Yes, GAHT and Walipini greenhouses offer lots of heating benefits. There are some nuances to these foundation types though.
If you are looking to grow fruits and vegetables year-round, but live in a variable climate, you may be considering options for energy-friendly heating. There is a general rule of thumb
that greenhouse lighting is much cheaper than heating, so let’s explore the idea of underground greenhouses!
Soil below your local frost line has a consistent year-round temperature and water barrels inside your greenhouse can quickly store solar heat gain which builds up during sunny days. You have varying options depending on your budget.
Disclaimer: Building a Walipini of GAHT (Ground to Air Heat Transfer) greenhouse is not your average greenhouse build. It is highly technical and not for the everyday backyard gardener.
Walipinis and Underground Greenhouses
Typically, a Walipini foundation will be recessed between 4 - 6 feet below ground level. The interior walls can be constructed from any material that is able to absorb heat well such as stone, mud bricks, or any dense and sturdy natural materials.
The greenhouse roof above ground captures radiant solar heat gain within the greenhouse. As the internal greenhouse zone heats up, the hot air will slowly transfer the heat into the materials inside of the greenhouse zone via convection and conduction, including the surrounding foundation and soil. At night, the heat is released back into the greenhouse. Effectively, during a sunny day the thermal mass inside the greenhouse will cool the greenhouse (by absorbing heat) and will start heating the greenhouse as the sun sets (by releasing that stored heat). Depending on the area you live in however you may still need to use an added heat source in cold snaps.
Ground to Air Heat Transfer Systems (GAHT) and Geothermal Greenhouses
Sometimes known as a climate battery, these air-tube circulation systems transfer heat that builds up inside the greenhouse and exchanges it with the soil under the greenhouse. The Ground to Air Heat Transfer System (GAHT for short) is installed underneath a greenhouse, roughly 2 – 5 feet below the floor. These systems use the soil to moderate the heating, cooling, and humidity of the greenhouse. Make sure you have good drainage around the exterior of your foundation, so you don’t have any pooling water filling your GAHT.
When solar energy is collected inside the greenhouse, fans move air from the interior peak of the greenhouse and circulate the warm air underground through perforated tubes and back
up the floor of the greenhouse. The heat gets slowly transferred to the soil for later use, and the air is then recycled back into the greenhouse to maintain a steady temperature. When
temperatures drop, the air continues to be recycled through the tube ducts, picking up the heat that was previously stored and what naturally occurs below the surface. While maintaining the temperature, the airflow also collects humidity in the air and transfers it directly to the ground as water droplets!
The ground in which the tubes store the heat should be prepared with a combination of crushed gravel and a screen-like material. The perforated tubes underground are wrapped in a screen and placed with crushed gravel filling around them. Air from the tubes can circulate around the spaces between the gravel and permeate the ground. Any condensation that occurs must have a way to drain out of the tubes into the surrounding ground.
If you are currently building a new home and interested in optimizing the greenhouse using geothermal energy collection, it’s a great idea to apply the same benefits to your greenhouse. However, it is recommended that gardeners also employ other methods to retain heat to reduce the size of the heat pump required. Some of these methods include reducing air gaps, installing
energy curtains, insulating the exterior of the foundation perimeter, using air circulation to prevent hot and cold air pockets, staging plants according to temperature ranges (cold loving
plants lower to the ground and tropical plants that require high temperatures above bench height) and installing electronic temperature controls to ensure optimum performance. Utilizing geothermal heat pumps and GAHT systems are a long-term benefit as it is expected that it will take approximately 10 years to payback the initial investment based on average fuel costs.