How to build a root cellar

We've been cleaning out the root cellar lately, using the last carrots, the sprouting potatoes, and jars of preserves.  It's getting too warm in the space, as we have it vented directly outside - i.e. this works perfectly in winter months to keep the room naturally cold, but as the air warms outdoors it's not so usable for longterm storage of root crops.  Last fall we installed this root cellar and cold room into out basement.  Unfortunately this house did not have an existing space set aside for winter food storage - we moved our washing machine upstairs (making it easier to reuse the greywater through a pond filtration system), and set aside about a third of the former laundry room as the root cellar, and another third as pantry shelving for preserves, canning supplies, jars, wine making equipment, etc.  The root cellar space was a very simple construction - insulated from the inside, then vented outdoors as mentioned, and closed with a solid, well insulated tightly sealing door.  So far so good.  Spring is the season to start thinking about new building projects for the coming months, so this may be one for your list.  Here is a great book we suggest as a resource -  Root Cellaring: Natural Cold Storage of Winter Fruits and Vegetables, by Mike Bubel.