1)  the fact that glass and other transparent / translucent buildingmaterials, and the earth’s atmosphere, are relatively transparent to the high frequency radiation of the sun’s rays, while being relatively opaque to the low frequency radiation (heat) given off by building materials, contents and inhabitants, etc. and the earth’s surface in general; 

2)  the global warming effect that climate researchers warn of in relation to the pollution of the earth’s atmosphere by CO₂ and NOx gases etc., and thus its increased opacity to low frequency radiation.