Don't know what type of boat you have or how much room, but on BR we have two layers of 1" styrofoam boards cut to fit between structural members. It works well -- even in the tropics the cabin will be much cooler than the deck. (I think the white hull and deck helps a lot in that regard as well). My only concern with this form of insulation is for the day we have a fire as I know styrofoam puts out nasty, toxic smoke. With that constantly in the back of my mind I keep the fire extinguishers close at hand and hope for the best.
I should add that the advantage of this type of insulation is that it is easily removed and thus access to the underside of deck hardware is much easier. As BR is a steel boat, removable insulation makes keeping track of rust much easier as well.
Last edited by billyruffn; 01-26-2009 at 10:22 PM.