My a/c is putting out about 9 lbs. of water per hour of run time. Despite this, the indoor humidity hovers around 50% (has ranged 48 - 53, but mostly 50+).

This A.M., measured the humiity in the basement and found it to be over 70%. The temperature was in the low 60s.

I know about stack effect and will have to wait for a cool spell to check the attic, but disregarding that, what other areas can I check?

Windows are double-pane, air tight; outside wall outlets are sealed. Is the sole plate (terminology?) a source of infiltration?

Would a minimal stack effect be sufficient to cause cool/humid basement air to rise into the conditioned area?

All suggestions welcomed. Thanks.