Before building an actual greenhouse, it is good to plan thoroughly and make a model of exactly what you want. Making a model avoids unforeseen problems during the building of the actual greenhouse. Some greenhouses are very simple walk-in cold frames, others are high-tech with automatic temperature and humidity controls, and the others are everything in between. Regardless of what you want in your greenhouse, making a model first is a great planning tool.
Make your model to scale. For example, if your greenhouse will be 10 feet by 12 feet, your model needs to be 10 inches by 12 inches, or 20 inches by 24 inches, or 5 feet by 6 feet. For every foot of greenhouse space, choose a scale measurement for your model that equals one foot.
Use graph paper to draw your model to scale. Draw the floor plan on one piece of graph paper, the sides on another, until you have drawn each section of greenhouse to scale.
Create the base with clay on a piece of cardboard. Use the clay to form a rectangular outline of the base of your greenhouse. Add the flooring. For example, if you are going to cover the floor of your greenhouse with pea stones (for proper drainage) line the model with small pebbles or sand.
Cut shelves out of pieces of cardboard, and cut out little legs. Glue or tape the legs to the shelves, and build as high (to scale) as you want the real shelves to be. Place your cardboard model shelves in the model greenhouse.
Build the frame with craft sticks. Press the sticks into the corners and sides of the clay, just as you would line up a real frame.
If you are building a model greenhouse with a pointed roof, the entire frame is built with craft sticks and masking tape. If your greenhouse is dome-shaped, use craft sticks for the sides, front and back, and pipe cleaners for the domed top. For larger scale models, twist the ends of two pipe cleaners together to make a longer pipe cleaner. See illustration.
Glue or tape toothpicks where you want windows (if any) and at least one door.
Stretch plastic wrap over the structure for a simple walk-in cold frame, just as you will stretch polycarbonate sheeting over a real greenhouse frame. If the real greenhouse will have glass or poly panels, cut sheets of plastic acetate to size and attach to the frame with glue. (You can buy clear plastic acetate at craft stores.)
Include in the model any temperature or humidity control planning that you want for your greenhouse. For example, if you will run electricity to your greenhouse, decide where you will run the wires, and indicate with pipe cleaners. If you expect to put a round fan in the back of the greenhouse, cut a circle of cardboard and attach it with tape where the real fan will be set.