Q&A for How to Build a Greenhouse

Typical costs for a 12'x12' greenhouse are about $3,590.00 - about $25 per square foot. For a 6'x8' structure, $482.00. For an 8'x8', $647.00.

Define energy performance index. Does food grow in it? That is really all you need to know. Don't over-complicate it.

You can, but it will not last as long. With wood in direct contact with the ground, it is best to use either rot resistant wood (cedar is commonly used in greenhouses for this reason) or treated lumber. Normal framing lumber will rot significantly sooner than either of these options. Even the lumber not in contact with the ground will not last as long in a greenhouse due to the highly humid conditions.

You'll need a minimum of one meter spacing in order to walk in between, perform possible repairs, allow for air flow, etc. You'll need more space if you're employing fans or other large equipment.

20x20 is not really enough information for calculating the EPI of a building. I think you are looking for a rating based on how much energy is used by kWh/annum but there are so many variables to calculating that it's almost impossible to determine unless you are a professional in the field and examining the building. A greenhouse that is used to grow native plants and is an actual building (as opposed to just plastic over a frame), that is insulated, vented, and well designed can be close to a zero energy user or even an energy producer with the use of alternative energy production on site and the reuse of waste materials. If poorly designed, however, it can be an energy hog.

I would strongly recommend a reinforced clear ploy film. You can find them on various greenhouse supply websites.

You will need an evaporative cooling system (aka swamp cooler) that can be put in front of the circulation fans. You may need to install an external evacuation fan as well to prevent over humidifying the room. Evan coolers are usually built with a frame that holds cellulose pads -from 1 inch to 1.5 feet thick, and a water distribution system to keep the pads wet. These should only be allowed to run when there is excessive heat, otherwise you can "freeze" the growth of the plants, or introduce leaf and root rot.

Tomatoes can be planted in soil within a greenhouse, but this may lead to pathogenic diseases that damage crops. Since the 1960s, tomatoes are often grown hydroponically to avoid soil-borne pathogens.

There are more greenhouses in the northern hemisphere than in the southern hemisphere, but your point is valid. In the southern hemisphere, a greenhouse would typically be oriented south to north, with sunlight coming from the north.

If you plan to use the greenhouse in a location with hot summers, the smoked polycarbonate will help shade the greenhouse and reduce summer temperatures. Clear polycarbonate would be better to maximize light for year-round plant production.