How heat and hot air affect buildings, a personal commentary

1 Two observations on relative temperatures and comfort
2 Background to the science
3 Current understanding and practice
4 My own case study
5 The Mitigation Options
6 Conclusion
7 Myths laid to rest
8 Related articles on Designing Buildings

[edit] Two observations on relative temperatures and comfort

Below are a series of personal thoughts about air temperature and buildings. These thoughts and research were largely encapsulated and triggered by two observations from the news and web described below:

The query in a home improvement chat 'Why does my house heat up, even when it's cool outside?'
“it’s going to be a muggy night tonight – overnight temperatures will be up to 18°C”. (Sarah Keith-Lucas (Meteorologist) was presenting the weather on the BBC News Channel in Autumn 2022)

I began to wonder what would happen if I transported my bed into the 18°C of the garden? I went and sat outside for a while, and found it was not hot or “muggy” at all. and, apart from midges, gnats, and mosquitoes, if I had camped out there, I would have enjoyed a comfortable, cool night!

[edit] Background to the science

Having spent a few years since the heatwave of 2018 writing and publishing papers about Metro Rail Network overheating, I have more recently turned my attention to the overheating of buildings. I really have to take issue with much of the advice being published in the media, as I believe this advice is based on a flawed understanding of the problem and is therefore potentially dangerous. It is all too easy to make assumptions about where the heat is coming from and arrive at the wrong conclusions, because most have not researched and understood the reality of how heat works. I happened upon the answer to this elusive question quite by chance, and now find it a fascinating subject!

In order to stay safe and comfortable, particularly during heatwaves, we need first to understand how heat affects our environment and only then can we understand how we can manage it to keep cool. School science teaches us there are three ways to transfer heat: conduction, convection and radiation. The first two are easy to explain and readily understood. However. understanding radiation is somewhat more challenging.

Most of us would assume that the air temperature in a shady wood is cooler than the air temperature in a street or car park exposed to the sun. This is not the case! In both scenarios the air temperature can be the same! However, in the shady wood a person would be screened from the sun’s radiation by the tree canopy, and be additionally cooled by the transpiration (aka breathing) of the vegetation as it releases evaporated moisture into the atmosphere to cool itself, whereas, in the urban location with relatively little shade, the people and all the solid, surrounding surfaces/objects are being irradiated by the sun, and some of the resulting absorbed heat is being re-irradiated back to the people – a double whammy.

When at home, keeping cool both in the day, and during the night is very important. Many sources, including some government websites, advise keeping windows open when it is cooler outside than inside and then closing them and keeping them closed when it becomes hotter outside than inside – but in fact the latter is a phenomenon that seldom occurs – though see later below.

Many articles comment about buildings heating up through the roof, walls and windows, but their subsequent advice predominantly or exclusively focusses on the transfer of heat through the windows. However, the one prevalent heat source that is difficult to mitigate in summer and cannot be “turned off” is the solar irradiated (super-heated) roof. On a middling summer’s day at 25°C outside air temperature, the heat generated by the roof is more than your boiler would generally be capable of generating in winter, when outside temperatures are low to negative. Hence, unless your house is completely shaded it will never be hotter outside than inside!!!

The example of a campfire in the open air helps our understanding of the three ways that heat is transferred; we know from experience that this type of fire only heats the side of our body facing it.

There is no conduction carrying heat to us because we do not touch the fire.
There is no convection carrying heat to us because the air that has been heated rises (skyward) and for the most part does not come near to us.
So, all we feel is radiation travelling through the intervening space and reaching us on the side of our body facing the fire.

However, translating that fire example into our lounge environment where the air is contained by the walls and the ceiling, the processes and outcomes are subtly different.

Again, there is no conduction carrying heat to us because we do not touch the fire.
However, here, contained within the room, there is now an element of convection contributing to this warming effect, as the heated air circulates in the confined area.
More significantly though, areas of the walls and the ceiling in the path of the fire’s radiant heat, are being heated by the fire. Then these in turn re-radiate that heat back into the room space and warm the surfaces of our body which are not directly facing the fire – and eventually, directly or indirectly, all the solid objects in the room are heated and transferring some of that heat into the air. Think of the heat like a ball continuously rebounding off all the surfaces in the room – so the heat continues rebounding around (and warming) the room, and its contents, as long as the energy source (the fire) lasts.

