Thermal mass
Thermal mass is the heat a building’s materials can store. When the air warms, the walls and floors absorb some of that heat, and when the air cools they give it back. The more mass there is inside the insulation, the slower the indoor temperature moves.
A useful way to compare houses is the time constant: the heat stored per kelvin (a one-degree change in temperature) divided by the heat lost per kelvin. It is a rough measure of how long a house takes to drift toward the outdoor temperature once the heating stops. A heavy, well-insulated house has a long one: hours or days. A light, leaky house has a short one.
Only part of the mass works each day
Heat from a daily cycle reaches only a limited depth. The standard measure is the periodic penetration depth (EN ISO 13786): the depth at which a 24-hour temperature swing has faded to about a third of its surface size. For concrete that comes to roughly 10 to 15 cm, depending on the mix. The top part of a typical 15 cm slab follows the daily swing, and the rest responds over several days. Extra thickness helps less than the heat capacity figure suggests, and a floor covering that insulates the surface would reduce the benefit.
What mass does to peak temperatures
In summer, mass absorbs sun and internal gains during the day, so the indoor peak is lower and arrives later than in a light house with the same windows. At night the stored heat comes back out. If the night is warm and the house is closed, the evening stays warm, and the next day starts with less room to absorb. Mass delays and flattens a heat load. It does not remove it. External shading, ventilation and cooling decide how much heat reaches the mass and how much leaves.
In winter, mass holds afternoon sun and keeps the house steadier through the evening. It also slows the response to the thermostat. With the default night setback (the thermostat target drops from 21 °C by day to 19 °C from 22:00 to 06:00; you can change the two temperatures, not the hours), a heavy house cools slowly overnight and takes longer to recover in the morning. Underfloor heating makes this clearer because the pipes sit in the slab. The heat source warms the slab first, and the air follows.
What the model does not do
The thermostat follows its schedule and nothing else. The model does not preheat the house ahead of cold or expensive hours, and the electricity dispatch (the step that decides where each kilowatt-hour comes from) does not use the slab as storage. The heat the house needs is worked out first, and dispatch takes it as given.
Limits of the model
- Construction mass is not an input. The walls are one generic masonry layer inside the insulation, whatever the construction year, so a brick house and a concrete-block house are modelled the same way.
- Timber-frame houses are not covered yet. A timber frame stores much less heat than masonry, so a run would overstate how steady such a house stays.
- Interior mass has a fixed size and material. Heavier or lighter interiors cannot be tested.
- Furniture, carpets, curtains and suspended ceilings are not modelled separately, though they affect how well the mass works.
- Each floor is one thermal zone, so mass is shared across the whole floor and not distributed room by room.
- To judge the effect, compare runs with the same weather and different thermostat temperatures or shading, and read the hourly indoor temperature.