I'm sat in between two fans as I'm typing this!
this post was submitted on 23 Jun 2026
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Casual UK
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30° here in Iceland is a national emergency
30^o^ abroad: Civilised humidity
30^o^ in the UK: Might as well be under water, being gently simmered into broth.
It's the humidity that makes it so unbearable
...and then combined with our entire architecture, diet, clothing and lifestyles being based around centuries of keeping out the cold, wind and damp.
If your house keeps out the cold, wind and damp, it will keep out the heat. There is no special way for walls, windows and insulation to let heat or hot air pass only in one direction.
The issue is thermal mass. Buildings are designed to absorb heat in the winter. Obviously when you aren't using heat they'll absorb whatever the temperature is.
First 2 days of a heatwave the building holds a cooler temperature. After that the walls begin to heat and it is simply too hot at night to dissipate all the heat the building has absorbed
Since I've explained this about twenty times today already, why don't you tell me what you think the temperature is like in a building with lower thermal mass?
Sure I can do that, I grew up in a trailer and we had the AC go out one summer in the southern USA.
Mid day was hot as fuck. Once the sun went down and we opened the windows it cooled off very quickly to the outside temperature and it was easy to sleep.
I would usually shut my windows around 12 to 1 am. Then I could sleep comfortably until about 10 or 11.
Here when I shut my windows to sleep I'm hot again by about 7am and feel the walls radiating heat.
So overall the peak of the day is better, but nighttime is significantly worse. Honestly I'd rather be a few degrees warmer during the day and sleep well than being consistently hot all the time
Well if we're reducing it to anecdotes, here in my house with thick brick walls, we open the windows as soon as the outside temperature drops below inside, use fans to exhaust heat, and end up with a bedroom in which it is comfortable to sleep. So I guess we're at an impasse, unless we're able to work out some general principle that doesn't rely on personal experience.
I don't think either of us have our building schematics on hand to run the math on this. If your room is cooling that effectively I suspect you have very favorable window placement and significantly less thick walls. Or perhaps just a smaller building. I'm in large apartment complex so I'm just surrounded by progressively warming concrete
I think in your apartment complex you're probably affected by a bunch of other people who are keeping their curtains open in the day and not ventilating in the evening, so you'll be screwed whatever. Your trailer will have been roasting in the day, but will also have been small and so easy to fully ventilate.
I think your suspicion misses other important variables: for example, I might just live somewhere cooler than what you're thinking about. That's why I don't think this personal experience swapping is very productive.
In contrast, the science is pretty simple: all other things being equal, a building with high thermal mass will maintain the same average temperature as one with low thermal mass. You may well be more comfortable during the worst nights, but you'll be less comfortable during the worst days. But special attention to cooling sleeping areas that can be performed in any house can mitigate that, but can't mitigate peak temperatures.
In contrast, the science is pretty simple: all other things being equal, a building with high thermal mass will maintain the same average temperature as one with low thermal mass.
This statement. Makes me unsure you know what thermal mass is. If all things are the same they will have the same thermal mass.
If you left a 10kg block of steel on the pavement and a 5kg block of steel on the pavement all day which would become cooler to the touch faster once the sun has set? Obviously the 5kg block of steel.
I said ”all other things being equal"
Okay so you've invented materials that can be exactly the same and have different thermal properties through sheer will alone. Honestly good for you, get that to market and you will be very wealthy
Everything is insulated and sealed shut. There are heaters inside the house. The sun comes through the window and heats the room up. The heat cannot escape.
Then nor can its occupants.
Or, perhaps... perhaps the occupants can open the windows when the inside temperature exceeds the outside temperature. They could use a fan to vent the hot air faster. They could even place some kind of opaque covering in front of - or even behind - the windows to prevent hot air from getting into the room itself.
I'm only being sarcastic because this is the tenth reply on the subject I've made today.
Well, you're kind of being a bit of a bell-end about it, but some of your suggestions are correct, in terms of that's what the buildings lack in their architecture.
