Q: I was recently asked about finishing a basement in a home located in a very cold climate. Plans would include cutting a doorway in the foundation for a walk out, with the grade retained on both sides of the opening. Is there a way to protect the footing and foundation from frost using exterior insulation once the opening is cut?
A: Steve Baczek, a residential architect from Reading, Mass., who specializes in building science, responds: The detail you would need to protect the door exit is not unlike a frost-protected shallow foundation. In this case, you’d have to excavate and insulate the foundation and the footing below the doorway. But one of my building mantras is “Don’t do anything stupid.” Creating a walk-out basement in the situation you describe gets pretty close. Here’s why: The insulated foundation and footing at the doorway would rely on heat loss from the house through the basement slab to keep it above freezing. So while gaining the convenience of a walk-out basement, the client would be incurring the cost of heating the outside ground at the door location.
That being said, I’d wrap the foundation and footing below the opening with a minimum of R-20 rigid insulation (a double layer of 2-inch foam), letting the foam extend outward horizontally at least 4 feet from the foundation. (Check out “Revised Builder’s Guide to Frost Protected Shallow Foundations,” at toolbase.org. Use the Air Freezing Index to determine the exact thickness and horizontal distance of the insulation.) Put the top of the horizontal insulation at least 12 inches below grade and extend it to either side of the doorway until it is into the grade at least 12 inches. The insulation below the door opening will break grade and require a protective covering. Or you could run the walkway over to the foundation and sacrifice the top inch or so of insulation, but that would make the whole system less effective.
Credit: Tim Healey