Phase change materials (PCM) have been proved to be an effective tool to regulate indoor temperature fluctuations and reduce the energetic demand of buildings [1,2]. Paraffins are among the most common PCM used for thermal storage in building applications. A broad range of melting temperatures, with an elevated heat of fusion associated, can be obtained varying the length of the hydrocarbon chain that forms the paraffins. Furthermore, they exhibit numerous benefits such as the nearly absence of supercooling, chemical stability, low vapor pressure during the phase change and no phase segregation [3]. These paraffins can be incorporated in several forms to building materials like gypsum or concrete [4]. Despite all these advantages, one major concern regarding the paraffins as PCM is their possible contribution to flame propagation in case of fire [5]. Several authors have tried to improve this aspect by combining the paraffins with HDPE polymers to obtain a form-stable PCM [6]. Blends of paraffin and polyethylene usually contain some inorganic filler, which can act improving both the thermal conductivity as well as the flame retardancy of the system [3, 7]. Microencapsulated PCM or paraffins adsorbed in a substrate are designed to be incorporated directly into the building material and, therefore blending with HDPE is less interesting. Another possibility to increase the fire performance of the PCM is the addition of flame retardants directly into the matrix-PCM mixture. Paraffins usually present their combustion between 220-320ºC and therefore the flame retardants selected should act in this temperature range. In this work we have studied the effect of different loads of PCM on the fire behaviour of gypsum boards. In order to improve the fire behaviour of the gypsum-PCM system, two different types of flame retardants have been added. We have chosen two endothermic flame retardants: aluminium hydroxide (ATH) and a synthetic basic magnesium carbonate, hydromagnesite (HM), obtained from an industrial by-product. Both FR have an endothermic decomposition starting around 200ºC and can be found among the cheapest alternatives to achieve flame retardancy.