Why this bedroom stayed at 24°C without air conditioning

Findings

1. 01

   The room does not overheat at night, it overheats between two and six in the afternoon

   The instinct on a hot night is to open the window at bedtime and hope. The room's evening temperature, however, has already been decided by what happened three to five hours earlier, when the west-facing glass took a direct solar load. In our worked example, keeping the curtains open and the window shut through the afternoon lets the room store about three kilowatt-hours of heat that then re-radiates through the evening. Every intervention that follows tries to prevent that store from forming in the first place; interventions that only address the room after it has formed lose most of their headroom before they start.

   Time the heat actually enters14:00 to 18:00
2. 02

   External shading is the single biggest lever, and internal blinds are not a substitute

   A well-fitted external shutter or an awning on a west-facing window reduces the room's solar gain by around seventy per cent at peak sun; a thick internal blind reduces the same gain by around twenty-five per cent, because the heat has already crossed the glazing before the blind can intercept it. In our worked example, adding a mid-range external roller-shutter to the back window brings the three in the morning peak down by roughly two degrees on the same August day. This is the intervention every subsequent step depends on; without it, the others do less than half of what they otherwise could.

   Overnight peak drop from shading aloneAbout 2°C
3. 03

   Closing the window in the afternoon is the counter-intuitive move that works

   In a summer heatwave the external air temperature between two and six in the afternoon is typically warmer than the room. Opening the window during those hours actively imports heat, in exchange for a small breeze the household is often not sitting in. Keeping the window closed and the curtains drawn during the afternoon, then opening at both ends of the house from around ten at night, is the pattern the CIBSE guidance recommends and the pattern our worked example needs to see the third intervention below actually deliver.

4. 04

   A well-timed night purge is where the last two degrees come from

   Once the walls and the floor of the room have not been heated during the afternoon, a cross-draught at night removes the residual heat surprisingly quickly. In our worked example, opening the bedroom window and a facing window elsewhere on the house from about ten in the evening until six in the morning, with an interior door held open, brings the three in the morning peak down by a further two degrees, to twenty-four. This is what a British bedroom is capable of on a hot night when the earlier steps have been done.

   Additional drop from night purgeAbout 2°C
5. 05

   The stack has a ceiling, and a top-floor bedroom sits at it

   The sequence above brings a first-floor west-facing bedroom into the low twenties on the night we modelled. A top-floor bedroom under a hot roof, especially in a chalet dormer or a Victorian loft conversion, will typically hold two to three degrees above the numbers here even after the same interventions, because the ceiling of the room is a horizontal surface facing the sun. That is the case where the honest answer is either roof insulation upgrades, roof shading over the rooflight itself, or, if neither is feasible, a small quiet split system sized for the room rather than a portable unit sized for the shop shelf. The stack is real; it also has bounds.

Headline figures

• Starting overnight peak, unmodified room28°C at 03:00measured
• After external shading and closed-window daytime26°C at 03:00modelled
• After adding a well-timed night purge24°C at 03:00modelled
• CIBSE overheating threshold, sleeping zones26°C for sustained hours is failurereported

Measured is monitored data from a real project. Reported is a publisher's stated figure. Modelled is our own estimate, worked from public references.

What this investigation cannot tell you

• One worked room in one archetype is not a national claim; solid-wall rooms with heavy internal mass and small windows behave differently and often overheat less.
• The overnight temperatures shown are modelled against a specific August week; a warmer week reads warmer at every step.
• The stack assumes the household is willing and able to keep windows closed in the afternoon; households where that is not practical will see a smaller effect.
• This piece does not argue against air conditioning; it argues that a portable unit is rarely the right first move on a west-facing British bedroom.

Where the figures come from

• TM59 methodology for the assessment of overheating risk in homesChartered Institution of Building Services Engineers (CIBSE)
• LETI overheating primerLondon Energy Transformation Initiative
• Passivhaus Trust summer comfort guidancePassivhaus Trust
• Approved Document O: Overheating in new residential buildingsgov.uk

How we built this

We took the CIBSE TM59 methodology as the anchor for what counts as overheating, and modelled a representative west-facing back bedroom in a 1930s semi against a warm August week. Each intervention was priced against published shading coefficients from CIBSE and the LETI overheating primer. The temperatures shown are modelled peaks at three in the morning; no specific household is described, and the room is a worked composite of the archetype's typical geometry.