$1.25-$1.50/sq ft of outside wall area (not house sq ft). I know you can buy fiberglas for $0.50/sf, but you still have to install it (they come out and spray it for you) which is never any fun.
It's an old house (1880s) with no vapor barrier, in order to have one I would have to tear out all of the siding (much of which is still in good shape, old cypress) and replace it. SO the foam also acts as a vapor barrier. And living in New Orleans, I've see what happens to fiberglas when it gets wet :O
Lastly, the foam actually adds structural integrity, something you can't have enough of in a hurricane prone area
They do make foam that you can apply with minimal sheetrock damage, but it's more expensive. The other route you can go is blown cellulose, which my old landlord did in my last apt. It works pretty well, but it tends to settle, so you lose insulation at the top, and rodents love to nest in it.
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