Is it normal for my AC seal to inflate like a giant balloon?

That ballooning effect is a perfect illustration of how much air a single-hose unit actually moves. You've essentially created a pressurized pocket because the unit is sucking air from inside the shed to cool its internal components and then blasting it out thru that exhaust hose. If that hot air doesn't have a clear path to the actual outdoors because the plastic sheet is blocking the way, it's just going to keep inflating like a giant lung til the tape fails or the unit overheats. The core issue with these single-hose setups is **negative pressure**. For every cubic foot of hot air the AC pumps out through the door, a cubic foot of air has to come back into the shed from somewhere else to replace it. Usually, it gets sucked in through the gaps in the floor, the roof, or around the door frame. Since your shed is likely not airtight, you're basially pulling warm outdoor air back in while trying to cool the room down. It is a constant cycle of fighting against itself. You definitely shouldn't let that hot exhaust air stay trapped behind the plastic. If the hose isn't venting directly through a cutout in the sheet to the outside, you're just heating up the space behind the barrier. That heat will eventually radiate back into the shed and could easily damage your wooden surfboard or curl your posters. You need a tight seal specifically around the nozzle where it exits the plastic, but the hot air itself needs to be gone. If you want to fix this properly, look into a **dual-hose conversion**. One hose pulls fresh air from outside to cool the machine, and the other blasts it back out. This stops the vacuum effect inside your shed and keeps the pressure neutral. If that's too much work, at least make sure your current exhaust has zero resistance. If the plastic is under that much tension, the fan is struggling to push air out, wich will kill your cooling performance and probably the motor too.

You're spot on about the pressure, but one thing people forget is the heat's effect on the tape itself. That exhaust air coming off a portable unit can easily hit 130 degrees. Most standard tapes or even heavy-duty duct tapes start losing their bond once the adhesive gets that hot.

Since the air is trapped and the sheet is under tension, that heat is basially melting your seal from the inside out. I had a similar setup for a workshop and the whole sheet peeled off in twenty minutes becuase the tape turned into a sticky slime. If you don't vent it properly, the heat will kill your adhesive long before the air pressure actualy rips the plastic.

I'm totally new to this, but the whole 'negative pressure' thing is blowing my mind. I always thougth ACs just magically made cold air. If it's pulling air in from the gaps in the floor, doesn't that mean the unit is just cooling down the hot air that's leaking back in? That seems so backwards! Also, I noticed my hose gets really hot to the touch. Is it possable that the hose itself is acting like a radiator and heating the room while it's trying to cool it? Would wrapping it in an old blanket help or is that a fire hazard?