Despite the large economic and ecologic costs, museum storage spaces are often equipped with extensive air conditioning, to provide the desired stable interior climate. The new "passive conditioning" paradigm aims at resolving these costs: a high-hygrothermal-inertia building with a high-hygrothermal-resistance envelope is to satisfactorily stabilise that interior climate, with no need for mechanical air conditioning hence. This paper first studies the reliability of full passive conditioning for museum storage spaces. It is shown to be an illusion, since it usually results in excessive interior humidities. Auxiliary dehumidification is thus required to provide good conservation conditions, and it is secondly investigated how the dehumidification load can be diminished. The study reveals the crucial impact of air tightness, while thermal insulation only has a minor influence. To further reduce the economic and ecologic cost of conditioning, the paper finally assesses "concentrated dehumidification": dehumidification during a part of the day only, while leaving the humidity free-running during the rest of the day. It is established that the hygric inertia of the interior air, building walls and stored objects keeps the fluctuations of the interior relative humidities to a minimum when considering adequately air tight museum storage spaces. (C) 2012 Elsevier B.V. All rights reserved.