At 30,000 feet, residential and commercial mortgage backed securities (RMBS and CMBS) look like they come from the same assembly line. Both start with mortgages, both go through the same securitization machinery—pooling loans, creating tranches, issuing through a trust. But when you get into the actual business of valuing these things, they behave like two entirely different species.
Let’s start with RMBS. These are your residential deals—pools of home loans spread out across the country. For agency RMBS backed by Fannie or Freddie, credit risk is mostly off the table, thanks to that government wrap. But prepayment? That’s the real issue. When rates fall, homeowners rush to refinance. That pulls principal back early, cuts into expected interest payments, and changes the timing of cash flows. If you’re an analyst, you spend a lot of time trying to guess how fast people will prepay. PSA speeds become your best friend, and you start building models to see how different rate paths affect average life and yield. And if you’re in the non-agency corner—jumbos, subprime, alt-A—you suddenly care a lot more about borrower credit quality, regional home prices, and default timelines. You build out loss curves, plug in FICO bands, and maybe even tie in housing price indices by zip code. But even then, it still all starts with rate moves and how homeowners react to them.
CMBSs flip the script. These are commercial loans—big properties like office buildings, malls, warehouses. There are fewer loans per deal, but they’re chunky, and when one goes bad, it hurts. The key here isn’t prepayment—it’s cash flow stability. Most CMBS assets have some kind of prepayment lockout or penalty baked in, so you’re not modeling optionality as much as you’re watching DSCR levels and lease rolls. You care about tenants leaving, rents dropping, cap rates shifting. That means modeling occupancy, lease expirations, and how the property performs in different macro conditions. You might have a hotel that looks great today but collapses in a recession. You might have an office building with an anchor tenant about to walk. Suddenly you’re knee-deep in special servicing scenarios and trying to guess how much value survives after a workout.
In practice, RMBS analysts build out prepayment models and apply them across thousands of similar loans. It’s a statistics game. CMBS analysts are often building out a property-by-property story. You’re running cash flow projections at the building level, plugging in assumptions about rent growth, market vacancy, and re-leasing costs. One is driven by rates and borrower behavior; the other is driven by property economics and sponsor quality.
The tranching structure—senior AAA down to equity—is similar on the surface. But the cashflow waterfalls behave differently. In RMBS, prepayments flow through fast and can be uneven. In CMBS, the payments tend to be steadier until something breaks. That makes the tail risks different—and how you discount those cash flows changes too.
So, while both RMBS and CMBS live under the MBS umbrella, analysts know better than to treat them the same. Getting valuation right means understanding what drives the cash flows—and what can derail them. In RMBS, it’s the speed of refinancing. In CMBS, it’s the durability of rent. Mix those up, and you’re not just off by a few basis points. You’re in the wrong model entirely.