-15 and here's why....the vents are in the perfect spot (can open and close at will also). I've sledded when it's been plus 30 and also minus 30 and the thing HAS NOT fogged up. My father-in-law (who used to sell sleds and parts) said that the HSO-classifieds
-15 is the first helmet he's ever gotten that doesn't fog up (beside electric shield). I'm a heavy breather and sweater and it's been GREAT for me. It's fairly cheap too. $125 I think or if you look around you can find a good deal. Check them out...they are really, really good.