index

<div class="row">
  <div class="large-8 column">
    <div class="main">My Main</div>
  </div>
  <div class="large-4 column full-height">
    <aside class="sidebar">
      My Sidebar
    </aside>
  </div>
</div>

script

$(".full-height").height($(".main").parent().height()); 

style

.full-height { display: table;  }

.sidebar { 
  display: table-cell; 
  background: #eeeeee;  
}