Thanks for all your replies... I think what I will do is to install some wireless thermometers as suggested by a couple of you, and I'll wait until summer before taking any action except data collection. I will also assess the amount of insulation. It's tough because the roof is not steeply pitched and so it's very tight and unpleasant up there, but I should at least go up there once to make sure there's nothing bad going on (mold, rot, bodies...). If the temperature measurements tell the story I expect them to, then I'll resume with the phone calls to contractors until I find one who seems to know what he's talking about. I'll try insulation contractors and general contractors as well as roofers.