A baffle for a heat exchanger tube extending between opposed header pipes. The baffle is a longitudinal member with a concave underside shape conforming to the convex circumferential shape of the heat exchanger tube, and has a length substantially equal to the distance between the header pipes. A lip having a convex top side shape conforming to the concave underside shape of the longitudinal member extends along one longitudinal edge of the member. Two such members can be nested together by overlaying the non-lipped longitudinal edge of a one member atop the lipped longitudinal edge of the other member. A plurality of longitudinally aligned heat vent slits extend substantially the entire length of an uppermost portion of the member.