Our 4Seasons greenhouse was installed on a new deck and attached to the house by a contractor 20+ years ago. It is unheated, it works great as a 3 season room and we love it. 
I noticed some ants and rot in the 2 corners away from the house. 
The deck bottom is 2 feet above the ground with good air circulation.
The frame is made of 2×10 pressure treated is all in good shape including the 2×10 joists.
The underside bottom of the joists were covered with 1/8″ non treated plywood that has rotted away at the edge.
The space between the joists was filled with fiberglass insulation then a plastic film barrier placed on top.
Then 3/4 ” untreated plywood topped by 1/4″ plywood routed together and screwed down. the greenhouse was placed on top of the flooring . Flashing on exterior edge of floor.
The plywood flooring has rotted under the greenhouse in several places. About 1/4 of the perimeter..the rest is solid. The greenhouse has never been totally waterproof.
We have removed the flooring within 4 feet of the effected walls. Removed wet, insect infested insulation.  Am in the process of removing rotted wood from under the greenhouse walls and stabilizing with plywood shims.
I plan to replace the rotted plywood and adjacent flooring with pressure treated. 
I also plan to seal off any remaining insulated floor with well caulked pressure treated wood so it is not open to the weather. It’s about 1/3 of the floor area next to the house. Seems clean, dry. The rest of the floor I plan to remove the thin plywood that was supporting the wet, infested insulation and leave that underside of the floor open to the air.
Does this sound like a reasonable solution?
I will also treat new wood with insect control. Continue to have regular exterminator visits and do some glass caulking in warm weather.
Our family loves this 14×16 space.
Any advice is welcome