When we discovered the problem, we consulted with a local hvac company specializing in conditioned, foamed spaces. They suggested that our first step be to have the attic re-foamed, since the foam was done wrong (sprayed on top of fiberglass insulation!), which we did. They also suggested putting a return in the stairwell to circulate the air, which we let them do. After both of these things were done, the problem was worse. What started as mold flecks on the ceiling of the stairwell that could be wiped off turned into visible condensation. We blocked the return and started with an oscillating fan at the top of the stairs, and problem solved. But we don't want to live with the fan, and the air in the stairwell is still very hot, just moving. When we had the visible condensation, the foam company came back to check their work and refoam anything they might have missed. There is a skylight in the stairwell, another in the hall with the attic hatch next to it, and another in the bathroom. The bathroom and hall temperature are somewhat comfortable, but bedroom is hot, and we suspect the heat from the stairwell keeps it that way. It was suggested that we completely or partially block a large return downstairs, 10 ft. to the left of the foot of the stairs, which would force cold air from the bottom of the house up the stairs. It was also suggested that we put an a/c vent in the stairwell to condition the space, which in turn would cool the bedroom. A/C installation done before we bought the house, but we suspect by fly-by-night people. All help, advice, and suggestions are SO appreciated! Thank you!!!!!!!!

Running the fan during, slightly before, and after the shower will help. Most bath fans are not sized properly when they are new construction. If the skylight is insulated, you have the proper size fan, and run it for the right intervals and it still does it, you can look at mounting a fan in the shaft. This is all dependent on the condensation happening on the skylight. Be sure it is not happening on the shaft and that the skylight shaft is properly insulated.

One other thing to check out is the filter. A clogged filter or low Freon can cause the coils to freeze and turn into one giant block of ice. Usually if this is happening you will notice that the unit is not cooling as well. If this happens when the coils thaw water can drip around the condensation pan in the unit and water will find a way out. Given when your unit was built, there should be a drip pan under the entire unit assuming it was built to what are considered to be normal codes and standards.

If the flashing on the existing curb was fine, you would not want to disturb it as a normal procedural process (assuming the curb size is correct for the new unit). Every home is different. I suggest you get a relative humidity gauge and see where your levels are and what the levels are inside the skylight shaft. That will take all the guess work out of the questions you are having. Amazon has them and I have used this one: https://www.amazon.com/RockyMars-Temperature-Humidity-Monitor-Hygrometer/dp/B01M3Q2T21/ref=sr_1_12?s=home-garden&ie=UTF8&qid=1500913462&sr=1-12&keywords=humidity+gauge  As a general observation, the tighter homes and installations will have more issues with humidity. If you neighbor's homes are getting more unintentional ventilation (i.e. doors opening and closing), the humidity level could still be higher in your home.