Well Pollock, it's very technical and little out of my depth but it's all related to the wavelength of the suns rays, which vary depending on the elevation of the sun in winter vs summer. The low E coating allows more of the winter suns warmth (which has a different wavelength) to penetrate the glass and into the room. Conversely the coating doesn't allow as much of the summer sun's rays to penetrate. Also the coating limits the heat loss from the interior space to the exterior space, as this heat has a different wavelength. That's my understanding of it anyway, maybe someone out there with a phsics/ mechanical engineering background might differ?