High Humidity...Help?!?

I have a 4-plex; 2 units on upper level and 2 units about half way below ground level on one side (the other side is about 1' below ground level). It's all brick - concrete floors - no crawl space beneath the building but there is a sump hole with 2 slats in the bottom of it. There is no pump in it, there has never been enough water (we've owned the building for approx 10 years - it was built in the late 70's). We have super high humidity in the building. It's creating mold issues. We're looking for a solution that won't break the bank (although we're probably grasping at straws). We are located in Northwestern Ontario so summers are short and winters are long and very cold. We have had some advice from a local basement person. They suggested digging up the perimeter of the entire building and putting foam insulation on the area that comes into contact with the earth. We did the whole backside of the building but the two ends are paved driveway (which we would hate to tear up) and the front is landscaped with a concrete sidewalk (which we would also hate to tear up). Does anyone have any suggestions as to how we can improve the air quality? We have provided all tenants with a dehumidifier but they don't like to use it because they pay their own electricity bill. I almost forgot to mention, we have electric radiant heat in the building, so there is not much air movement.

Thank you in advance for any advice.