An Energy Efficient Season-Extension Bunker
An Instruction Manual
Prepared for: Farm Energy Working Group
Prepared by: Jason Gomes, Rainbow Ridge Farm
Date:
January 2011
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Table of Contents
INTRODUCTION
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PROJECT NEED
PROJECT CONCEPT
DEMOLITION
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4
5
WALK-IN COLD STORAGE
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MATERIALS SELECTION
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6
CONSTRUCTION
DRYING/CURING CHAMBER
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CONSTRUCTION
12
GREENHOUSE RECONSTRUCTION
13
MATERIALS SELECTION
CONSTRUCTION
13
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RESOURCES
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INTRODUCTION
The purpose of this project was to remodel/retrofit two existing on-farm bunkers to
serve as an underground cold storage chamber for produce on one side, and a
curing/drying chamber for produce on the other side. The remodeling project was
be designed around the following principles:
 A greater reliance on on-farm energy resources
 Energy efficiency and conservation
 Renewable energy usage (solar)
 Re-purposing of existing farm materials
 Passive Solar Design (operable windows and vents, South-facing
structure, critical eye toward material selection: concrete/foam mix
for walls)
The project was designed to more efficiently utilize two existing underground
bunkers on the farm. These framed in storage units are located under the
coordinator’s garage – with one side formerly used as a woodshed and the other
side with a degraded greenhouse-type structure attached to the south. Because of
their positioning – underground, facing south, and with only one exposed side, they
were excellent candidates for a repurposing/remodeling project with a mind
toward increasing energy efficiency. Using minimal resources, some skilled labor,
and the right building materials, this makes an excellent demonstration project for
many farmers with under-utilized outbuildings or even walkout basements. The
remodeling project resulted in an energy efficient cold-storage chamber and a solarpowered curing/drying chamber for season extension of sustainably produced food.
PROJECT NEED
Curing/Drying Chamber Primary Needs:
Curing Potatoes
Curing potatoes requires warm, humid temperatures – preferably in the 75 to 85
degree range. Potatoes must be protected from sources of direct or indirect sunlight
to prevent them from turning green.
Curing Sweet Potatoes
For curing sweet potatoes, late fall sun is usually sufficient to partially cure skins,
but the outcome is highly dependent on favorable weather (sunlight and warm
temps). The tubers tend to cure either more slowly (at best) – or only partially (at
worst) when the drop in nighttime temperatures can be significant, or when
daytime highs are in the mid 70s or below. A goal of the proposed curing system
was to capture solar heat during the day and maintain a sufficient curing
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temperature during the night (roughly 75 to 80 F) either through use of
supplemental heat or through conservation of daytime solar heat.
Drying Onions/Garlic
Onions and garlic require a warm, dry environment with good air circulation to cure
for storage. Both crops are harvested at least 3 to 4 weeks prior to potato/sweet
potato, so use of the chamber for drying bulbs will not interfere with its’ use as a
curing chamber for tubers. Fans will be used as needed to maintain airflow and
reduce humidity.
Curing/Drying Chamber Secondary Needs:
Starting Curcubit Seeds
The passive solar unit will also serve as a small greenhouse for starting plants.
Melons and Squash are transplanted around the 3rd or 4th week of May, so seeds will
be started around mid-April. The greenhouse will have the capacity for 2 to 3,000
plants started in 72-cell trays. Use of the unit to start plants will not interfere with
its’ primary purpose as a curing/drying chamber.
Cold Storage Bunker/Walk-in Refrigerator Primary Needs:
Root Vegetable Storage
The basic need served by the cold storage bunker is the storage of root vegetables to
extend the marketing season of the farm. The walk-in refrigerator in this bunker
will cool efficiently and allow for durable storage of potatoes and sweet potatoes
through the months of August/September/October using a refrigeration unit.
Potatoes/Sweet Potatoes will be stored at high humidity at approximately 40
degrees. Use of the refrigeration unit is not anticipated for the late October through
the winter months. Insulation and use of heat from the curing/drying chamber is
expected to produce in the walk-in from freezing over the winter months.
PROJECT CONCEPT
The walk-in was designed for late-summer, early fall use at moderately cool
temperatures (low to mid 40s). These temperatures extend the marketable life of
potatoes by 4 to 6 weeks.
During the late fall and winter months, the walk-in is intended to function more like
a root cellar. It’s design is to provide sufficient insulation from the outside to
maintain storage temperatures in the mid-30s.
The original concept for the project involved the use of concrete infused foam for
the walk-in refrigerator walls. Use of concrete for the walls was determined to be
too cost prohibitive as it would have resulted in the project going considerably over-
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budget. In addition, the concrete infused foam – though possessing excellent
insulation properties – was determined to be excessive for the rather basic project
need of short-term storage for root vegetables.
DEMOLITION
The east bunker was originally purposed as an over-wintering room for outdoor
plants. Its’ concrete walls were lined with glued-on white Styrofoam and secondary
heat was provided by a black Coleman outdoor grill connected to the homes LP
supply line via a steel lined hose.
A greenhouse attached to the south side of the east bunker had a partial concrete
base and was framed with storm-windows attached to 2x4s with 2x6 base and
corners. The roof was a lean-to covered with clear fiberglass roofing and lined with
a layer of clear plastic on the inside.
