After getting sticker shock from Western Mountaineering's $99. reflective VBL (nice as it is) I now have decided to discard my jerry-rigged and fragile mylar VBL and make my own. Heck two to four seams and a little sealing and I'm done.

QUESTION: Where can I get reflective, waterPROOF ripstop nylon?

Eric

Edited by Danepacker on 12/02/2007 19:44:30 MST.

Plus is the seam sealing really even needed for VBP useage?

"QUESTION: Where can I get reflective, waterPROOF ripstop nylon?"

1.3 oz. Heat and Solar Reflective Ripstop:
http://www.seattlefabrics.com/nylons.html#1.3%20oz%20Reflective

Thanks Bob. Looked there once but not hard enuf I guess.

Eric

Despite what marketers would like you to believe, reflective materials have no discernable effect on retaining human warmth. I once posted the formula for heat transfer on this site. Once you see it, it becomes immediately clear that conduction is the primary heat transfer conduit at low temperatures (98.6 degrees). Convection takes over at mid temps and radiation kicks into high gear with high temps. When your body hits 800-1200 degrees F, that's when reflectivity will pay off.

Put your face 4' from a blazing bed of coals. place a 1' square of aluminum foil halfway between your face and coals. Notice the heat from the coals hitting your face go to zero.

On a cold night hold your bare arm in front of you. Now place your other bare arm 1/2" away from the first. Observe no discernable radiation from one arm heating the other. Now slide a piece of foil between the arms. Feel the radiation not being blocked by the foil because it doesn't exist. Your body is far too cool.

Hm... Have you ever slept under a space blanket? It gives me at least a 10 degree comfort boost. Of course, to know for sure what's causing that I would need to try sleeping under a thin plastic sheet for comparison.

Edited by Legkohod on 03/08/2008 06:12:16 MST.

Other very subjective tests -

Singlepane versus thermalpane windows:

There is a very noticeable difference up to say 3 or 4 feet.
I "Feel the Cold" - I sense that radiant loss easily.


Lack of ceiling insultaion:

I bought a house with no insulation in the attic. (don't ask.)
While sitting at the dinning room table I could feel a dramatic difference when my legs came out from under the table. The sense of a cold ceiling was real. It went away with 18" if fiberglass.

There may be a difference between sensing incoming radiation, say from the fire place, and sensing a Loss of heat, say to a window pane. The quantative number are certainly very difference.


Al, your formula indicated "Radiant loss is small compared to convection", but in terms of sensible comfort, I believe controlling radiant loss makes a difference.

And to muddy the waters a bit, I also believe that much of the "space blankets are warm" sensation result from their vapor barrier effect.

Now - to sort out all of the science....

Edited by greg23 on 03/08/2008 07:43:14 MST.