The summer solstice sun hits the south wall at a very oblique angle. I don't think it's imparting very much energy to the wall; when I put my hand on the wall just now (1:30pm), it wasn't unpleasantly hot, although I singed my feet on the relatively light stone walkway. Earlier and later in the year we'll get more energy from the sun on the wall. So it seems like the main thing is to keep the summer sun from shining into the interior of the house through the windows during the hot season, but allow it to shine through during the cold season. At the depths of winter, the sun will be striking the wall at a much less oblique angle as well, so it will absorb a much larger percentage of the sun's energy.