Mortgage Prepayment as Investment: Another Look
November 3, 2003, Revised July 21, 2009, Reviewed August 28, 2011

"Should not your analysis of when mortgage repayment is a good investment distinguish between two different sets of circumstances? One is where you pay off the mortgage in one fell swoop by liquidating assets. The other is where you allocate surplus income each month between mortgage prepayment and asset acquisition."

Yes, it should, I have been remiss. This article will make the distinction.

Mortgage Repayment as an Investment


Repaying a mortgage is an investment that yields a return equal to the mortgage interest rate. The portion of the monthly payment that consists of principal is such an investment, but it is required. Additional payments, including repayment of the entire balance, are discretionary. In deciding whether to make them, the borrower should compare the mortgage rate with the yield available on other investments.

It doesn't matter whether the comparison is made before-tax or after-tax. If you are comparing repayment of a 6% mortgage with acquisition of a 5% bond, for example, the before-tax comparison is 6% versus 5%. The after-tax comparison, assuming the borrower is in the 40% tax bracket, is 3.6% versus 3%. If mortgage repayment earns the higher return before-tax, it also earns the higher return after-tax.

What I failed to do in previous articles on this subject is distinguish two quite different situations: case 1 involves investing cash flows over an indefinite future period; case 2 involves investing a lump sum right now.

Investing Cash Flows


A young homeowner is most likely to be faced by the question of how to allocate excess current cash flow. Should the cash be used to make additional payments to mortgage principal, or to acquire financial assets?

The answer is that the owner should allocate excess cash flow to mortgage repayment if the mortgage rate is higher than the rate that can be earned that month on newly acquired financial assets. The owner confronts a new investment decision every month, and the action taken could change from one month to the next.

For example, the borrower with a 6% mortgage who has excess cash flow would do well to use it to pay down the mortgage balance if the alternative is investment in assets that yield 2%. But if two years down the road the same assets yield 7%, the borrower can stop allocating excess cash flow to the mortgage and start accumulating financial assets.

Investing a Lump Sum


A senior is most likely to be faced with the question of whether to liquidate financial assets in order to repay the entire loan balance. The owner makes a single investment decision that is irrevocable. Either the assets are liquidated to pay off the mortgage, or they aren’t.

While the principle, that the decision should be based on comparison of the mortgage rate and the investment rate, is the same, the borrower can’t adjust to future changes in the investment rate. He has to look ahead and anticipate what these changes might be. He also has to anticipate how long he will be around.

To help deal with this problem, I developed a spreadsheet which allows a borrower to enter any scenario for future interest rates, and compare his wealth in every future month in the two cases: where he liquidates his assets to repay the mortgage at the outset, and where he retains both the mortgage and the assets. To access the spreadsheet, click here.

For example, assume the mortgage rate is 5% while the current investment rate is 3%, but the borrower assumes that in two years it will jump from 3% to 7%, and stay there. The spreadsheet shows that for the first 51 months, the borrower’s wealth would be greater in the case where he repaid the mortgage. After 51 months, his wealth is greater in the case where he didn’t. The borrower then must decide whether he is likely to be around for more than 51 months.

In general, the sooner that interest rates increase, the larger the increase when it happens, and the longer the borrower expects to live, the weaker the case for liquidating assets to pay off the mortgage. Seniors having to make this decision may find it instructive to play with the spreadsheet.
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