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Does the order you put blankets on matter for insulation?

  1. Jul 28, 2013 #1
    I've always wondered this. I have blankets of different materials and thicknesses. In very cold nights I sometimes have to use three or four of them when sleeping. Traditionally I put the thinnest blankets (least insulating) ones first (close contact to body) and the best blanket last, because the weight of the last ones should help keep the thinner ones from slipping off with my nightly movements. But now I'm obsessed with the question of whether putting the better ones first is more effective for heat insulation.

    My educated guess is that it doesn't really matter. Anyone's got a light on this?
     
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  3. Jul 28, 2013 #2

    mfb

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    I'm sure you can construct scenarios where it does matter, if we consider "blanket" as a general "some passive layer of material with some properties". For realistic blankets, I would not expect any significant effect.
     
  4. Jul 28, 2013 #3

    turbo

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    My wife and I use a very porous "hospital blanket" over the top sheet, and if we need more blankets, we put them over the "hospital blanket". The porous bottom blanket traps body heat and keeps it next to your body, and the upper blanket(s) help keep that warm air from being lost. Nothing scientific, just a few years of experience.
     
  5. Jul 28, 2013 #4

    OmCheeto

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    Not really, but this is a very good question, as I'm always doing the same thing every winter.

    My first recommendation, is to buy a "Physics Forums" t-shirt. This will make you think, no matter where you are, and try and solve problems.

    I was wearing one two weeks ago, and maximized the efficiency of ice retention, by constantly looking at the materials involved.

    I had a 1/2 gallon cooler, a metal liquid container, and a plastic bottle.

    The 1/2 gallon cooler was awkward to drink out of, but was the most efficient means of keeping the ice in its phase. I later decided that the high thermal conduction of the metal container made it the least efficient of all of my devices, so I simply stopped using it.

    So, in answer to your question, I would recommend that you use the least thermally conductive materials closest to you. And since I have not a clue how thermally conductive your different blankets are, my guess is that they are all made out of very similar materials, and it will have no effect on the order upon which you layer them.

    Oh wait.

    A hat. I heard somewhere that the head loses the most heat. Wear an insulated hat to bed.

    And socks. Your feet are the furthest away from your heart, and I would imagine receive the least blood flow.

    When I was camping two weeks ago, in the very cold mountains, I wore socks, and a cashmere scarf around my head. I was most comfortable. :smile:
     
  6. Jul 28, 2013 #5
    Doesn't the insulating effect mainly come from the layers of air between the blankets and pockets of air inside the blankets? In that case, the heavier blankets would compress the lower, lighter blankets and provide less insulation. On the other hand, the thicker blankets probably breaks airflow better (if you have draft, for example) and having one of the lightest blankets closest to the body would be good for leading away sweat, which conducts heat better than air.

    I mean, it probably doesn't really matter practically - I can even see being still vs. moving about a lot being a more important factor. (Movement would "pump" cold air under the blankets.)
     
  7. Jul 28, 2013 #6
    Nothing special about the head, really. You lose heat from any exposed part of the body. It's just that we tend to cover the other parts of the body before considering donning a hat.

    From the British Medical Journal 2008;337:a2769:
     
  8. Jul 28, 2013 #7
    *Reads answers, wonders if a 'sandwich' style approach (thick-thin-thin-thick arrangement of four blankets or thick-thin-thick of three) would be best*
     
  9. Jul 28, 2013 #8

    OmCheeto

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    hmmm.... I'll have to delve over to the medical sciences forum and ask why I have this misconception of Brain vs. Body blood flow thermal conductance.

    (ref)

    It may have been a picture I saw somewhere in the past of someone who needed a radiator on their skull that reinforced this misconception.

    User-Ich_with_Mohawk.jpg
     
  10. Jul 28, 2013 #9
    Since pressure increases with depth, you should put the least compressible blanket on the bottom and the most compressible (ie fluffiest) blanket on the top. That way, the fluffy blanket can fully expand, maximizing the amount of trapped air, which maximizes its insulation abilities.

