Instructions for Building a Solar Box Cooker

Read the instructions in the previous section, Supplies and Equipment, for information about the materials. After all the supplies are gathered, the first step in constructing the solar box cooker is cutting the cardboard and foil-backed foam board insulation into the correct sizes.

Once the cardboard and foil-backed foam board insulation are cut into the correct sizes, the actual construction begins. The box knife is used to score the cardboard to make neat folds. Elmer’s glue adheres the foil to the cardboard. Aluminum foil tape holds all the pieces together.

Components in completed solar oven: Inner box lid constructed from a five inch strip of cardboard Inner box bottom constructed using the lid of an office paper box Cardboard and foil reflector made of two large boxes Plate glass, 12 1/4” by 18 3/4” Outer box, approximately 15” by 22” and at least 6” deep

Figure 6: Diagram of complete solar oven.

The instructions cover constructing the three-part inner box, which is insulated to trap the heat used for cooking. The reflector, four pieces of sturdy cardboard covered with foil, is constructed next. The last stage is to use the outer box and bungee cords to hold the reflector and inner box together. Use Figure 6, a diagram of the complete solar oven, to help identify the parts.

The instructions are broken down into the following sections:
Preparing Supplies by Cutting Them to the Correct Sizes
-- Cutting cardboard for the inner box
-- Cutting insulation for the inner box
-- Cutting cardboard for the reflectors
Constructing the Inner Box
-- Constructing the inner box bottom
-- Preparing the glass
-- Constructing the inner box lid
-- Insulating and taping the inner box lid
Constructing the Reflector
Assembling the Solar Box Cooker

Notes:

1. Refer to the  Skill Tip:  Gluing Foil to Cardboard  and  Skill Tip:  Scoring and Folding Cardboard  for extra hints.
2. Cut cardboard and insulation on a protected surface the box knife will not damage.  

Caution:

Use care when cutting with the box knife. Avoid cutting yourself by keeping one hand on the handle and the other hand well out of the way.

Preparing Supplies by Cutting Them to the Correct Sizes

Instructions for cutting the cardboard and insulation for the inner box are first. The cardboard for the reflectors is next. The reflectors are an odd shape, and big, but the illustration for them is carefully made to scale.

Cutting cardboard for the inner box. This first piece is the easiest! Cut on a protected surface, and use the straight edge and T-square to make strips of even widths. (I suggest using aluminum foil tape to tape pieces together throughout the project, as all other tapes dry and crack when exposed to high temperatures.)

1. Cut 5” wide strips of cardboard until you have 62” when they are placed end to end

2. Use the aluminum foil tape to tape the pieces together to create the 5” wide and 62” long strip.

Cutting insulation for the inner box. Now that you have practiced cutting 5” wide rectangles on cardboard, cutting the pieces of foil-backed foam board insulation will be easier. I prefer to wear leather gloves when handling insulation to avoid itchy hands.

1. Cut two strips that are 5” wide by 18” long and two strips 5” wide by 12 1/2” long.

2. Next cut a rectangular piece 12 1/2” by 19”.

3. Check the fit; the five pieces should fit around the office paper box lid as shown in Figure 8 in the next section.

Cutting cardboard for the reflectors. The hardest cutting was saved for last. The four trapezoid shaped reflectors form

the reflector illustrated Figure 6 when fastened together. The inner box is rectangular, so two of the reflectors that surround it are wider than the other two. The illustration in Figure 7 shows one of the wider reflectors, with a width of 20” at the narrow base, on the left. The reflector on the right is one of the two narrower reflectors, with a width of 14” at the narrow base. The instructions will cover cutting each size separately.
Figure 7. Cutting cardboard for the reflectors. Cut two of each size.

To create the two wider reflectors, as shown on the left:

1. Cut a rectangle 40” by 35”. Mark one of the 40” sides as the top of the reflector.

2. Measure and mark a horizontal line, parallel to the top, 20” from the top of the reflector. Make a mark 10” from each side along this line. This marks where the next line should intersect.

3. Turn the cardboard so the top is facing towards you. Use the protractor to measure and mark a 22 1/2 degree angle at each end of the 40” width. Make the resulting lines long enough to intersect the horizontal line at the marks drawn in step two.

4. Check the measurements. You should have a trapezoid with a long side of 40” marked as the top, a short side of 20” and a height of 20” drawn on the cardboard.

5. Mark the bottom rectangle shown under the trapezoidal shape in Figure 7. Draw a straight line from the short side of the trapezoid to the bottom of the reflector. The rectangle should have two sides of 20”, and two 15” sides.

6. Cut along the outlines marked on the cardboard and shown in Figure 7. Do not cut along the solid line that connects the trapezoid and the rectangle.

