Expert: Ted Barnhill (Principal, Design45 LLC) - 9/6/2007

Question
My vacation house in Northern NH has a crawl space about 4 feet high with a dirt floor. The foundation is cinderblocks. On each side of the foundation, the builder laid 2 of the cinder blocks on thier side, to provide ventilation holes. This summer I laid down a 6 mil vapor barrier on the ground, and made screens to fit over the outside openings in the cinder block vents to keep out the critters. With the cold weather coming I would like to fill the vent openinings from behind with some type of insulation to prevent drafts and cold air coming in to the crawl space. I have though of stuffing the holes with closed cell foam, which would be easy to remove next spring. I have also thought of using an expanding spray foam insulation, but I would need to be able to remove it and I'm not sure how easy that would be. I was thinking that I could let the foam dry then cut it around the perimeter of the hole, and be able to re-use the foam 'block' each winter. Do you think that will work or is there a better method? thanks for any ideas...

Answer
Before filling the holes, be sure that you have fully prevented soil gases from entering the crawlspace.  The poly seams should be taped, and it should be sealed to the foundation walls at the perimeter.

Depending on whether the crawlspace is otherwise reasonably airtight, it may (or may not) be worthwhile to go to much trouble to make your plugs airtight.  It seems likely that simply stuffing batt insulation into the holes would suffice for a simple draftstop.  You can get treated cotton batts if you don't like handling fiberglass.  The idea of reusable closed cell plugs is good, but you'll have to use a removeable caulk each year (or a great tape job) to make it draftproof.  Keep in mind that your uninsulated block walls have a negligible R-value, so the plugs are primarily to stop air, not heat.  Good luck.