Abstract: The walls of a stable building bound an animal shelter in the building and normally separate the shelter from the ambient atmosphere. A vertically extending wall portion includes an outer shell and an inner shell which define a space therebetween. An opening in the inner shell connects the space with the shelter. A heat exchanger mounted on one of the walls defines first and second, separate conduits. The first conduit connects a portion of the afore-mentioned space remote from the opening with the atmosphere for discharge of exhaust air from the shelter. The second conduit connects the atmosphere with the shelter for flow of fresh air into the shelter in thermal contact with the exhaust air.