Air conditioning a conservatory: the hardest room in the European house

Updated September 8, 2026 · Claude AC · 2 sources cited

Britain counts several million conservatories, the Low Countries their glazed serres, France its vérandas, Germany its Wintergärten. They share a seasonal biography: adored in April, abandoned in July, cold again by October. The room was sold as extra living space and behaves as a solar collector. Fitting an air conditioner is the obvious response, and often the right one — but a conservatory is the single room in a European home where the usual sizing rule leaves you short by a factor of two, and where buying the machine before treating the glass guarantees an expensive disappointment.

Why glass changes the arithmetic

An insulated wall in full sun passes a handful of watts per square metre into the room behind it. Glazing passes hundreds. On a clear European summer day the solar radiation reaching a surface square to the sun is of the order of 700–900 W/m²; ordinary clear double glazing has a solar factor (g-value) around 0.6–0.7, so roughly 400–600 W of heat crosses every sunlit square metre of it. A modest conservatory with 15 m² of glazing, a third of it in direct sun at any moment, is taking on a couple of kilowatts of heat before anyone opens the door.

Three further properties make it harsher than a living room with large windows:

• The roof is glazed as well. A roof faces the sky and meets the midday sun almost square-on, at the hour it is strongest. Polycarbonate roofs, standard on older builds, transmit generously in both directions — a greenhouse in July, a cold store in January.
• There is no thermal mass. Glass, aluminium or uPVC frames and a thin floor slab store almost nothing, so the indoor temperature tracks the sun within minutes instead of lagging hours behind it as a stone-walled room does. Nothing carries the room through a cloudy half-hour.
• The envelope leaks. Conservatories are usually the least airtight part of a house, and every air change delivers outdoor heat the machine has to remove again.

These are planning figures for a conversation with an installer, not measurements of your own room: orientation, latitude, frame ratio and the shade of a neighbouring tree all move them. What survives every variation is the ranking — stopping radiation outside the glass beats every measure taken inside it.

Treat the fabric before buying the machine

1. Deal with the roof first. It is the largest gain and the easiest to fix. Replacing a polycarbonate roof with an insulated panel system, or fitting external roof blinds, changes the character of the room more than any appliance will. A solid or partially solid roof also stops the winter heat loss that made the room unusable at the other end of the year.
2. Shade the sunny elevations from outside. Awnings, external venetians or even a well-placed pergola intercept the radiation before it becomes indoor heat.
3. Ventilate while it is still cooler outside than in. Roof vents and opening lights at high level exhaust hot air by stack effect in the morning and late evening; the full passive sequence for a European home is in our guide to cooling without air conditioning.
4. Keep the doors to the house closed. This is free, and it is what separates a hot conservatory from a hot house. It also matters legally, as set out below.

Sizing a conservatory honestly

Our guide to sizing an air conditioner in kW works from about 100 W/m² for an ordinary European room and 130 W/m² for a difficult one such as a loft conversion. A conservatory is not a difficult room in that sense; it is a different category. Realistic planning bands run from about 150 W/m² for a north-facing, well-shaded conservatory with a solid roof, to 250 W/m² and beyond for a south-facing glass box with a polycarbonate roof and no shading.

The third column is the argument for shading, expressed in equipment. A 5 kW machine for a 20 m² room means a large outdoor unit, a heavier electrical supply, a bigger purchase and a bill that repeats every summer for fifteen years. Fitting external blinds and an insulated roof moves the same room a column or two to the left — to a 3 kW split that costs less to buy, less to run, and holds the temperature more steadily because it is not permanently at full output.

Which unit can physically be fitted

• High wall split. The default European answer, and the one that most often has nowhere to go: a conservatory has a dwarf wall a metre high and glass above it. The workable position is the house wall that forms the back of the room, where there is masonry, structure and a short route for the pipework.
• Floor console. Sits on the dwarf wall line under the glazing, which is dead space anyway. It is also the better format if you intend to use the heating mode, because it delivers warm air at floor level instead of into the highest part of a glazed volume.
• Cassette or ducted. Almost never possible — both need a ceiling void, and a conservatory has a roof instead. A slimline ducted unit hidden in an adjoining utility room, ducted through the house wall to a grille, is the exception where the layout allows it.
• Portable. Honest verdict: it will not hold a sunlit conservatory. A single-hose machine of 2.5 kW nominal is fighting a load two or three times its output, and the hose has to leave through glazing that is awkward to seal. The reasons are set out in our comparison of portable and split air conditioners in Europe.

