Two-person bivy for use with quilt and Neoair 3/4 length pads

[Elliott Wolin]

On our last trip to RMNP it got snowy and windy and we were slightly cold with a two-person quilt, draft stopper fabric all around, Neoair 3/4 length pads and empty packs under our feet. We were under a three-person tarp (our daughter used a sleeping bag) with the sides pitched down to the ground and the ends partially blocked off. We used a pair of polycryo ground sheets on saturated soil in a designated camping spot (sigh…).

I'm thinking of making a two-person bivy sack to help cut drafts for trips where we expect some cold and wind but where a tarp is still appropriate (e.g. below treeline, temperatures above say 25 degF or so and not too windy).

Anyone have plans, suggested fabrics and/or dimensions, or pictures? Do they typically have bathtub floors? I'm thinking of silnylon for the bottom, not sure about the top nor what to use in the head area, but I'm thinking of maximum breathability as we always take a tarp.

[Mole J]

Hi Eliot

I made something like you describe last year. Used for a 10 day trip. We're very happy with it.

details here:

http://www.trek-lite.com/index.php?threads/double-bathtub-groundsheet-bivvy-type-affair.202/

[Elliott Wolin]

Wow, some nice ideas there. I was thinking of something simpler, but now I'm really intrigued, probably will make something like it.

What are the dimensions? Did you make the netting hood? Any design changes you'd suggest now that you've used it?

[Roger Caffin]

Hi Elliot

You do know that doing this in the snow will result in LOTS of condensation inside the bivy?

About all I can suggest is that you add some arched poles and have some ventilation at BOTH ends. It's going to look a bit like a very low tunnel tent.

Cheers

[Mole J]

Eliot
Dimensions weren't planned exactly but based on a groundsheet fabric start point of 2.5m long. So base length is that -seam allowances and the 6" bathtub walls. (We are both tall) . The sil and pertex quantum left at full width.

I haven't yet made the netting hood , but plan to make it in a similar way to this:
http://www.trek-lite.com/index.php?threads/another-custom-mod-zippered-head-net-on-a-bananabivi.262/

But with a different zipper/fastening arrangement across to still allow pertex to still roll down.

And, will make the end panel trapezoidal to accommodate the width. maybe put a 2mm cf strut across the top of the trapezoid to hold it's shape with just one tie out..

re condensation. We don't plan to use this bivi in snow as we're use a double skin tent then. (On my own I just use a warmer full sleeping bag if under a tarp in freezing temperatures)
But opening the lower end of side zips should help and maybe another zip across the foot?. If required, consider light plastic pipes for occasional hoops (like used by Milesgear bivis). Could be fixed in place by thin velcro tabs at hoop ends and centre.

[Elliott Wolin]

Although I wasn't planning on using it much, if at all, on snow, I am wondering about Roger's comment concerning condensation/humidity.

I understand lots of people use bivy sacks under tarps, although I've never used one. Is condensation or humidity a problem if the top fabric is nice and breathable? Is it much worse when camping on snow? How about on saturated soil?

My main reason for considering a bivy is to cut drafts and add a little warmth in the colder weather. For warmer weather I'll probably just bring a polycryo ground sheet. If it's cold enough for a tent I probably wouldn't bother with the bivy, I'd rather bring a second quilt.

[Katherine .]

Look up John Abela's piece on bivies. He validated my experiences w/condensation. In short: he recommend a mesh-top bivy, with which you get a deep bathtub floor to cut drafts. I plan to go this route at some point.

I think some of it is regional — I did fine w/a bivy in the SW. But get condensation in the PNW — even in an arid part East of the Cascades. Curiously the one in the SW had a silnylon floor, while the two I've used in the PNW both have had cuben floors.

[Steve Staloff]

Katherine what were the colors of the tops of the bivies and of those that did not?

[Elliott Wolin]

Thanks Katherine, maybe a noseeum netting top is the way to go. I started reading Abela's stuff on the web, more to go…

I actually have net tents for under my tarps (RayWay style), but they are a bit confining and usable space under the tarp is hard to access from inside. To some extent a bivy with a bathtub floor and net top is like a mini net-tent, but you aren't as confined since your arms can easily reach outside the netting.

My wife insists on using a full net tent when it's buggy, though.

[Katherine .]

"Katherine what were the colors of the tops of the bivies and of those that did not?"

I think the one I used in the SW was blue (no condensation)

The other two, used in the PNW, are/were black (condensation)

Maybe you're on to something….?

[Steve Staloff]

Black radiates well. We need some color experiments. Any light white fabrics available?

[Elliott Wolin]

It took forever but I finally got around to making a fairly generous sized two-person bivy.

Bottom is one piece silynlon, finished 60″ wide x 94″ long with 5.5″ bathtub sides (extra-wide 72″ silnylon from RBTR allowed for no bottom seams, just short side seams up the bathtub corners).  Corners have short dowels and are staked out and tensioned to maintain the bathtub shape.

Top is breathable with perimeter of 1.1 oz ripstop and a center 0.9 oz netting section.  The wrap-around bathtub + ripstop perimeter should keep out drafts, and the netting top should eliminate condensation while still keeping the wind out to some extent (the netting will lie on top of our quilt, and the quilt should block the wind).

Finally, YKK #3 zipper along three sides of the netting with two double zipper pulls to allow opening from the inside and out.

Total weight not including stakes is 456 grams, almost exactly 1 lb.

Still to do:  add a thin line to pull the netting up off our faces if we zip the netting completely closed due to bugs (note that if we expect bugs we typically take a net tent that hangs below our tarp, this bivy is more for colder weather).  Also, it takes a lot of tension to keep the bathtub sides vertical in the center of the long edges, maybe some sort of stiffening or a dowel would help.

Could have used lighter fabrics on top but I had a bunch of fabric left over from previous projects.

 

[Elliott Wolin]

Just got back from a 15 degF night in a lean-to in the Adirondacks, near Marcy Dam.

Although the bivy perhaps is a bit larger than needed (I allowed about 10 inches all around our sleeping footprint of approx 40 inches by 72 inches, giving a 60 inch by 94 inch footprint with a 5 inch bathtub all around), it did a great job of blocking side drafts under our doubled quilts.  It further kept the spare clothing we stuffed around our heads and shoulders in place throughout the night, and kept the water bottles I placed outside the quilt but within the bivy from freezing.

My wife particularly liked how the bivy allowed us to organize lots of gear and easily find everything in the morning.  Finally, we zipped the netting up over our faces and it did a decent job of keeping warmth in and allowing moisture to escape.  The only icing we saw was on the outside of the quilt directly in line with our breath (not much you can do about that I suppose).

Note that in the lean-to we didn’t try to stake out the bivy to get the bathtub to stay vertical as there was no need.

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