We decided to build an underground cold frame as nursery for the springtime garden. During the last weeks of winter is necessary to start planting seeds that will later be transplanted into the garden. Temperatures here can lead to freezing during the night. The cold frame must have enough insulation to maintain a higher indoor temperature and prevent the plants from die.
The idea evolved from a design that includes a trench inside, using the terrain as support. The place where is doing to be done faces the south, and has the old channel wall just behind. The foundations of the wall are stones, in which we excavate. Therefore, the thermal mass of the wall heated by day, should provide certain temperature into the cold frame. To isolate more at night, we will place a door with double glazing.
Keep reading to know more…
We dug a trench in the chosen place following the measures that we designed. It was hard digging because the foundation stones of the wall channel. The place is a slope, so the sun rays will get the inside of the box. After that, we built a wooden structure to support the door. This structure fit into the trench we dug before. To fill the gap between the ground and the structure, we use the same soil and rock already we dug out. The door consisted of a wooden frame made of pine slats 8×8 cm and a handmade double glazing system to generate a more effective insulation. On the back wall, we put brilliant steel sheets, recovered from an industrial refrigerator, to reflect sunlight back inside. This is effective when the sun is low on the horizon and the angle of incidence on the coldframe is small.
Inside the cold frame was created a trays system to manipulate and move outside the plants easily when the outdoor temperature is mild.
This winter hasn’t been particularly cold. Temperatures in the early months of the year have just been less than 10 degrees. In tests we could measure a temperature difference between outside and inside about 10 degrees. More than enough to keep plants warm on colder mornings.
Visit the complete photo album: