this post was submitted on 23 Jun 2026
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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.
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