R16/18 Insulated Garage Door?

[athuswal]

I've been looking at insulated garage doors on-line and at the local HD and I don't get how the units on-line can be R16 and R18? I don't know how to make a 2x4 stud wall anything more than R14 so how do they get an even bigger R value in a garage door? The HD only has doors that are R6.5 and R9 (if I recall correctly). Also, if you have any recommendations for door manufacturers I'd appreciate knowing that too. Thanks!
Andrew

[jbergstrom]

They use foam not fiberglass in the core of the door...

Foams have a different R rating than fiberglass.

[wbrooks]

Extruded polystyrene (pink foam) 3.5" = R17.5 but I have never seen a 3.5" thick door. I have a Clopay 2", it is a decent door and comes with good weather stripping. Home Hardware also has a good 2" (much cheaper than Clopay) door but you need to toss the weather stripping and buy better.