    I'd only expect this to make a noticeable difference if you have a blanket stuffed with some very fluffy down.
     
  11. Jul 29, 2013 #10
    It does matter, but in a subtle way. Consider the air gap between each blanket as you layer it. Static air is a very poor conductor of heat. Therefore, you want to put the heaviest blankets on the bottom and the lightest on top to maximize the air gap between each sheet. This is assuming the properties across each blanket are uniform (IE constant conduction coefficients).
     
  12. Jul 29, 2013 #11
    Heat flow rate depends upon the thermal resistance. This resistance doesn't depent on the order of blankets. So ideally speaking, it shouldn't make a difference. Keep the material your skil like close to you!
     
  13. Jul 29, 2013 #12
    Yes it can affect the heat transfer rate .
    heat transfer rate depends upon the thermal resistance offered to the heat flow.
    Modelling a human body equivalent to a cylinder can approximate the calculations,than one can perform calculations
    in insulating a pipe for preventing heat transfer.
    Let us model a human body as a cylinder of radius 10 cm.
    You have a good thick blanket of approx. 5 cm of good quality wool(thermal conductivity k1=0.029 w/m.k)
    and another thin blanket of approx 1 cm (inferiour quality wool)(Assuming its k2 = 0.04 w/m.k)

    case I : thick blanket first and thin one next
    calculating thermal resisitance neglecting convection resistance.

    we get Resistance= ln(R2/R1)/2∏k1l + ln(R3/R2)/2∏k2l
    substituting values in it we get approx R value = 1.43

    Case ii :Thin blanket first and thicker one next
    we get R value to be 1.39

    so it can be understood that good insulator when covered first helps in reducing heat transfer rate from our body.
     

    Attached Files:

  14. Jul 29, 2013 #13
    Mate, you don't really roll yourself in the blankets do you? Don't consider the blankets as cylinders. They're walls.
     
  15. Jul 29, 2013 #14

    sophiecentaur

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    This is a question which can be treated totally theoretically or at great practical depth. Some of the answers can be contradictory, as a result.

    If you don't "roll youself" in a blanket then the cold (air) comes creeping in through the gap as you move. For this reason, a sleeping bag is more effective than the same total weight of duvet when you really need to keep warm. All good bags have a very effective seal along the zip for this reason.

    The basic thermal conductivity equations (just as the Series Electrical Resistance equations) will tell you that the heat flow is not affected by the order of the layers. But this is an ideal, laboratory case and, once there is any air flow through the bed covers (in a windy tent or draughty bedroom, etc) the rate of heat loss can become dominated by air circulation (convection). The most windproof layer would be better right on the outside to limit convection effects in lower, 'open weave' layers.

    If you don't cover your head and shoulders well then you will never get a good night's sleep. The brain consumes much more power than other organs, when you are at rest, and will account for more heat loss because there is no (significant) insulating layer of fat round your (most people's) head(s). Thermo-regulation will reduce the surface temperature of the body in regions other than the head and upper torso. The surface temperature of the skull (short hairstyle) will be much higher than the surface temperature of the legs. I would sacrifice one blanket round the lower bits to keep the top bits and head warm on any cold night. Look at the endless discussions about the practical aspects of posh sleeping bags, sleeping mats etc etc on the camping and backpacking forums. They may not all have the elite level of technical know-how of PF but they certainly have loads of experimental evidence.
     
  16. Jul 29, 2013 #15
    Practical and ideal two very different cases indeed. Lots of heat loss due to convection in practical case.
     
  17. Jul 29, 2013 #16
    Thick below, thin on top...

    Anyone who has camped out in cold weather will have discovered that you will be warmest with the heavy thick blankets under you, not over you. The pressure of your body weight makes most of your heat loss conduct below you into your sleeping surface - that is where you want the most insulation.
     
  18. Jul 29, 2013 #17

    sophiecentaur

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    Roll mats and carry mats rule!
     
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