelope). IF the lien is lifted and we finish the refi with no more problems, what is the interest rate the IRS charges? I'm trying to decide where the money would be most wisely placed to save us the most in interest dollars and I can't seem to get a layman's answer to the interest rate question.