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I heard a famous physicist (was it Feynman?) argue that blankets do not keep you warm by trapping heat but by trapping air next to the body. Is this true?

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  • $\begingroup$ This is true. When backpacking you don't want to carry too much. One useful trick is to bring two thin shirts. One doesn't do much, but wearing two is surprisingly warm. Even better under a windbreaker. $\endgroup$
    – mmesser314
    Feb 27 '15 at 14:01
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It may be worth pointing out that blankets also (surprisingly) act as (thermal) radiation shields. This is the reason that "emergency blankets" can sometimes be found in survival kits that appear to be nothing more than thin shiny plastic. But they really make a difference in the amount of heat lost by a warm (37 °C) body on a cold night (cloudless sky - assume 0 °C).

For a body with an area of 30 cm x 180 cm facing the sky, the area is roughly $0.5 m^2$. Assuming an emissivity of 0.3 (just picking a number), the heat loss is given by

$$E = \epsilon \sigma (T_1^4-T_0^4) = 53 W/m^2$$

Or 25 W for the human I just mentioned. That is a not insignificant amount of heat... especially when you consider that the basal metabolic rate ("doing nothing" which is a good approximation of sleep) is around 60 W. And that's not counting the heat you will lose by breathing (heating cold air, and filling it with vapor).

Heating cold air (still going with 0 °C as our baseline):

250 ml per second, heat capacity 1020 J/kg/C, $\Delta T = 37 C$, you get about 12 J

Evaporating water:

Saturated vapor pressure of water at 37 °C around 47 mm Hg, and breathing about 250 ml per second (900 liter per hour) with an effective fraction of 47/760 per volume of water, this takes another 25 W.

So surprisingly, these three mechanisms result in similar amounts of heat loss - and protecting yourself from radiative heat losses is indeed significant. Because of this, a good blanket (which will reflect some of that heat back to you) is indeed "keeping heat in".

The above underlines that the most significant form of heat loss is evaporation. A good blanket stops circulation, and will keep the air near your body "moist". This will slow down the rate of evaporation, helping you stay warm. Stopping the air from circulating also stops it from carrying away "heat" - but the amount of heat carried by moist air is significantly greater than "just air", as the above example demonstrates.

There is more to this question than meets the casual eye...

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  • $\begingroup$ The inclusion of heat loss due to breathing is interesting to include, but convection is a pretty big deal. Even with no wind, you'll have significant natural (buoyant) convection. With and $h$ of 5 , you'll be losing $185 W/m^2$. $\endgroup$ Nov 18 '14 at 22:40
  • $\begingroup$ @user3823992 completely agree that it matters. I would be interested in your comparison of h with and without evaporation - the impact of moisture gradient. And lying down presumably affects thermal convection... $\endgroup$
    – Floris
    Nov 18 '14 at 22:48
  • $\begingroup$ 250 ml/sec is however not a resting rate. Minute volume at rest is usually estimated at 6 to 8 liter, giving a loss of about 9 to 13 W due to evaporation (assuming exhaled air at 95% humidity). $\endgroup$
    – Previous
    Jun 1 '16 at 15:41
  • $\begingroup$ @Previous you are right, my number for respiratory rate is a little high. The same scaling applies to the heating-of-air factor. At high altitude (or when your metabolism is increased because you are cold) the rate will go up again... This is more about estimating than about hard values. $\endgroup$
    – Floris
    Jun 1 '16 at 15:54
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The thermal conductivities of a range of materials is given here. I can't find figures for the thermal conductivity of solid wool or cotton (i.e. a solid block with no air voids in) but the thermal conductivities of organic materials seem to be around $0.25$ Wm$^{-1}$K$^{-1}$. By contrast the thermal conductivity of air is $0.024$ Wm$^{-1}$K$^{-1}$, so given a constant body temperature and external temperature you would lose ten times less heat when insulated by air than when insulated by most solid materials.

The trouble is that air won't stay in a static layer surrounding your body. Air currents and thermal convection created by the heat of your body cause the air to move. This will replace the warm air that you've heated to your body temperature with cold air, and increase the rate of heat loss.

Ideally what you want is some stuff that can hold air in place around you so the air can't move and carry heat away. And you want this stuff to be as insulating as possible. The best such material I know of is silica aerogel, which is such a good insulator it was used on the space shuttle as a heat shield for re-entry. Glass is actually a rather poor insulator, but aerogel contains only a few percent glass by volume and the other 90 odd percent of its volume is air. Hence it's excellent insulating properties. However aerogel is a brittle solid and a poor choice for bedclothes.

Blankets are something of a compromise. They contain a lower percent of air by volume than aerogel, and they also trap the air less tightly, and both factors lower the insulating properties. However they're a lot more comfortable than aerogel would be.

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  • $\begingroup$ Aerogel blankets lol ☺ $\endgroup$
    – Geremia
    Nov 18 '14 at 18:20
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    $\begingroup$ @Geremia: There have been attempts to make flexible aerogels. See for example this article. If you Google aerogel clothing there are lots of hits, but how successful it is I'm not sure ... $\endgroup$ Nov 18 '14 at 18:29
  • $\begingroup$ This is why down clothing is warm. Feathers have little mass, but trap air well. $\endgroup$
    – mmesser314
    Feb 27 '15 at 14:04
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Normally your body heat would dissipate in the air, so when it's cold, your outer body cools down, because you are losing your body heat to the air near you. So when you cover yourself in a blanket, you stop your body heat from escaping, and as it is trapped, and your body continues to produce heat, you feel warmer and warmer under the blanket. Overall, the blanket prevents convection of heat through the air by greatly slowing the movement of air. Thus, your body heat is trapped inside the blanket.

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  • $\begingroup$ Air convection is one factor but, more importantly, in my opinion, is the heat insulation properties of the material. If you cover yourself with a poor heat insulator but that still traps air completely, it won't be efficient in keeping you warm. $\endgroup$
    – Diego
    Nov 18 '14 at 17:48
  • $\begingroup$ @Mara: Yes, I think that's what he argued: it stops convection by preventing air movement. $\endgroup$
    – Geremia
    Nov 18 '14 at 18:17
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This is a big topic in our house at the moment because it's winter here and we don't heat the whole place.

Shivering in bed, think about how heat moves by any of the following:

1) Convection (air moving)

2) Conduction (touching)

3) Radiation

An ideal super-blanket will address all three:

1) Stop air moving. It can do this by enclosing you (like a plastic bag), but more practically, it can be fuzzy. Fuzziness makes lots of little difficult to move air pockets.

2) Reduce conduction - by being a material that doesn't transfer heat well (e.g. plastic vs. cotton) it will keep heat next to your body. Even better, if it's fuzzy, it will be touching your body less. Less surface area contacting means less conduction.

3) Reduce radiation - if it can "shine" the heat back at you, like those mylar emergency blankets, then the heat you're radiating can be returned to you.

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