The room
One typical conservatory: a 14 m² floor (4.0 m by 3.5 m), a 15.4 m² roof, 13.2 m² of double glazed walls on three sides above a 0.6 m dwarf wall, and the fourth side against the house, which is held at 19°C. Volume 36 m³. The floor slab and furniture give it thermal mass of about 1.6 MJ per °C, which is why the hottest hour comes in the late afternoon, not at noon.
| Part | Value | Why |
|---|---|---|
| Polycarbonate roof | U 2.4 W/m²K, lets in 50% of the sun | A 25 mm multiwall roof that has aged |
| Glass roof | U 2.8 W/m²K, lets in 62% of the sun | Double glazed roof units of the 1990s and 2000s |
| Insulated panel roof | U 0.43 W/m²K, lets in no sun | 75 mm foam core between aluminium skins |
| Glass walls | U 2.8 W/m²K, 60% of the sun, a third of the sky's sunshine reaching them | Older double glazing, three sides lit in turn |
| Air | 0.8 air changes an hour shut, 8.8 once over 25°C | Doors or vents opened when it gets warm |
| Facing | South 100%, west 92%, east 85%, north 60% of wall sun | The share of the sky's sunshine reaching the walls |
The sum
Every 15 minutes the room temperature moves by the heat coming in minus the heat going out, divided by its thermal mass. Heat in is sunshine (half of what the roof and walls let through stays in the room, the rest reflects or radiates back out) plus a little from the house wall. Heat out is the roof, walls, dwarf wall and air, each in proportion to its U-value, area and the difference between inside and outside.
That gives an hourly room temperature for each roof. We then count waking hours (8am to 10pm) in five bands, average each month and hour for the calendar, and add up the heat a heater would need to hold 19°C during waking hours.
The heat loss of the whole room, per degree of difference, comes out at 91 W/K with a polycarbonate roof, 97 W/K with glass and 60 W/K with panels. At 0°C outside and 19°C inside that is the 1.7 kW, 1.8 kW and 1.1 kW heaters quoted around the site.
The bands
| Band | Room temperature | What it means |
|---|---|---|
| Too cold | Under 12°C | Coat on. A heater fixes it, slowly. |
| Chilly | 12 to 17°C | Jumper weather. A heater fixes it. |
| Comfortable | 17 to 25°C | Sit and read without thinking about it. |
| Warm | 25 to 28°C | Doors open, fan on, still sticky. |
| Too hot | Over 28°C | Most people leave the room. |
The weather
Hourly air temperature at 2 m and shortwave sunshine on a flat surface, from Open-Meteo's historical weather archive (ERA5 reanalysis), for every hour from 1 January 2024 to 31 December 2025, at each town's coordinates. That is 17,544 hours per town and 1,824,576 hours in all. Weather data by Open-Meteo.com, licensed CC BY 4.0.
Worked example: Sheffield
Average room temperature by month and hour under each roof
- 1,236
- hours over 25°C
- 47°C
- hottest hour
- 32°C
- July, 3pm
- 1,376
- hours over 25°C
- 50°C
- hottest hour
- 33°C
- July, 3pm
- 286
- hours over 25°C
- 34°C
- hottest hour
- 24°C
- July, 3pm
Blue: heater weatherPale: comfortableAmber to plum: too warm
What it does not do
- It does not know your room. Size, shading, blinds, colour of the floor and how often you open the doors all change the result.
- It treats sunshine on the walls simply, as a share of the sky's sunshine, rather than tracking the sun round each wall.
- It ignores wind, which cools a draughty room, and assumes the house next door stays at 19°C.
- It does not heat or cool the room when it reports temperatures. Heating is only used for the heat cost figure.
So read the numbers as a fair comparison of roofs in your weather, not a forecast of your thermometer. If you test your own room with a thermometer, the shape of the day should look familiar: cool mornings, a late afternoon peak, a slow evening fall.
Use the data
The results for every town are free to use under CC BY 4.0 with a credit and a link to Warmlid. Download the league table as a CSV, or see it on the overheating league table page. The per town hourly averages behind the calendar are also published as small JSON files used by the Comfort Calendar.