Air Conditioning for a Conservatory: Sizing, Cost and What Actually Works
Published: May 25, 2026
A conservatory is the hardest room in the house to cool. It’s mostly glass, so it soaks up solar radiation all day — a south-facing one can pass 40°C in summer. That means the usual BTU-per-m² sizing rule undershoots badly: you have to cool the room and fight a continuous solar heat load. Here’s how to size it, what type actually works, and what it costs.
Why conservatories overheat
Glass has a poor insulation value (high U-value) and is transparent to solar radiation, so sunlight passes straight through and heats everything inside — the greenhouse effect. Unlike a normal room, the heat doesn’t just leak in slowly through walls; it pours in through the roof and walls as direct radiation. That’s a large, continuous heat load the air conditioner has to remove on top of the normal cooling demand.
Size up — a lot
The standard rule (~450 BTU per m² / 20 BTU per sq ft) is for a normal insulated room. A conservatory typically needs 1.5–2× that because of solar gain:
| Floor area | Normal room | Conservatory (allow 1.5–2×) |
|---|---|---|
| 10 m² | ~4,500 BTU | 7,000–9,000 BTU |
| 15 m² | ~7,000 BTU | 12,000–14,000 BTU |
| 20 m² | ~9,000 BTU | 14,000–18,000 BTU |
Lean toward the higher end for south/west-facing rooms, lots of glazing, or a glass (rather than solid/tiled) roof. Use the BTU calculator for a baseline, then size up. Undersizing is the usual mistake — the unit runs flat out and never wins.
Cut the heat load first
Every watt of solar gain you block is cooling you don’t have to buy or run:
- Roof blinds or a solid/tiled roof conversion — the roof is the biggest gain.
- Solar-control film or blinds on the glass.
- Ventilation to purge built-up heat before cooling.
Reducing the load lets you fit a smaller, cheaper-to-run unit — and means a portable might actually cope.
Portable vs fixed in a glass room
- Portable — no installation, but the exhaust hose must vent hot air outside, which is awkward when every wall is glass (you’ll need a window-kit panel in an opening light or a vent through a frame). A single-hose unit also fights itself against the solar load — choose dual-hose and oversize. Best for renters or occasional use.
- Fixed split / mini-split — rejects heat outside through its condenser (no in-room hose), is more powerful, quieter and far more efficient, so it actually holds temperature on a hot day. Needs professional installation (indoor unit mounted, pipes routed through a frame or dwarf wall) but it’s the only type that reliably tames a conservatory.
Running cost
A big unit working hard in a hot glass room is at the expensive end to run — easily £0.30–£0.70/hr at UK rates for a 12,000–18,000 BTU unit, more if it never reaches setpoint. Two things keep the bill down: reduce the solar gain (so the unit cycles instead of running flat out) and choose an efficient inverter model. Compare units by their actual cost per hour rather than sticker price.
Verdict: treat a conservatory as a much bigger cooling job than its floor area suggests — block the solar gain, then fit an oversized, efficient unit (a fixed mini-split if you can install one, a dual-hose portable if you can’t).
Next: what size AC do I need? · portable vs split · single-hose vs dual-hose
