Why do buildings, including our homes, overheat even on seemingly cool days? To answer that, we need to unravel the mysteries of heat transfer.
My thoughts and research on heat transfer in buildings have been triggered by the two observations:
It got me thinking about how little heat transfer is understood. Having researched Metro Rail Network overheating, I have turned my attention to building overheating, a fascinating subject where myths abound.
We need to understand how heat affects our environment to manage it and stay safe and comfortable. School science taught us three ways of transferring heat: conduction, convection and radiation. The first two are easy to explain, but not radiation.
It is assumed that the air temperature in a rural wood is cooler than in an urban area exposed to the sun, where hard surfaces/solid objects reflect the irradiated heat of the sun. Not so. They are the same. The impression of being cooler is that tree canopies both screen us from the sun’s radiation and also cool the surrounding air by transpiration (aka breathing) releasing evaporated moisture.
Commentators may mention heating up of building fabric but mainly focus on the windows. However, the principal summer heat source (the sun irradiating the roof) cannot be “turned off or thermostatically controlled”. With the typical, current configuration of building stock, where the roofs will absorb this heat in summer, this will often produce greater heat than a heating boiler can generate in winter, when outside temperatures are low to negative.
To clarify the ways that heat is transferred, consider first a campfire:
With a fire in our lounge where the air is contained by the walls and ceiling, the results are subtly different.
It is the same with the sun – which irradiates (super-heats) everything solid on the earth’s surface, including us.
Government and other advice are contradictory and changeable, and consist of:
Other advice mentions that “homes with opening windows on just one side of the property are more likely to overheat because there is less or no cross ventilation”.
New Building Regulations on overheating mitigation support Part O of Schedule 1 to the Building Regulations 2010, effective from 15 June 2022, sets out mandatory requirements to mitigate building overheating. It suggests limiting unwanted solar gains in summer and designing dwellings with openings to enable cross-ventilation – this being the most environmentally effective (unforced) way of getting the coolest thing (the air) circulating; (see case study below). Clearly cross ventilation supports the rationale for keeping windows open – closed windows equal no cross-ventilation.
For example, when arriving back at your solar-irradiated car (one of the most obvious examples of radiant heat in action), you would almost certainly open all the windows. So, closing windows when you are in the house during the day is illogical.
My family are lucky as we have openings, delivering first-class cross ventilation on all four sides plus Velux roof windows. With rising temperatures this summer, I undertook some late-morning temperature monitoring in four separate locations.
The first reading was in the shade of some trees; the air temperature I recorded was circa 22°C.
The second reading was on the unshaded lawn area and registered around 32°C. Vegetation, such as grass, is the only surface under the sun that you can walk on in bare feet without sustaining first or second-degree burns – or even potentially third-degree burns when on other surfaces.
The third reading’s location was on my light beige, unshaded sandstone patio. Not long ago, I had sat down on this patio to attempt French door maintenance. The sun had been shining on it for a while, and I can assure you I got up quickly. Given the subsequent temperature reading of around 44°C, my earlier, untypical athletic response was quite predictable.
The fourth reading’s location was on my unshaded medium-coloured terracotta roof tiles. This reading was 48°C. Why?
For the reasons set out above, a building with all openings closed will get very hot when the sun shines on it all day.
Where does all the above leave us?
To experience and understand fully the above processes, be aware of the difference between being in the sun and shade, and of any cool breeze on the shady side of your body. Spend time outside in the evening; and compare the difference to the inside rooms – it is very, very unusual that in our temperate geographical location, the outside air temperature at night is going to be “muggy” – only the inside of our houses will be “muggy” if we do not manage the heat correctly.
Premature closing of windows in the evenings allows the rooms to heat up again, as the walls and ceilings continue to irradiate the day’s heat, warming the cooler air now trapped inside.
Hopefully, this overview will give people a sense of how they can most effectively keep themselves and their loved ones safe and cool.
[Read more: Why is London’s Tube so hot?]
