Our 2"x6" foam panel walls have an R value of approximately 22. The R value of the glass in front of our solar heaters is approximately 2, and the 4 inches of air space between the heaters and the glass is about 1, for a total R value of 3, so compared with a solid wall the R value is substantially reduced. Given this rather large difference, why won't these solar heaters cause more heat loss at night than heat gain during the day?
The first thing to keep in mind is that most of the heat we capture from the sun comes in the form of light, actual photons that contain energy that is absorbed by our specially coated water tanks inside the window. Photons are one of the three ways heat energy can be transferred and are the only active method, and of course, there are none emanating from our heater water tanks.
The other two methods of heat transfer are conduction and convection. WikipediA defines these as follows:
Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems.and
Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid.In the case of a home, convection is by means of the air circulating in the room.
Our solar heater tanks are mounted such that the air space between the tank and the window is sealed, so there should be no air movement to transfer heat from the tank to the glass, which leaves only the possibility of conduction. During the dark hours conduction will transfer some heat to the dead air between tank and window, but because the air is not moving it will be progressively colder the further from the tank. The interior face of the tank does have air moving, so heat will be transferred to the room by this moving air.
Most of us have experienced how this works when standing by a campfire on a cold evening. The side that is facing the fire will be warm, often too warm, while the side not facing the fire will feel as cold as if the fire was not there at all.