I've been agonizing over the selection of a lightweight winter bivy bag for a couple of weeks and I thought I'd come to a final decision when I ordered the Integral Designs Overbag Bivy. This bag is completely made using eVent fabric and is considered to be one of the most breathable bivies available. But, it doesn't have a zipper! I realized this a little late in the game.
But let's start at the beginning….
This year, I decided to switch from a tent to a floorless pyramid tarp for winter camping, augmented with a bivy sack for moisture management and extra warmth. I've gotten really used to sleeping in a bivy this year under all kinds of different tarps and I've really come to like them.
After sifting through the various winter bivy bags available, I narrowed my decision to the Integral Designs Overbag ($240) or an Alpine Bivy from Mountain Laurel Designs ($290 – extra long with a side zipper). The Overbag is made using eVent while the MLD Alpine Bivy has an eVent top and a silnylon floor. Upon request, MLD will also sew a custom, all-eVent version of the Alpine Bivy, for a hefty premium.
Ultimately, I decided to go with Integral Designs Overbag despite the fact that it is heavier than the Alpine Bivy (18.7 oz vs 13.75 oz) in the configuration and long sizing I wanted. In the past, I've experienced some condensation where a sleeping bag touches a bivy made with silnylon, something that would be avoided using the Overbag.
Mummy Hood with side compression loops. Top hang loop is my modification threaded through eVent Label
To confirm my reasoning, I posted a thread in the Gear Forum at Backpackinglight.com. I came away after an informative discussion with the understanding that an all-eVent bivy was likely to perform better in snow shelters because eVent has a higher hydrostatic head than silnylon, so I went ahead an ordered the Integral Designs Overbag from Amazon, where it is heavily discounted to about $170.
A week later I received a package from Amazon containing an Integral Designs Sil Tarp 2, and not an Overbag. I was not happy. Moreover, when I tried to return it, Amazon couldn't get past the fact that the product they'd sent me didn't have an ISBN. I eventually managed to get a full refund, but that's the last time I'm going to order a bivy bag from Amazon.
I found another retailer, Backcountrygear.com, that carries the Overbag and ordered it, paying full price. The OverBag arrived after a week and I tried it on in my living room with an Exped Downmat and a Western Mountaineering Puma -25 (F) sleeping bag. Man, is it big! It swallowed the Puma and the Downmat without causing any loft compression in the sleeping bag. So far, so good.
I inspected the stitching around the bag, which is nicely taped, and checked out the eVent fabric. But as I lay there on the floor, I realized that the Overbag bivy was going to be hard to get into and out of at night, in the dark, under a tarp, in the dead of winter. Let's just say that I drink a lot of fluid in the evening when I go winter backpacking to rehydrate and prehydrate, and I can't hold it for 12 hours at night.
OverBag: Two side cord locks on either side of a peaked mummy hood
Without a side zipper, the only way to get out of an Overbag is to do something that looks very much like a wet exit from a kayak.
If you have no idea what I'm talking about – the motion is similar to taking your pants off while sitting down. I just can't see doing this in a snow shelter and staying dry. The same issue arises with getting your sleeping bag into it all the way: you need to be wearing the sleeping bag first, and then slide into the bivy.
Hence the need for a zipper.
I admit it. I have been completely spoiled by the zipper on my 3 season MLD Superlight bivy sack. So, I'm returning the Integral Designs Overbag and I've bought the MLD Alpine Bvy instead, in a size long, with the zipper option. On hindsight, I can live with a little condensation if it happens (and it might not in dry cold), given that my winter trips are at most 2 nights long. Having a zipper is probably not ideal on a winter bivy bag, but I will also be levitating on a 2.5 inch inflatable down pad, so even if snowmelt does enter the bivy sack, it will pool well underneath me.
Size comparison with 32 oz Hunnersdorf Water Bottle
I'm always looking to understand the lesson behind my experiences, and I guess the lesson here is that their no use resisting the CRACK that Ron Bell sells at Mountain Laurel Designs! He should set up an automatic payroll deduction plan for his customers so we can dispense with paying him.
No seriously. It was good that good that I tried using the Imtegral Designs Overbag at home in my livingroom and envisioned using it before taking it on a trip and nullifying the 60 day return policy. I obviously overlooked the need for a zipper until I was forced to get into and out of the OverBag without one.