Hi v194,
Air sealing will have the double benefit of increasing your moisture level and reducing both heating and cooling costs. How much yours is leaking can be measured but your dry air is already telling us it is leaking too much. I'll add a link below that will describe the many leaks a home can have.
However, you are on the right track. Joe is correct in that the porch is not necessarily a problem. The exception would be if the wall between the porch and house was never completely sealed. That can happen when they know it will be covered so sometimes the exterior sheathing is not complete at the top.
The link below will go through many possibilities, but since your home is relatively new I would focus on something big that was overlooked. Examples would be plumbing vents from basement to attic, areas around any chimneys, drop ceilings above cabinets, openings below sinks, tubs, and showers.
The other big contributor could be any ducts outside the conditioned space, like an attic. Those ducts need to be well air sealed (and checked for disconnects) as any air forced out of the house will result in outside air coming back in.
As a note, a typical home will leak about 1/3 of its air every hour. Tight would be every 4 hours and leaky every 2 hours. These numbers aren't exact but they give you an idea where all of that moisture is going. People and their activities usually provide plenty of moisture as long as the house or heating system isn't too well connected to the outside.
Bud
http://www.efficiencyvermont.com/stella/filelib/TBC_Guide_062507.pdf
