Air Conditioning a Converted Basement or Garage in Paris

"We turned the garage of our house in Fontenay-sous-Bois into a teenager's bedroom with a shower room. The work was finished in March: plasterboard, floating floor, one transom window. In summer the room hit 29 °C even though it's half below ground; in winter the convector heater runs non-stop and the place smells stale. Two installers came round: the first suggested a 2.5 kW split system within ten minutes, the second turned the job down, saying 'you don't do that in a basement'. Who's right?"

Both are wrong, and that is exactly why we get calls like this one. A partly below-ground space is not just an ordinary room with fewer windows: it is a microclimate, where humidity drives everything and where air conditioning is only the last building block of a wider system. Installed on its own, it often creates more problems than it solves.

This article does not revisit topics already covered on this blog — building management approvals, outdoor unit noise, repair costs, dehumidifier versus air conditioner. It describes the specific working method for garages, cellars, basements and semi-buried levels converted into living spaces in the Paris region: bedroom, rental studio, home gym, home office, workshop, utility-cum-shower room. A fast-growing segment in the suburban housing of the 92, 93 and 94 départements, and in older Parisian buildings with vaulted cellars.

Why a basement behaves in exactly the opposite way to an upper floor

In a typical Paris-region house, heat comes from above: roof, loft, south-facing windows. We set this out in detail in our article on the hot upper floor of a house and treating the loft. A basement works in exactly the opposite way, with four distinctive features:

• ground inertia: buried walls stay between 13 and 17 °C all year round in the Paris region. A basement warms up slowly, but it also cools down very slowly;
• internal heat gains dominate: there is almost no solar gain, so heat comes from occupants, lighting, appliances — and very often from the adjoining plant room (boiler, hot water tank, washing machine, freezer, internet router, server);
• the air is the most humid in the house: cold walls + warm air coming down from upstairs = condensation on cold spots. This is the mechanism described in our feature on damp, condensation and mould;
• air renewal is virtually nil: one transom window, sometimes a vent, rarely a mechanical ventilation system that actually serves the level.

The practical consequence: a converted basement almost never has a cooling capacity problem. It has a humidity and fresh-air problem, and a winter heating requirement that is greater than owners imagine.

A rule we apply on every technical survey: if the measured relative humidity exceeds 70 % under normal conditions, no installation goes ahead until the cause has been dealt with. An air conditioner fitted in those conditions becomes an odour diffuser within six months.

Step zero: measure before sizing

Before any quotation, we ask for two to three weeks of readings. Not to buy time: because the figures almost always contradict what people feel.

What should be measured, and what the owner can do themselves:

A simple €15–25 digital thermometer-hygrometer left in place for a fortnight produces a more useful curve than any estimate. In a healthy Paris-region basement, you would expect 18–22 °C and 50–60 % relative humidity. Above 70 %, there is water ingress, a significant thermal bridge or a ventilation fault — and sometimes all three.

The three causes of damp that air conditioning will not remove

1. Lateral water ingress or rising damp. An undrained buried wall absorbs water from the ground. No thermal equipment corrects this: it is structural work (perimeter drainage, tanking, water-repellent render). We decline installation in such cases, and we put it in writing in the survey report.
2. Condensation on cold walls. It disappears once you insulate on the inside with a ventilated air gap, or once you heat the space continuously rather than in bursts. This is where an air-to-air heat pump becomes relevant.
3. Internal moisture production with no extraction. An unvented tumble dryer, a shower with no extract fan, laundry drying indoors. A remote humidistat paired with mechanical extraction solves 80 % of these cases for a few hundred euros.

What equipment actually works in a below-ground space

The wall-mounted split remains the baseline — but not just any model

In the vast majority of cases, the technical solution is a reversible single-split R-32 unit of 2 to 3.5 kW, with two specific requirements:

• a genuinely low operating range in heating mode: the room must be heatable in January, when the outdoor unit — often sited at ground level in a yard or against a boundary wall — is working at -5 °C. Check the heat output delivered at -7 °C, not the nominal rating;
• a dehumidification mode independent of cooling (often labelled dry), able to run in the shoulder seasons without dragging the room down to 19 °C.

We set out the calculation method in our guide to room-by-room sizing in kW. In a basement, we systematically increase the heating requirement and reduce the cooling requirement: the opposite of what an online configurator does.

The case of the Parisian vaulted cellar

Cellars built of millstone or rubble masonry in Haussmann-era buildings present a different problem: the walls are breathable and thermal inertia is enormous. Fitting a split system there without ventilation turns the cellar into a condensation trap as soon as warm summer air is introduced. In such spaces we almost always work in two stages: first controlled mechanical ventilation, and only then the thermal equipment. The façade and courtyard constraints are the ones described in our article on the Haussmann-era apartment and heritage architect constraints.

What to rule out

• The portable monobloc air conditioner: it exhausts through a hose run out of the transom window, so it continuously draws hot, humid outdoor air into the coolest space in the house. It is the worst possible environment for it, for the reasons set out in our analysis of portable monobloc units and their real alternatives.
• Electric supplementary heating on its own: it dries the air at the surface without dealing with the water vapour, and costs three times as much to run as a heat pump.
• A dehumidifier alone in summer, if there is no extraction at all: it dehumidifies by reheating the air, which can raise the perceived temperature.