Materials & Construction

Terminology & Diagram

The following diagram denotes basic terminology associated with high tunnel framework.

LINK: How to Build a High Tunnel, by Amanda Ferguson, University of Kentucky

1. Rib, Hoop, Arch, Bow
2. Purlin , Ridgepole
3. End Wall
4. Hip Board
5. Side Wall
6. Baseboard

Frame & Framing Materials

• The frame supports the structure and must withstand various stresses or “loads”

  For more on Loads go to:
  Orientation& Structural Concerns

• To add strength to the structure, a number of strategies can be employed
  
  • Drive posts 18 to 24 inches into the soil
  • Use heavier gauge metal pipe for the ribs
  • Add more than one purlin to the structure
  • Decrease the spacing between the hoops
    • Decreasing rib spacing from 5 feet to 3 feet intervals increases the amount of load that the structure can carry by 66%
  • Fasten plastic glazing tightly to the structure
• The type and shape of the structure also influences its strength
  • A gothic arched roof sheds snow better than aflatquonset roof, increasing structural carrying capacity by 15%
    • The gothic arch can be achieved in high tunnel designs by bending the typical hoop shape with a pipe bender or bending jig
  • No sidewalls or short sidewalls are stronger than structures with higher sidewalls
• Metal pipe is the strongest framing material for rib construction
  • Quality ranges from electrical conduit on the weakest end to 1.9 inch OD (outside diameter) thick steel or schedule 80 water pipe as strongest materials used
  • Schedule 40 (3/4″ galvanized water pipe is commonly used but results in a 20% weaker structure compared to using Schedule 80 pipe)
  • Metal ribs make the structure very heavy and difficult to maneuver
• PVC (polyvinylchloride) plastic pipe is an inexpensive material for rib construction
  • This lowest cost material is one of the weakest
  • PVC is lightweight and easy to maneuver
  • High tunnels made from plastic hoops are vulnerable to wind uplift and snow load collapse
  • PVC expands and contracts with temperature changes
• Wood is not typically used for rib construction, but is often a component of end walls, hip boards, and baseboard construction
  • Treated lumber should be used for baseboards and any other wood that will come in contact with the soil

Covering Materials

Polyethylene

• 

  Picture courtesy of Farmtek.com

  Greenhouse-grade polyethylene, a plastic film, is the most common material used for covering high tunnels

• “Poly” is sold by thickness in mils (1 mil = 1/1000 of an inch) and rated for longevity in years
  • Typical specification for a high tunnel covering would be a single layer of greenhouse grade 6-mil poly rated for 5 years
  • Traditional greenhouses usually have two layers of poly, separated by air blown between the layers, to reduce heat loss during cold season production
• Polyethylene glazing contains additives designed to enhance durability and performance, but these also increase cost and reduce light transmission
  • Photosynthetic Active Radiation (PAR) is light that is utilized by the plant in photosynthesis and occurs in the 400-700nm wavelength range
• Ultra-Violet (UV) stabilizing additives block UV light to slow degradation and hardening of plastic
  • Never purchase construction grade plastic because it does not contain a UV-inhibitor and will only last one growing season
• Anti-Fog surfactants makes poly covering less repellant to water
  • Condensate will sheet and run down to the sides of the structure rather than bead and drop on foliage of plants below
  • Condensation is undesirable because it decreases light intensity
• Infared (IR) heat blocking additives reduces radiant energy heat loss
  • Polyethylene alone is a poor barrier to radiant energy loss, but IR treated poly can block IR heat loss by half, which is 15-25% of total heat loss at night
  • A single layer of IR-absorbing poly decreases PAR transmission to 82%
• Photoselective films absorb or reflect a specific range of light wavelengths to manipulate plant growth
  • For example, installing a poly with photoselectively that blocks 280-320 nm UV light, which is essential for sporulation of Botrytis cineraria may prevent this disease

Shade Cloth

• Shade cloth is made from knitted polyethylene strands or woven polyester, and it is water permeable
• Shade cloth is used to reduce light intensity, temperature and plant exposure to wind
• Shade cloth is often used in combination with plastic covering but in some applications such as cut flower production in Southern California, it is used as the sole covering for a high tunnel
• Black, white and various shades of green and brown can be used
  • White shade cloth may have a more profound cooling effect because it reflects more light than black and other darker colors of shade cloth
• Shade cloth is purchased by percent light blocked, 20-90% are commonly available
  • Seasonal and crop species requirements dictate what percent shade cloth would be chosen
  • For example, in spring little or no shade would be required for lettuce greens production: however, during the warmest summer months, 60% shade may be required to produce a quality product
• If used in conjunction with polyethylene, shade cloth may be installed on the inside or outside of the high tunnel

Attachment Techniques

• Secure attachment of the covering material is necessary to avoid tearing and excessive wear that shortens its longevity
• Fastening poly between two boards such as the hip board and batten board is the least expensive method
  • However, this method may be the least secure, depending on the type of screws, spacing and installation technique
• Wiggle wire nestled in an aluminum channel offers continuous force along the surface of plastic and is relatively easy to install
• Wiggle wire is a continuously s-curving wire that is placed into the channel by “wiggling” it back and forth
• Many channel and clip systems are manufactured, including Poly Clip,Agrilock,andSurelock
  • These are the most secure and most expensive poly attachment systems
  • Some designs are reported to rip the plastic at the attachment site

End Wall and Side Wall Options

End Walls

• End walls offer access and ventilation for the high tunnel
• A major consideration for doors in the end wall design is ease of access
  • Openings must be wide enough for a person to walk through with tools, supplies, or produce
  • Large equipment access is also needed for roto-tiller, tractors, and other bulky equipment
  • Door designs vary from manufactured storm doors, and sliding doors, to a non-framed plastic curtain
• To increase ventilation in the summer, some high tunnel designs utilize completely removable end wall sections
  • If end walls are removable, sufficient structural support must be maintained
  • Large screen sections, windows, and winged panels can also be installed in endwalls for summer ventilation
• If winter production is a goal, endwalls that minimize air leakage and accessibility during snow cover should be used

Side Walls

• Either fixed or roll-up side walls designs can be constructed
• Fixed side walls are applicable for winter production or in a climate where less ventilation is required
• Roll-up side walls offer a simple way to manage temperature and promote ventilation
  • Ventilation is accomplished by rolling up the sides and allowing warm air to dissipate to the outside
  • Temperature can be further manipulated by such actions as rolling up each side wall to different heights at different times of the day
  • There are many designs for hand-powered handles and cranks that can be inexpensively installed to roll up the length of the plastic wall
  • Purlin metal or fence toprail can be used for rolling up the length  of the sidewalls