Building a Cold Frame

Cold frames aid growing plants in two ways. First, the
devices admit sunlight to enable seeds to sprout
into up-and-coming edibles. And, in much the same fashion
as do solar collectors, they convert solar energy to heat
… in this case to maintain air and soil temperatures which
are conducive to growing. Consequently, the two major
concerns when designing a cold frame are to let the sun’s
rays in and to retain heat.

In keeping with state-of-the-art energy efficiency, we have
designed a passively solar, earth-sheltered, and
well-insulated cold frame. The box is built from a sheet of
inch-thick, foil-backed foam insulation board. Using the
technique developed while constructing MOTHER EARTH NEWS’ Heat
Grabber and used
frequently in subsequent solar projects, we trimmed out 7″
corners from a 39″ X 61 1/4″ section of the insulation, and
carved 90° grooves in the foam so that the sides and
ends could be folded up without our having to cut the
exterior foil. (Note that — in order to achieve the proper angle
— the slicing tool must be drawn first along one side
of the half-round-molding guide, then reversed to
cut along the other side.)

Once the insulation board was shaped to the proper dimensions,
we further insured a good seal
by lining the joints — inside and out — with
metal tape.

Two triangular frames built from 2 X 2 lumber hold up the
corrugated fiberglass lid, and the 18″ X 18″ X 25″ (outside
dimensions) sides describe an isosceles triangle. Once the
pieces have been mitered and screwed together, tack on
18″-long strips of ripple board, and connect the two frames
by butting 44″ lengths of 1 X 2 between them.

The glazing adds additional stiffening to the lid assembly,
and both of the 19 1/2″ X 50″ sheets and the 4″ X 50″ ridge
cap can be cut from a single 10′ piece of corrugated
translucent fiberglass. Attach the sheets to the ripple
board by drilling 1/8″ holes in the material and sinking
No. 6 X 1/2″ sheet metal screws — fitted with rubber
washers from aluminum roofing nails — into the
openings. Then pop-rivet the fiberglass cap over the joint
between the two sheets at the peak of the roof and
seal each fastener with a dab of silicone caulk. An
additional application of sealant between the ripple board
and the fiberglass, and along the seam where the material
contacts the horizontal 1 X 2’s, will help to retain heat.
To close in the ends of the cold frame’s lid, cut out two
18″ X 18″ X 25″ triangles of 3M Flexigard (or a similar
material) and staple the plastic — spacing the
fasteners three inches apart — to the ends of the
frame.

Though you could simply set the lid atop the box,
we decided that a trio of hinges and a handle would provide
a more convenient arrangement for opening the cold frame
during the heat of the day. Three standard hinges were
attached to the outside of the insulation box and the 1 X 2
on the frame, and the 1 1/2″ screws and nuts were backed
(on the inside of the box) with 2″ X 3″ pieces of 1/8″
plywood. On the other side of the lid, we fastened a 1″ X
8″ chunk of leather to the 1 X 2, using panhead screws, to
serve as a handle.

To put the cold frame to work, simply prepare a 25″ X 48″
hole that’s 6″ deep, cut three 1″ holes in the bottom of
the box for drainage, and set it into the pit. Then open
the lid and fill the container with about five inches of a
combination of the earth that was removed from the hole and
whatever fertile soil you prefer. With the ground and
insulation preserving a favorable environment for growing
and the sun beaming down in the daytime, you should be able
to start seeds safely three weeks before the last frost
predicted by your agricultural extension office. And once
the final freezing chill has passed, you can set out
thriving seedlings in your garden, ready to resist
pests and grow their way toward your dinner plate!