In order to predict the variation in heat transfer coefficient setup by natural convection, independent CFD simulations have been performed for various tall slender vertical geometries (100 mm < H < 1000 mm) with varying gap widths (5 mm < L < 84.7 mm) and temperature differences (5 K < Delta T < 90 K) covering the ranges reported in the literature (Batchelor, G.K., 1954. Heat transfer by free convection across a closed cavity between vertical boundaries at different temperatures. Quart. Appl. Math., 12, 209-233; Newell, M.E., Schmidt, F.W, 1970. Heat transfer by laminar natural convection within rectangular enclosures. Trans. ASME C: J. Heat Transf., 92, 159-167; Yin, S.H., Wung, T.Y., Chen, K., 1978. Natural Convection in an air layer enclosed within rectangular cavities. Int. J. Heat Mass Transf., 21, 307-315; Elsherbiny, S.M., Raithby, G.D., Hollands K.G.T., 1982. Heat transfer by natural convection across vertical and inclined air layers. Trans. ASME J. Heat Transf., 104: 96-102; Lee, Y., Korpela, S., 1983. Multicellular natural convection in a vertical slot. J. Fluid Mech., 126, 91-124) and compared with their own experimental and numerical studies. A good agreement of Nusselt number (+/- 10%) has been found between the CFD predictions and the literature data. Further, simulations were carried out for gap widths (5 mm <= L <= 25 mm), heights (100 mm <= H <= 1000 mm) (4 <= AR <= 200) and temperature differences (20 K <= Delta T <= 90 K, 5.99E + 02 <= Ra <= 3.15E + 05) and a correlation for the estimation of Nu has been proposed. The sensitivity analysis shows %deviation of heat transfer coefficient only in the range of 10%. A generalized correlation based on all the above results encompassing the effect of height, gap width and temperature difference has also been proposed, which can be used to accurately estimate heat losses from a vertical air gap acting as an insulation. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.