How do non-agency RMBS differ from agency MBS in terms of credit risk, structure, and analysis?

Question

AcadiFi · Accepted Answer

Non-agency RMBS are mortgage-backed securities issued without government agency guarantees (no Ginnie Mae, Fannie Mae, or Freddie Mac backing). Investors bear full credit risk, making the analysis fundamentally different from agency MBS.

**Key Differences from Agency MBS**

| Feature | Agency MBS | Non-Agency RMBS |
|---|---|---|
| Credit risk | Guaranteed by agency | Borne by investors |
| Prepayment risk | Primary risk | Important but secondary to credit |
| Underlying loans | Conforming (meet agency guidelines) | Non-conforming: jumbo, alt-A, subprime |
| Structure | Pass-through or simple CMO | Senior/subordinate tranching |
| Spreads | Tight (near Treasuries) | Wide (credit + liquidity premium) |

**Credit Enhancement Structures**

Non-agency RMBS use multiple layers of protection:

1. **Subordination** — senior tranches are protected by junior tranches that absorb losses first
2. **Excess spread** — the difference between weighted average coupon on loans and coupon paid to bondholders creates a cash flow cushion
3. **Overcollateralization** — pool principal exceeds bond principal
4. **Reserve funds** — cash accounts set aside to cover losses

**Worked Example — Grandview Mortgage Trust 2025-1:**

| Tranche | Rating | % of Deal | Subordination Below |
|---|---|---|---|
| A-1 (Senior) | AAA | 75% | 25% |
| M-1 (Mezz) | AA | 10% | 15% |
| M-2 (Mezz) | A | 8% | 7% |
| B-1 (Sub) | BBB | 5% | 2% |
| B-2 (First loss) | NR | 2% | 0% |

For the AAA tranche to experience a loss, cumulative defaults (net of recoveries) must exceed 25% of the pool — a very severe scenario for a well-underwritten pool.

**Why 2008 Was Catastrophic:**

The pre-crisis non-agency market was dominated by subprime and alt-A loans with features that amplified losses:
- Teaser-rate ARMs that reset to unaffordable levels
- No-documentation ("liar") loans with inflated incomes
- High LTV ratios (100%+ with piggyback seconds)
- Concentrated geographic exposure to bubble markets
- Rating agency models dramatically underestimated correlated defaults

Actual loss rates reached 30-50% in subprime pools, blowing through all subordination levels and impacting even AAA-rated tranches.

**Modern Non-Agency Analysis:**

1. **Loan-level data** — analyze individual borrower FICO scores, LTVs, DTIs, and documentation type
2. **Prepayment modeling** — voluntary prepayments reduce credit exposure but also reduce excess spread
3. **Loss severity assumptions** — depend on property values, foreclosure timelines, and geographic concentration
4. **Structural waterfall analysis** — model how cash flows are allocated under various default and prepayment scenarios

Explore mortgage-backed securities in our CFA fixed income course.

How do non-agency RMBS differ from agency MBS in terms of credit risk, structure, and analysis?

Master Level II with our CFA Course

Related Questions

Practice Questions