Heat capacity is a measure of how much heat different types of material can hold. For a given building element, it is found by multiplying the density of its material by its overall thickness, and then by its specific heat. Specific heat is the amount of heat a material can hold per unit of mass. The greater the specific heat, the more energy is required to heat up the material. Water has one of the highest heat capacities at around 4.2 kJ/kg°K, while most building materials vary from around 0.8 to 1.3 kJ/kg°K. Another property of a material, as discussed in the thermal resistance topic, is its conductivity or the ease with which heat can travel through it.

Both heat capacity and conductivity give rise to what is known as the thermal mass effect. In large heavyweight materials, it can take a significant amount of energy to heat up their surface. This is because much of the energy is actually absorbed deeper into the material, being distributed over a larger volume. With a lot of energy incident on the surface, this absorption can continue until it travels through its entire width, emerging on the inside surface as an increase in temperature. This conduction process can take a significant amount of time, sometimes in the order to 10-12 hours with a thick masonry wall. If the energy incident on the outside surface fluctuates, this can set up 'waves' of temperature flowing through the material.

Figure 1 - The thermal mass effect for some example building materials. To view a dynamic simulation, click the clock icon and drag the outside and inside temperatures to different values. Choose from the four materials to see how the effect varies.

The illustration above shows how different materials respond to surface temperature fluctuations. You should note that, due to the surface air film resistances, the actual surface temperatures often do not match the adjacent air temperature. This is also an important aspect of the thermal mass effect.

Thermal Lag and Decrement

The time delay due to the thermal mass is known as a time lag. The thicker and more resistive the material, the longer it will take for heat waves to pass through. The reduction in cyclical temperature on the inside surface compared to the outside surface is knows and the decrement. Thus, a material with a decrement value of 0.5 which experiences a 20 degree diurnal variation in external surface temperature would experience only a 10 degree variation in internal surface temperature.

The result is an overall reduction in the level of heat flow as well as a delay in its occurrence, as illustrated immediately below. This example uses a wall comprising two layers of 10mm plaster either side of an increasing thickness of brickwork. In the animation, the brickwork increases from 10mm to 200mm. The lag therefore varies from 0.5hrs to 6hrs and the decrement from 0.97 down to 0.4. The flows are based on a simple rectangular zone with a single south-facing window and a highly insulated roof.

Figure 2 - A comparison of hourly heat flow in a small space with increasing thickness of masonry wall (from 10mm to 200mm).

Designing for Thermal Mass

This effect is particularly important in the design of buildings in environments with a high diurnal range. In some deserts, for example, the daytime temperature can reach well over 40 degrees. The following night, however, temperatures can fall to below freezing. If materials with a thermal lag of 10-12 hours are carefully used, then the low night-time temperatures will reach the internal surfaces around the middle of the day, cooling the inside air down. Similarly, the high daytime temperatures will reach the internal surfaces late in the evening, heating the inside up.

In climates that are constantly hot or constantly cold, the thermal mass effect can actually be detrimental. This is because both surfaces will tend towards the average daily temperature which, if it is above or below the comfortable range, will result in even more occupant discomfort due to unwanted mean radiant gains or losses. Thus in warm tropical and equatorial climates, buildings tend to be very open and lightweight. In very cold and sub-polar regions, buildings are usually highly insulated with very little exposed thermal mass, even if it is used for structural reasons.

Related Links

Thermal Mass and R-Value

Making Sense of a Confusing Issue: http://www.buildinggreen.com/features/tm/thermal.html

Passive Solar Design - Thermal Mass

http://www.epsea.org/mass.html

Good Residential Design Guide - Technical Manual - 1.7 Thermal - Mass

http://www.greenhouse.gov.au/yourhome/technical/fs17.htm

Thermal Mass - Energy Savings Potential in Residential Buildings

http://www.ornl.gov/roofs+walls/research/detailed_papers/thermal/banner.html