Indoor Humidity and Building Materials?

[Moirae]

Does anyone know if plaster keeps moisture/humidity in better than drywall? As opposed to the summer, when temps are high and RH plummets, I've had no problems keeping humidity up (45-65%) in my room this winter. Even when it's the same temp inside as outside, I can have humidity be 20-30% higher inside. I've noticed that if I open up my room for a couple hours for fresh air at night when humidity is higher here and then close my door and windows. The moisture will stay in the room for at least a day as long as I don't leave my door open to the rest of the apartment (if I do, the humidity will start dropping rapidly; more than 5% in a matter of minutes).

I've never experience this before, but I used to live in the desert and my house had drywall. The apartment I've been in for the last year has thick plaster walls and I'm wondering if this could be the reason the room holds moisture in so well. Does anyone else here have plaster walls and notice this effect?