What glycerin and other humectants do is draw moisture to themselves. The traditional thinking is that if the humectant is in direct contact with hair, it will draw water to itself, and the hair in contact with it will be the first to benefit from excess water the humectant absorbs. Up to a point, that's right. However, the evidence shows that humectants will first and foremost maintain a minimum amount of water in the humectant, and that if the amount of water in the air (which is what dew point indicates) goes too low for it to draw moisture from the air, it will actually draw that moisture from hair it is in contact with, drying out the hair.

It's science: just the facts, ma'am.
Peace,
Morgan

Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008