If you don't have enough cash to put 20% down, or have weak credit, you'll need private mortgage insurance to buy a home.

The policy guarantees that your mortgage will be repaid if you default on the payments. Virtually every lender will insist on it.

But falling home prices and soaring foreclosure rates have all of the major mortgage insurers raising premiums and issuing new, more stringent qualifying standards.

That's wreaking havoc for many home buyers, Michelle Collins, director of mortgage lending for ShoreBank Corp. told the Wall Street Journal.

For a popular conventional loan package, "easily 70% of the previous set of borrowers will not be able to buy," she says.

The biggest insurer, for example, is now charging as much as 0.75% of the loan balance for fixed-rate, 30-year mortgages with a 10% down payment, up from 0.67% earlier this year.

It's also not uncommon for insurers to require a 10% down payment, a substantial increase from the 3% to 5% they used to accept.

Those costs can make a home more expensive than you might have expected, so make sure you know all the insurance you'll need before buying.

Use this calculator to determine how much home you can afford.