Hammock Camping Re: Oh Boy, Cold Wars II, winding up!
Good morning. I think you are right and wrong about wind and foam.
There is no doubt that wind blowing across closed cell foam that is
something like .25-.5 in thick will quickly cool your skin. However,
the physics of the materials does not allow the air molecules in that
wind to penetrate the foam.
I looked into closed cell and regular latex foam rubber when I was
building kayaks more. An engineer friend that runs one of the
mattress production companies helped me out on the science. Closed
cell foam is built of very many small gas filled spheres, tightly
melted against each other. The air molecules in the foam are basicly
stuck there and have been there since the foam was built. This is
very different from the air cells of the foam in a mattress, most of
which do have connection with the outside air. By the way, closed cell
foam is much more expensive to make than foam rubber.
My experience with thin pads is that whenever wind is blowing the skin
on my back cools and it may even feel like the wind is coming through
the pad... But it is not. I have proved that by creating an air
tight chamber out of closed cell foam. Even under pressure, air does
not get through the closed cell.
What really happens, as I understand it, is this: If I lay directly
on the foam, heat from my back warms the first layer bubbles in the
foam. THis is mostly due to conduction of heat directly, but there is
a little radiatiant heat loss as well. That layer of bubbles conducts
and radiates heat to the next 1/16 inch or so of foam bubbles. Inside
each bubble, there is a little convection current (an internal wind in
the bubble) in which the warm air rises and is replaced by the cool
air from the other side of the bubble. The structure of the rubber
concucts heat reasonably well, but the air does not conduct heat well,
almost all the heat that makes it across each air space is from
convection in the bubble. This process is carried out layer after
layer away from my body -- as long as the air outside the foam and my
hammock is cooler than my skin.
What is happening from the outside is that heat is lost to the
environment. The rate at which heat is lost depends on a number of
factors... Mostly, the air on the outer surface of the hammock, which
has been warmed by my body, through all those layers of bubbles,
drifts away and is replaced by colder air, which is more efficient at
collecting more of my precious heat. If the air is blowing by the
hammock, more heat is absorbed from the surface. In effect, the
temperature down 1/16 in the foam and 2/16 and so on, is colder. It
ends up being colder against my skin, and more of my heat is lost to
that first layer of bubbles on the inside.
If the goal is to keep body heat in, then anything which makes that
effective distance from the outside air to the skin bigger is good.
It is also good to make the distance full of lots of small chambers,
instead of large chambers. (Heat travels across large chambers by
convection almost as well as it travels across small chambers) This
is the reason that an old fashioned air mattress (no foam inside) is a
pretty poor insulator even if it is an inch or more thick. It is only
one chamber thick. This is the reason that the Garlington insulator
is improved a lot when the spaces are filled with some material which
breaks up the space into a lot of little spaces. (newspaper,
styrafoam peanuts, leaves, crinkled space blanket).
Another small aside: It is important to keep that cold outside air
from getting inbetween any layers of the insulation. If wind is
blowing and inflates a Garlington Insulator by getting between the
hammock and the insulator, all the effect of the insulation is
temporarially lost. I learned this one windy snowy night when the GI
kept getting blown downwind like a sail.
Your idea of a tent like chamber beneath the hammock has a lot of
merit in that it can keep cold air from blowing across the hammock.
However, it has the problem that it can be a large volume. To be
effective as insulation, my body will need to begin to warm the air
inside that large bubble and the ground beneath.
I look forward to your experiments this winter. I enjoy the give and
take on this group. I look forward to continued discussion.
--- In firstname.lastname@example.org, "o123david" <o123david@y...> wrote:
> Some ideas and responses. I hope they help.
> 1. Cold wind DOES blow through closed cell foam. I have felt it.
> Closed cell foam is just one more form of insulation where, to the
> extent that cold air blows into it, it loses its ability to
> insulate. This is both my experience and what is predicted by
> Newton's Law of Cooling.
> In other words, closed cell foam does not protect you from more than
> a little bit of wind.
> 2. Thank you for pointing out a serious mistake that I made.
> While it is true that convection results in stagnant air being less
> that a perfect insulator, it is not true that that this prevents
> stagnant air from providing any insulation. If obviously does. It is
> definitely warmer in a two layer tent. Stagnant air definitely
> provides what can be a significant amount of insulation.
> In other words, if you have a windproof material surrounding the
> hammock and hold it away from the hammock you will be warmer than if
> you let the windproof material blow up against the hammock. And if
> you connect this to a tarp above and a floor below (or just bring it
> down to the ground) you will be warmer still. This can easily be
> done using a diamond-shaped tarp, the two stakes already used to
> hold the tarp down, and a cord at each end going around the bottom
> of each tree. Condensation could be controlled with a couple of
> vents at the bottom to let in dry cool air and a couple of vents
> at the top to let out warm moist air, as Stephenson has done with his
> Warmlite tents. Since this is very similar to his tents it is clear
> that it would work. Netting protecting each vent would eliminate the
> need for add netting over the hammock. The entrance could be through
> the floor held up by velcro since any other design would probably
> result in accidents and damage to the tent. It appears that this
> would work very well with a Peapod or a thicker bag filled with down.
> I don't like this idea because it is unnecessarily heavy and
> But it isn't that heavy and it might be a good idea.
> I spent a lot of time while thruhiking the AT last year ('02)
> thinking up this design. It would be nice if somebody would try it.
> Maybe it is a mistake to include the hammock as one layer of a two
> layer tent and it would be best to build the tent as a two layer
> Whatever, if you try it, please post a message on this list so I can
> hear about how it works (or doesn't work).
> 3. One other idea. This would make it so you wouldn't need a separate
> barrier to block the wind.
> You could construct the hammock from three parallel pieces of
> material going lengthwise. The two outer pieces of material would be
> windproof, such as the 1.9 oz ripstop which is then silicon
> impregnated. The inner piece would be breathable, such as uncoated
> 1.9 oz ripstop. It would be interesting to see if this is both
> sufficiently windproof and provides sufficient fresh air to prevent
> condensation. --David