Correlation between thermal conductivity and the thickness of selected insulation materials for building wall has been analyzed. The study has found that a relationship between the thermal conductivity (k) and optimum thickness (x(opt)) of insulation material is non-linear which obeys a polynomial function of x(opt) = a + bk + ck(2), where a = 0.0818, b = -2.973, and c = 64.6. This relationship will be very useful for practical use to estimate the optimum thickness of insulation material in reducing the rate of heat flow through building wall by knowing its thermal conductivity only. (C) 2006 Elsevier B.V. All rights reserved.