The west bunker was used as a wood shed and for miscellaneous storage. One wall
was partially lined with rusted, steel shelving. The east bunker was lined with
coarse, 1” gravel flooring; the west bunker floor was a mixture of coarse gravel and
sand.
Demolition required two full dumpster containers for the foam, wood, windows,
shelving and miscellaneous junk. Two woodchucks and numerous bats were evicted
in the process.
Figure 1: Greenhouse site after initial concrete work, prior to construction.
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WALK-IN COLD STORAGE
MATERIALS SELECTION
Scaffolding (to hang refrigerator):
4x4 posts
2x6 cross supports
Deck screws
Wall:
2x6 (base)
2x4 (wall)
1 ½” Rigid Foam Insulation (R-Value 7.5)
2” Rigid Foam Insulation (R-Value 10)
4 mil plastic moisture barrier
Mold resistant sheet rock (interior)
Vinyl siding (exterior)
Insulated Steel door
A refrigeration unit from was selected that was sufficient to cool a 12x9 foot room.
The unit & compressor were purchased and installed using a local commercial
refrigeration company.
CONSTRUCTION
A simple scaffolding was constructed to hang the refrigeration unit from the center
of the walk-in. The unit was installed using an accordion lift from a local rental
company.
The wall on the south side was a basic 2x4 construction. Studs were placed
approximately 2 feet apart and the interior was insulated with stacked rigid foam –
one sheet of 1 ½” and one sheet of 2”. A plastic moisture barrier was tacked to the
interior and exterior of the wall.
The interior siding used was mold-resistant sheet rock, with vinyl exterior siding
used on the outside wall. The door to the walk-in is a standard, steel exterior door
with insulated core.
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Figure 2: Scaffolding to hold refrigeration unit, 4x4 & 2x6 frame.
Figure 3: Wall base of 2x6, buried in gravel filled trench.
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Figure 4: 2x4 wall frame.
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Figure 5: Interior wall lined with 4 mil moisture barrier and mold resistant sheet rock.
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Figure 6: Walls are filled with rigid foam insulation; 2 inch R-10 and 1 ½ inch R-7.5.
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Figure 7: Completed walk-in with exterior moisture barrier. Outside will be finished with vinyl exterior
siding.
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DRYING/CURING CHAMBER
Figure 8: Photo facing towards back of east bunker. Styrofoam siding was removed during demolition
phase. The small outboard motor at the lower left of the photo is a Johnson, circa 1960s.
MATERIALS SELECTION & CONSTRUCTION
The Curing/Drying chamber required a lot of clearing out, but nothing in the way of
additional materials. A small electric shop heater was purchased to provide a
secondary heat source to allow for curing during cool fall weather and to maintain
heat during night time.
The original Styrofoam siding was removed to allow for power-washing of the
bunker walls. The importance of cleaning the structure to prevent mildew or fungal
disease was deemed a higher priority than preserving the minimal insulation value
offered by the siding.
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Figure 9: Photo of drying/curing chamber from the rear facing south. Pipe sticking up from floor is
drainage tile.
GREENHOUSE RECONSTRUCTION
MATERIALS SELECTION
Cement blocks and mortar
2 “ steel arches
2” steel poles with mounting feet
1” steel cross supports
2x6 Trex composite decking material for base
Twin-wall polycarbonate sheeting
Teks screws/neoprene washers
Aluminum Tape
Aluminum flashing
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CONSTRUCTION
An existing cement block base was extended on the south and west sides of the
greenhouse. The composite-decking base was mounted to the base using anchor
bolts set in concrete. Composite-decking was also mounted to the garage with an 8”
strip of aluminum flashing to extend over the greenhouse roof.
Figure 10: Greenhouse framing. Cement walls were repaired/extended on south and west sides. Base of
greenhouse was constructed of 2x6 composite decking material.
Steel arches and poles were mounted to the composite decking with mounting feet.
The arches were modified from a pre-fab set purchased at Nolt’s Midwest Produce
Supply. Cross supports were installed at approximately 4 foot intervals, and the
polycarb sheeting was attached to the cross supports using self-drilling screws.
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Figure 11: Finished greenhouse exterior.
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Figure 12: Finished greenhouse interior.
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RESOURCES
Canada Plan Service, 6000 Series.
(http://www.cps.gov.on.ca/english/fv6000/fv6319.htm) provides some basic
principles for walk-in cooler design, along with some other plans for vegetable
storage structures (http://www.cps.gov.on.ca/english/fv6000/fv6000.htm).
http://web1.msue.msu.edu/imp/modp1/morefile/E0104.pdf
Numerous states extension services provide root cellar or cold storage design. This
plan from Michigan (Extension Bulletin No. 104) is from the 1930s, and still relevant
for DIY storage.
http://www.storeitcold.com/
CoolBot has some technology that can use a simple window AC for walk-in
refrigeration. I’m not sure how – or whether – it works, but they do also have some
good DIY notes for cooler design available on their website.
Root Cellaring, Mike & Nancy Bubel. Available through Fedco Seeds
(fedcoseeds.com)