I have a 4,600 square foot home (which include the basement). I have a heaating/air unit that serves the basement and main level, the unit resides in the basement. I hae a heating/air unit that resides in the attic serving the upstairs. Our house is extremely dry. We have ahrdwood floor separation, my families skin is very dry, etc. Is a humidifier worth the investment? If so, should I get the kind that are attached to my heating/air system or should they be portable. Additionally, if they should be attached to the heating/air system, should I get one for each of my units?