A heat exchanger for transferring energy from a second fluid to a first fluid has a plurality of tubes to carry one of the fluids and a layer of thermal buffer members on one side of the tubes to act as a buffer to reduce high thermal stresses that may be caused by global temperature patterns and localized high gradients that occur in system transients such as start up and shutdown. Conventional heat exchangers often have a short lifespan due to high thermal stresses acting on the core of the heat exchanger. The heat exchanger uses the thermal buffer members as an increased thermal mass to cause the temperature of the heat exchanger core to change in a more gradual manner, thereby reducing thermal stresses.