Some years ago I bought a duvet which said "[This duvet] is designed to keep you cool in summer and cosy throughout winter". I didn't think much about it at the time because it seemed like typical advertising bluster, but then after using the duvet it's become apparent that it actually works. I need to wrap the duvet tighter during winter (because any exposed body part definitely feels cold), but aside from that, it is OK to use the duvet during both summer and winter.

Thermodynamically, how does this work? Naively I'd have imagined that something that keeps heat in (important during winter) would also cause the user to overheat during summer - which is why winter jackets are not worn during summer.

If it matters, the box says the composition is "outer: cotton & inner fill: polyester".

Related: How do blankets keep you warm? which however does not deal with the question of why the blanket works during both summer & winter.

  • $\begingroup$ Good question. I read up on it with an idea at the back of my mind that clothing is a thermal insulator, so it must work for extremely hot temperatures as well. It needs to be hot enough out there to warrant insulating yourself with your own body heat, which isn't really applicable in your case anyway. But I didn't find much. Bedouins wear thick clothing, but that's just to protect from the sun, and they wear it loosely so air can flow and let sweat do its thing, not for its thermal insulation capacity. $\endgroup$
    – Diego
    Sep 29 '20 at 6:33

This claim is based on the way the duvet reacts to sweating.

If you are cold you don't sweat so the only heat loss is convective, and since the duvet stops the air trapped within from moving convection is prevented. This means the rate of heat loss from you is small so you warm up.

Now suppose you get too hot and start sweating. The idea is that the sweat evaporates and cools you. The sweat then condenses in the material of the duvet and is wicked to the outer surface of the duvet where it can evaporate again into the air in your bedroom. So the end result is that your sweating cools you just as if the duvet were not there. This works because wicking transports the sweat through the duvet despite the duvet preventing convection.

  • $\begingroup$ Perhaps it would be useful to phrase the question as "does not allow you to get humid in summer" rather than "keeps you cool in summer" since the duvet is preventing the mixing of air inside and outside it (then the wicking action is also there - I did not know of this, then again, I've never seen a duvet.) $\endgroup$
    – Sidarth
    Sep 29 '20 at 9:43

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