First off, I'm talking about temperatures where you have liquid precipitation.
My understanding of how the layers work is really my understanding of how they work for me and how I use them.
I normally have a pack and only wear the base/insulation layers I need, and try to minimize sweating. Extra layers stay in the pack. Besides layering I use zip-front base layers and shirt to ventilate, gloves, and hat that go on/off without stopping to fine tune heat retention. I stick to none or lightweight long john bottoms because you can't adjust those on the move.
Base layers should wick sweat away from the body, and both base and insulation layers need to be hydrophobic, not clump or collapse when wet, and insulate when wet.
Unless you're sitting on the ground where you lose heat through conduction, or at night where you lose it to the sky through radiation, you'll be losing most of your heat through convection and evaporation.
Good hydrophobic base and insulation layers will insulate when wet, although not as good as when dry, as long as you don't have significant convection, which equates to being cooled by moving air. If you've got wet insulation and can't get out of the wind you'll be losing heat through both convection and evaporation, not a good combination. So a windproof shell over wet insulation will cut down on both and keep you warm, and if you're already warm and/or on the move you obviously take off the shell and let the moisture evaporate.
If you have a windproof shell, you may as well make it waterproof or water resistant and not get your insulation soaked in the first place. It's easier to stay dry (most of the time) than to get dry.
Like a lot of others I don't put much stock in waterproof breathable shells for keeping inner layers from getting wet with sweat. Water vapor from your body is warm and the outer shell is cold so no matter how permeable you'll get some condensation in the shell.
The reason I carry a shell is to cut down on both convective and evaporative heat loss. And to keep my insulation as dry as possible.
To be clear I responded to you because you quoted me, not because I necessarily found that you were wrong. A rain shell is one of the last clothing items that I would leave on a backcountry trip.
Really the whole thing is about equilibrium. You, me, the environment- all want to keep everything balanced. In SE Alaska and environments like it there is no way to stay dry if you are outside. I have never been able to stay dry, no one that I have ever talked to has been able to stay dry, and there is no possible way with what we have to stay dry. In that environment all I care about is clothing that will keep me alive while I'm wet as there will be no comfort involved. For sure it may be possible to be less than sopping wet, but you will always be damp.
I don't think that is what you're referring to however. Instead I believe that you are referring to the Rocky Mtn. area of relatively low humidity, high elevations, and cold. If we are sitting still it is possible to stay mostly dry in this environment during an afternoon storm. If we move we will perspire and if we are wearing a "waterproof, breathable" outer layer at some point the wet from inside matches the wet from outside and then all we have is a windbreak, which may be very important. All this is to say that you are generally correct, however my initial post was more to do with 175rltw's assertion that rainwear isn't needed, though it sounds like he learned clothing and comfort from the same local that I initially did.
I use a simplified 4 layer system for about 95% of what I do.
Synthetic or merino wool Base layer- main purpose is to help move moisture away from skin, secondary is for added insulation.
General outer layer- Mainly used to protect base layer or skin. Can be synthetic, wool or Cotten depending on conditions.
Insulation layer- puff pants and jacket either synthetic or water resistant down- for warmth and to help provide a micro climate.
Waterproof, breathable shell- princapply used for those who particapate in exertive activities not to block rain, but to create a micro environment within the clothing system to "pull" moisture away from their skin and base layers.
Going back to the equilibrium thing, the base layer absorbs the moisture from direct contact. Next to skin is a high humidity environment. Between base layer and outside layer is a lower humidity environment. Because the air wants to equalize the high humidity next to skin is "pushed" in to the lower humidity of the general outer layer. The goal is to get the moisture as far away from the skin as possible, so while hiking in you soak yourself with sweat, your base layer is absorbing that moisture and pushing it to the outer layer. That's all good except that if it is cold enough- when you stop it can do to good a job and produce cold injuries. So the moment you stop throw the insulation (puff suit) top and bottom on AND the waterproof, breathable outer layer on even while still hot- in fact because you are still hot. You're using that heat to help "push" the moisture away. Again the environment between the general purpose layer and insulation layer is lower in humidity and will pull the moisture into the insulation layer. The humidity between the insulation layer and waterproof, breathable layer is again lower and will pull the moisture from the insulation layer into the waterproof, breathable layer and then out into the air.
There is of course more to it and while it seems that you know what you're doing, it is very clear from a lot of the responses in threads that a lot have no idea that clothing IS a system and understand how that system works.
