David, Rick is right about the wind not penetrating closed cell foam. Heat transfer on a detailed basics can be difficult to understand. You are absolutelyMessage 1 of 57 , Sep 7, 2003View SourceDavid,
Rick is right about the wind not penetrating closed cell foam. Heat
transfer on a detailed basics can be difficult to understand. You
are absolutely right about it being cooler when the wind blows across
your insulation. Without the wind the convection heat transfer is by
a process caused natural convection, which occurs because the warm
air rises and is displaced by cooler air. When the wind blows, the
process is called forced convection and is a much more efficient
means of heat transfer than natural convection because it moves more
air and moves it quicker, effectively removing your layer of
insulating 'dead air'.
--- In firstname.lastname@example.org, "Risk" <geoflyfisher@y...>
> 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.
> the physics of the materials does not allow the air molecules inthat
> wind to penetrate the foam.basicly
> 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
> stuck there and have been there since the foam was built. This iscell
> 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
> foam is much more expensive to make than foam rubber.skin
> My experience with thin pads is that whenever wind is blowing the
> on my back cools and it may even feel like the wind is comingthrough
> the pad... But it is not. I have proved that by creating an airdoes
> tight chamber out of closed cell foam. Even under pressure, air
> not get through the closed cell.is
> 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
> a little radiatiant heat loss as well. That layer of bubblesconducts
> 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 windin
> the bubble) in which the warm air rises and is replaced by the coolwell,
> air from the other side of the bubble. The structure of the rubber
> concucts heat reasonably well, but the air does not conduct heat
> almost all the heat that makes it across each air space is frommy
> 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
> hammock is cooler than my skin.which
> 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,
> has been warmed by my body, through all those layers of bubbles,at
> drifts away and is replaced by colder air, which is more efficient
> collecting more of my precious heat. If the air is blowing by theis a
> 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)
> pretty poor insulator even if it is an inch or more thick. It isonly
> one chamber thick. This is the reason that the Garlington insulatorwhich
> is improved a lot when the spaces are filled with some material
> breaks up the space into a lot of little spaces. (newspaper,and
> 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
> take on this group. I look forward to continued discussion.wrote:
> --- In email@example.com, "o123david" <o123david@y...>
> > Some ideas and responses. I hope they help.the
> > 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
> > extent that cold air blows into it, it loses its ability tothan
> > 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
> > a little bit of wind.less
> > 2. Thank you for pointing out a serious mistake that I made.
> > While it is true that convection results in stagnant air being
> > that a perfect insulator, it is not true that that this preventsIt is
> > stagnant air from providing any insulation. If obviously does.
> > definitely warmer in a two layer tent. Stagnant air definitelythan if
> > 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
> > you let the windproof material blow up against the hammock. Andif
> > you connect this to a tarp above and a floor below (or justbring it
> > down to the ground) you will be warmer still. This can easily bebottom
> > 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
> > of each tree. Condensation could be controlled with a couple ofwith his
> > 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
> > Warmlite tents. Since this is very similar to his tents it isclear
> > that it would work. Netting protecting each vent would eliminatethe
> > need for add netting over the hammock. The entrance could bethrough
> > the floor held up by velcro since any other design would probablydown.
> > 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
> > I don't like this idea because it is unnecessarily heavy and
> > complicated.
> > 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
> > Maybe it is a mistake to include the hammock as one layer of atwo
> > layer tent and it would be best to build the tent as a two layercan
> > tent.
> > Whatever, if you try it, please post a message on this list so I
> > hear about how it works (or doesn't work).separate
> > 3. One other idea. This would make it so you wouldn't need a
> > barrier to block the wind.would be
> > You could construct the hammock from three parallel pieces of
> > material going lengthwise. The two outer pieces of material
> > windproof, such as the 1.9 oz ripstop which is then siliconuncoated
> > impregnated. The inner piece would be breathable, such as
> > 1.9 oz ripstop. It would be interesting to see if this is bothprevent
> > sufficiently windproof and provides sufficient fresh air to
> > condensation. --David
Looks very nice. _____ From: Chet Clocksin [mailto:firstname.lastname@example.org] Sent: Friday, September 12, 2003 5:55 AM To: email@example.com IMessage 57 of 57 , Sep 12, 2003View Source
Looks very nice.