The trade-offs between formats are compared in our guide to wall, console, cassette or ducted indoor units, and the equipment families themselves on our European split air conditioner pages.

The rules that catch people out

A conservatory is often exempt from building regulations, and that exemption has conditions worth knowing before an installer starts drilling. In the United Kingdom the usual test is that the room is single storey, at ground level, under 30 m² of floor area, separated from the house by external-quality walls, doors or windows, largely glazed, with safety glazing to the relevant standard, and — the clause that matters here — served by a heating system with its own on/off and temperature controls, independent of the house system. A reversible air conditioner controlled from its own remote satisfies that condition comfortably. Removing the separating doors to open the conservatory into the house is what changes the picture: the room then becomes a habitable extension and the exemption falls away.

Two more permissions apply everywhere. The outdoor unit is a fixture on an external wall, so local consent rules apply — heritage areas and street-facing elevations are where they bite, and the national picture is in our guide to planning permission for air conditioning in Europe. And the refrigerant circuit may only be commissioned by an F-gas certified installer, whatever the country: how to verify that is in our guide to choosing an installer. Country-by-country context, including the equipment rates and heat records that make this a live question in the UK, sits on the United Kingdom page and across the Europe hub.

What it costs to run

Running hours, not tariffs, are what make a conservatory expensive. Unlike a bedroom unit that idles most of the time, a conservatory machine works near its rated output for the sunniest hours of the day — and in southern Europe those are also the most expensive hours on a time-of-use tariff. As indicative arithmetic: a 3.5 kW split drawing roughly 0.9–1.2 kW while working, run for five afternoon hours, uses something like 4.5–6 kWh a day; at European retail prices of about 0.15 to 0.35 € per kWh that is roughly 0.7 to 2 € a day, so a summer of daily use is a bill you will notice. The method for your own case is in our guide to air conditioning running costs in Europe, and the SEER figure that governs it is explained in our guide to the EU energy label.

Frequently asked questions

Will air conditioning make a conservatory usable in a heatwave?

With the roof and the shading dealt with, yes — a correctly sized unit will hold a shaded conservatory at a comfortable temperature through a 35 °C afternoon. Without them, it will run flat out and still lose ground on the worst days, because the load rises with the sun faster than the machine can grow. That is not a fault in the equipment; it is the arithmetic of several kilowatts of solar gain in a room with no thermal mass.

Do I have to keep the doors to the house shut?

For comfort and for cost, yes. Leaving them open turns the conservatory into a heat source for the whole ground floor and asks one machine to condition a volume it was never sized for. It can also affect the building-regulations exemption that many conservatories rely on, since separation from the dwelling is part of the test.

Can I heat the conservatory with the same unit in winter?

A reversible split will heat it, and it is by far the cheapest way to do so — but a glazed room loses heat as readily as it gains it, so expect long running hours for modest comfort unless the roof has been insulated. Our guide to heating with air conditioning in Europe covers the running-cost comparison, and the independent controls it comes with are exactly what the exemption rules expect.

Is solar-control film worth fitting before buying a machine?

Frequently, yes, because it is the cheapest measure that acts on the roof, where the gain is largest and where external blinds are most awkward. It will not do the job alone — a film that removed half the gain still leaves a 20 m² conservatory needing several kilowatts of cooling — but it can be the difference between one capacity step and the next.

How many kilowatts do I need for a 15 m² conservatory?

Between roughly 2.3 and 3.8 kW depending on orientation, roof type and shading, which is a wide band precisely because those three factors dominate. Take the low end only if the roof is solid and the glazing shaded from outside; assume the high end for a south-facing room with a polycarbonate roof. An installer should confirm it with a room calculation rather than a rule of thumb.

Read next

• Getting rid of an old air conditioner in Europe: the legal way out
• Putting a European air conditioner on Wi-Fi: modules, apps and what they really save
• Cooling a European home without air conditioning: the order that works
• The room under the roof: cooling a loft conversion in Europe
• Buying a second-hand air conditioner in Europe: what the price tag hides

All Europe guides →