I own 2 Martin guitars and 1 Guild. One of them is a Martin DRS1, the cheaper, Mexican-made version of your D15. Although it is not the same wood, I believe it shares the same porosity and density properties as the D15. My DRS1 is a lot more sensitive to humidity than the 2 other guitars. I had to install a de-humidifier with continuous drainage in my music room to make sure the guitars don't suffer from humidity.
Also, if your guitar is in a "safe" room but your guitar case is not, you might need to dehumidify the case before using it otherwise the guitar will absorb the case moisture. This is only true for the cheaper Martin cases though because the are made of some kind of wood or cardboard covered in vinyl. I'm not sure whether the D15 comes with the higher end ABS case.