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A Backyard Greenhouse from Old Windows

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1Where it goes matters as much as what it's made of

A greenhouse used mainly for starting transplants or growing plants to full size needs maximum sun and good air drainage — skip low spots boxed in by trees or buildings. One meant more for summer propagation does better with some shade, since a glass or plastic box in full sun overheats fast; the north side of a big deciduous tree is a genuinely good, sourced compromise site.

2What a comparable small greenhouse costs

A standard 12-by-14-foot (168 square foot) DIY hoop greenhouse in PVC pipe and plastic film runs about $100 in materials and lasts 10 to 12 years before the frame needs rebuilding (the plastic film covering wears out sooner than that). A salvaged-window build trades that purchased film for free glass, at the cost of more cutting, fitting and caulking labor — no extension source prices that trade-off directly, but the frame-and-siting rules below apply either way.

3The fogged-up window is a humidity problem

Warm, moist greenhouse air condenses the moment it touches a cooler surface — exactly what fogs up glass or film. Humidity above 80% is where fungal problems like tomato leaf mold start taking hold; below 70% the risk is slight. The fix is airflow, not a drier plant: even in winter, a greenhouse needs two to three full air changes an hour to keep humidity down without overheating from the extra heating load that more ventilation demands.

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