[jQuery] accessing offsetHeight

I'm new to Javascript and jQuery, so I'm sorry if this is basic
stuff.  How do I access offsetHeight for an element?  This is what I
have, but offsetHeight comes up undefined:

        var headerElem = $("#header");
        var headerHeight = headerElem.offsetHeight;

whereas this works:
     var docHeader = document.getElementById("header");
     headerHeight = docHeader.offsetHeight;


Thanks --MERC