It is just the same as with the SUN – it irradiates (super-heats) everything solid on the earth’s surface, including us ….. The Micro heat source process mimics the Macro heat source process!

[edit] Current understanding and practice

Government advice on keeping cool is contradictory – some suggests shading/covering windows but does not always mention windows themselves and, when it does mention the windows, it often advises leaving them open “only when its cooler outside” and closing them “when it is hotter outside”! However, the advice also mentions that “homes with opening windows on just one side of the property are more likely to overheat, as this means there is less or no cross ventilation through the home.” Referring to the Building Regulations approved document on overheating mitigation, it supports Part O of Schedule 1 to the Building Regulations 2010 (https://assets.publishing.service.gov.uk/media/6218c5aad3bf7f4f0b29b624/ADO.pdf).

This approved document took effect on 15 June 2022 and is sets out mandatory requirements to mitigate building overheating by limiting unwanted solar gains in summer and designing dwellings with windows and (patio) doors to enable cross-ventilation.

The degree of success of such measures will be dependent on:

the dwelling’s orientation and the placement of windows and doors;
whether for instance there is any shading overhangs or awnings, or trees or vegetation nearby providing shade;
the type and colour of the building materials used; and
in practice… how the heat is managed – whether the occupants are at home in the day, or whether the house is usually unoccupied and closed for security reasons.

The coolest thing around is the temperature of the air (see case study below), so this is the most effective cooling measure and the general rationale for keeping the windows open to get that ambient air to ventilate your house. On the back of that, cross ventilation is the most effective (unforced) way of getting this cooler air to flow through your house, but you cannot achieve that if the windows are kept closed!

Considering that your house is most intolerably hot when you return after needing to be out all day and having left it closed up for security! The first thing you would probably do is open all the windows – the same as you do when you arrive back in the car and open up your solar-irradiated car – one of the clearest examples of radiant heat in action! So why would you close the windows when you are in there during the day. You, being in the house all day with the windows closed is not going to reduce the overheating below the level that happens when you are out of the house.

[edit] My own case study

My family are extremely lucky as we have doors and windows on all four sides of our dwelling, and Velux windows in the roof, all of which deliver first class cross ventilation. However, as the rising temperatures in summer motivated me to think more deeply about this topic.

[edit] Selected temperature measurements

I began to monitor the late morning temperatures on a moderately warm day in four separate locations around our own house. Here are the four readings:

The first reading was in the shade of some trees, measuring the air temperature, with the thermometer not being irradiated by the sun. In this shaded area the recorded temperature was circa 22°C.
The second reading was in the unshaded lawn area and registered around 32°C. Now it is worth noting, vegetation such as grass is the only heated up surface under the sun that you can walk on in bare feet without sustaining first or second-degree burns – it is even possible in extreme circumstances to sustain third degree burns on other surfaces!
The third reading’s location was my light beige, unshaded sandstone patio. Previously I had occasion to attempt some maintenance at a low level on my patio doors so, in order to reach the area, I sat down on the patio without thinking. The sun had been shining on it for a while, and I can assure you I got up rather quickly. Given my more recent temperature monitoring of this unshaded patio being around 44°C, my untypical athletic response was quite understandable. (I then got out an old carpet off-cut to sit on before resuming my maintenance task…!)
The fourth and final reading’s location was on my unshaded medium-coloured terracotta roof tiles. This reading was 48°C and is particularly interesting.

[edit] Observation on the results

These roof tiles being darker than the patio tiles consequently absorb more heat than lighter coloured building materials. It is worth pointing out that most roofing materials (clay, slate, and concrete look-alikes), are usually dark in colour and more vulnerable to absorbing considerable amounts of heat.
The roof area had the highest temperature, BUT not only because it was darker…
Unlike the walls and windows, the roof temperature is also compounded in that the WHOLE roof is irradiated for the WHOLE of the time the sun is shining, whether brightly or defused by cloud (cloudy bright). (Hence my comment about the solar irradiated heater that is the roof of the building, which in summer can cause the house to overheat but cannot be turned off!)
Naturally the sun does not strike all the roof at the same angle, so the intensity of the radiation will vary. However, unlike the roof, the windows and walls (using building materials of variable colours), will likely experience maximum irradiation for part of the day, with a lesser amount at other times of the day, to almost zero radiation where they are northerly facing.

