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404RE: Hammock Camping Franken pad and bride of Frankenpad

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  • Ernest Engman
    Feb 2, 2003
      If reflective pads are so good, then why not a sleeping bag that is made
      from them? They aren't. Can a space blanket really work? Somewhat, but
      I've never bet my life on one and will never do so because the theory
      that tells you they will is very flawed. But people buy them, so people
      will sell them.

      My position that radiant heat reflectors are snake oil has grown over
      time, and after a couple of debates with some people smarter than I, I'm
      sure of it.

      The science is this, and don't take it is as just me, this is from an
      aerospace engineer working for NASA and another engineer that have both
      convinced me of this:

      Heat isn't as simple as saying 25% of your body heat is lost thru
      radiant heat loss. It is much more complicated than that. On a night
      with high humidity but low temperature, you will be a lot colder than on
      a night with the same temp with lower humidity. Why? Because the
      moisture in the air is conductive, same principle as sleeping in a wet
      bag in a way, but you are loosing more of a percent of heat this way and
      less thru radiant heat. Same thing in high winds. How can higher winds
      make that big a difference if more is lost thru radiant heat loss? It
      couldn't but the real problem is demonstrated by this is convective heat
      loss. I hope this isn't too much of a tangent for the discussion, but it
      just shows how one factor can change the percentages.

      Radiant heat reflectors (actually insulators) were made to work in
      space. In space there isn't any air flow at all to worry about, so
      absolutely no convective heat loss to worry about. In space they not
      only have to protect the person from heat loss, but excessive heat gain
      from solar radiant heat. Ever hear that in space one side will freeze
      while the side facing the sun would burn up?

      So what is heat? Heat is energy, cold isn't the opposite, it is the
      absence of heat. Heat energy, by its nature, will try to equalize the
      energy from places of higher energy to places of lower energy. If your
      body is the point of higher energy (which it normally is at night) then
      your heat energy will attempt to displace outward.

      There are about 4 means of heat loss that you must worry about.

      1. Evaporation. When you sweat or breathe, the water loss from your body
      carries heat energy with it. To prevent this you can wrap yourself in a
      vapor barrier. But to make this work, you will most likely want to be
      totally undressed or you will end up in soggy clothing which is also
      bad. Ed talks about always allowing your breath a way to get out - very

      2. Radiant heat loss (the main topic) which is heat transfer based on
      how IR opaque or transparent barriers between you and the object are. If
      you were to stand naked in a freezer, you would loose a great deal of
      body heat thru radiant heat loss. Wrap yourself in a space blanket, and
      you would reflect about 50% of your heat loss back at yourself because
      the air will still carry away some through convection. But to reduce
      this loss, you can wear clothing. Since the clothing is essentially IR
      opaque, you can reduce this loss of you radiant heat energy down to
      about 5% of your original loss. So if you have a given (which I disagree
      with) of 25% heat loss thru radiant heat, and reduce that to 5% of the
      original loss by wearing clothing, then you are down to less than 2% of
      your total body heat loss. Now use a radiant heat reflector to capture
      50% of that, and it is returning less than 1% of your total body heat
      lost. This 95% (give or take) heat loss prevention occurs for every
      layer of IR opaque material between you, and the colder objects around
      you. So if you wear clothing, then a sleeping bag (inside layer) then
      you already have reduced your IR heat loss to less than 1%.

      For a radiant heat reflector to work (in the atmosphere) you would need
      to be inside it without clothing and not touch it (which is where I'm
      going next) for it to be of any help. Interestingly Stephensons
      Warmlight sleeping bags understands this and explains it quite well for
      someone looking at their bags.

      To prove this, feel free to make a shower curtain of mylar then turn
      your heat off. Stand inside the shower with the curtain nude and see if
      it (the curtain) gets warm. It won't. You are not a big enough an
      emitter of IR to make it work. The sun is a huge IR heat emitter and
      does this quite well, unfortunately our bodies don't come with a huge
      supply of self heating hydrogen.

