Overbag for Deep-winter use

Do you have any sense of how much of your 70″ width gets tucked under you and compresses?

You could likely stick w/ the 60″ width of the insulation and just extend the outer layer of fabric from the edge of the shell 5″ on each side to tuck under and seal out drafts.  For the foot box, you want to be sure it will fit over the full dimensions of the inner quilt.  Consider the width of your inner quilt at the foot (assuming it opens flat w/ a draw cord), use that as a circumference and calculate the corresponding radius.  Then add 1.2″ (thickness of 5osy Apex) and calculate the new circumference.

Since this is an over bag, you’ll have little to no contact with the inner liner so you shouldn’t be too concerned w/ how it feels against the skin.  Since this is for winter use, you have less concern for how the outer shell handles liquid water.  That really just leaves breathability.

There are a number of fabric property charts posted on the Reference Resource Index ;)

Rene the Gerry Mountain Sleeper is like a quilt but it is stitched all the way around in the original idea and you wriggled you way into it. Gerry C used fibre insulation in the base and down in the upper part.
With the modern insulated air mats I want to use my current combination of CCF pads and insulated air mats in the base and that would make wriggling in very hard if not impossible and leaving a gap would be very cold at very low negative temperatures. So I need to have at least one side openable with a zipper.
I was intending to use lightweight EPIC as the outer shell fabric, it is only 32GSM and I already have it and the lightest possible most breathable fabric for the liner but if that fabric wasn’t suitable I’d change.
I’m a restless sleeper and tend to sleep on my side mostly so I would say that a third of the sleeping bag is under me at any one time
It is the zipper installation I am having trouble visualising and whether to have a 1200mm zipper on one side only or two shorter zips on each side

Edward, is that lightweight EPIC available anywhere?

Not anymore that I am aware of although Jan at Nunatak has stocks for use in the factory I’ve had mine a while waiting for a project

OK, thanks Edward.

Sorry, I’m not following what you want to make: zippered vs zipless; over-bag vs stand-alone.  I’m not familiar w/ that brand, so maybe I’m the wrong person to be advising on your design.

32gsm is about twice the weight of Momentum 10 and Argon 67, both commonly used for quilt/bag shells.  If you’re inside a tent or bivy, I don’t see the need for a heavier shell.

I can envisage circumstance where being weatherproof is a survival need and if I have an overbag with an EPIC shell I won’t be using a bivvy, saving a kilo+ there. I have tried a standard overquilt and it doesn’t work for me, too many drafts

Here is a link to Big Agnes Rene so you can see what a Mountain Sleeper is all about

https://www.bigagnes.com/Gear/Sleeping-Bags/Big-Agnes-System-Bags

BA do not make UL gear tho and what I am planning is going to be very light for what it is even with EPIC as the outer layer; or at least i hope so

Climasheild recommend using scrim between layers of APEX if double layers are being used. I cannot find UL scrim [ or even LW ] here. so I am wondering if No-See-Em mesh would be too heavy?

I know people do not usually use scrim when using APEX in DIY projects but if the makers say you should then I think I should use a separating layer