7. Repeat steps 1 through 6 to create the second wide reflector.

The instructions for the two narrower reflectors are very similar, but the entire set of instructions is repeated to avoid confusion. To create the two narrower reflectors, as shown on the right:

1. Cut a rectangle 34” by 35”. Mark one of the 34” sides as the top of the reflector.

2. Measure and mark a horizontal line, parallel to the top, 20” from the top of the reflector. Make a mark 10” from each side along this line. This marks where the next line should intersect.

3. Turn the cardboard so the top is facing towards you. Use the protractor to measure and mark a 22 1/2 degree angle at each end of the 34” width. Make the resulting lines long enough to intersect the horizontal line at the marks drawn in step two.

4. Check the measurements. You should have a trapezoid with a long side of 34” marked as the top, a short side of 14” and a height of 20” drawn on the cardboard.

5. Mark the bottom rectangle shown under the trapezoidal shape in Figure 7. Draw a straight line from the short side of the trapezoid to the bottom of the reflector. The rectangle should have two sides of 14”, and two 15” sides.

6. Cut along the outlines marked on the cardboard and shown in Figure 7. Do not cut along the solid line that connects the trapezoid and the rectangle.

7. Repeat steps one through six to create the second narrow reflector.

You have now cut the pieces necessary to construct the solar box cooker. The next three stages of the construction will not require extensive cutting, but you will need the box knife to score the cardboard so it folds easily.

Constructing the Inner Box

The inner box of the solar box cooker traps the heat used for cooking. The food to be cooked is placed inside the inner box, which is covered with the piece of glass. The sun’s rays enter through the glass top. The heat from the sun’s rays is trapped, heating the inside to temperatures over 200 degrees. It is important that the inner box be very well-insulated and constucted of material that withstand high heat.

The inner box has three parts: the bottom, the glass cover, and the lid. The bottom is constructed by insulating the lid from an office paper box. The glass is checked for fit and strips of adhesive-backed foam insulation are applied to the edges. The inner box lid is constructed by scoring the 5” wide strip of cardboard so it will fold easily, lining it with foil, then folding and taping it in place.

Constructing the inner box bottom. The bottom of the inner box is constructed by insulating the lid of the office paper

box with aluminum foil and foil-backed sheathing. Follow these four steps to create a well-insulated inner box bottom:

1. Line the inside of the box with aluminum foil. Glue the foil to the inside of the box following the instructions in the Skill Tip:  Gluing Foil to Cardboard below.

2. Fold any foil that extends above the top edge of the box to the outside.

3. Fit the 5 pieces of foil-backed sheathing to the outside of the box. Figure 8 illustrates how the five pieces fit around the box. Use a few dabs of unthinned white glue to hold them in place temporarily.
Figure 8. Fit the five pieces of sheathing to the bottom and sides of the box.

4. Permanently attach the sheathing to the outside of the box by taping in place with the aluminum foil tape. Figure 9 illustrates where to tape the pieces in place.

Note: The sheathing has friable fiberglass that will flake away over time. Cover all exposed edges with the aluminum foil tape.

Figure 9. Tape the sheathing in place with aluminum foil tape.

Skill Tip:  Gluing Foil to Cardboard

Follow these steps to glue foil to cardboard neatly: Mix 1/3 part water to 2/3 parts glue. Wet the brush with glue. Apply a thin even coating of glue to the cardboard. Lay a sheet of foil in place. Use a damp paper towel to smooth the foil out. Start in the middle and work out to the edges. Gently press foil into folds Repeat steps 2 through 5 until all surfaces are covered. Wipe up any glue that seeps out at seams.

Preparing the glass. The piece of glass lays on top of the inner box bottom constructed in the last phase. There are two steps to preparing the glass to use in the inner box: (1) check the fit, and (2) apply weatherstripping.

1. The glass which covers the top is 12” by 18 1/2”. Now that the 1/2” thick sheathing has been attached to all four sides of the box, lay the glass on the inner box bottom and check the fit. It should cover the inner box bottom completely. If it does not fit, carefully measure the outside length and width of the inner box bottom and get a piece of glass to fit those measurements.

2. Once you have checked that the that the piece of glass is the right size, apply the 1/2” wide adhesive-backed foam weatherstripping. Place it along the four edges on one side of the glass where the glass rests on the top edge of the inner box bottom, as shown in Figure 10. The weatherstripping seals the gap between the inner box bottom and the glass and keeps the glass from sliding around. The weatherstripping also makes the glass easier to hold on to when lifting it off the inner box bottom so you can get your hot food out of the oven.

Figure 10. Apply weatherstripping to the edges of the piece of glass.

Constructing the inner box lid.  The cardboard lid for the inner box holds the inner box bottom and glass together. The open center allows the sun’s rays to enter the oven. The 5” wide strip of cardboard is first cut and scored. Then foil is applied to the cardboard, which adds extra insulation to the outside of the inner box. Finally, it is folded into a rectangle and taped in place. Follow these five steps to cut and score the cardboard for the inner lid:

1. Use the straight edge, T-square and marking pen to mark a line the length of the cardboard and 1” from one of the long edges of the 5” wide strip of cardboard.

