John: A vbl's purpose (while sleeping) is to prevent moisture from entering and affecting insulation loft.
Thanks for the clarification. You are half right.I was thinking about VBL clothing's use to retard heat loss, but inside both a sleeping bag and insulative clothing VBLs can also keep the body's moisture from invading the insulation. I was talking about one benefit of VBLs and you have pointed out the other.
Whether sleeping or active, VBLs allow us to use significantly less insulation due to both not wetting our insulation and shutting down Evaporative Heat Loss (EHL). My use of raingear as VBL demonstrated a dramatic decrease in insulation needed to remain comfortable. With or without the VBL, my insulation (both down and synthetic) felt dry and lofted well in the dry Sierra air.
If a vbl didn't trap moisture, how is it going to stop evaporative heat loss Al?
A VBL does trap moisture. That is how it shuts down EHL. See my post above.
John: There can still be conductive and radiative heat loss going on though.
Heat loss from the human body through radiation is negligible. If you look at the formula for heat transmission from conduction, convection and radiation (which I have posted elsewhere on this forum) it is immediately apparent that at low temperatures (98.6 degrees), conduction is the primary heat transmission agent and radiation has almost no effect. That is why aluminized mylar space blanket marketing is pure hyperbole.
Conduction is present with or without VBLs.Fortunately, when we take EHL out of the equation, this conductive heat loss becomes much more manageable.
If VBLs did not dramatically decrease perspiration, as you posit, the ensuing moisture build up would, among other problems, increase conductive heat loss greatly through the puddle of liquid trapped in the clothing and flooding into gloves and socks.
Fortunately, I have observed no such occurrence.
Thanks for keeping me honest.