You're right most of the time; the exception shows up when the cash you need is large relative to the balance you're protecting. The variable in play is that second mortgages carry meaningfully higher rates than first mortgages, so keeping the low first and adding a second isn't automatically the cheaper path. The ratio decides it. If you owe a large balance at a low first-mortgage rate and want a modest amount of cash, the blend wins easily. The big low-rate balance dominates, a modest higher-rate second barely moves your overall rate, and leaving the first alone clearly beats refinancing everything at current rates. Flip the proportions and the answer flips. If you owe a small amount at the low rate and want to pull out a large sum, the low-rate balance is too small to protect the blend, and the big new borrowing happens at a high second-mortgage rate either way. In that case a full cash-out refinance of the entire balance, all at one first-mortgage rate, can come out cheaper than keeping the small first and bolting on a large, higher-rate second. So there's no flat rule that the blended rate always wins. Run the blended-rate math on your actual numbers, and when the two paths land close, weigh the flexibility differences between them too.