Check around the foundation & basement windows for air leaks, and seal up to not allow air flow. My basement stays around 67-69 in the summer, with just two vents & one return, while the upstairs stays around 72. House is air tight, but in my case, no insulation in the walls, due to older home built in the late 30's. Just go around and check to make sure storm windows are closed, no air leaks around doors & windows, and that bathroom fan vents have dampers on them to keep air from leaking out or outside air coming back in when the fans are off.
Also find out what the humidity & temp is down there, along with the main & attic, which you can use a remote temp/humidity unit to allow you to see this on the main floor, or in your office.
Also find out what the humidity & temp is down there, along with the main & attic, which you can use a remote temp/humidity unit to allow you to see this on the main floor, or in your office.