So Webflow can do this natively, but one must set it on each page individually - it cannot be configured site wide. However, the piece of javascript below allows it to be configured site-wide very quickly.
Check out the jsfiddle of it in action - Scroll Up to show Nav Bar (Jquery) - JSFiddle - Code Playground
var position = $(window).scrollTop();
$(window).scroll(function() {
var scroll = $(window).scrollTop();
if (scroll > position) {
$('.nav-bar').css('top', '-120px');
} else {
$('.nav-bar').css('top', '15px');
}
position = scroll;
});