Last Updated on September 13, 2026
How to Build an Automatic Vent for a Greenhouse, High Tunnel or Hoop House
I built a high tunnel over my raised beds to grow cool-weather crops through late winter and early spring. The first problem it gave me was overheating: on a sunny day in February the inside gets hot fast. This is the inexpensive automatic venting setup I built, and why I chose it over a roof vent.
Why not a roof vent?
I looked at the usual options. Wax-cylinder automatic vent openers are manual in the sense that they only open one flap, they are fairly expensive, and I read they may not work reliably in freezing weather. More importantly, a single roof vent lets warm air out but does not move fresh air through the tunnel. Circulation matters as much as temperature for the crops I grow in there.
The setup
- An inexpensive HVAC-style power vent (a few dollars on Amazon) mounted inside a wood frame I attached to the front of the tunnel at the door frame. It pushes hot air out.
- An intake opening on the opposite end, so cool air is pulled in and the whole tunnel gets a change of air. A box fan set over the intake blows a little extra cool air in.
- A wireless thermostat controller, about $22, that works for heating and cooling. I set it to 60°F: when the tunnel hits 60, the vent fan comes on; when it drops, it shuts off.
Build steps
- Build a simple wood frame for the vent and fasten it to the tunnel frame at the door.
- Mount the vent to the frame, then cut the hole through the plastic. Tape around the cut with vinyl tape first so the hole cannot tear and spread.
- Zip-tie the fan to the tunnel frame.
- Create the intake on the far end of the tunnel.
- Wire it: power comes into the tunnel, into the thermostat sensor box, then a splitter feeds the power vent and the box fan.
Setting the temperature
I am not trying to heat this like a greenhouse. I am trying to hold a stable, cool temperature, roughly 40 to 64°F, for spinach, arugula and other cool-weather crops in February, March and April. Big temperature swings stress those crops and make them bolt, and you lose the harvest. Stable beats warm. A Bluetooth temperature sensor that logs to my phone lets me check the history and trends, which is how I know the setup is holding.
Two more things I run in there
A second thermostat controller is set up for heating, to give one bed supplemental warmth overnight. And because daylight only returns to about 10 hours in February, when growth is still slow, I add a grow light early in the season.
Items mentioned
- Inexpensive power vent / duct fan
- Wireless thermostat outlet controller (heating and cooling)
- Bluetooth temperature logger
- Wiggle wire and channel
Building the hoop house itself (the build videos)
The vent above sits on a hoop house we built over the existing raised beds. The three build videos are here, with the notes that matter.
- Frame: attached to the existing beds (two 4×8, a 4×6 and a 4×4). Six-foot studs with doubled 2x6s and a 2×4 on top, about 7.5 feet tall, and 10-foot PVC bows with 45-degree fittings at the peak so New Hampshire snow slides off.
- Cost when we built it: about $150 in lumber and PVC, $150 for the plastic, $100 for wiggle wire and channel. Two people, roughly 12 hours over two very cold days.
- Plastic should not touch bare PVC (it breaks the film down over time), so tape the PVC wherever it contacts the plastic.
- Wiggle wire and channel (ours were Bootstrap Farmer, from Amazon) hold the plastic tight and made the wrap fast: only five 6-foot channels per side. We mounted furring strips along the PVC with one-inch brackets and machine screws, then screwed the channel to the strips. Do not bolt through the PVC: drilling the bow put all the tension on the hole and it snapped. Tape any sharp edges before the plastic goes on. It is a two-person job.
- Sealing the bottom: for a temporary winter structure, snow banked against the bottom edge does the job. Ours comes down in spring and stores as a few pieces.
