Welcome to the forum, Mike
It is the real channel number that determines what antenna is needed.
VHF-Low, real channels 2-6
VHF-High, real channels 7-13
UHF, real channels 14-51
The virtual channel number (like 5.1) is a holdover from the analog TV days to maintain the identity of the station, and is what the TV displays.
One VHF-High/UHF combo antenna to consider is the Winegard HD7694P.
http://www.winegard.com/kbase/uploads/HD7694P.pdf
It would be suitable outside, but the signal loss from the trees and the attic location is difficult to predict. I suggest you try it in the attic in a temporary setup before drilling any holes. If the signals are too weak, try adding a preamp like the Antennas Direct Juice, which has an LTE filter.
The next model up is the Winegard HD7698P. It is much larger because Winegard discontinued several models in between.
Do you have an aluminum foil thermal barrier in your roof?
Do you have solar panels?