Bivvy for use under a Trailstar

Jamess

Section Hiker
Hi there.

I'm looking for some advice from TS users about a bivvy to use under a TS.

I'm currently using my old alpkit hunka which is OK for overnight use but doesn't really breathe well enough for multi night use with a down bag.

I was interested in Martin Rye's comment on Matt Holland's borah gear bivvy review. Martin suggests that so long as you have a groundsheet then the silnet/cuben bottom on the MLD and bora bivvies isn't necessary and using a more breathable material on the base would be a better bet.

I'm thinking do I buy one of the above or do I myog using a more breathable fabric top and bottom?

What advice would the more experienced TS users amongst you give?

If taking the myog route what would be the best fabric to give high breatheability, enough water resistance, and enough strength for use on the bottom?

Sleeping mat could be inside or outside the bivvy as I'm using a groundsheet - may impact the fabric choice for the bottom.
 

EM - paul

Thru Hiker
My experiences have led me to these conclusions.

1. E-vent is the only upper ive had that actually breathes well enough to prevent moisture build up inside the bivy in most conditions.
2. The base fabric isnt that critical as breathability from it is inherently compromised when on a ground sheet or mat.
3. If i was buying now id go for the borah gear e-vent or As Tucas Millaris. I currently have an integral designs e-vent and its superb but at nearly 800g its overkill within a shelter but great for a mountain bivy
4. The Alpkit is a mobile sauna :D
5. Quality fabrics for myog will be no cheaper than stumping up for off the peg

Ive yet to try the Shoeller FTC fabric that As Tucas is using but i hear good things. perhaps @Marco A. Sánchez or others could offer some insight as to how it compares to E-vent.
 

Mole

Thru Hiker
I've never used a bivi with my Trailstar.

I haven't really felt the need as my sleeping bag is warm enough and have DWR. It's big enough not to need it really.
Maybe in snow.

I used to have a titanium Goat bivi. no side zip meant hard to get in and out, and it got condensation trapped at foot under the top fabric.

I plan to make a sil based bivi with pertex top, mainly for use with a quilt and for insect protection . It will have a side zip.
 

Whiteburn

Thru Hiker
I've used both the MLD Superlight & MYO bivi with the TS.
Both have a breathable DWR Pertex upper & silnylon base, haven't experienced condensation issues.
I prefer to trust the DWR on a bivi rather than rely on the sleeping bag's cover fabric with all the stitching providing an easy route for droplets to get inside.

I personally don't like the idea of a pertex top & bottom bivi in conjunction with a ground sheet, is water gets in top of the ground sheet, droplets being rattled off the TS in a high wind (I've had it happen) there'd be enough pressure if you move around to force water trough the fabric.

The bivi is probably one of the easiest MYO projects: I'd look at a ~ 50g/m2 silnylon with a hydrostatic head of 3000mm for the base (or 1oz cuben); ~30g/m2 DWR nylon for the top; ~30g/m2 mesh;#3 zipper. Simple French seem to join the top & bottom is you're using silnylon base.
 

Munro277

Thru Hiker
I've used my Trailstar for last 4 yrs,less so this last year as I got hold of a s/h Cuben Duomid..I recently had my tub floor modded and had 30" of fabric added to the bottom half ..no condensation at wknds camp..I don't think it matters that much how breathable bottom is ..if buying a bivy I'd get the AsTucas one..would love to try one..but funds tight or a bivy with event top think Ron at Mld does one now too..that said first two years I just used plastic sheet with Matt on top..
 

Graham

Thru Hiker
My setup for Winter from top down. TS then Borah Gear side-zipper Momentum top, quilt/bag, air mat, bivy silnylon base then polycryo. Occasionally 3mm ccf between polycryo and bivy. I have woken up a fair number of times with spindrift or rain splatter that the bivy stopped but probably would have dampened the quilt/bag. Probably if I'd orientated myself differently would have avoided that.

I'd say the BG Snowyside or any eVent bivy would be overkill with the TS. Nominally you have your w/p mat or pad inside the bivy so can't see the point of an expensive w/p/b material under that?
 

Graham

Thru Hiker
The bivi is probably one of the easiest MYO projects: I'd look at a ~ 50g/m2 silnylon with a hydrostatic head of 3000mm for the base (or 1oz cuben); ~30g/m2 DWR nylon for the top; ~30g/m2 mesh;#3 zipper. Simple French seem to join the top & bottom is you're using silnylon base.

Good to hear as I'm starting one next week.:) I'm building one that can (hopefully) take some brutal weather, so it'll be a different spec (and weight).
 

Jamess

Section Hiker
1. On sizing, providing the bivy is big enough not to compress the down, is it better to have a smaller bivy to minimise any condensation issues inside the bivy, or doesn't it make much difference? - in which case having a bit of extra space for a few bits and bobs sounds a good idea.

2. Presumably if you go for the oversize option you need to stake out the floor to maintain the bathtub walls on the MLD and BG bivys or is that not a problem in practice? I notice whiteburn has built proper bathtub corners in his myog bivy.
 

