Bivvy bag VS waterproof sleeping bag

As in all of life, depends on your usage.

If just the outer shell is waterproof, then moisture can accumulate in the down degrading it's performance anyway. Over a few days in freezing temperatures this becomes a severe problem as the down loses loft and the moisture trapped by the WPB membrane freezes, increasing the weight while decreasing warmth. If the entire bag is water proof (like the cuben bags from some cottage manufacturers) this is less of an issue, although the VBL effect may be unwanted.

As for bivies, 550g is a lot. MontBell makes a 6 oz WPB bivy that uses their breeze tec membrane that is often compared to just a touch less breathable than eVent. May be worth looking at.

Like so many others have asked in times past… why would you consider a bivy? What exactly do you believe a bivy was originally designed for? Why do you believe that bivys should be used by ground hikers?

A SUL bivy can weight in the 150-170 gram region. A XUL bivy can be in the 66-85 gram range. But again educate yourself on what a bivy was designed for than ask if using it on the ground is the right way to approach your situation.

A polycryo ground sheet can weigh 45 grams.

You posted this in the SUL category of the BPL forums so I can only conclude that you are looking to be in the SUL/XUL category of hikers and thus you have to ask yourself the above type of questions. If weight reduction is your goal why bother with heavy gear?

On the matter of a sleeping bag that has WP material or not. That is an answer that can only really be decided on whether you want to use a bivy or a tarp/ground sheet or cowboy camping. And of course the extra weight for the WP sleeping bag. Do the numbers on your end and see which is the lightest for you.

Thanks to all for the comments.

I ended up keeping the bivvy bag and getting rid of the sleeping bag. Just sleep in my down jacket instead. This works down to zero degrees Celsius.

I like the bivvy because it keeps me and my bedding clean and dry. It also blocks the wind and adds some insulation, especially if I find a way of lifting it from my body to create some dead air space.

More about this on my blog at http://www.piano-tuning.co.uk/lifestyle

I gave that a go for a bit earlier this year… or maybe it was the end of last year… anyway, it was an interesting study on my part. What I found is that I was carrying more weight in heavier jackets and clothing than what an appropriately rated sleeping bag weighed, and actually look a little bit more bulk space. It was fun giving it a try though.