They need more air than just goes through the zipper.
Hydrohuts had a problem with the plastic outgassing but the manufacturer has fixed it. They are actually a VERY good product.
You will need a proper exhaust fan to draw hot, stale, moist air out and bring in fresh air. Huts should have a passive intake near the bottom. But you need an exhaust to make it work the way it was designed.
Plants shrivel, get dark green, and DIE with not enough CO2. Bringing 'fresh' air from within your home is great, since you breathe OUT CO2 that the plants can use.