Sticky navigation element jumps during scroll

You need to have a placeholder when your nav goes from relative to fixed. Therefore you need to make a new div.

jQuery(".nav").wrap('<div class="nav-placeholder"></div>');
    jQuery(".nav-placeholder").height(jQuery(".nav").outerHeight());


    jQuery(".nav").wrapInner('<div class="nav-inner"></div>');

Remember to change ".nav", "nav-inner" and "nav-placeholder" to your desire.

For a fully functional sticky nav, check my website: http://www.swegre.se/

Made it this way now: Added an element before the nav:

<div class="nav-placeholder"></div>

And the jquery:

<script type="text/javascript">
    $(document).on("scroll",function(){
        if($(document).scrollTop()>150){
            $(".nav-placeholder").height($(".nav").outerHeight());
        } else {
            $(".nav-placeholder").height(0);
        }
    });
</script>

When I scroll down to 150 the placeholder gets the height of the nav, when i scroll up again I set it's height to 0.

Here is a fiddle: https://jsfiddle.net/herrfischerhamburg/562wu62y/

After seeing you asking for help on another answer, I will try and explain more clearly for you.

The Problem

Your problem is when you add position:fixed to the navigation bar, it removes it from its place and sticks it at the top of the page. This is why the rest of your content jumps up - because the navigation bar is not where it was anymore.

How To Fix

You can get around this by wrapping your navigation element in a new div - let's call it nav-wrapper - and set its height to the same as your navigation element. These are known as placeholder elements. This new wrapper and your original navigation bar must always be the same height for the 'jump' to disappear.

<div class="nav-wrapper" style="height:80px;"> <-- add this

    <div class="your-original-nav" style="height:80px"></div>

</div> <!-- add this

Now, when you set the navigation bar to fixed and it disappears to the top, the new wrapper we created with the same height keeps the page's content the same. When the fixed class has been removed, it sits back in the wrapper again, without pushing the content down.

A Suggestion

From what I can see on your site, there will be a big gap where the navigation bar was until the new fixed navigation reaches that point and covers it. What you want, is a little jQuery to figure out where to make the navigation fixed and where to hide it. I'll explain:

// cache the element
var $navBar = $('.your-original-nav');

// find original navigation bar position
var navPos = $navBar.offset().top;

// on scroll
$(window).scroll(function() {

    // get scroll position from top of the page
    var scrollPos = $(this).scrollTop();

    // check if scroll position is >= the nav position
    if (scrollPos >= navPos) {
        $navBar.addClass('fixed');
    } else {
        $navBar.removeClass('fixed');
    }

});

You may want to add further functionality to this example, as it is very, very basic. You would probably want to recalculate the offsets on window resize as one addition.

A Demo

This is a little demo which might help you - I was bored and feeling helpful :)