Many high tunnel operators who focus on winter production insulate the base of their structure with 2” foam insulation. This allows soil temperatures to be maintained at an elevated level inside the tunnel longer into the winter. The insulation is attached to the ground posts and extends 2-4 feet below the soil surface.
The high tunnel is usually covered with a 6 mil greenhouse poly that contains a UV-inhibitor. Most greenhouse plastics are guaranteed for four years but can last longer if properly installed. Many growers who use a two-layer poly with an inflation fan see five years of use before the poly must be replaced.
Ventilation is very important to prevent death of your plants due to high temperatures or increased chance of disease due to high humidity levels.Ventilation will be especially critical in the early spring. Outdoor temperatures can fluctuate between the 40’s and 70’s while temperatures inside an unventilated tunnel could exceed 120°F .
There are a variety of methods available to provide ventilation to your high tunnel. The size of the tunnel, the amount of time you are available on site, and the amount of money you want to invest will determine the method chosen.
The simplest method of ventilation is the use of manually-operated vents in the end walls and by opening and closing the high tunnel’s doors.
Many high tunnel owners install roll-up sides in order to provide ventilation. Most roll-up sides can be raised either partially or up to the “hip board”. Automatic roll-up sides, which are thermostatically controlled, can also be installed at a greater cost.
In smaller tunnels, thermostatically controlled solar powered attic fans or temperature activated openers, such as Univents® attached to louvered vents or windows, can provide ventilation. The advantage of these devices is that the tunnel will be automatically ventilated when no one is available to open doors.
Ventilation fans can be used to increase air circulation around plants. However, this will result in higher initial costs as well as higher operating costs.
Bracing and Support
Check with the manufacturer of the high tunnel frame, but most tunnels wider than 20 feet will require some type of internal bracing.
Many manufacturers suggest cross bracing on every other rib. However, if you plan to trellis plants such as tomatoes and cucumbers, you may want to consider installing cross braces on every rib. This will allow more places to attach your trellis material.
Fastening Poly to Frame
Another cost is the channel used to secure the poly to the frame. This channel usually has a “spring wire,” which secures the poly to the channel. If used, the channel will be attached to the entire length of each of the end ribs as well as the length of each kick board.
If roll-up sides are being installed, then a double row of channel must be attached to the hip board. The upper channel keeps the cover taut while the lower channel allows you to replace only the side plastic if it is damaged by tools or the wind.
Poly can also be attached to the kick board and end walls (depending on material selected) using wooden lathe strips and screws. However, care must be taken that the poly does not “pull” from the screws during high winds.
We’ve covered many of the major material costs, but there will be other costs such as floating row covers, supports for row covers, hardware, thermometers, etc. While most growers are “do-it-yourselfers,” extra labor may be required when erecting a tunnel. If so, this cost must also be taken into consideration. Calculate all costs to get the true picture.