thermodynamics - How do blankets keep you warm?

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?

Answer

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...