What is the outside humidity level? You will have a tough time keeping it lower than the atmospheric humidity.

Spray foam rim joists and sill plate, and use watertite on all walls and floor. Pretty much the only way to get what you want.

No matter what the outside humidity level is, even on dry sunny days, even with the air conditioning on and all windows closed, indoor relative humidity levels never get below 60%. Like I said, before the water damage occurred, RH never got above 55%. In the winter with the heat on, RH was hard to keep above 35% without the use of a humidifier.

Vith, all the materials (other than the structural concrete slab and walls) have been removed and disposed of 22 months ago, within 72 hours of the sewer backflow. Are you saying the concrete would have absorbed the water, and to this date it would still hold most of it?

All I know is that you have a humidity problem and the watertite will block it for the most part. Whatever had backed up would have most likely dried by now with a dehumidifer running all the time. Something has changed, perhaps a neighbor has adjusted slope towards your yard or something. Is there a sump pump that is not sealed properly? If you can find the reason for the high humidity go that route but as of now I would deal with the symptoms since we cant find the reason.

Also, the rim joists/sill plate does leak quite a bit if not sealed properly, if it is real humid out that would get in through there. I remember before I sealed mine, I could feel the wind though it.

watertite would block it for the most part, providing the humidity is coming from the basement walls. Besides, why did it work for 43 years without watertite, and suddenly it's absolutely needed? None of the immediate neighbors have regraded their yard, nor did my mother. They dug a trench to replace the root-clogged sewer pipe, and it was backfilled flush with the surrounding soil. No changes were made to the downspouts, they still discharge about 3 feet away from the foundation walls. The interior concrete feels dry to the touch, and there is no white efflorescence anywhere. There is no sump pump, and of course the sewer pipe from the main stack to the street is all brand new PVC. All toilets, sinks, tub drain properly with a quick swirling action.

There was never more than 2 inches of water in the basement, and it was addressed immediately. Everything higher than perhaps 3 feet up to the ceiling was not damaged and left untouched. A hygrometer http://www.amazon.ca/General-Tools-Instruments-MMD4E-Moisture/dp/B00275F5O2/ref=sr_1_1/179-1872848-1950766?ie=UTF8&qid=1445032321&sr=8-1&keywords=general+moisture+meter shows between 5 and 8% humidity in the wood.

Could there be another possibility for the increased humidity levels? I am no expert, but it seems to me that enough moisture coming from the basement to cause a 20% increase in RH levels would be noticeable with the naked eye. We bought two hygrometers, one analog and one digital, and I brought a third one from my house so I know it's not just a faulty hygrometer.

I am not discrediting you or saying that you are wrong, or saying that I know better than you, I am just trying to understand where the excessive moisture would be coming from. Thank you for taking your time to help me.

My only other thought is that she is drying laundry down there or the vent for her dryer has a disconnect causing all the moisture to stay down there.

The dryer and its vent line have been checked, the range hood works fine and so does the bathroom fan. The only laundry she dries inside are sweaters that have to lay flat to dry.

I was there today and I noticed a bathroom cabinet made of particle board that has a swollen shelf and smells like a mixture of brand new particle board and pva glue. No plumbing pipes go through that wall. Next time there is a heavy rain, I will get back in the attic and look for water coming in from the roof or around the drain vent. Exhaust fan exits through the wall.