There is not enough detail in your question.
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Bob Tranchell Total Mortgage Services LLC (508) 367-5731 cell
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One of the issues though is the second mortgage typically being a Home Equity Line of Credit. A HELOC is basically a credit card with a term limit. It has an interest only payment which makes the payment of the two loans favorable to one loan with PMI. The danger is in only paying the interest only payment which means your loan balance never decreases.
In the past most who went the two loan (to avoid PMI) route could epxect their home value to grow over a couple years and eventually consolidate the two loans together with no PMI. These days, home values are not growing as fast.
PMI factors/rates have come down a bit in the past year or two. There is also the option of a single premium PMI where the cost is built into the rate. I would suggest looking at all three:
1. two loans to avoid PMI
2 One loan with PMI
3. One loan with PMI built in (also know as Lender Paid PMI).
its easy enough to get figures.
rshapiro at assetmortgage dot net