Basement moisture/humidity issues
My wife and I just bought a house ~6 months ago in Hudson Valley, NY. It is a split level built ~1987, and has a finished basement (that is semi- below grade) and has a thin layer of old carpet on the floor. We are having some issues with humidity/moisture in the basement and are trying to work out the best way to address it.
Here is a timeline of our experiences so far, which should give a better indication of the nature of our issue:
We first inspected the home in late-fall last year, and were happy to see that the basement seemed very dry and without much odor (other than that usual basement smell). This was despite a VERY wet late-summer, and that other homes that we inspected around that time exhibiting significant water in the basements. This was a significant reason for choosing the house mentioned above - we did not want to deal with a wet basement!
We finally closed the sale and moved in around 6 months ago (early-spring). The basement still had that typical basement smell, but seemed quite dry and similar to how we remembered it during the initial inspection. However, since then the mustiness in the basement has slowly increased over the spring/summer, to the point where humidity is ~70-80% and we have found minor amounts of mold on the surface of the interior drywall.
In response to this we have cleaned the affected areas of the walls with bleach and bought a dehumidifier. This has reduced the humidity (and associated smell) significantly, but we would like to explore a more permanant fix to give a healthy, warm living space that doesn't require constant use of a power-sucking dehumidifier.
Attached are 2 photos. One is of the outside of the house showing the grade (water flow direction from right to left of the picture). The other picture is of the inside of the basement. The divide in the drywall is roughly in line with the outside grade. To put the inside/outside in perspective, the photos show the same window (the window seen in the photos is one and the same).
1) Given the photos and timeline described above, what is the likelihood that there is water coming through the wall (and possibly mold behind the drywall)? From the timeline I feel that this it is only water vapor pushing through the basement wall/floor due to differences in hydrostatic pressure. Is this too big an assumption? Should we consider ripping out the lower portion of the drywall (below grade), apply a type of sealant/waterproofing, and then refinish??
2) What is a good flooring option? We would like to keep some type of carpet as we plan to use the space extensively during the winter. At the moment we are considering an indoor/outdoor carpet (100% synthetic) on top of some type of dimpled sub-floor product (such as Seperseal All-in-one). Does this sound like a good idea? Is it absolutely neccesary to place a layer of ply between the carpet and the subflooring?? Should a layer of waterproofing paint (used for the walls) be applyed to floor before laying the other stuff down???
3) From looking at the outside photo, what would be the best way to landscape the area to prevent water from pushing against the basement wall?
4) We are considering an infrared quartz heater. Is this a good option for a basement? I know they are supposed to be more energy efficient, but is this the case for basements? I am concerned that all of the energy is going to be absorbed into the walls and floor (a massive heat sink given that it's underground) and that this may not be a good option. Is this another bad assumption??
A long post and a lot of questions, but thanks for any advice on all of this!