Large garbage bag as bivy?

Avoiding condensation is possible if you can assure a good amount of ventilation.

Some ways to do this:
1) Cut a good sized hole in the foot end

2) suspend the top with a line so there is a gap between your bag and the top of the bag-bivy

If your trying to save cost, you could look into something like an Oware Drawstring bivy.
It has a waterproof bottom with highly breathable top.

I know people who have had good luck using an extra long tyvek groundcloth, folding and sewing the the extra length over the bottom half to protect against spray and splash. Kind of a half bivy.
I guess Tyvek breaths enough to help reduce condensation and being only half covered helps.

Won't work well for a bivy. Moisture from your clothes and body won't escape so your bag will get wet. I normally have used my DriDucks jacket over the foot of my bag. One night I decided to keep it on and tried the trash compactor bag over the foot. That area of the bag was wet in the morning. I was honestly surprised it made that much of a difference – like you, I figured moisture would just migrate out the top of the trashbag (and I toss a lot). Depending upon your pack's material and inner circumference, it might work.

However, since you're only doing it on overnighters, you'll survive.

Edit: Steven's 2 ideas would help.

I've done this with a synthetic bag before, and without a tarp. There was plenty of condensation, but I was warm. I don't think I'd try it with a down bag. I don't plan to try it again either.

It would probably work ok if you used a vapor barrier liner to keep perspiration next to your skin and out of your bag.

How big is your tarp? You're probably better off adjusting your tarp pitch, or getting a bigger tarp. Even breathable bivies trap some condensation.

If you have a larger groundsheet you could also just fold that over the top of your bag when the weather picks up. Like the garbage bag it will tend to trap condensation but if it's loose and partly open and you only use it when you need to it should work well enough for the short term.

On the other hand you could just have a larger tarp. Then you won't need any extra protection! I've found a 7×9' tarp in an A-frame with one end closed can protect very well against wind and rain.

not recommended for down bags especially in humid conditions

just put yr rain jacket over one end of the bag, and the windshirt over the other …

Only in a dire emergency! There will be almost as much moisture from your body inside as there will be from the rain outside, and your insulation (whether down or synthetic) will be severely compromised.

Been there, done that, except it was a plastic tarp laid directly on top of my sleeping bag, rather than a garbage bag. Anything covering your sleeping bag needs to be breathable.

If it's below freezing, you'd be better off to use the plastic bag inside your sleeping bag as a vapor barrier. Don't wear your insulating clothing inside the plastic bag, though, or the same thing will happen to your clothing. Just wear a base layer inside the plastic.

Someone suggested to me a Dri-Ducks poncho over the sleeping bag. It's pretty breathable. I haven't tried it.

A mate of mine and myself were up at Mt Stirling (Vic/Au) for New Year.
Yair took the opportunity to show me where last winter he rescued a lady that thought of using a garbage bag as a 'bivy".
She was under some trees, the last on the mountain before the bald area, over thick snow, about 300' from a refuge that she had passed getting to her spot.
She was totally wet,shivering and incoherent; and that is why she wasn't capable of going back to the refuge.
Took them 1 hour to warm her up (got her totally undressed, wrapped her up in dry clothing, inside my mate's bivy (eVent…) gave her progressively warmer drinks and stabilised her.
The rescue squad took her away not long after that.
Now if my mate and his students had not been in the area or if visibility was low as often is there in winter , she would have died.

Franco

My tarp size is roughly 7×9 but I was thinking of using a MYOG plastic garbage bag bivy on it's own too. I'm just cheap and didn't want to invest in a bivy since I've never used one before. Maybe I'll try making a small enclosure for the foot end of my tarp instead. Thanks everyone!

D Johnson
You can easely test your garbage bag as a bivy at home .
Turn the heating off, put your mat on the ground , get inside your sleeping bag and bivy and see what happens.
Franco

I think how much of a problem condensation is has a lot to do with how warm your sleeping bag is. The one time I wrapped my ground sheet over me it made a big difference in blocking the wind but later on the condensation made me cold again because I only had a blanket on. Another time in my gortex-like (ie: not very breathable) bivy I've been damp and cold with a blanket and then damp but warm with a thicker sleeping bag. If the bag is plenty warm for the conditions then it can handle being somewhat damp (even a down bag). If not then the dampness will make you cold.

By the way, if your only worried about some spray and splash, most modern sleeping bags are good at shedding that. They might absorb a little moisture, but probably not as much as the condensation trapped inside a plastic garbage bag.

I have carried a 45 gallon low density trash can liner for emergency use and it will cover to my armpits. The idea being to use it with a rain jacket and hole up under a tree to make it through a storm or a wet night out, or to cut arm and head holes for an emergency rain shell– as much for others as myself. Condensation is to be expected, as with a space blanket type bag or MYOG Tyvek bivy. The AMK Thermo-Lite bivies have a Velcro opening at the foot to *help* with condensation. I would expect a damp sleeping bag with any of those options. Cutting some vents in the bag would help, or maybe open it up all the way and drape it over the top.

It would be better than nothing in a tight spot, but not a best choice for regular use. If these things were tolerable, you would see them used a lot, along with tube tents and the like.

I think the least expensive option would be a MYOG bivy from light ripstop with a good DWR coating. A 2-way zipper would allow adjustable venting options. From there you go to the more exotic breathable fabrics for the top and truly waterproof bottoms. No free lunch, I fear.