Should I insulate a crawl space from the outside?

Ryan Argente
Updated: March 9, 2021

I've recently purchased a 1960s home in southern Quebec. The house was build with a dirt floor crawlspace. When I purchased the home the crawlspace had taped eps foam on the walls and the floor was open. The humidity in the crawlspace was in the 60s. I have since put 6 mil poly down on the floor which has brought the humidity down to a reasonable 38% (within 5% of the rest of the house). 

At this point I'm planning upgrades to the insulation of the house. I know that one solution is to remove the eps and poly and spray foam the walls and floor but I'm concerned about having the vapour barrier on the inside of the concrete wall. My concern is trapping water in the concrete, a serious problem in our cold climate. Also the floor joists are cast into foundation walls which means trapped water could lead to rotting joists. 

My preferred option is to digout the foundation from the outside, repair any damage, waterproof  and then insulate from the outside. As I need to replace the french drains excavation doesn't really add any costs. My concern here is the giant thermal bridge caused by the foundation wall being inside the conditioned space. My thought would be to replace the eps with rockwool to allow any moisture drawn into the wall to diffuse out into the crawl space. Easily taken care of with a dehumidifier if needed.

Does this make sense? or am I missing something obvious?


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