Best bivy bag for managing condensation

Has anyone found a good one? I know it’s dependent on the local environment more than the bag, but I’d love to know which one performs the best. I’m currently looking at the Mont Bell Breeze Dry-Tec, but the internet has very conflicting opinions on how well it stops condensation building up, with some saying it’s the best and others saying it’s the worst.
I’m hoping for something really lightweight rather than robust, the Mont Bell is a light-but-fragile 190 grams as a marker

Does it also need to be waterproof?

Yes, it does really. I often cowboy camp, without a tarp, because I love the open sky, but if you can’t find a convenient bit of woodland then the unpredictability of the UK weather has always made me pop my sleeping bag in something waterproof. Plus, a waterproof bivy just seems like a more versatile and useful thing to have in your pack! It can double up as an emergency shelter, or a standalone shelter for camp if you can’t find a footprint big enough for a tarp

Yeah – I have much the same issue. The most breathable bivvy bag I’ve used is a TiGoat Kestrel but it’s barely showerproof. Other than that I rely on keeping my breath out of the bag, so something with an airy entry/exit (with bug mesh). Currently using a OR Helium to good effect. It can still get a bit overwhelmed when the air is very damp of course, at which point a tarp tends to help.

Yes, I heard that a tarp really helps, something along the lines of the moisture settling on the tarp instead of onside the bag.

I can’t really recommend the ‘best’, but I taped together three bin bags a few years ago and can confirm that it was less than ideal

I’ve got a Deathlon one which is supposed to be breathable but condensation is an issue. Let’s face it, with a good sleeping bag there’s going to be next to no temperature difference across the bivy bag to drive moisture out. A two-man tent is only 860gm and I can use a lighter sleeping bag for the same comfort in a tent. So now I have two stategies 1/ just a sleeping bag and keep walking to somewhere dry if it rains. 2/ A tent.

I have a proper goretex bag that is super breathable, minimal condensation in extreme conditions and only on the foot area. The goretex is quite fragile but has lasted incredibly well with no leaks. It is basically a cover for the sb only so minimalist.

I have also used a pertex bag that is good but not waterproof. It is effective at keeping wind off and some moisture but will wet out. I had intended to wash it in grangers but haven’t got round to it yet as an experiment

Tarps are excellent, even the thinnest will stop the few falling. But of course you can’t see the stars in the same way and you get into the point ed is making about weight and performance vs a light tent.

For me, Bivi bag only trips are one nighters where I am confident of the forecast. Otherwise it’s a tarp plus Bivi if rain is forecast. Multi day it depends

Two different bivi setups depending on terrain and weather, either tarp and light bivi bag or hooped bivi, but only on shorter trips and in conjunction with a synthetic sleeping bag. It’s not a lightweight alternative to a tent for me; it’s a different way of experiencing the hills.

Longer term with variable weather it’s a tent all the way. I’m in the NW Highlands at the moment and last night was horrendous – no problem in my tent, but a bivi bag would have been rather unpleasant.

I have a proper goretex bag that is super breathable, minimal condensation in extreme conditions and only on the foot area

Which one is that?

Let’s face it, with a good sleeping bag there’s going to be next to no temperature difference across the bivy bag to drive moisture out

Lightwave make a bivy with their own, proprietary material called X-tex, which apparently has an ‘activated carbon coating’, the theory being that it helps to pass water through (hopefully in the right direction!) even when the temp differential isn’t enough for it to pass through as vapour. Marketing ahoy, of course, but the reviews are very favourable. I’m very tempted, but their bivy bag is expensive (£300) and heavier (approx 325 grams) than the Mont Bell (£150 and 190 grams) so I would need to be really convinced that it manages condensation considerably better before choosing it

Activated carbon really is the new miracle material isn’t it… recalling a recent thread discussing it’s use to modify fork behaviour by absorbing/releasing air.  Wouldn’t the carbon just bind water, or at least for a few minutes till it’s capacity to bind is used up absorbing any smoke , pollen, other small air particles?

My inital thought is that you’ll always struggle without a heat/humidity gradient, and that with a decent sleeping bag that’s hard to generate.  Also , whenever someone comes along with something claiming to be better than GoreTex it never is.  I’d go for Goretex, and make sure to look after it and keep it clean to help it breathe

Activated carbon really is the new miracle material isn’t it… recalling a recent thread discussing it’s use to modify fork behaviour by absorbing/releasing air

Maybe that’s why it’s good for flatulence!

I have an old Rab Superlight bivi, been the nest i have used, Event fabric, and it has a guy attachment above head, so you can lift the top of the bag away from your face making it feel less claustrophobic when it is zipped right up.

They don’t make the Superlight any more, but i presume the new Rab bags would be as good

I’ve just bought a proper tent, and my current bivi is a RAB Storm which isn’t particularly light or ventilated once zipped in so can’t help!

I think realistically any bag is going to be full of condensation if you zip up the opening. I find the Rab Storm works well for me as a side sleeper because for 90% of the night, unlike a mummy bag you can just sleep with the top ‘flap’ open, and just pull it over when it rains or the sun comes up.

Let’s face it, with a good sleeping bag there’s going to be next to no temperature difference across the bivy bag to drive moisture out.

