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Dear Bridgett, here's the response to your question:

Hi Bridgett, 

These types of greenhouses are a good fit for a backyard season extension growing program. There are a few things you can do to help with the cooling and heating. Below you will read the main suggestions to help with these two components. The main thing you are running into is the three feet of aluminum.  Aluminum features high thermal conductivity (heat transfer), which means aluminum conducts heat effectively. However, while aluminum conducts heat effectively, it has a low rate of heat radiation. If you change up the aluminum to the polycarbonate you will see a reduction in heat loss at night and heat gain during the day. 

Cooling:

         Even though the greenhouse is only 150 square feet, you will still want to install air circulating fans to move the hot air out of the greenhouse. With windows and doors on the outside walls without circulating fans, the heat gets trapped in the upper part of the greenhouse and doesn't move. If you rather not add circulating fans, then a suggestion would be to add roof vents to allow air circulation throughout the greenhouse. 

        These added measures will help with cooling only to a certain point before evaporative pads or other cooling devices would be needed. I'm not sure of your location in the state, but if you are on the plains of Colorado mid May would be the time to turn the greenhouse off and move plant outdoors. If you are in the mountains this could be extended to the end of May depending on temperatures of the current year. 

Heating:

"Most heat loss occurs by conduction with the addition of a small amount from convection and radiation. These usually occur simultaneously from the surface of the greenhouse. This component of heat loss can be calculated from the equation:

Conductive heat loss = SA x U x TD

SA is the glazed surface of the greenhouse exposed to the cold. Calculate this by adding the areas (length x width) of the roof, sidewalls and end-walls.

U-factor represents the overall heat transfer coefficient of the glazing (Btu/hr – sq ft-° F). It includes a small value for the convection and radiation component. Typical value for greenhouses with a single layer of glass, polycarbonate or polyethylene is 1.15. Use 0.7 for double poly and 0.6 for double wall polycarbonate or acrylic.

TD is the temperature difference (maximum inside night temperature - winter design temperature) (° F). A good inside night temperature is usually 70° F as this provides heat for melting snow." (greenhousemag.com)

If you would like to send us the dimensions of the greenhouse we can send you the recommended size of heater to use to promote plant growth for season extension. Also do the doors and windows have weather stripping to help reduce heat loss at night?

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