A greenhouse foundation built in a growing zone with below-freezing temperatures will need insulation to protect the greenhouse in spring during frost heaving. A well-insulated foundation will help plants grow in the best conditions all year round.
Step 1: Plan Your Greenhouse Insulation Before You Construct the Foundation
If you know you will need to insulate your greenhouse, include a foundation that you can insulate easily and inexpensively in your greenhouse construction plan. Avoid a wood foundation that will corrode aluminum greenhouse frames as it becomes damp. Insulation around a wood foundation will be ineffective due to the expansion and contraction of the wood. Use a structural solid plastic material for your foundation and sill plates, and foam board greenhouse insulation.
Step 2: Lay Out and Level the Foundation
Confirm how deep the soil freezes in your area in winter, and dig your foundation to below the deepest frost line. If it is greater than 12 inches, dig and build a supported foundation, where the solid plastic beams rest on perimeter piers made of concrete. Measure, compress and smooth out the foundation until it is level. Spread a 1-inch deep layer of sand over the entire foundation base. Install the perimeter and area piers and the solid plastic foundation beams. You will add spray foam insulation, either polystyrene or polyurethane, under the greenhouse floor in the next step. Leave a 1-inch trench 12 inches deep around the external edge of the foundation.
Step 3: Insert the Greenhouse Insulation and the Exterior Sheets
Inside the perimeter of the foundation, spray the foam insulation into the foundation till it is level with the top of the beams. In the trench around the outside of the foundation, insert the 1-inch thick foam sheets. Make sure they are in contact with each other all the way around the outside edge. Pack in dirt snugly around them so they press firmly against the foundation's perimeter beams.
Step 4: Attach the Greenhouse Structure to the Foundation
Using the aluminum sill strips and large bolts, attach the greenhouse walls and door frames to the foundation. Add the roof, and put in the plastic or glass panes.
Step 5: Build the Greenhouse Interior Floor
Place a layer of landscape cloth over the entire floor of the greenhouse before installing the floor. This will prevent weed and grass growth inside. The floor material must maintain correct drainage inside the greenhouse. Gravel, concrete or brick pavers, lava rock (pumice) or white landscaping gravel are the most functional, to ensure drainage and secure footing for the gardener. Light colored pavers and the white gravel also reflect light coming into the greenhouse, reducing the number of artificial light sources needed. Lava rock, brick and concrete pavers will store solar heat, keeping the interior warmer for you and your plants.