I don’t think that’s 100% true in the way you are thinking. The heat loss per meter square is the (roughly, obviously the circumference is getting bigger) same through the sleepingbag and bivi. So there’s still the same enthalpy to drive the process. The problem is that what you actually need is the dew point inside the bag to be slightly lower than the temperature (i.e. so it doesn’t condense). Which is nigh on impossible below a certain threshold temperature because to keep the temp in the bivi up, you need a thinner sleeping bag, which means you end up cold. The thinner the bag the cooler the outside can be before you get condensation.

Which is why:

the internet has very conflicting opinions on how well it stops condensation building up, with some saying it’s the best and others saying it’s the worst.

If you used a 3-season bag in summer, or a 4 season bag in Spring/Autumn you would get lots of condensation compared to using the ‘correct’ 1-3 season bags.

So this:

So now I have two stategies 1/ just a sleeping bag and keep walking to somewhere dry if it rains. 2/ A tent.

An/or save the bivi for trips I know will be dry-ish with a synthetic sleeping bag so I can just get them both out and throw them over a fence to air once the sun comes out.

I’ve ordered the Mont Bell Breeze Dry-Tec. I’ll let you know how I get on when I camp next to a lake on a cold and rainy night!

I’ve a rab superlite and an alpkit hunka.

The rab I’ll take anywhere anytime and have done for nearly 15 years.

The hunka- I can’t even bring my self to give it away and inflict the rubbish experience on them .

A good bivy worth its weight in gold.

The dew point will probably be somewhere in the middle of your sleeping bag insulation at the high humidity levels inside a bivvy bag. In cold damp climatic conditions you need a pretty much perfectly breatheable bivvy bag to avoid that, or better still no bivvy bag at all. Even without a bivvy bag sleeping bags get damp because people perspire and the dew point in many conditions is likely to be somewhere in the sleeping bag insulation.

Montgomery says he uses a synthetic sleeping bag in a bivvy which is sensible because it will get damp. A solution if you really do want to see the stars but you’ll wake up damp. My sysnthetic sleeping bag that is comfortble in cold conditions and my bivvy bag weigh more than a warmer down bag and MSR tent.

I’ve used a bivvy bag and down sleeping bag at altitude in dry air at 10°C and woke up slightly damp but warm. I’ve used the same bivvy bag with a Winter down sleeping bag in freezing damp valley air and woke up cold before dawn with a soggy limp and useless down sleeping bag so got up and blumdered on in the dark to warm up.

Oops lost some content: no bivvy bag plastered with frost ice on the outside and frozen condensation on the inside will beathe. In the same conditions a tent is fine, just avoid touching the outer so you don’t get showered in ice.

The dew point will probably be somewhere in the middle of your sleeping bag insulation at the high humidity levels inside a bivvy bag.

I view them as occasionally useful but extremely limited bits of kit. I’ve owned one for over 20 years and maybe used it a dozen times whereas I’ve used a lightweight tent hundreds of times in that interval.

The expectation of a bivvy bag and the reality are quite different.

Let’s face it, with a good sleeping bag there’s going to be next to no temperature difference across the bivy bag to drive moisture out

It depends on the fabric – up to a point – and how it works. If it uses a hydrophilic coating, attracts moisture, it will be less dependent on a temperature/humidity gradient to work than perforated membrane-type material.

The reality of all this is that breathable bivi bags work pretty well in very cold, very dry, high altitude mountain environments, but are a lot less effective in relatively warm and damp UK conditions. Use one with a tarp and it’s more pleasant and allows easier venting of the bag without getting soaked, but personally I usually choose a lightweight tent for UK use.

Throw in the joys of trying to dress/undress/pack/unpack/store kit etc in a bivi bag on a wet night for maximum misery. Arguably some sort of hooped bivi gives you more versatile venting options, but at that point you’re almost in a small tent anyway. YMMV etc.

I totally get why people prefer tents, they definitely offer more comfort and protection from the elements, with the wider choice of pitch site (footprint permitting!) that that allows, with not really any weight penalty any more now that there are DCF trekking pole tents. I’m just much more of a fan of sleeping in a bivy, with a tarp at hand if I need it. It might be partly due to the fact that I’m tall, so I lose a some of that extra comfort that a tent is able to provide, but it is definitely mostly because I just absolutely love being outdoors, it’s the reason why I hike, and so being able to sleep with a night sky as my ceiling rather than a tent will always win hands down for me. Plus, I really like a set up that can be squeezed into a ridulously small footprint! On balance I actually find that this it gives me a wider range of places I can sleep, and it gives me peace of mind as the end of the day starts to loom, because I know can stop pretty much anywhere (within reason!) and I’ll be able to get warm and some kind of sleep for a few hours.

Old Goretex one. Bought from Wintergear in Llanberis in about 1986. Original style Goretex with a yellow, slightly soft inner surface. Envelope style allows clothing to be used as pillow and full zip across face allows midges/rain to be kept away. Never had a touch of condensation, even when, once,  wrapped in a heavy plastic tarpaulin as well. Mate next to me then was soaked in his Alpkit Hunka which is really more of a sleeping bag cover.

Was it made by Snowdon Mouldings who were based in Llanberis?