[edit] What do the results tell us?

If the occupants of a dwelling are absent and unable to manage opening / closing of windows and doors during the day to reduce the effects of the sun’s irradiation, what are the processes we need to combat and to mitigate the overheating?

[edit] The Process

For the reasons set out above, a building with all access points closed and secured will get very hot when the sun shines on it all day.

The significant area of irradiated darker roofing materials readily absorbs the heat, which is conducted through the tiles into the attic and subsequently throughout that space by radiation.
This heat is then conducted through the building fabric down into the upper rooms (generally the bedrooms) and onward.
The external walls too will have absorbed heat progressively according to the sun’s direction.

[edit] The Analysis of the Problem

Most people assume that the attic gets hot because heat rises from within the house, but on reflection – no pun intended, whilst one can still relatively comfortably spend time in the dwelling’s two or three storey living spaces, with the extreme loft temperature build-up on a hot summer’s day, staying in the very high temperatures of an enclosed attic for more than say 15 minutes could potentially expose one to considerable harm.
Despite the heating system keeping your house warm in the winter, that heat does not permeate into the loft past the loft floor insulation so the loft temperatures in winter are closer to the outside temperature than the house’s internal temperature!
Contrary to popular belief, the required insulation installed to protect us against the winter cold is ineffective against radiation, so a specific solar barrier within the loft would be needed to have any effect. Radiation does not just rise like convected heat!

One of the best inventions mankind has developed that does not require any energy supply to keep a room cool, is the humble, “external” shutter – widely used in warm “Mediterranean” countries for example. Usually made of wood, as wood has low thermal conductivity compared to metals. Its sloping slats allow cool air in whilst also providing shade, preventing the sun’s radiation overheating the room. Unfortunately, most UK (double-glazed) windows generally open outwards and this precludes the installation of such “external” shutters, albeit we can attempt to mimic their functionality.

[edit] The Mitigation Options

Where does all the above leave us…?

One option to protect the loft from overheating, would be to install a solar barrier like an aluminium foil, but this material is delicate and somewhat impractical for retrofitting.
When the system of integrating solar panels into the roof was introduced, it looked much neater and seemed to make economic sense because the tiles were replaced with lightweight inexpensive trays under the photovoltaic panels. However, after further thought and when contrasted with the planted-on panels, the superior advantages of the latter in the context of shading became more obvious to me.
Dwellings that have planted-on solar panels have an advantage with regard to keeping the building cool. They are usually installed with a healthy gap between the panels and the roof. This gap allows the cooling air (cf cross ventilation mentioned earlier above) to flow through the gap between the solar panels and tiles, and also shades the dark-coloured heat-absorbent tiles from solar irradiation. Currently I can see no more functionally elegant way of keeping a tiled roof cool as the use of a planted-on system, which not only provides that effective shading, but also generates free energy.
The use of highly reflective “cool-roof / 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 and less beneficial option than planted-on solar panels. Also many products tended to be light in colour, potentially raising objections with the local authority planning department. and certainly more disruptive to retrofit.
The use of a solar reflective roof paint could be considered as a cheaper and less disruptive option.
If none of the above can be made to work for you because of cost or practicality, the best we can do is aim for VENTILATION with SHADE, using our curtains in a similar way to shutters and to create, if possible, some cross ventilation, with windows or doors open, on opposite sides of a room or area of the dwelling.

[edit] Conclusion

If all the technical solutions are impractical for YOU, and you need to close and secure the house whilst you are out, just focus on the following when you come home from work to a stuffy and very hot dwelling.

Personal security permitting – throw open all the (bedroom) windows, and do not shut them again too soon. The windows need to remain open until the actual overheated FABRIC OF THE BUILDING has cooled down. (This may not fully occur until quite late at night, if at all).

Premature closing of the windows will just allow the rooms to heat up again as the walls and ceilings continue to irradiate the heat of the day into the rooms and warm the trapped air. Remember, if you were to spend some time outside late in the evening, you will feel and understand the temperature difference compared 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 inside our houses if we do not manage the heat correctly.