Free cooling means cooling a home with an outdoor source that is cooler than it, night air or the ground, without running a compressor: only a fan or a circulator draws power. In housing it takes three forms (night purge ventilation through the windows, the bypass of an MVHR unit, geocooling on a ground-source or water-to-water heat pump) and it only works if the night drops at least three degrees below indoor temperature, according to the study by Artmann, Manz and Heiselberg (Applied Energy, 2007).
Summer comfort has exactly one regulatory set of figures in France, the RE2020 DH indicator, and it applies to new build only. The RE2020 guide published by the Ministry for the Ecological Transition sets a lower threshold of 350 °C.h, below which the building is deemed comfortable during a heatwave, and an upper threshold DH_max of 1,250 °C.h for a detached house with no external constraint, above which the building fails the regulation. The counter starts as soon as operative temperature exceeds 26 °C at night, or an adaptive threshold between 26 and 28 °C during the day. In a retrofit no text sets an equivalent threshold, and that is precisely why your sizing note and your handover report are the only documents anyone can hold you to.
Free cooling: definition, three families and what they are worth
The term comes from commercial buildings, where an air handling unit switches off its compressor as soon as outdoor air is enough. In a house, three sources share the principle, and what separates them is the flow of coolness they can carry: the power removed by air equals 0.34 × flow rate (m³/h) × temperature difference (K), in watts.
| Family | Source of coolness | What draws power | Output, order of magnitude for 100 m² | Deciding condition |
|---|---|---|---|---|
| Night purge ventilation through the windows | outdoor night air | nothing, or the ventilation on high speed | 1.5 to 3.5 kW (5 to 10 ACH over 250 m³, 4 K difference) | cross-ventilated home and a cool night |
| MVHR bypass | outdoor air, routed around the heat exchanger | the two fans | 0.15 to 0.35 kW (100 to 250 m³/h, 4 K difference) | automatic, properly controlled bypass |
| Geocooling on a ground-source or water-to-water heat pump | ground or aquifer, around 10 to 14 °C at depth | the circulators | up to 3 kW through a floor (below 30 W/m²) | existing ground loop and low-temperature emitter |
These orders of magnitude, worked out with that formula, settle the main point: two open façades carry ten times more coolness than an MVHR bypass, and only geocooling also works during the day. The subsurface temperature comes from the BRGM and ADEME geothermal portal. Adiabatic cooling, which lowers the supply air by evaporating water into it, belongs to none of these families and only works in dry air: see its principle, its limits and the buildings where it fits.
When free cooling works: day/night swing, tropical nights and thermal mass
You need a cool night, an indoor mass to store the coolness and a day during which the sun stays out, all at once.
| What you survey | Favourable | Unfavourable |
|---|---|---|
| Indoor to outdoor difference at night | 3 K or more for several hours | under 3 K, or only at the end of the night |
| Night-time minimum in hot spells | well below 20 °C | tropical nights, minimum of 20 °C or more (Météo-France definition) |
| Accessible thermal mass | masonry, slab or cross walls left exposed on the inside | bonded lining, timber floor, carpet, suspended ceiling |
| Daytime solar gains | closed external shading on exposed glazing | west glazing and roof with no protection |
| Outdoor air humidity | dry night air | warm, humid air, coast in stormy weather |
Useful thermal mass is thin: the daily wave only penetrates about ten centimetres into a masonry wall (√(a·P/π), with a ≈ 7 × 10⁻⁷ m²/s for concrete and P = 24 h). A stone house lined on the inside therefore stores almost nothing, see thermal mass, from very light to very heavy. The Artmann study finds a very high potential in northern Europe and a more limited one in the south, where night ventilation alone is not always enough.
Talk about gains in hours of discomfort, not in degrees. The Confort d'été tool simulates the warm season at the site address, with and without shutters closed by day and windows open at night. In its teaching model, those two measures cut the hours above 28 °C by about four in a house heavily glazed to the south, and by less than two in a converted attic: the building sets the ceiling on the gain.
