Heat loss from a sleeping bag.

Sleeper cranking out 100 watts.
Temperature difference of 50degK, or 90 degF.
A surface area of 2 square meters.

So you need a Rsi value of 1, or 5.75 in english units.
A clo value of about 6, more or less.

Shouldn't you be able to achieve R 5.75 with 1" of loft, maybe 1.5"?
If say half the heat is lost by other means, like breathing or leakage from aroud the head and neck area, then perhaps 2" or 3". But aren't those losses reduced to 20% or so with a good design?

So how effective is 600fill or 900fill down, per inch of loft?
How much does it depend on the shell material?
How do bivies and ground pads tie in?
How about wool liners?

I guess I am curious about how much of the 60-100watts go out through the shell and insulation to the air, and how much through the shell and insulation and ground pad, and how much through respiration, and how much sneaks out by other means. How much weight should go into the ground pad?

Also, I am curious just how much thicker lightly knit or woven wool or fleece layers might need to be, if covered with a gortex bivy or light nylon bivy. Would 1" be enough for clo 6, if alternating knit wool and fleece layers? Still too heavy? What about 1/2" of knit wool blanket inside of a blue foam bivy, like a mobile snow shelter?

Unfortunately, if the outer sleeping bag shell isn't breathable, there will be condensation on the inside of it, especially in cold weather! This does mean unavoidable heat loss to avoid wetting out your insulation.

A vapor barrier between your base layer and the sleeping bag helps, but it appears that everyone reacts to this differently. For me, the point at which I'm comfortable in a VBL inside my sleeping bag is in the low 30's F. For others, it has to be a lot colder to avoid the VBL's becoming a sauna inside.

Right now I have a 2 pound army surplus gortex bivy, and an old Kelty 3 pound synthetic mummy. I use them with a 72"x28" blue foam pad, outside the bivy, because the pad fits the 30"wide bivy very well once the bag and I are in it. The bivy is a little oversized at the foot though. It has a semicircular flap at the top that tightens up around the neck and head area of the mummy pretty good. I've been thinking about tapering down the part below the hips though. At 5.5 pounds it is not a bad setup, and is comfortable to 0F with clothing, socks, mitts, necktube, and hat. Below 0F it gets very sketchy. I have done -20F falling to -25F in the backyard under a spruce tree, and I survived to 6am. That was alot of clothing also, like everything I would wear walking at that temperature.

So I am going to try adding some synthetic batt insulation inside the bivy and outside the sleeping bag. I am thinking of giving it a light nylon shell on just one side, for some durability, but initially I might not even bother with that. It would fold to have one layer under the bag and me, and two layers over top of the bag and me. The gortex bivy will hold it all together. I am going to add some sort of strings so I can snug the bivy up a little, so there is no large air pockets between the bivy and the synthetic batt, or the synthetic batt and the sleeping bag. If it works well I might use just the quilt in summer; just the quilt and bivy or just the sleeping bag in Spring/Fall; the sleeping bag and bivy in the less extreme parts of winter; and all three mid-winter on longer trips when I might get hit with 2-3 days of a -25F cold snap, which can happen Jan-Mar, usually once or twice each year.

Not sure about the best way to mess with the bivy yet. I might just keep it as is because I can't see removing all that much material and still having a foot box, and room for the extra insulation. If I really went to town on it I could just use the blue foam pad as the bottom. That might save some weight, at least as much as the blue foam pad, half a pound. I am not sure of the best way to seam a blue foam pad to a gortex bivy. Wrestling matt tape maybe. I would like it all to be able to roll up flat when done. If I used the bivy as my pack cover also, with a light pack frame, or on a toboggan, then I might just as well leave it the size it is so I can dump stuff into it, with the blue foam pad as a cylinder, and then roll it all up like a cigarette, and then tie it up It would be expandable that way, but 28" tall which is not a bad height for me. My jam2 is that height now when fully extended with the blue foam pad inside. So I would save 20oz in not bringing the jam2, except I would add at least half that back in shoulder and waist straps and any frame, if needed. With the blue foam pad, probably not. Bottle holders on front straps maybe. I would need some sort of mesh pocket for clothing layers going on and off. I like a front pouch in my overfleece for warming hands and holding mitts and items, including my water bottle if I make tea and keep moving.

Just some ideas to work on. Not all that different than the old packs before modern packs and sleeping bags. Gortex instead of canvas. Synthetic batt and sleeping bag instead of wool blankets. No long fire and lean to, though that is still an option for -25F.

A bag liner can add 10 degrees to your rating and only add a 4 to six ounces.

You will probably get more warmth in the cold from additional pads than from lofting insulation underneath you. My memory is that the blue foam pads are between R2 and R3 and that you need at least R5 under you for frozen ground. My winter pad set up is a full length 1.5" Thermarest (the uncored kind) over a Ridgerest for R value over 7. Last winter, I realized it was finally time to throw away my old CCF pad because I was getting cold from the ground even with the Thermarest.

This thread makes me like quilts much more.

Have you looked at overbags at all? I saw some on the EMS website, I think...