A lot of enclosed buildings stay 38-40 all winter depending how tight they are, regardless of having insulation or not. I proved this to a few friends a couple years in a row with a thermometer, a glass of water and a 24' Baja that I didn't winterize. I did turn it over a few times because the impeller will bond itself to the housing. I think you're ok but I'd at least put a trouble light in the engine hatch for cold weather. Like I said, this depends how tight the building is!