Summer comfort (overheating)

Summer comfort (overheating) refers to a building's ability to remain comfortable during hot periods without excessive air conditioning or a decrease in usability (unproductive offices, uninhabitable living spaces at night). This involves controlling indoor temperatures by addressing the building envelope, solar gain, thermal inertia, ventilation, and system regulation. In France, the RE2020 regulation introduced the DH (degree-hours of discomfort) indicator to quantify exposure to overheating: the lower the DH, the higher the comfort level.

Summer comfort: operational definition

Overheating occurs when indoor temperatures exceed a comfort threshold (often 26–28°C depending on usage) too frequently or for too long. Recurring causes include: unprotected glazing (west/south-facing facades), poorly insulated roofs, low thermal inertia (lightweight structures), significant internal heat gains (lighting, computers, occupancy density), and poorly managed ventilation. 

Prevention follows a simple hierarchy:

• Reducing solar gain: external solar protection (BSO, blinds, awnings), choice of glazing (solar factor g) and joinery (Uw/Sw), light colors on roof/facade.
  
  
• Buffering the peaks: thermal inertia (heavy walls, partitions), thermal bridge corrections, roof insulation.
  
  
• Heat removal: night ventilation (cross or mechanical), free/night-cooling.
  
  
• Precise control: Building management systems (BMS/BAS), weather compensation, variable flow rates, adaptive setpoints, and automated opening/closing strategies.
  Dual-flow mechanical ventilation improves comfort by limiting unwanted heat air intake and recovering cool night air. Ceiling fans enhance perceived comfort (wind chill effect) with very low energy consumption.

Advantages, limitations and points to consider

Advantages: The right combination of building envelope and control system reduces the building density (HD), lowers the required air conditioning capacity, stabilizes operating expenses (OPEX), and improves occupant well-being (productivity, sleep, satisfaction). Passive solutions (shading, thermal inertia, night ventilation) are energy-efficient and resilient to power surges.

Limitations: Interior shading devices (interior blinds) only act after radiation has entered the building: they are useful for glare reduction but less effective against overheating than exterior shading devices. Nighttime ventilation requires favorable conditions (lower outside temperature, noise, safety, air quality). Thermal inertia is more effective if the structure can be relieved of heat at night; otherwise, it can trap heat. Finally, adding a reversible heat pump should only be a last resort: it addresses temperature issues but can worsen the energy efficiency and carbon footprint if a passive design strategy is not prioritized.

Key considerations: balancing acoustics (nighttime openings vs. noise), security (intrusion detection), maintenance (exterior blinds), control (BMS scenarios, sensors), and actual usage (times, density, internal heat gains). STD/SED allows for objective assessment of these trade-offs by simulating DH, temperature peaks, and energy consumption under various scenarios.

Refreshing anecdote

In a school building constructed in the 1990s, the attic rooms suffered from stifling afternoons. Before considering air conditioning, the project team tested a simple combination: external adjustable sunshades on the west-facing windows and nighttime ventilation controlled by sensors (opening when the outside air is cooler). The result after the summer: temperature peaks subsided, and the nights allowed the stored heat to dissipate. An unexpected bonus: teachers noticed fewer blinds being drawn during the day, resulting in better natural light and less artificial lighting. The moral of the story: sometimes, two well-implemented low-tech solutions are better than a brand-new cooling system.