page of posts sort by custom field

Found this simple line of code to display posts from a single category:

query_posts( array ( 'category_name' => 'mycategory', 'posts_per_page' => -1 ) );

this sorts on title by default. I want the found records to be sorted by one custom field named ‘zipcode’. Did a lot of digging but it’s getting more and more complex for me. Anyone for a simple solution.
thank you.