      3. Convective heat loss. When air passes around your body, it carries
      away heat; this is the biggest problem keeping warm in an atmosphere.
      The trick is to stop air from passing around you and create pockets of
      still air that can be warmed and kept into place. Sleeping bags and pads
      are designed to do this. Again, if radiant heat reflectors were truly
      beneficial, then that is how we should have been staying warm for the
      last 50 years in the hiking world, but so far we all still use sleeping
      bags and pads. Ed has done a great job of figuring out how to make this
      happen with a pea pod. And to make it even better would be to surround
      the pod with a waterproof material that totally stopped any air from
      getting out and totally stopped any moisture from getting out. Foil wrap
      does a great job of this, by the way - that is what a space blanket is
      made from. But it also has a drawback that it traps moisture that
      destroys loft in a down bag. The best solution would be some material
      that could surround your pea pot and prevent air flow in, strip heat on
      the way out for moisture, and wouldn't suffocate you.

      4. Conductive heat loss. This is a problem because some people try
      laying directly on the reflective surfaces which are also excellent
      conductors. Some spots under these conductors are warm because they
      prevent evaporation or convection, but they also conduct heat away. This
      is also how the moisture carries away your body heat.

      So to conclude this lengthy post and spark the debate I don't mean to.
      Radiant heat reflectors are not that inside the atmosphere, they are
      serving as barriers to convective heat and if used right, evaporative
      heat. I've never said that people using a system that involves a "Heat
      Rflector" aren't staying warm (heck I have one) I'm just saying they
      should get smart about why they work and figure the best applications
      for these materials. Aluminized plastic (mylar) is very good at blocking
      moisture, and it is what all really long shelf life packet foods are now
      packed in.

      Ernest Engman
      AKA SGT Rock

      -----Original Message-----
      From: starnescr <starnescr@...> [mailto:starnescr@...]
      Sent: Sunday, February 02, 2003 1:37 PM
      To: hammockcamping@yahoogroups.com
      Subject: Re: Hammock Camping Franken pad and bride of Frankenpad

      If a reflectic pad is snake oil then why are "emergency blankets
      made of reflective material. Of course you wrap a space blanket
      around you and are not laying on it only. Would a reflective pad be
      more effective (no mater how unpracticle) if it went all the way
      around you. I think the reflective pad is somewhat usefull if you
      are not laying directly on it. From what Tom Hennessy and others
      have said there needs to be a little space between the heat source
      and the reflective surface. Hence my fleece covered reflectic pad.
      A 1/2 inch closed cell foam pad the same size as mine would be
      heavier but I'm not sure how much. I'm not saying a reflective pad
      is the answer either. I just believe given the same R value a
      reflective pad would be slightly warmer.

      Coy Boy

      --- In hammockcamping@yahoogroups.com, "Ernest Engman"
      <ebengman@h...> wrote:
      > The pad I use serves a couple of purposes.
      > 1. Wind/air barrier. It is made of aluminum coated close cell
      > pad. I can use it alone in cool but not cold temps by turning it
      > side down and preventing air circulation.
      > 2. Since it is also foam, it adds a light and small layer of
      > insulation.
      > 3. It serves as a ground pad under the hammock when I lay it on
      > ground to stay warm. It is wide and long enough for that. Often
      > having to do this, the ground isn't dry and no sense getting the
      > wet.
      > 4. Closed cell foam pads work better against a non-breathable
      > barrier than against a breathable material like than hammock. See
      > excellent post by youngblood:
      > http://hikinghq.net/forum/showthread.php?
      > ff2
      > <http://hikinghq.net/forum/showthread.php?
      > bff2&threadid=359> &threadid=359
      > I say heat reflectors are snake oil, because the science that says
      > are a radiant heat reflector for someone sleeping in a hammock is
      > flawed.
      > Ernest Engman
      > AKA SGT Rock
      > sgtrock@h...
      > <http://hikinghq.net/> http://hikinghq.net
      > -----Original Message-----
      > From: David Chinell [mailto:dchinell@m...]
      > Sent: Thursday, January 30, 2003 9:25 AM
      > To: hammockcamping@yahoogroups.com
      > Subject: RE: Hammock Camping Franken pad and bride of Frankenpad
      > Sarge: Thanks for the R&D work. I'm dreaming along the same lines.
      > That's what the evazote "shawl" is about -- extra width between
      the hips
      > and shoulders.
      > But I need to make a cloth casing to hold the wings so they're
      > detachable, rather than permanent, so I can roll it all up in a
      > 24-inch wide roll. How do you pack the Frankenpad?
      > And if, as you say, "heat reflectors are snake oil" why do you use
      > windshield reflector? Why not just another pad?
      > Bear
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