You know greenhouses, like you grow tomatoes in etc? They gather all the sun's heat and trap it inside, so it's hot and humid - but even in cold weather, they stay warm enough so seeds can germinate and plants don't die? That's what our houses are like.
Office buildings are especially bad for it. Big fancy glass thing. Triple glazed. No openable windows. No shutters. No curtains.
Houses likewise, you can open all the windows in the house, but there's high humidity and no air movement, so it just sits. A fan can help a little in a localised space.
Even shutting the curtains during strong sun doesn't do much - external shutters would help - so the heat is stopped before it's trapped behind the glass - but it, or air conditioning things are a lot to go through for 3 weeks of too-hot weather, compared to 8 weeks of temperate weather and 41 weeks of too cold weather.
Internal shutters and curtains absolutely do a lot. The difference is most noticeable here in the kitchen, simply because I am more likely to forget to close the blinds there, but it is several degrees cooler after a day of sun intercepted by the blinds versus sun pouring directly into the room. The difference comes from the fact that visible light is reflected straight back out of the window (rather than bouncing around inside the room, heating up each surface it hits), and that a layer of hot air develops behind the blind - this hot air passes its heat slowly into the room, yes - but it also conducts into the outside. Because the air directly next to the window is significantly hotter than it would be with no barrier, it conducts outside much faster than it would be (because rate of heat transfer increases with the difference in heat). Meanwhile the heat transfer into the room is slower (because it's instantaneous if you allow the sun in).
I hope we start putting external shutters on buildings faster. Everyone getting A/C is unsustainable, and as you say, it's a lot for just a few weeks each year.
Absolutely - I'd love it if some of the energy efficiency grants would also go towards shutters (which can work for cold in the winter too). Not so useful for rooms you're actually in, but great for keeping a room at home dark and cool whilst you're at work.
Blinds are probably a lot better than curtains at this time of year as well. Most people (myself included) tend to leave rooms with "whatever they already had in them", but it wouldn't be a great hardship to swap curtains out for blinds for some of the year.
Not so useful for rooms you’re actually in
I disagree. Turning the lights on during the day for a few weeks every year isn't a big problem! And I'd certainly rather do that than run A/C.
(As an aside, my desk is right in front of a window so I actually have to draw the curtains and have a light on a lot of the time anyway to be able to see my screen)
There is no special way for walls, windows and insulation to let heat or hot air pass only in one direction.
Hot air, no, but heat from sun, very much yes. That's a one-way kind of transaction
Quite easy to mostly block with curtains. Our houses could definitely do with external shutters though, which would be more effective. I would say this architectural difference doesn't fall under "our homes are designed to keep heat in" though.
Once the sunlight has passed through the window, the battle against the heat is mostly lost. Awnings, external shutters and to some extent solar protection film are some of the better options in this instance.
These are things we've had the luxury of not having to care about in the past, instead being able to harvest some solar heat during the seasons where heat is scarce. Less so these days.
In any case, I think "our homes are not designed to keep heat out" would be an accurate statement for northern Europe.
We have the oldest housing stock in europe and some of the most leaky. In the winter the heat bleeds out and in the summer the cold does too.
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The bricks and wall thickness are designed to absorb heat and release them in the evening though. Not sure if it can work the other way around.
Really they're designed to prevent the passage of heat from inside to outside, so that your heating (or in the past, your fire) didn't pointlessly heat the outdoors. But this design can't work only in one direction, so it also slows the passage of heat from outside to inside.
There are three main ways this slowing happens:
- Any solid barrier prevents the passage of air from the environment to the place where you are, and vice versa. Obviously this is symmetric.
- Cavity walls have an air-gap in them. Air is a poor conductor of heat, so while hot air on one side of the wall heats the wall's bricks up, this heat then travels only slowly to the other side of the wall. Again, this is obviously symmetric.
