How to keep a south-facing room cool
A south-facing room in Britain is a solar collector for most of the day, and the standard heatwave advice — "close the curtains, get a fan" — is half right and half backwards. Here's the honest ranking of what works, ordered by impact per pound, from free to installed. The organising principle behind all of it: heat you keep out is worth three times heat you try to remove later.
Free and immediate
- Get the ventilation timing right — most people have it backwards. Windows open when it's cooler outside than in (night, early morning), then everything shut before the outside temperature crosses over, usually by 9–10am on a hot day. Opening windows at 2pm "for a breeze" imports 32°C air into your 27°C room. The night purge — windows open from evening to early morning to flush the day's heat — is the single most effective free tactic there is.
- Close blinds and curtains before the sun arrives, not after. Once sunlight has crossed the glass and hit the room's surfaces, the heat is already inside. For a south-facing room that means shading deployed by mid-morning.
- Kill internal heat sources. Ovens, tumble dryers, gaming PCs, even incandescent-era lighting all add heat. Cook late, dry outside, and let the desktop sleep.
- Use fans on people, not rooms. A fan doesn't lower temperature — it accelerates sweat evaporation off skin. Point it at where you sit and sleep; a fan "cooling" an empty room is 1p an hour spent on nothing.
Cheap and permanent (£20–£300)
- Reflective window film on south-facing glass rejects a substantial share of solar gain before it enters, works every sunny day forever, and costs £20–£60 per window in DIY form. The trade-off is a slightly tinted view and less winter solar warmth — worth it on the worst window, debatable on all of them.
- External shading beats internal every time. An awning, sail shade or external blind stops the sun before the glass; an internal blind fights it after some heat is already through. If you can rig anything outside the window — even temporarily for heatwaves — it outperforms the fanciest internal blackout.
- Thermal or blackout blinds/curtains are still worthwhile as the internal layer, especially pale or reflective-backed ones. Dark blinds absorb heat and radiate it inward — colour matters more than people think.
- Draught-proofing helps in summer too — the same gaps that leak heat in January leak it inward in July once outside is hotter than in.
When the passive measures aren't enough
Everything above might hold a south-facing room 3–5°C below where it would otherwise be. In a normal British summer that's often the whole answer. In a proper heatwave — the kind arriving more often — a room that would have hit 33°C instead hits 29°C, which is still miserable to work or sleep in. That's the honest limit of passive cooling: it lowers the peak, it can't set the temperature. The two options that can:
- A portable air conditioner (£300–£600) — right for renters and few-weeks-a-year use. Combined with the passive measures above it's markedly more effective, because you've shrunk the load it fights. Current picks here, and don't skip sealing the hose properly.
- A split system (£1,500–£3,500 installed) — the fix-it-properly option: quiet, a third the running cost per unit of cooling, and it heats the same room cheaply in winter. South-facing rooms are, not coincidentally, exactly where installers fit most single-room systems. Costs in the cost guide; size it here with sun exposure set to "sunny".
The combined play: film or shading first, night purging as routine, then size the AC for the reduced load. You'll buy a smaller unit and run it less — the passive measures pay for themselves inside the AC's first summer.
Quick answers
Should I leave windows open all day in a heatwave?
No — only while outside is cooler than inside. On a hot day that window (so to speak) closes by mid-morning and reopens in the evening. All-day open windows in a heatwave heat your home.
Does putting foil on windows work?
It reflects solar gain, yes — it's the crude version of proper reflective film. It also looks like it, can void glazing warranties by creating heat stress in double glazing, and annoys landlords. Proper film does the same job without the drawbacks.
What temperature is too hot to sleep?
Sleep quality measurably degrades above roughly 24°C for most people, and badly above 26–27°C. If your bedroom holds 27°C+ overnight through a heatwave, passive measures alone won't fix sleep — that's the use case where cooling earns its keep most clearly.