Physicist: A greenhouse is a little like a “light pump” that works because of the picky transparency of glass. The heat you feel from radiators, fires, or people is so red that it’s below the red we can see. Glass appears transparent to us because visible light passes through it, but visible light is only a tiny piece of the spectrum and no material is transparent to all light. Warm glass conducts heat directly into the air around the greenhouse and also radiates light like any other material. So, the theoretical limit to how hot a greenhouse can get is limited by the color of the glow of the things inside as they heat up.