jquery slideUp/ slideDown xui equivalent

Question

Hello im trying to create the same effect of jQuery's slideUp and slideDown function with [xui]http://xuijs.com)

so far i have created the fade effect

xui.extend({
           fadeOut:function(dur, callback) {       
           this.tween({opacity:'0',duration:dur}, callback);
       },

           fadeIn:function(dur, callback) {       
           this.tween({opacity:'1',duration:dur}, callback);
       }
});

which has been successful:)

But I am trying to master the slideUp/Down but have no luck. I have tried:

   slideUp:function(dur, callback) {       
       this.tween({height:'0',duration:dur}, callback);
   }

which works on elements without padding and a border. Is there anybody who can help me solve this problem any advice will be greatly appreciated :)