An autonomous mechanical testing robot cycled silicone and urethane building sealants through a daily strain cycle based on external temperatures while recording the response loads for each individual sealant specimen. Twice weekly it stopped cycling and measured the apparent modulus of the sealants by applying a step strain followed by a stress relaxation period. The robot was installed outdoors in Madison, Wisconsin, at a 45° angle to the horizon for maximum solar exposure. Next to the robot was a 45° inclined plane with the same sealant placed in racks for static (non-moving) exposure. Thus we had simultaneous outdoor exposure of building sealants with strain cycling and with no imposed movement. In addition, one-third each of the specimens had a tensile prestrain, zero prestrain, and a compressive prestrain. The tensile prestrain simulated a summer installation such that the sealant was installed at its smallest gap due to substrate expansion in hot weather. The compressive prestrain simulated a winter installation when the bond gap was its largest due to substrate contraction during cold weather. These three prestrain conditions had significant impacts on the stress levels the sealants experienced during strain cycling. This research indicates that for some building sealants the installation condition can significantly affect its stability with tensile stress/summer installation being preferable to compression stress/winter installations. Also, the results highlight the overwhelming importance of cyclic strain as a key factor in durability evaluations.