I ride in shells and I'm fine down to 20 at night with just a base layer and some b-ball shorts on. It's mostly important to keep your upper warm. Your legs are putting a ton of heat anyways since that is what you are using while boarding.
Then again, my socks come about to my knee pads and my knee pads come about to my underwear so I have two layers on my whole bottom half.
2011 Smokin Buck Ferton 155
2013 Flow NX2-SE
2013 Flow Hylite boa
Burton Stagger pants/jacket