Crawlspace encapsulation
Venting the crawlspace is important. It helps to reduce moisture by allowing air to circulate and by equalizing the temperatures inside and out and thereby preventing condensation. I don't know about Ontario, but I'm in Ohio and I believe the building codes here (about the same climate) specify venting crawlspaces at specific ratios 


  This is a good article which explains vented and non-vented
  crawlspaces and how they are supposed to work better than I can..


Open up the vents when it gets warm in the spring and close them back up in the fall when the air cools down. There are even automatic vents available that will do this on their own at appropriate temperatures without using electricity.



Additionally, you should probably install the plastic vapor barrier on the dirt floor and walls to keep out additional moisture (we do it here) and some screens over the vents would be good to keep insects and other uninvited guests out of there.