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That really doesn't seem possible.

If in fact your room is built correctly, then your humidifier is malfunctioning.

If the vapor barrier were working too well and construction was very tight (and I've seen European construction where infiltration was so low that humidity really did build up) then your humidifier should sense that and stop emitting water. It has a humidistat, or should.

It's like if your room temperature went to 100 degrees F, and your builder told you it was too well insulated. Nope. Your thermostat should know what temperature it is, and turn the furnace off when setpoint is reached.

Either your room is built incorrectly, or your humidifier is working wrong, or both. Correctly constructed rooms do not have condensation with a properly functioning humidifier.

In my experience (I'm a mechanical engineer, and this is what I do) about 90% of the time it's the humidifier. But in your case I'd have to see it.

As I have watched this thread, I tend to agree that the humidifier has a problem with its humidistat. Perhaps it is cycling too widely - going too wet before stopping the addition of water to the air. If this assumption is correct and we had a long-term graph, it would show severe peaks in the relative humidity during times when the heat was being used.

Of course, vapor barrier application is important - being very different here in the south than in the northern part of the country. In this case, however, it seems that something beyond the vapor barrier issue alone is in play. The Dampp Chaser system can't solve the whole-house-system puzzle, but it can take care of your piano, regardless of the possible quirks which may be found in your home system's humidistat.

Where is the humidistat located? If it is on the cold air return duct near the furnace and has a knob for adjusting the humidity level, I'll send you a new one free. I have several lying about. Takes 10 minutes to replace, which you can do yourself (2 wires, 24 volt system).