basics on high thermal mass walls and floors
In sunny climates with hot days and cold nights, strategies involving high thermal mass walls and floors should be considered to get energy efficient homes.
Thermal mass and materials
High thermal mass materials - stone, cement, etc. - can store the heat during the sunny hours (preventing overheating), and release it slowly when temperatures are lower (at night). These materials can be used with advantage to avoid overheating (and to heat the house in colder periods).Materials like concrete, aerated concrete, ceramic tile, brick, stone and masonry have high thermal storage capacities: they are good at storing heat during the sunny hours (acting like a sponge), and at releasing it slowly in a later moment, typically during the night.
On the opposite side, wood (timber floors…) and insulation materials are good examples of materials with low thermal mass: they do not absorb/release heat or cold in a significant degree (that's why strategies based on high thermal mass materials and principles are unsuitable in many climates and are in opposition with strategies using insulation: see Thermal Mass vs. Insulation).
High thermal mass materials
Materials with high thermal mass (materials able to store the heat in the sunny hours, and to cool and release it slowly, at night) can be used in walls and floors...
- to absorb solar heat during the sunny hours, avoiding home overheating;
- and to release the stored heat during the night, when the house is cooler, heating it.
Sun-facing surfaces
To get the best results, in northern hemisphere countries, high thermal mass floors and walls should be mostly located in south and east and west-facing floors/walls (or, in the southern hemisphere, in north and east and west-facing floors/walls).
Colors
Color matters when using thermal mass strategies. Moderate-dark or dark colors are best suited for solar thermal storage. In other words: floors, tiles, walls or roofs with dark colors will help thermal storage. White or close to white colors will have the opposite impact.
Amount of Wall and Floors thermal mass
Some studies recommend specific percentages of thermal mass, according to the type of building and climate zones. But the exact solution depends mostly on budget reasons. In places and climates where the use of thermal mass is advantageous, the rule is generally a simpler one: the more thermal mass the better.
How to Use thermal mass
The figures below (from the Australian Government) show how high thermal mass walls and floolrs can be used in southern hemisphere countries, in suitable climates. In northern hemisphere countries instead of north-facing walls, we should use a south-facing sun orientation. The strategy uses large overhangs and sufficiently large windows on the sunny side of the home, and is based in a floor with high thermal mass.
Obviously, such strategy isn't advantageous in most climates. It demands sunny days and cold nights, during most of the year. Only in such conditions adding thermal mass to walls or floors may help reduce extreme indoor temperatures, making the home more comfortable.
High Thermal mass floors and walls vs. Wall insulation
The use of high thermal mass walls and floors can be contradictory with the use of insulation and the principles of modern super-insulated and airtight homes. In other words: in most climates, namely colder ones, where large amounts of insulation are crucial, the use of thermal mass in walls is at odds with insulation-based strategies.
Thermal mass principles are inadequate in colder climates and also in tropical countries or in regions with low day-night amplitude of temperatures.
Trombe walls
In colder climates with a reasonable number of sunny days, trombe walls are one of the few advantageous strategies based on thermal mass principles.
See also: