I used sliding doors in a studio I built in 1996, and while the glass double glazed panels were great, the leakage came from the outer frames. While supposed to be real wood, I think they were some kind of fibre/composite material. Wood on the outside but much softer material inside. Maybe to produce good heat figures? The metal frames that were the heavy duty contact with the glass were also extruded and contained voids and these didn't do much for sound isolation either. A real let down on the drum room. Not awful, and because we had two sets, fac to face, it was workable.