Greenhouse

A greenhouse is a covered structure holding a growing space inside the temperature range a crop can grow in, for more of the year than open ground does. Every crop has a minimum below which it stops growing, an optimum where it grows fastest, and a maximum above which heat damage begins; those numbers differ crop by crop, so there is no correct inside temperature, just a crop and its numbers. A plastic tunnel warms and cools on its own; a greenhouse runs fans and heaters to do it. A tunnel disappoints; a greenhouse breaks down.

Overheating is the failure that catches people, and it is a sunny-day problem, not a summer one: unvented, a covered structure has topped 49 C (120 F) on a clear day in early spring. Venting by eye is not enough: the openings must be large and built into the frame, as they cannot be cut later. Humidity is the failure nobody designs for: shutting the house tight on a cold clear night to save heat is the same act that leaves every leaf wet by morning, because water condenses once a leaf drops below the dew point, a margin that narrows as air gets damper. Wet leaves carry disease; the answer is a short vent at dusk and sunrise.

Fans and heaters are why it needs power and water: an outage in a cold snap fails the building at once. Water drums are the usual heat store, holding about three times the heat of masonry by volume; a full 200 litre (55 gallon) drum weighs over 200 kg (450 lb) and stays cabled to the frame. What that buys is modest: about a month of extra season. The covering ages out before the frame; swapped for shade cloth the same building becomes a shade house for summer, and built against a dwelling a sunspace working with the mass, glazing and banked earth of that wall. Upkeep is daily temperature checks against the crop's numbers rather than comfort, venting morning and evening, replacing the covering and servicing fans and pads.