My thoughts and research on heat transfer in buildings have been triggered by the two observations:
- When a Florida resident asks, “It’s cool outside but my AC kicks in – why?”
- When a qualified meteorologist says, “It’s going to be a muggy night tonight – overnight temperatures will be up to 18°C”
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:
- No conductive heat is carried to us because we do not touch the fire.
- No convective heat is carried to us because the heated air largely rises.
- We only feel the radiation travelling through the intervening space on the side of our body facing the fire.
With a fire in our lounge where the air is contained by the walls and ceiling, the results are subtly different.
- As above, there is no conduction.
- Being contained within the room, some convected air circulates and reaches us.
- More significantly, the fire’s radiation heats areas of the walls and ceiling that are directly in its path. These surfaces re-radiate heat back into the room, warming the solid objects and surfaces, including our bodies, and eventually, some heat is transferred to the air.
It is the same with the sun – which irradiates (super-heats) everything solid on the earth’s surface, including us.
Current understanding and practice
Government and other advice are contradictory and changeable, and consist of:
- “shading/covering windows”, “leave windows open”
- leaving windows open “only when it’s cooler outside” and closing them “when it is hotter outside” – the latter unlikely unless your house is entirely shaded
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.
Building heat transfer study
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?
- Roofing materials (clay, slate and concrete) are mainly dark in colour and consequently absorb more heat (and were certainly darker than our patio slabs).
- The roof area had the highest temperature, but not only because it was darker.
- The whole roof will be irradiated for the whole of the time the sun is shining, whether brightly or defused by cloud (cloudy bright), albeit the intensity of the radiation will vary as the sun does not strike the whole roof at the same angle.
The process
For the reasons set out above, a building with all openings closed will get very hot when the sun shines on it all day.
- The significant area of darker roofing materials readily absorbs the sun-irradiated heat, which is conducted through the tiles into the attic and subsequently throughout the attic space by radiation.
- This heat is then further conducted through the building fabric down into the upper rooms and onward.
- Depending upon the sun’s direction and intensity, the external walls and windows will also be irradiated.
The analysis of the problem
- Many people assume, wrongly, that the attic gets hot because heat rises from within the house, but unlike the upper floors, the dwelling’s habitable spaces seldom reach the elevated attic temperature. Staying in an overheated attic for more than say 15 minutes could expose one to considerable harm.
- Despite the heating system keeping habitable spaces warm in the winter, that heat does not permeate through the attic floor insulation so the attic temperatures in winter are never elevated.
- Contrary to popular belief, the insulation required to protect against winter cold is ineffective against radiation.
- One of the best passive inventions humankind has developed to keep a room cool is the “external” shutter. Of low thermal conductivity (wood) and with sloping slats, it allows cool air in while providing shade, preventing the sun’s radiation from overheating the room. Unfortunately, most UK windows open outwards, which precludes their installation, albeit we can attempt to mimic their functionality.
The mitigation options
Where does all the above leave us?
- One option to protect the attic from radiative overheating would be to install a solar barrier like aluminium foil, but this material is delicate and impractical for retrofitting.
- Planted-on solar panels (not as economical and elegant as integrated panels) have an advantage regarding keeping the building cool because they are installed with a generous gap between the panels and the roof, which allows both the cooling air (cross ventilation mentioned earlier above) to flow through the gap, and the panels to shade the heat-absorbent tiles from solar irradiation. There appear to be few more functionally elegant ways of keeping a tiled roof cool, and it also generates free energy.
- The use of highly reflective “cool-roof tiles” on new builds or to replace existing tiles is now a more feasible option but it is likely to be a more costly, more disruptive and less beneficial option than planted-on panels. Moreover, many products tended to be light in colour, potentially raising objections with local authority planning departments.
- The use of solar reflective roof paint could be considered a cheaper and less disruptive option.
- If none of the above is cost-effective or practical, the best we can do is aim for ventilation with shade, using curtains in a similar way to shutters and creating; if possible, some cross ventilation, with windows or doors open, on opposite sides of a room or area of the dwelling.
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?]
