In the last article we looked at the high tunnel frames. Since there is more then just a frame to a high tunnel, we need to look at the other parts of these units. Different manufactures offer different types of ventilation as well.
So lets look all the options available. Tunnels can be vented from the side’s walls, end walls and also the roof. They can also be vented at more then one location. Ventilation can be manually or can be done automatically with either electric or mechanical.
Now which option is best for you? First of all you need to know what crops you are going to be growing in your high tunnel. Is it tomatoes? Cucumbers? Peppers? Leafy greens? Strawberries? Or a combination of several different crops?
Since we have not made up our minds on which crops we are going to be growing in our high tunnel, let’s look at the different options. What are there good points and the not so good points?
SIDEWALLS: On sidewalls you can have either drop curtains or roll up. They can be manual or automatic operated with electric. Each has their good points and also the not so good.
DROP CURTIANS: Just as the name says, the curtains open from the top down. In the photo above, curtain is open. This unit is not finished as shown in photo. Auto curtain machine was installed later to make it automatic. Drop curtains are opened and closed by a series of ropes, pulleys, Curtain machine is controlled by an electric controller according to temperature and time set for curtain to open/close. Manual operated tunnels use a small winch that you would find on a boat trailer.
To open curtain, weather manually or automatic, the cable that runs along the outside of the tunnel moves away from the winch end towards the weight located below the strap pulley. By doing this, it lengthens the ropes that are attached to the cable and curtain, which lets down the curtain. To close the curtains, the cable is rolled up which moves the cable towards the winch end. This pulls the ropes upward to close the curtain.
If tunnel is set up for automatic control, the opening and closing of the curtain is done with a curtain machine. This machine is controlled by a controller that tells when to open or close the curtain according to temperature setting. The controller also has a time control on time of the movement of the curtain. In either opening or closing, the machine will run for a set amount of time. Most of the time this is in seconds. 15 to 20 seconds is the most popular setting. The machine will open or close the curtain for this time and then pause for the same length time before resuming operation again. The reason for this is because if there are any clouds around that get between the sun and your high tunnel, temperatures can and will change quickly. This time control will keep the curtain machine from opening or closing the curtain all the way and then the cloud moves away or in front of the sun causing the machine to either close or open the curtain again.
The good points of a drop down sidewall is that you can grow warm loving crops like tomatoes, cucumbers, melons, eggplants, peppers and others. The cool air comes in over the top of the crop and cools off the top part of the tunnel. It may not be the best choice for cool weather crops, put it will work. The draw back is in putting together the drop curtain system. There are pulleys, eye screws. Stay on clamps, cable and a winch assembly to put together. This maybe more then some people want to tackle. However many do for the benefit they get from there tunnel.
ROLL UP SIDES: This is one of the most popular styles of tunnel ventilation sold by companies that need to sell a low cost unit. A roll up sides can be build very economical is so desired. Roll ups if done right with no opening for the wind to come in, will costs as much or maybe a little more then a drop curtain sides.
So how is a roll up side different from a drop curtain sidewall? A roll up sidewall opens from the bottom to the top. This is done by steel tube that rolls up the poly. By rolling up the poly. It continues till it reaches the top of the opening. The roll up tube is either operated by hand using a crank that you turn and need to lock into something to keep it in place. This is the economical way. A better choice is a gear assembly attached to the tube. This gear assembly rolls up and down on a hanging tube. The good thing about a gear assembly used on a roll up is that it stays where you stop cranking. No need to have something there to lock it unto.
Roll up sides can also be made to be open and closed by a machine. This machine is controlled by a controller just like the drop curtain machine is. By using a machine to open and close your tunnel, you do not have to be there to open or close it when needed. The machine will do it for you.
Roll up sides work great for crops that like it cool like lettuce, spinach and greens. It is also the choice if you are growing crops on benches. If fact, roll up sides are the most popular tunnel ventilation sold by the companies that sell on tunnel cost only.
The down fault of roll up sidewalls is that there are not recommended for warm season crops that like it warm. This would include tomatoes, cucumbers, eggplants, pepper, melons and etc.
RIDGE VENT: This a ventilation system that few consider. If you need to keep the top of your tunnel cool, a ridge vent will cool it off fast.
So what is a ridge vent? Just like the name says, it vents out the top of the tunnel. A ridge vent has a roll up on the front of it. By rolling up the poly here, you can leave out the hot air. Ridge vents can be opened and closed by a gear crank from the ground or they can be made to open and close automatically by a machine.
All ridge vents need either a drop curtain or roll up sides. When the ridge vent is open, the sides need to be open also. If not the air will not flow. The front of the ridge vent needs to be faced away from the wind.
Ridge vent add to the cost of a tunnel. On a 30’ x 96’ tunnel, by adding a ridge vent you will add around $1500.00 to $2000.00 to the cost of the tunnel. It will also add 1 more day to constructing the tunnel for a crew of 4-6 guys that know what they are doing.
END WALL VENTILATION: End wall ventilation can be as simple as just 2 doors on each end of the tunnel. Several other ideas are a exhaust fan of fans installed in the end walls. The draw back with exhaust fans is that if the sides are opened up. They pull the air from the easiest and closes opening. This will be from the first few feet of the side wall opening. Fans work great as a ventilation system if you are not at home. Since the fans are controlled by a thermostat, they will come on when the temperature inside the tunnel hits the high temperature set on the thermostat.
Some suppliers also supply a vent called a butterfly vent. This is a square opening usually in the top part of the end wall. There will be one on each end of the tunnel. A butterfly vent is easy to make. In a square opening, build another square to fit inside the first square opening. In the center of the sides of the both squares, drill a hole through both. Now but a long bolt through the holes, with a flat washer in between both squares. You will need to put a lock nut onto the bolt to keep it together. Cover inner square with either poly or double wall poly carbon sheets. Butterfly vents open by pushing out the bottom of the inner square. This will cause the upper part of the inner square to come in.
I am sure that I missed some other ventilation ideas. Tried to cover the most popular ones. High tunnels are being build on a daily basic. Some one some where is coming up new ideas on how to make the better.