Expert: Ted Barnhill (Principal, Design45 LLC) - 11/29/2007

Question
I'm planning an 18' x 18' extension to the kitchen of our old farmhouse here in Eastern Canada.  Can I sink concrete piers (below frost line, about 4' around here) and build on them, and if so, how many will I need, or do I need to put in footings and walls for the entire perimeter? In both cases I plan to leave a crawl space underneath, but how best to insulate the space under the floor?  Should I put a vapour barrier between the ground and the new floor?  Should I put rigid insulation between the joists?  Or should I enclose the area and keep it heated to above freezing in the winter?

Answer
Tony,

You can use piers, a vented crawlspace, or a heated crawlspace.  Current building science favors the heated crawlspace: it is typically more energy-efficient, as the heat loss through the perimeter foundation walls (which would of course be insulated) is less than through the entire floor into a vented crawlspace.  Also, there may be less moisture problems as it will be easier to seal against air infiltration.  In any crawlspace you should use a taped poly film or similar on the ground to seal out any soil gases or moisture.

If you choose to use piers or a vented crawlspace, you need to provide a vapor retarder on the warm side of the floor system (and insulation of course).  Spray-applied urethane insulation is a good choice for this area, if it fits in your budget, as it provides a good vapor barrier and high R-value.

Enjoy your new kitchen, and feel free to post any follow up questions you may have.  -Ted