A DIY, ultralight, multipurpose bivy sack

In the past, on some multi-day climbs, I slept under a tarp, with my quilt and air mattress inside a bivy sack.  I also had other nights in a tent, in alpine country, with condensation  wetting out my exposed quilt.  Sometimes in shoulder seasons, my quilt isn’t quite warm enough.  Then this year I discovered a great source of ultralight fabric.  These were all motivations to try another project – an ultralight bivy, with options for bug protection and a cold weather liner.  I finished it and have now tried it out successfully on a cold night.

The fabric source is Ripstop by the Roll.  They offer a wide assortment of coated and uncoated nylon and polyester at weights as low as 0.56 oz. per square yard.  My go-to coated fabric has been 1.3 oz. per square yard sil-nylon, so it looked as though these alternatives would be good for even lighter-weight projects.  I still occasionally use a very old Early Winters Gore-Tex bivy, which, after washing and retreating, continues to be serviceable.  At one point, years ago, I made a velcro-attached bug screen for it, letting me sleep out for a lot of nights in the Greenland summer outback.  I recently added a zipper to it after admiring the utility of a zipper on a newer, used REI bivy.    Unlike the REI bivy, the older Early Winters model had a hood that extended to cover my head for rain, drip and wind protection.  I decided I wanted all these features in my new design with the addition of an insulated liner blanket attaching by velcro to the bivy top cover.  Internet discussions of bivy designs also suggested that I increase the bag circumference to better match the total volume of my liner, air mattress and my quilt..  Using lighter weight fabrics, it looked like I could have it all, with weight savings compared to the two 1 lb. bivys I owned.

Since the goal was never to use the bivy without at least a tarp, I selected a 0.9 oz./SqYd breathable polyester top fabric, but one with a durable water resistant (DWR) finish.  The bottom is 1.3 oz./SqYd ripstop sil-nylon for water and abrasion resistance.  The liner is a layer of 2.5 oz./SqYd APEX Climashield insulation sandwiched between two 0.75 oz./SqYd uncoated ripstop nylon covers.  While keeping the overall length and hood dimensions of my old Gore-Tex bag, I enlarged the new circumference from 72″ to 80″.  Matching velcro tape outlines the perimeter of the inside of the bivy top layer and one side of the liner.  With the bivy turned inside out, the liner is easy to install.

A 36″ skirt of bug netting velcro’s to the front of the hood.  The hood also has three sleeve sections for insertion of a Nite-Ize flexible 64″ Gear Tie.  This oversized “twisty-tie” supports the hood and netting away from my face.

I finished the project making two stuff sacks, one for the bivy (and netting if carried) and the other for the liner (or bivy with liner installed).  The bivy alone comes in at 9.2 oz.  The liner adds another 8.2 oz.  Bringing the bug netting and Nite-Ize hoop adds 3.7 oz.  I estimate that the liner will be worth about 10 degrees of bag warmth, plus whatever the bivy alone adds.  I tested the bivy bag and liner on a night that got down to the upper 30’s.  This would have been too cold for my quilt alone, but the new combination was warm. There was overnight condensation and again my bivy bag surface was wet in the morning, but nothing inside was even damp.

So I have a system that gives me condensation, drip protection and a little warmth (the bivy bag alone) for 9.2 oz.  If I need bug protection while sleeping under a tarp, I will be carrying 13 oz. including the netting and Gear Tie. For cold weather, leaving the bug netting home and carrying the liner, I have a total of 17.4 oz., about an oz. heavier than my older bivy sacks, but much warmer.  It’s a nice set of options for my local varied backcountry conditions.

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