Understanding passive cooling: principles, limits and real gains
What RE2020 quantifies, and what it leaves out of scope
Before promising a result, you need to know what the regulation is able to measure. The table below reproduces the RE2020 guide figures, which remain the only reference framework available even where they do not legally bind your job.
| What RE2020 sets | Value | Scope |
|---|---|---|
| Night-time discomfort threshold | 26 °C, non-adaptive | operative temperature |
| Daytime threshold (7am to 10pm) | 26 to 28 °C, adaptive, capped at +2 °C | operative temperature |
| Lower threshold | 350 °C.h | building deemed comfortable in a heatwave |
| DH_max, detached house, category 1 | 1,250 °C.h | regulatory limit |
| DH_max, detached house, category 2 | 1,850 °C.h | regulatory limit |
| Between lower threshold and DH_max | compliant | cooling allowance added to energy use |
| Scope | new construction | no DH requirement in retrofit |
Two practical consequences. First, the counter only increments during occupied hours, which is why a home left empty during the day mechanically accumulates fewer degree-hours. Second, the daytime threshold depends on the preceding days: after a run of hot days, tolerance climbs to 28 °C. What the regulation calls comfort is therefore not a fixed temperature, and that is exactly the argument to put to a client demanding a single number.
Difference between cooling and air conditioning: what is settled on the quote
The enquiry almost always arrives worded as a request for cold air. Passive cooling produces no cold, it delays and shaves the temperature rise by working on shade, ventilation and air speed. Air conditioning holds a set point and dehumidifies, at the cost of dedicated cooling plant. Confusing the two is the leading cause of dispute on this kind of job, and it is settled on the quote, not at handover.
So it is on the quote that the distinction is written, descriptions, quantities and mandatory wording carried over from the work plan.
When passive is enough (and when an active solution is needed): real-world cases
Passive measures hold when solar gains are controlled and night purge ventilation is genuinely possible. When that is not the case, an active solution becomes more realistic, for example via a reversible air conditioner.
- A house with exterior solar protection and few west-facing windows, backed by a shading survey.
- A top-floor flat with large south-west bays and overheating already observed in a previous summer.
- Nights with no drop in outdoor temperature, humid air, vulnerable occupants flagged.
When the active solution is needed, the demand of each room and the coverage of the indoor units are worked out before the equipment is chosen.
Free cooling: capturing available coolness without over-consuming
Air free cooling: cooling with fresh air when conditions allow
When it is cooler outside than inside, ventilation increases the supply of fresh air and reduces, or even stops, active cooling. A set point tied to a fixed hour rather than to a measured difference will pull warm air into the home one night in three.
Free cooling with an MVHR unit: the bypass, useful but limited by flow rate
The bypass, or thermal bypass facility, routes fresh air around the heat exchanger so that the extract air does not warm it up. The ecodesign Regulation (EU) No 1253/2014 requires it on bidirectional units, but the flow rate stays that of hygienic ventilation, around 100 to 200 m³/h in a house, or about 200 W removed with a 4 °C difference (0.34 × 150 × 4).
At commissioning, check that the bypass is automatic and controlled on an indoor set point and a minimum outdoor temperature, otherwise it never opens or opens in the middle of the day, and that night-time high speed stays below 30 dB(A) in bedrooms. Record those settings on the handover report. The matching settings are covered in commissioning an MVHR unit.
Geocooling: ground-source, water-to-water heat pumps and cooling floors
Geocooling, or geothermal free cooling, circulates the borehole fluid or aquifer water through a plate heat exchanger and then the floor, with the compressor off. With the subsurface at around 10 to 14 °C, coolness is steady day and night for the circulators' consumption alone: 2 kW of cooling for 100 W of pumps gives a ratio of 20, against 5 to 8 for a recent air conditioner. On vertical boreholes, the heat injected also recharges the ground.
The limit is the emitter: a floor does not cool beyond about 30 W/m², and flow water does not go below 18 to 22 °C depending on the zone, with a safety cut-out at 12 °C, as detailed in the heating and cooling floor and its CPT. On an aquifer, add the abstraction and discharge formalities, water quality and heat exchanger fouling.
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, a night-time window, safety interlocks and automatic resume if comfort drifts. Every setting you apply gets recorded: that is what separates an installation you answer for from one the occupant has quietly altered.