Graham

Thru Hiker
My BG bivy seems huge (off the top of my head it's 0.85m wide x 2.3m long, laid flat). I went for the regular width/length. However, put a deep mattress inside of that, an average sized bloke (me) who side-sleeps and my Pipedream 600 and it's about right. In Winter you'll want a bit extra like you say. I've never had a condensation issue, providing I'm breathing out of the mesh. I'm not building a bathtub corner in my own MYOG bivy but that's because a) I tend to build up a 'wall' with the polycryo and b) I sleep on a NEOAir Trekker mat which is a full-length, 2.5" deep mat. I'd wake up before any puddle over-topped that.
 

Marco

Ultralighter
My experiences have led me to these conclusions.

1. E-vent is the only upper ive had that actually breathes well enough to prevent moisture build up inside the bivy in most conditions.
2. The base fabric isnt that critical as breathability from it is inherently compromised when on a ground sheet or mat.
3. If i was buying now id go for the borah gear e-vent or As Tucas Millaris. I currently have an integral designs e-vent and its superb but at nearly 800g its overkill within a shelter but great for a mountain bivy
4. The Alpkit is a mobile sauna :D
5. Quality fabrics for myog will be no cheaper than stumping up for off the peg

Ive yet to try the Shoeller FTC fabric that As Tucas is using but i hear good things. perhaps @Marco A. Sánchez or others could offer some insight as to how it compares to E-vent.

eVent and our schoeller-ftc fabric are two different fabric classes. eVent is a waterproof and breathable fabric, so you can use an eVent bivy alone to withstand rain. Our schoeller-ftc fabric is not waterproof, but weather-resistant. It's windproof and has a really good DWR finish, so it handles dew and rain splash very well. Not being waterproof, it breathes much more than eVent. If you plan to use the bivy under a shelter (e.g. a Trailstar) I highly recommend a non-waterproof bivy, as breathability is the key factor. If you choose a Millaris Bivy Sack, even better ;).

If you really need a waterproof bivy (you plan to use it without a shelter too), go for an eVent bivy. But take into account that there are many, many versions of eVent. For a bivy you want the version with the highest breathability. As far as I know, the most breathable eVent bivy sacks are those by Locus Gear. Those are made with a special (an very difficult to source) eVent fabric. But as I said previously, I would only choose this if I really, really need it to be waterproof.

Marco.
 

Graham

Thru Hiker
The w/p/b material I've just had delivered for my MYOG bivy is *only* rated at 15,000g per m^2 per 24 hr. That's only 15 litres per day. Lucky I don't sweat like the proverbial OR am I missing something?
 

EM - Ross

Thru Hiker
Sorry @paul - I still don't get it. I can see the Pnema Bivvy & it's transmission rate but Marco tells us there are many different versions of eVent & we should choose the one with highest breathability for a bivvy. As far as I'm aware most gear makes don't list the transmission rate so how is a punter supposed to evaluate one eVent bivvy against the next?
 

Graham

Thru Hiker
Why do we need the HIGHEST breathability. 61 litres of water vapour transmission sounds like over-engineering for a bivy - unless there's some intense physical exercise and heavy respiration going on that I haven't thought about ;)
 

EM - paul

Thru Hiker
Sorry @paul - I still don't get it. I can see the Pnema Bivvy & it's transmission rate but Marco tells us there are many different versions of eVent & we should choose the one with highest breathability for a bivvy. As far as I'm aware most gear makes don't list the transmission rate so how is a punter supposed to evaluate one eVent bivvy against the next?


All the bivys ive ever seen in e-vent quote it. You wont tell by looking at a fabric. You need the supplier to give you the numbers.
 

EM - paul

Thru Hiker
Why do we need the HIGHEST breathability. 61 litres of water vapour transmission sounds like over-engineering for a bivy - unless there's some intense physical exercise and heavy respiration going on that I haven't thought about ;)

There is a lot of confusion surrounding the humble bivy. To me a proper bivy is a bag i can roll out on a mountain without a tarp, jump in and zip up. A lot of UL bivy bags are designed to be used in conjunction with shelters and feature mesh panels. When you are zipped in against the weather 61,000 would be the height of luxury and not at all over engineered.
 

Graham

Thru Hiker
When you are zipped in against the weather 61,000 would be the height of luxury and not at all over engineered.
I'll continue to use my Borah Gear side-zipper bivy under my tarp/TS. Plenty breathable.

The 15k per m^2 per 24hr rated fabric I'm referring to will be for a standalone bivy open to the elements. I'm expecting to be comfortable with that but could be wrong.
 

EM - paul

Thru Hiker
I'll continue to use my Borah Gear side-zipper bivy under my tarp/TS. Plenty breathable.

The 15k per m^2 per 24hr rated fabric I'm referring to will be for a standalone bivy open to the elements. I'm expecting to be comfortable with that but could be wrong.

at 15k with bivy closed you will generate more condensation that you can shift through it. With 60k you would be dry. humans generate huge amounts of moisture in an enclosed space

Borah gear is fine under a shelter but i class bivys like that as slepping bag covers. not designed to repell the elements but good at keeping condensation, wing blown water and spin drift at bay.
 
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