2. The 62” length of cardboard is divided into 4 sections. Mark a line the width of the cardboard 12 1/4 ” from one end. Mark a second line 18 3/4” from the first. Mark the third line 12 1/4 ” from the last line. This last line should be 18 3/4” from the end.

3. Use the box knife to cut through the cardboard at black lines shown in Figure 11.

Figure 11. Cut and score the 5" strip of cardboard for the inner box lid.

4. Score cardboard along the yellow lines indicated in Figure 11. Refer to the hints in the Skill Tip: Scoring and Folding Cardboard below to score the cardboard for neat folds.

5. Check the fit of the inner box lid. Fold it into a rectangle, and slide it over the inner box bottom. It should slide over the bottom easily, but without too much extra space.

Skill Tip: Scoring and Folding Cardboard

Follow these steps, demonstrated in Figure 12, to make neat folds in cardboard: Cut through one layer of the corrugated cardboard with the box knife Completely retract the box knife blade. Use the blade end of the closed box knife to make a channel along the cut surface of the cardboard. Place a straight edge along the channel and gently fold. Figure 12. Score cardboard for neat folds.

Insulating and taping the inner box lid.  Now that the piece of cardboard used for the inner box lid is cut and scored, you can cover it with foil, tape it into a rectangle, and apply a strip of weatherstripping. Follow these steps, using the illustrations for more information:

1. Lay the cardboard out flat, and glue aluminum foil to both sides. Press the foil gently into the score lines.

2. Use aluminum foil tape to seal the top and bottom edges. Cut through the tape at the three 1” deep cuts along the top edge.

3. Fold into a rectangle, and tape as illustrated in Figure 13.

Figure 13. Fold the 5” wide strip of cardboard into a rectangle. Tape the ends together.

4. Fold the top flaps down, and tape into place. Figure 14 shows the completed inner box lid with its open center, which will allow the sunlight into the oven.
Figure 14. Fold the flaps down to make a box lid with open center.

5. Turn the lid so the folded-in flaps are face down on the table. Apply a strip of the 1/2” adhesive-backed foam insulation to the inside corner of the box lid.

Congratulations! Figure 15, from top to bottom, shows the inner box lid, the piece of glass, and the inner box bottom. Check the fit of the three pieces you have made which fit together to create the inner box of the solar box cooker.

Figure 15. The inner box lid, the glass, and the inner box bottom pieces fit together to create the inner box.

If the sun is straight overhead, this inner box alone will heat to temperatures of about 180 degrees in twenty minutes. The reflector, which is built next, increases the amount of sunlight which strikes the box.

The reflector is easy to finish, though a bit cumbersome. The final step of constructing the reflector requires a very large flat space to lay the four pieces out before taping them together. A clear space on the floor is probably best for this last part.

1. Get the four pieces cut for the reflector, and lay them out.

2. Glue aluminum foil to the top trapezoidal shaped parts of the cardboard reflectors, as illustrated by the gray areas in Figure 16. Refer back to the  Skill Tip: Gluing Foil to Cardboard  for tips.

3. Mark a line 5” below the base of the foiled trapezoid, as illustrated by the yellow line in Figure 16. Score and fold the cardboard along this line. Refer back to the Skill Tip: Scoring and Folding Cardboard  if necessary.

4. Lay the four reflectors out, alternating wide and narrow, in the shape shown in Figure 17. Using aluminum foil tape, tape the reflectors together along three of the angled sides.

Figure 16. Glue foil to the four reflectors. Score the cardboard along the yellow line, 5" below the base of the trapezoid.

Figure 17. Tape the four reflector pieces together.

The reflector is now completed. The final step of folding it into the rectangular funnel shape that reflects light down into the solar oven is included in the last stage of construction, assembling the oven. Follow these four steps to assemble the solar box cooker:

The inner box and the reflector are now completed. The outer box does not require any construction. If it is bigger than 16” by 22” or deeper than 6” you may need to use some wadded up newspaper or scrap cardboard to hold the inner box and reflector in place. The bungee cords and clothespins are used to hold the reflector closed around the inner box.

1. Fold the reflector along the taped seams to form a rectangular funnel shape, as illustrated in Figure 18.

Figure 18. The reflector folded into a rectangular shape.

2. Fold the bottom of the reflectors along the score line to form a base.

3. Place the reflector in the outer box and ask someone to hold it in place.

4. Place the inner box into the center of the reflector. It is heavy enough that it will anchor the reflector in place. Fasten the reflector closed with bungee cords at the base and clothespins and string along the top edge.

The construction of the solar box cooker is now complete. The next section, Tips for Use, will guide you through the first heating of the oven in the sun, which will drive off any fumes from the construction materials, and allow you to check the oven’s performance. After the box cooker’s first heating, it is time to cook and experiment.

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