Congress finally drove a stake in the heart of private mortgage insurance policies that never die - the bane of buyers who put down less than 20 percent on a home.

But the new law requiring lenders to eventually cancel PMI doesn't give much extra ammunition to alert homeowners who already knew that they should ask their lender to drop the insurance against default once their equity reached 20 percent.You've always been able to ask your lender to drop PMI, either because your payments have raised equity to the 20 percent mark or because your home's value has risen. The legislation requires lenders to cancel PMI automatically when your equity reaches 22 percent (based on regularly scheduled monthly payments). That may take nearly 10 years with a 10 percent down payment and about 12 years if you put only 5 percent down.

The lender may still demand that you pay for an appraisal to verify that the home's value has not declined since you took out the loan.

Even for the highest-risk, slowest-paying homeowner, however, PMI will be dropped automatically halfway through the loan term. That's after 15 years of payments on a 30-year mortgage - longer than most people ever keep one mortgage.

The new law will cover mortgages taken out starting next summer, but lenders are required to start sending current customers annual notices of their cancellation rights within the next few months.

There's no need to wait, however. If your monthly payments plus real estate appreciation have boosted your home equity to at least 20 percent, write to your lender and ask that it cancel the PMI. Some investors in mortgages will still refuse to drop PMI on their mortgages until the law forces them to. Expect to pay $250 to $300 for an appraisal.