-From BPL Thermoregulation article.


Convection
Convective heat loss occurs in response to movement of a fluid or gas. In outdoor clothing systems, convective heat loss occurs when warm air next to the body and in the clothing is displaced by cool air from the outside environment. The biggest factor contributing to convective heat loss, of course, is wind.

In addition to wind-induced or “forced” convection, “passive” convection occurs via the “chimney effect” that draws cool, dense air into our clothing system from pants cuffs and waist hems, displacing warm, light air that exits out of our neck hems and other vents.



If I understand correctly, I think if you have an airight suit you could prevent convective heat loss completely. Air inside will be heated by body but since it cant be displaced it will form an insulative layer. Body heat will turn sweat into vapour but since it is air tight it and warm inside it will condense and give out energy. Anything wrong with my logic?

SUL air tight suit? cuben fiber with stretchy neoprene cuffs, hems, and neck collar.

You still get convection and resulting heat loss in a sealed system... eg. an air-core sleeping pad. So in a sealed suit there would still be internal air currents from warmer to cooler regions. A completely sealed suit would stop outside air from flowing in, so would obviously be warmer. But the idea of stretchy cuffs and hems to keep warm air inside is nothing new.

Nothing new here. They are called Sauna Suits, available from under $10. They work as advertised.
Franco

BTW, you would be unlikely to want a full body-suit. To cut down on convection you would want a separate jacket and pants... the same way that dividing a sleeping pad into many smaller internal cells will help lower convection currents.

ok it may not be a new idea but I am interested in knowing why isnt the concept being used by UL backpackers to save on insulation.

here is an idea to prevent internal air current- a close fitting down jacket/pants with nonbreathable inner and outer shell and stretchy neoprene cuff, hems. you may need a valve. This jacket would be much more warmer then a breathable jacket with same amount of down.

Huzefa,

If you have on a VB as you describe, then what about the air between the VB and your down? What about between your down and your shell?

Though convection is occurring in all these places, a model of organized swirling currents is unlikely to be useful within the clothing system.

It is mainly the convection of the air outside your clothing and moving against it that is of concern. If you want to work with that outside air, I can imagine fins or ridges sticking out from your shell to act as wind-breaks. Of course they would have to be non-conductive, so as to not be a heat-sink. But now I think my insomnia is designing some silly stuff...

Why isn't it being used more while sleeping? Because you will wake up with moist/wet skin. If used much above freezing, some may even begin sweating (other than insensible) inside a VB causing you to lose even more water in addition to your hike.

Edited by jshann on 06/19/2008 08:05:44 MDT.

doesnt down by 'trapping' tiny packets of air basically preventing internal air current?

This thread is certainly not wasted. I understand conduction and convection much better now. But there is a lot more I still need to understand. For eg. since we are talking about clothing not a fluid system isnt convection and conduction both taking place?

Huzefa, the reason is, it would be miserable to wear such an air-tight down suit while exercising.

This technique is used by wrestlers and their ilk to 'make weight' when they are over the weight limit of their weight category. They don these airtight suits, work out, sweat like pigs and loose so much weight (water) than it nearly makes them pass out and die.

Putting your body in anything with very limited air flow, even if your head is outside, is foolhardy at best. Even space suits and hazmat suits have air inlets and god knows they want to protect you from the elements.