I have a flat roof house and don't want to tear out my ceilings or roof but am considering installing some sort of insulation because my house temp is so uncomfortable. I've seen that there is spray foam that can go over existing roofing, roofing systems made of foam board, spray in cellulose which goes in the space between the ceiling and roof (though i'm not sure how to get it in there w/o removing roof or ceiling), and i think that's it. What is the least expensive and what would the cost be approx. for a 1100 sq ft home? eh?