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I live in the mid-west and have a 2 story home built in the 1950's.  Over time, the home was remodled.  The remodle resulted in 1 AC/Furnace unit for the basement/main level and a 2nd AC/Furnance for the upstairs (the upstairs was previously attic space).  In the summer the humidity is frequently in the 70's (yuck!) on...

...lower humidity? That is odd, since your humidity isn’t all that high ‘relatively’ speaking. Houses in cold climates should maintain indoor Relative Humidity levels of around 30-40% in general, that's a good level for the house but also good for occupants. And Guelph Ontario is actually A pretty mild climate for Canada....