Night purge ventilation: the field method for evacuating heat
Prerequisites for the home: cross-ventilation, solar protection, airtightness
Night purge ventilation only works if the home is genuinely cross-ventilated and solar gains are controlled during the day. At night, cooler air can then discharge the structure. Survey the cross-ventilation potential opening by opening rather than assuming it: it is the one prerequisite that cannot be recovered later through commissioning.
Natural ventilation: wind effect and stack effect
Natural ventilation moves air without a fan, through two drivers: wind, which pressurises the exposed façade and depressurises the opposite one, and the stack effect, warm air leaving at the top and drawing cool air in at the bottom. The stack effect works without wind, provided there is a height between air inlet and outlet (stairwell, roof window above a low window). Hygienic natural ventilation through ducts, by contrast, does not have those flow rates.
Flow rates and purge plans: mechanical ventilation, openings, bypass and noise management
No text sets a night purge flow rate, so the figure you keep is the one you measure, not a catalogue value. On site, the approach combines opening two facades and running mechanical ventilation at high speed for a short window, then stabilising. With a heat-recovery unit, activate the summer bypass to avoid reheating the incoming air. The sizing constraint is acoustic: the order of 30 June 1999 caps individual equipment noise at 30 dB(A) in main rooms and 35 dB(A) in the kitchen, and a noisy bedroom gets the whole arrangement switched off by the second night.
The achievable flow, on the other hand, depends on the opening configuration rather than on the equipment. The orders of magnitude below are what measurement finds, and they explain why the same advice works in a house and does nothing at all in a single-aspect flat.
| Configuration recorded on survey | Air change rate achievable on night purge | What it buys |
|---|---|---|
| Mechanical ventilation on boost only, windows shut | 0.5 to 1 ach | Nothing noticeable on temperature, only on humidity |
| One opening, one facade | 1 to 2 ach | Marginal, not to be sold as cooling |
| Two openings well apart on the same facade | 2 to 4 ach | Partial discharge, useful in a small room |
| Dual aspect, two opposite facades | 5 to 10 ach | Genuine discharge of the fabric over one night |
| Dual aspect with stack effect, stairwell or duct | 10 to 15 ach | The best case, with no electrical input at all |
The operating rule that goes with it is short enough for the handover notice: open when the outside temperature falls two to three degrees below the inside temperature, close as soon as the gap reverses in the early morning, and do not expect mechanical ventilation alone to do the work. A single-aspect dwelling with no cross-ventilation option needs a different answer, not a night purge.
Common mistakes: humidity, over-consumption, discomfort and client complaints
Opening up when it is warmer or more humid outside produces the opposite of the intended effect. Monitoring humidity levels avoids condensation and odours. Another pitfall is leaving the ventilation at full power all night: consumption rises and the noise gets the system turned off. Set a schedule, close up early in the morning, and leave the list of applied settings in the file.
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 the fabric. Focus on exterior protection (blinds, shutters, brise-soleil) and on installing joinery without air leaks or thermal bridges. Suitable glazing and continuous seals make the difference on west-facing bays, the ones that drive the late-afternoon peak.
Working with thermal mass: insulation, thermal lag, materials and good site practice
Insulation is not 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 holds its flow rates. Don't switch off the ventilation during a heatwave, including when the occupant asks you to. Offer interlocked night purge ventilation when the 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: what you can promise, and what you must not
Sell cooling as lasting, bounded summer comfort. You reduce overheating and shift the peak, but you do not hold a set point. The right wording for a commitment covers a measured reduction in discomfort hours, not a guaranteed temperature. Without insulation, solar protection and ventilation, no device rescues fabric that absorbs sun all day.
Study and evidence: site surveys, diagnosis, energy audit and results commitment
Before pricing, do a simple site survey. Surfaces, orientations, solar gains, airtightness, ventilation, humidity. Back your offer with a diagnosis or an energy audit if the fabric is complex. Attach a sizing note, the assumptions used and the commissioning settings. Since the DH indicator does not bind retrofit work, that note is what fixes the level of service you commit to, and it is what an expert will read if the job is disputed.
Grants and rules: points to check in 2026
In 2026, check real eligibility before pricing. Some cooling-only solutions do not qualify. Certificate schemes require precise documentation, and 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 requires overheating to be tackled through the fabric before equipment is added, and the cooling allowance added to energy use between 350 °C.h and DH_max exists precisely to make added cooling expensive in the calculation.
