Using corrugated fiberglass is less costly over a long period, but it is nice to see thru the clear 8 mil plastic. If I build another one, I will use the fiberglass except 8 mil on the door and vent panels. Without a walk-in greenhouse, a cold frame is excellent. I have not used mine as much as I should have. I know I can have plants ready to transplant in April.
Larger structures require the most waterings.
A greenhouse may require water daily. The tunnels usually do not require water. Let's look at circulations in the tunnel and see why. Two cycles move both air and water each day. As this happens, air is drawn into the soil to replace it.