- What you're talking about is that bricks and stones have a high thermal mass. This is a bit more complicated so I'll explain: it means that if you take a given lump of heat (say, all the heat coming off a fire for one minute) and apply it to the bricks so it's fully absorbed, they'll heat up less than if you were heating up a wall made out of steel. The energy is still in there, though, and when the temperature of the air next to the wall drops below the wall's temperature, that energy is released back into the air. But this too is symmetric: it applies just as much to the energy of a fire as it does to the energy of, say, one cubic metre of air at 35 degrees celsius. The same property that releases heat into the house on a winter evening releases heat into the outside air on a summer evening. This is what you want!
Each of these properties is symmetric, because physics doesn't "know" which side of the wall is inside and which is outside - it only "knows" which is hotter and which is cooler. So the exact properties which keep you toasty in winter help keep you cool in summer.
You speak with a lot of confidence for one who is not well travelled.
We use the Dehumidify option on our units, it uses less energy (also less €) and makes the place fresh as can be.
Source: in Spain with my nuts stuck to my leg from May-October
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30° in Slavic countries is called the apocalypse.
Just been to Tesco. Was in there about 15 mins. When I got back into the car, the dash board said 41 🥵 Now parked up waiting to collect wife from work and it’s a pleasant 33 😒
Hmm, both feel acute to me.
If you paid a months salary for the chance to go to Sainsbury’s you’d bloody well take it in 30°
I need my AC unit upstairs to sleep, but I need it downstairs so that I'm not peeling myself off of my chair every 5 minutes while working. The struggle is real.
Genuinely considering buying a second, despite barely having the space.
I'm sure I'm stating the obvious here, but have you considered
- working upstairs?
- sleeping downstairs?
- lugging the unit up and down the stairs?
Those are very sensible suggestions tbf; it's just that my house is really small! I live alone, renting. My room upstairs doesn't fit any more than my bed and the AC (floor unit). The only way I can work up there is with a laptop on my bed, which is an ergonomic nightmare that will have me cramping in about 10 minutes.
I have a small sofa that is comfortable enough to sleep on downstairs, that I would probably use if I did not have the AC
I can lug the AC up and down the stairs but one of those 2 areas will be so warm that by the time I've moved it I'm immediately in need of another shower... and then again when I move it back.
I'd probably just spend the next few days living downstairs, seems like the minimum amount of discomfort!
Don't know about their unit, but my one is 48KG, so lugging it isn't the easiest.
I would've though have it setup at the bottom of the house, have it running and only open a window at the top of the house...
Assuming no major air leaks in the house, the cold will displace the heat starting at the bottom, the rising heat then able to escape out of a cracked window at the top of the house,
Otherwise have it at the top of the stairs and hope that the falling cold will displace the heat
You would want it at the top of the house if it's purely for cooling. Cool air at the top of the house will sink down to the bottom. Rising hot air will then be cooled by the A/C unit. If you put it at the bottom, all that cool air will just sit there and never move upstairs. The open window upstairs will allow hot air out... if it's cooler outside than inside, otherwise it will allow hot air in. In either case, that won't promote much mixing of the upstairs and downstairs air, so your A/C won't improve the temperature upstairs much.
Yeah, if running it isn't too expensive then using it to boost the old 'diagonal air exchange' strategy seems like a good plan
In my loft conversion that I spend most of the day in it gets quite hot. Nevertheless, scrupulously keeping curtains closed, keeping windows closed when the outside temperature is hotter than inside, and using a fan has kept me comfortable in all but the hottest days of the last few years. I always encourage this before AC due to the power usage.
Blackout curtains + closed windows do not keep my first floor anywhere near cool enough unfortunately; I think it's really poorly insulated as it is also really cold up there in the winter (though this is much more manageable to me.)
My problem with sleeping with a fan is they trigger my myofascial pain syndrome quite badly. My only relief is still air that's cooler. I also run quite hot in general :/
I have noticed, however, that I can do a single top up at around 13:00, where I run the AC for a bit to get the upstairs room slightly cooler than downstairs. Then I turn it off and close the window and it stays somewhat bearable for the rest of the day, opening the window again at night.
I completely understand your concerns about power usage as I am eco-conscious myself, and I really wish I had a ceiling fan to use instead. Getting my landlady to sign off on that is a bit unlikely.
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