I am installing a bathroom exhaust fan in a portion of the house which is a small addition to the original house. The attic above the addition is separated from the rest of the attic by the original roof (though portions were cut out for the A/C duct and wiring). I know that soffit vents are not typically recommended for bathroom exhaust fans, because roof vents cause air to be drawn into the soffit and exhaust from a soffit vent would get sucked into the attic. However, the attic above the addition does not have roof vents (thus air should not be sucked in through the soffits), so I am wondering whether it would be okay to use a soffit vent for the bathroom exhaust fan. I am in South Florida, where the temperature doesn't go below freezing in case that is relevant.
Avoiding penetrating the roof with a bath fan vent is the wrong disposition. Given current materials and techniques, a weather tight seal can be expected every time. And the trade-off is clearly on that side. Their just too many risks when dumping the vent fan anyplace but through the roof. Keep the run as short as possible and as vertical as possible. Happy venting.