Seasonal heat storage technologies are the key for a widespread use of solar thermal energy in residential applications. This can be achieved using hygroscopic salts encapsulated in a porous matrix with a high pore volume. Using strontium bromide encapsulated in the mesoporous MIL-101(Cr) Metal-Organic Framework, a heat storage density of 233 kW h/m³ could be achieved, by using water vapor at a partial pressure as low as 1.25 kPa. This excellent result is partly due to the high salt content (63 wt. %), but also to unexpected modifications of the water sorption isotherms of SrBr2 once it is encapsulated. At a temperature of 30 °C, ideal for space heating in low energy dwellings, it may be suggested that the salt is partially soluble upon water sorption.