steamed up windows

hey everybody! this is my first post and would love some advice. my 2002 yukon xl foggs up the windows in the rear when i have it full of people. it only does this with people in the back and no matter how the rear controls are set it will not clear up. is this normal? i live in a northern climate and its really bad the colder it gets. should i be concerned? this is the first yukon i have owned so i nothing to compare to. thanks alot.