Passive heat transfer from enclosures with rectangular fins is studied both experimentally and theoretically. Several sample enclosures with various lengths are prepared and tested. To calibrate the thermal measurements and the analyses, enclosures without fins (“bare” enclosures) are also prepared and tested. Surface temperature distribution is determined for various enclosure lengths and heat generation rates. Existing relationships for natural convection and radiation heat transfer are used to calculate the heat transfer rate from the tested samples. The theoretical results successfully predict the trends observed in the experimental data. It is observed that the contribution of the radiation heat transfer is on the order of 50% of the total heat transfer for the tested enclosures. As such, a new correlation is reported for calculating an optimum fin spacing for vertically-mounted uniformly finned surfaces, with rectangular straight fins that takes into account both natural convection and radiation.