I've tried various goggles (some with more ventilation than others) and resorted to swimming goggles, but they all steam up after 60 seconds at the most. Maybe I sweat too much(!), but whatever I try doesn't seem to be very effective (i.e. licking the lenses, rubbing with a potato, etc.)
They make a spray for Hockey visors that prevents them from fogging up; I have never tried it on goggles, but it should work on goggles too. It always kept my visor fog free, and I was always hot and sweaty playing hockey. You should be able to find it at your local sporting goods store.
Actually I can't believe I have never tried this, I'm going to spray my safety glasses right now.
I've never had much luck with goggles. I just use the regular safety glasses that look like sunglasses. Should provide enough eye protection for doing fiberglass insulation anyways.