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The term "thermal mass" comes up time and time again on building related forums and discussions, yet as a parameter it has one notable feature - it does not really exist. There is no such thing as "thermal mass" and never has been. Mass is a simple physical property, in simple terms it's approximately how much a given volume of something weighs at the surface of the earth. This, in turn, depends on the density of the material. For example, here are some densities for some common building related materials, in terms of the weight at the Earth's surface for a 1m square cube of the stuff (1m2? Brick ~ 2000kg/m2 Concrete~ 2400kg/m2 Plaster and plasterboard ~ 2700kg/m2 Water ~ 1000kg/m2 Structural softwood ~ 550kg/m2 Typical hardwood ~ 700kg/m2 Granite ~ 2700kg/m2 On its own the mass of a given volume of material isn't that useful for working out how much heat it would take to either raise the temperature of the stuff, or for it to give off heat as it cools down. What we need to know is the specific heat of the material, expressed as the amount of heat energy (called sensible heat, which can be measured in Joules, J) needed to change the temperature of a certain mass (lets say 1m3 to match the data above) by 1 deg C (or more correctly a deg K, but it's the same thing for this purpose). So let's list the same materials as above, with the amount of heat energy we need to put into increase the temperature of 1kg of it by 1 deg ? Brick ~ 840 J/deg C Concrete ~ 880 J/deg C Plaster and plasterboard ~ 1080 J/deg C Water ~ 4200 J/deg C Wood ~ 1700 J/deg C (This is an average value, as the true range is dependent on variety, with a wide range, from 1200J/degC/kg to around 2300J/deg C/kg) Granite ~ 790 J/deg C So, if you want to create a house with the highest "thermal mass" (i.e. Heat capacity per unit mass, if that's a reasonable way of trying to define this unknown term), then here is a list of materials, with the highest heat capacity for 1 kg at the top, and lowest at the bottom: Water Wood Plaster or plasterboard Concrete Brick Granite You may well spot a few odd things here. The first is that you cannot build a house with water (but you can include water as a heat distribution or storage system). The second is that concrete, brick and stone aren't great materials in terms of storing heat for a given mass. Surprising, isn't it? Even more so when building professionals keep harping on about the virtues of so-called "thermal mass".