Phase shift / Thermal inertia

Thermal lag and thermal inertia describe the ability of a wall or building to slow down and mitigate temperature variations. Thermal lag is the time difference between a peak in outdoor temperature (or sunlight) and the perceived peak indoor temperature; thermal inertia is the "thermal mass" that stores heat (or cooling) and releases it later. When properly designed, these two factors smooth out daily temperature fluctuations, postpone discomfort peaks in summer, and stabilize heating in winter.

Phase shift & inertia: operational definition

In a wall, the heat wave propagates through the insulation and load-bearing materials with a delay (phase shift, expressed in hours) and an attenuation (damping factor, or decrement factor). Dense materials (concrete, stone, solid bricks, screed, raw earth) offer high thermal capacity; heavy insulation (wood wool, dense wadding) provides better summer phase shift than lightweight insulation with the same R-value. On the building side, the effective thermal inertia depends on the surface area in contact with the interior air (exposed slabs, partition walls), nighttime ventilation (which removes stored heat), and building management system (BMS) control (openings/external blinds).

Concrete examples:

Façade: frame + wood wool on the outside (phase shift ↑) + external BSO;

Roofing: high specific heat capacity insulation + reflective screen + attic ventilation;

Floors: active slabs (thermal activation) or exposed concrete ceilings;

Offices/schools: night cooling (night ventilation) to recharge the “cold” in the mass.

STD/SED (simulation) allows us to estimate the internal ΔT, degree-hours of discomfort (DH) and the effect of variants (materials, thicknesses, protections).

Phase shift and inertia in projects: advantages, limitations, points to consider

Interests

• Summer comfort: indoor peaks pushed back to evening/night and attenuated.
• Less refrigeration to install/operate (power and operating hours ↓).
• Winter stability: more regular temperatures, reduced heating cycles.
• Robustness against heat waves and urban heat islands.

Boundaries

• Mass = weight & inertia of construction site (structures, drying times).
• Reduced responsiveness: very massive buildings = slower temperature rise upon restart.
• The effect will be degraded if sun protection and nighttime ventilation are absent.
• A very lightweight insulator can have a low phase shift despite a good R-value.

Points to consider

• Combining: inertia + external protections + night cooling; one without the other = limited gain.
• Roofs = priority (strong solar gains); pay attention to light colours and ventilation.
• Materials: aim for high specific heat on the interior side (exposed slabs, partition walls).
• Insulation: for facades/roofs, prioritize products with high phase shift (dense wood wool/fibre, compact wadding) if summer comfort is critical.
• Control: BMS/timers to open at night when the outside air is cooler; locking during nighttime heatwaves.
• Measurement & validation: use STD/SED to quantify DH, cooling power and verify the actual benefit.

Anecdote — “The class that kept its cool in Avignon”

In Avignon, west-facing classrooms overheated every June. Rather than adding split systems everywhere, the team installed exterior blinds, replaced the roof insulation with denser wood fiber, and left the concrete ceilings exposed. A building management system (BMS) activated nighttime ventilation as soon as the outside temperature dropped below the inside temperature. The result: a significant decrease in summer humidity, reduced cooling capacity, and more attentive students at 4 p.m. The headmistress made the decision: "We didn't air condition... we shifted the schedule and absorbed the heat."