Bivvy bag as standard gear

I am just getting into the idea of using a tarp and a bivvy sack as a shelter in inclement weather. Have you experienced any downsides to the use of a bivvy?

if i were to name one problem, perhaps the major complaint, it would be…

in one word: condensation

in twelve words:
condensation – sometimes bad; sometimes nearly none, depending upon humidity conditions and breezes

Condensation: My hiking partner has used a waterproof/"breathable" bivy in cold weather, and had huge problems with condensation. I've been using an old Mountain Hardwear MicroMesh bivy — very breathable, not waterproof — for about seven years, in all kinds of weather in the Southeastern US, from very hot humid summer nights to deep winter, and I've never had any condensation at all. So I think this depends on the type of bivy to a great extent, along with weather conditions etc.

General Usefulness: I like the bivy/tarp combo. I find it to be a most versatile combination: I can sleep in the bivy by itself, either under the stars or in an AT shelter. If the weather threatens, I can set up the tarp.

The top on mine is water resistant & bottom is waterproof. Frankly condensation hasn't been a major problem, though it has felt clammy when used without sleeping bat in mild weather. In very wet conditions, the bivvy is highly protective regarding dew, etc.

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I'm not experienced with gore-tex bivvies and am rather suspicous of them.

I have used a bivvy only once. It was an REI one that had a watgerproof bottom and breathable top. Weather conditions were dry and 25 degrees in the Sierra. No condensation in the tent, but lots at the foot and on the sides of my bag. I returned it to REI. But I would like to hear about a bag that does not cause condensation. As far as use goes, I would only be using it unde a tarp type of shelter like a Ti Goat tipi or in Lunar Solo tent.

Sounds like the dew point was inside of the bivy and the temp inside of the tent was above freezing – hence wet and not frozen condensation. Water vapor driven through the sleeping bag condensed upon contacting the colder inside surface of the bivy fabric.

What was your rational for using a bivy inside of the tent?

Also, there will nearly always be some situation in which condensation will occur in nearly any sleep/shelter system.

I don't see a need for a bivy in a SMD Lunar Solo e tent, or most other tents for that matter. In floorless shaped tarps lacking bugnetting like some tipi's, i'd probably rig a small bug net of some type instead of using a bivy. Other's will just use a bug headnet.

I've been testing a bivy with Epic top and silnylon bottom under a variety of conditions. I get condensation almost every time.

Last night: temp 42 F, dew point 40 F. IOW dry air, moderate temps. Result, wet bag. Loft sagged about 3am.

In other tests, condensation did not always occur at temps above 50F or with a dropping barometric pressure. At temps below 50F or with a rising barometer, condensation almost always occurred. Test temps ranged from 30F to 60F.

My conclusion: I'm going back to good, permanent DWR for outer shell of quilt and no bivy. I hammock much of the time, but was using the bivy as backup for cold weather.

Vick, what fabric are you referring to that has a perm. DWR? Or is it an aftermarket application which gives a fabric a perm. DWR?

I made my own Bivy using DWR nylon with a Silnylon bottom. I've used it quite a bit with both a MB #3 sleeping bag and a Polarguard quilt, Temps from high 20's to Low 70's.

Condensation has never been an issue … even in cases where the outside of the sack frosted up and I had to tuck inside the sack to keep warm.

My first bivy was an Epic-topped model from MLD, and I had a lot of trouble with condensation. I've since switched to a DWR nylon topped bivy, and I've had no condensation problems whatsoever. It may have been a coincidence, but the experience has made me suspicious of Epic in general.