Guide · Passive cooling

How to cool an office without air conditioning.

Six evidence-based tactics that work in a real UK office, drawn from CIBSE TM52 passive design principles, WHO fan guidance and HSE thermal comfort duty.

Modern office interior with vertical blinds diffusing sunlight and a ceiling fan overhead
In short

Close sun-facing blinds before 10am, run night purge cooling, turn off unused PCs and monitors, add ceiling and pedestal fans, flex working hours to avoid 11am–3pm, and provide chilled water on every floor. Together these hold a typical UK office 3–5 °C cooler — enough for most summers, but not for a red Heat-Health Alert.

The six tactics

In order of impact.

  1. 01

    Close blinds on sun-facing windows before 10am

    Solar gain is the biggest single heat load in most UK offices — external shutters block up to 90%, internal blinds 30–40%. Get them closed before the sun hits the glass, not after.

  2. 02

    Purge cool air overnight and early morning

    Open every window when outside air is cooler than inside. This is 'night purge cooling', the standard passive strategy in CIBSE TM52. In most UK offices this alone knocks 3–5 °C off next-day peak temperature.

  3. 03

    Turn off heat sources you're not using

    Every desktop PC produces around 100–150 W of heat, monitors another 30–60 W. Unused meeting rooms, printers on standby and legacy AV racks add up. Turn them off.

  4. 04

    Use ceiling and pedestal fans

    Fans don't cool the air, they cool the person — air movement over skin allows sweat to evaporate. WHO advises fans give useful cooling below 35 °C air temperature; above 40 °C they can actually accelerate dehydration.

  5. 05

    Flex working hours

    Start early, finish early — Acas guidance explicitly supports flexible hours during heatwaves. Getting the demanding work done before 11am avoids the 11am–3pm peak flagged by NHS and UKHSA.

  6. 06

    Provide cold drinking water on every floor

    HSE Regulation 22 requires an adequate supply of drinking water. In a heatwave, chilled water within 30 seconds' walk of every desk is a low-cost, high-impact intervention.

Where passive fails

The limits of not having AC.

  • —Passive cooling can't hold operative temperatures below 25 °C on days above ~28 °C outside.
  • —Night purge doesn't work in dense urban sites where outside night temperatures stay above 20 °C.
  • —Fans stop being useful once air temperature exceeds skin temperature (~35 °C).
  • —Vulnerable workers still need active cooling regardless of the tactic mix — HSE thermal comfort duty is absolute.

Related reading: AC vs fans in a heatwave, employer HSE guidance, office AC installation, cost guide.

Passive cooling FAQs

What works, what doesn't, and when passive isn't enough.

How do you cool an office without air conditioning?
The highest-impact tactics are: close blinds on sun-facing windows before 10am, night purge cooling by opening windows when outside air is cooler than inside, turning off unused heat sources like PCs and monitors, using ceiling and pedestal fans, flexing working hours to avoid the 11am–3pm peak, and providing chilled drinking water on every floor. In combination these hold a typical UK office 3–5 °C cooler than baseline.
Do fans actually cool a room?
No — fans don't cool air, they cool people. Air movement across skin accelerates sweat evaporation, which is what removes body heat. WHO guidance says fans give useful cooling below 35 °C air temperature; above 40 °C they can accelerate dehydration and should be avoided, especially for vulnerable people.
What is night purge cooling?
Night purge cooling is opening windows overnight and early morning when outside air is cooler than inside, letting cool air flush the building. It's a standard passive strategy documented in CIBSE TM52 for non-domestic buildings and TM59 for homes. On typical UK summer nights it drops next-day peak indoor temperature by 3–5 °C.
Can plants cool an office?
Marginally. Plants add a small amount of evaporative cooling through transpiration and shade sun-facing glass if placed carefully. The measurable temperature drop is under 1 °C in typical offices — a nice-to-have, not a primary control. Air quality benefits are more meaningful than the cooling effect.
When do you actually need air conditioning in a UK office?
When passive tactics can't hold operative temperature below CIBSE TM52's 25 °C benchmark for more than 3% of occupied hours in a typical summer. In the UK that increasingly means: any south-facing office with full-height glazing, any city-centre floor without operable windows, any space with high IT or occupancy density, and any building housing vulnerable workers. HSE thermal comfort duty applies either way.
References

Sources for every claim on this page.

When passive isn't enough

A one-hour audit tells you what an AC upgrade would actually cost.

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