Interest Rate Differential on Mortgages

When homeowners try to break a fixed-rate mortgage, they can be blind-sided by a whopping interest rate differential (IRD) penalty. It’s important to understand what you are agreeing to when you sign for a mortgage.

Periodically, your bank will send you a statement telling you what your current mortgage balance is, but this balance is not an accurate reflection of your obligation to the bank. To understand this, consider the following fictitious example.

Bob and Sue Ward bought a house a year ago and got a $250,000 mortgage with a 5-year fixed term at 6% interest. With a 25-year amortization, the payments were $1599.52 per month. After 5 years, the remaining mortgage balance would be $224,592.

Sue’s job is moving to another city and the Wards have now sold their house. Their latest statement says that the remaining mortgage balance is $245,501. However, this figure has no connection to the Wards’ actual obligations.

When they signed for their mortgage a year ago, the Wards agreed to make 60 monthly payments of $1599.52 and a lump sum of $224,592 after 5 years. A year into the mortgage, their obligation is 48 more monthly payments plus this lump sum.

The remaining balance shown on their mortgage statement assumes that interest rates have remained the same. But, what if rates have dropped? Suppose that the current rate for a 4-year term is down to 4%. This means that their supposed mortgage balance is a meaningless number. What the Wards really owe is 4 years of monthly payments of $1599.52 plus the lump sum.

To figure out what the Ward’s really owe, we need to find the mortgage balance that would give a monthly payment of $1599.52 and a lump sum after 4 years of $245,501 based on 4% interest. Their debt turns out to be $262,574! This is $17,073 more than the amount on their mortgage statement.

Such a large interest rate differential penalty can come as quite a shock. The Wards didn’t realize it, but when they signed for their mortgage, they were betting against an interest rate drop and lost this bet.

There are a number of potential complications when calculating an actual interest rate differential (IRD). Prepayment privileges, like doubled payments or yearly lump-sums, should lower the IRD (but may not in practice). There are various ways of estimating the IRD, and most of these overstate the amount you get using the proper calculation.

For example, one might say that the bank will be missing out on 2% interest for 4 years on an average balance of about $235,000. This works out to a penalty of $18,800. This is $1727 too much. The problem with this estimate is that it takes future amounts of bank losses and doesn’t discount them before collecting them in the present.

An important input into an IRD calculation is the currently prevailing mortgage rate.  Some banks play games with posted rates to come up with a lower currently prevailing mortgage rate.  This increases the size of the IRD they calculate.

Before taking on a fixed-rate mortgage, it's important to consider the possibility of having to break that mortgage, and to understand how much the lender would charge as an IRD.  Different lenders have very different IRD calculation rules.