I built this 10’x20′ greenhouse 20 years ago, from my own design, using recycled tempered glass (old sliding door windows) and cedar lumber. At the peak of each gable end, I installed a small triangular window connected to a Univent heat-sensing hydraulic arm. This opens the windows wide for ventilation if the inside temperature gets too high, allowing adequate temperature control in all but the hottest weather. The entire cost of construction (including the expensive hydraulic arms) was $400. The structure has held up remarkably well over the years.