Need advice redesigning my Bivvy

I don't have any solid suggestions, but I do have some observations. I agree, keeping that cuben fiber layer around your feet is likely to severely limit foot ventilation. I understand your desire to avoid having condensation from the inside of the shelter to rub off against your sleeping bag. I don't know exactly how you would do this, but consider a square of Reflectix about 16 inches on a side. Somehow prop that up against the shelter where your sleeping bag would otherwise rub up against the shelter. Since your sleeping bag foot is now ventilated, that is not a problem. With the Reflectix in place, there won't be any moisture rubbing.

Now convert that into a practical solution.

I just stick some rain jacket or other garment between my feet and the shelter wall.

–B.G.–

what works for me:
according to the principle of the VBL
put your legs in a trash bag. (e.g. up to level of the knees)
The vapor of the feets goes outside where there is more body heat produced for evacuating moisture.
Then you must not modify the bivvy

It looks like you've "painted yourself into a corner."
Problem:
Your feet are much too close to the tarp. You want a minimum 6" space between your highest point and the tarp. If your feet are touching, then you need a larger tarp. I usually check this by placing one foot on top of the other. If you are touching the tarp skin, you will have problems with condensation dripping down onto your bag/bivvy.

Soo, one solution is to remove the cuben on the bivvy, replace it with uncoated nylon. The uncoated nylon will breath pretty well. Then add about 12" or so all around the bottom of your tarp, making it a bit larger and taller. Also, it looks like you are sleeping sideways in the pyramid. You really want to be against the back wall. A few mitten hooks and a rain jacket will close off much of the door from spray. But I think

If you don't want to fix the problem, you can also add some gortex or other breathable water resistant fabric removing the cuben. This will be a compromise between runoff and ventilation. But it should be tolerable in bad weather. Cuben is not breathable. (Well, they are working on it.) Ventilation is at direct odds with convection as far as heat loss. So it might feel cooler all the time.

I would simply drop wind break. A loose piece of netting (http://www.equinoxltd.com/the-gear/fabric-and-cord/no-see-um_mosquito_netting.cfm) will cover your head/arms fairly effectively. I believe it can be treated with Permethrin, but check. Many are falling back to polyester(cannot be treated) rather than using nylon(can be treated.) I have some older stuff around here that I *know* takes permethrin. Yes, you might get a bite or two. Try some plastic collar stiffeners (or milk jug plastic can be molded with boiling water) to support it away from your face. I think the silk would be too warm by trapping your breath.

Generally, I don't use a bivvy under my tarps. Whatever weight savings is taken up by having a larger tarp. It is simpler(1 piece of gear to carry) and I avoid the run-off & condensation issues you are having. Still have to deal with bugs, though.

Former long time poncho/tarp and bivy user/advocate.

I no longer use a bivy.

IMO, if you need to supplement your shelter with a bivy, then your shelter is inadequate. You (and me) are trying to compensate for a poorly designed shelter for the conditions you are in.

Several years ago I bought a 8 X 10 Cuben tarp and a Marmott Essence rain jacket. This set-up was lighter than my old poncho/tarp and bivy combination. The larger tarp eliminated the need to protect my quilt or sleeping bag from wind driven splash from rain. Soon I replaced the Essence with a Cuben poncho, because the Essence wasn't truly waterproof/breathable. This was an even lighter system.

If you need a bivy because your sleep system is touching the walls of your shelter, then your shelter is too small.

If you need a bivy to stop wind from infiltrating your quilt or to add additional warmth to your sleeping system, your sleeping system is in adequate.

If you need a bivy as a groundsheet, there are lighter options, such as polycro.

The only thing left to address is mosquitoes. Over the past 6 years or so I have backpacked in over 30 states in the lower 48, and have not needed "bug" protection with my shelter. A few times a headnet was required. In conjunction with proper clothing, the headnet was sufficient. I haven't been in Minnesota, so that might be a different story.

I normally sleep without my shelter. The shelter is for bad weather only and on the majority of my trips it stays in the bottom of my pack.

I know my thoughts here are not going to be popular on BPL, but IMO Cuben tarps have made bivies obsolete.

thank you for the feedback. very valid concepts. Because I run my own business all summer I needed this bivvy to acompany my diy 5oz apex quilt down to 15 degrees but because I love the cocoon concept so much, I wanted to see if it can be practical in warmer conditions with more moisture management and bug control, but still add warmth to a 2.5oz apex quilt that I plan on making.
The cuben at the feet is already cut off and with your comments I think focusing on not touching a wall over adding moisture protection material is far more practical. I do sleep long ways with the tarp, being on the right side was just for photo purposes. What made me try the elivated cuben foot protection is that anytime I share my shelter with someone, I get squeezed out, generally with my feet in the weather.

To bad about the silk bug protection being to warm. Glad I get a moment from you guys to save me wasting time and material.

The bivvy weighs 7.3oz without cuben, probably go down to 6.5 or so if I remove the wind hood and go back to using my face net this summer.

much appreciated!