FAQ:General:HeatFlowThroughExteriorSurfaceIsNotRealistic

(20): Heat Flow Through Exterior Surfaces Is Not Realistic

I calculated the sum of the heat flows through the exterior and the interior surfaces during one year and I noted that the heat flow out of the building component through the exterior surface is much larger than the heat flow into the component through the interior surface. But shouldn't they be nearly equal? How can more heat flow out of the component than into it? No heat can be created in the wall.

The solar radiation incident on the exterior surface is electromagnetic radiation and not heat flow; it is therefore not included in the heat flow data.
However, after absorption it is converted to heat so that there exists indeed a heat source in the wall. Since the heat source is close to the exterior surface, most of the generated heat flows outward through the exterior surface, only a small amount flows inward through the interior surface. This asymmetric heat flow is superimposed on the usual transmission heat flow (which in colder climates alway goes from the indoor side to the outdoor side of the building element).

Please note that in the film display the heat flow arrow at the exterior surface does include the solar radiation. Otherwise it would look very strange to see the sun shining on the wall surface but a lot of heat flowing out of the wall. This is a concession to the intuitive expectations of the audience.

Also note that there can be a heat source or sink in the wall when water condenses or evaporates. In some cases these latent heat effects can be non-negligible (e.g. drying of a wall wetted by driving rain).