Passive cooling: free cooling & night purge ventilation (2026 practical guide for tradespeople)

Understanding passive cooling: principles, limits and real gains

Difference between cooling and air conditioning: what the client really expects

Clients often expect to "make it cold." Passive cooling instead aims to limit overheating. You work with shade, ventilation and air speed to gain a few degrees and help occupants sleep better. Air conditioning, on the other hand, produces a stable temperature, often dehumidifies, but consumes more and requires a dedicated system.

Summer comfort: perceived temperature, humidity and building inertia

Summer comfort doesn't depend on air temperature alone. Wall surface temperature, humidity and air movement all change the perceived temperature. Good thermal mass and continuous insulation slow down heat entry. At night, cross-ventilation can recharge the building with coolness, except when nights stay hot or very humid.

When passive is enough (and when an active solution is needed): real-world cases

Passive measures are often enough if the home is well protected from the sun and can be ventilated at night. When that's not possible, an active solution becomes more realistic, for example via a reversible air conditioner.

• A house with exterior solar protection and few west-facing windows.
• A top-floor apartment with large south-west facing bays, recurring overheating.
• Nights above 22 to 24°C, humid air, vulnerable occupants.

Free cooling: capturing available coolness without over-consuming

Air free cooling: cooling with fresh air when conditions allow

The principle is simple. When it's cooler outside than inside, ventilation increases the supply of fresh air and reduces, or even stops, active cooling production. The goal is useful cooling at night or during mid-season, without draughts or indoor pollution.

Water-based free cooling: hydraulic loop, groundwater, geocooling (points to watch)

On the hydraulic side, coolness is recovered via a heat exchanger and a loop connected to a dry cooler, cooling tower, groundwater or geocooling. Points to watch: water abstraction permits, water quality, fouling, condensation risk and frost protection. The priority remains energy sobriety and equipment lifespan.

Control and automation: sensors, schedules, safety and comfort

Without proper control, free cooling quickly becomes counterproductive. Plan for temperature and humidity sensors, dew-point logic, schedules (night-time), safety interlocks and automatic resume if comfort drifts. You gain in efficiency without over-consuming.

Night purge ventilation: the field method for evacuating heat

Prerequisites for the home: cross-ventilation, solar protection, airtightness

Night purge ventilation works if the home is well cross-ventilated and the sun is controlled. During the day, blinds, shutters and shading limit heat gains. At night, cooler air can finally bring down wall temperatures. Without proper airtightness, you end up ventilating when you should be closing up, and cooling becomes unreliable.

Flow rates and scenarios: mechanical ventilation, openings, bypass and noise management

On site, the approach combines opening two facades and running mechanical ventilation at high speed for a short window, then stabilising. With a dual-flow system, activate the summer bypass to avoid reheating the fresh air. Think about mosquito screens, safety and noise. A partial opening on the street side is often enough if extraction is effective.

Common mistakes: humidity, over-consumption, discomfort and client complaints

Opening up when it's warmer or very humid outside has the opposite effect. Monitoring humidity levels avoids condensation and odours. Another pitfall is leaving mechanical ventilation running at full power all night. That tires occupants and increases consumption. Set clear schedules, and close up early in the morning to keep the coolness in.

Designing effective cooling: complementary levers to offer

Reducing heat gains: solar protection, glazing, blinds, shutters and careful installation

For cooling that lasts, the most cost-effective move is blocking the sun before it heats up the rooms. Focus on exterior protection (blinds, shutters, sun breakers) and on installing joinery without air leaks or thermal bridges. Suitable glazing and continuous seals often make the difference on west-facing bays.

Working with thermal mass: insulation, thermal lag, materials and good site practice

Insulation isn't just for keeping heat in during winter. Well designed, it improves summer comfort through thermal lag, especially in roofs and lofts. Keep thermal mass on the indoor side where possible, ensure layer continuity, avoid compressed insulation and treat hard points around floors, boxed-in areas and load-bearing walls.

Ventilation and air quality: don't degrade indoor air while chasing coolness

A tighter home needs ventilation that actually works. Don't switch off mechanical ventilation during heatwaves. Offer safe night purge ventilation when weather allows, and regular maintenance (vents, filters) to keep healthy air while gaining cooling.

Pricing, sales and compliance: securing your cooling offer in 2026

Client pitch: summer comfort, savings, and limits explained simply

Sell cooling as lasting summer comfort. You reduce overheating, improve sleep, and avoid "full blast" usage that drives up the bill. Also explain the limits. Without insulation, solar protection and ventilation, no solution will work miracles. The right promise is more stable comfort, not a "guaranteed 19°C."

Study and evidence: site surveys, diagnosis, energy audit and results commitment

Before pricing, do a simple site survey. Surfaces, orientations, solar gains, airtightness, mechanical ventilation, humidity. Back your offer with a diagnosis or an energy audit if the home is complex. Attach a sizing note, the assumptions, and a settings plan. Commit to measurable indicators, such as a reduction in discomfort hours, rather than a single target temperature.

Grants and rules: points to check in 2026 (MaPrimeRénov', CEE, RGE, RE2020)

In 2026, check real eligibility. Some "AC only" solutions don't qualify. CEE require precise documentation, and RGE certification must be valid for the right trade area. To avoid non-compliance and unpleasant surprises during inspection, rely on precise documentation. For new-build, RE2020 pushes for addressing overheating (solar protection, ventilation, thermal mass) before adding equipment.