different goggle models have different designs to get air behind the lens, I guess neutralizing with outside air and reducing fogging. check if your intake vents on the goggle are interfacing well with the helmet. Smith helmet / goggle systems do this admirably, as do others I'm sure.
A check you can do. Use a simple anti-fog cloth or comparable. Then try to fog the inside of the lens by breathing on it while it's a little cold. like you do to clean your regular sungasses or reading glasses. If you can't make the lens fog that way, you're good to go. If it still fogs, you have a serious moisture and air flow issue.
This week I have noticed many people skiing with their goggles up on their helmet, so the problem is widespread I'd say, and really messes with one's skiing. gaper? can't go there.