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I have the normal Hunka. Maybe you're too fat for it ;) but I seemed to fit in it with the pad and sleeping bag. It wasn't super comfortable though which is why I want to try ditching the pad. 600g is only a couple of hundred over what the Hunka weighs so it isn't that heavy.
Why am I using a bivvy? Because I want my expensive sleeping bag to last. Because it might get cold enough to require a bivvy?
That PHD cover looks just like a bivvy, just 300-500g lighter. I want to be warm though. I was in polypro thermals last night and could definitely be warmer but I presume I will also have a down jacket in my kit for the race.
I was looking at the Minim 200, on sale at the mo'
http://www.phdesigns.co.uk/minim-200PHD also do a quilt but it's not much lighter than their sleeping bag:
http://www.phdesigns.co.uk/single-ultra-down-quilt
What size hunka have you got? I hate the hunka, could never sleep well in it. And you really need the pad outside the bivvi, unless you've got a big bivvi. The Rab ascent I have is huge and so nice to sleep it, but it's 600g!
Why are you using a bivvi? You won't be sleeping in the rain? Why not just get a lightweight sleeping bag cover to insulate from the wind. If you're sleeping in the rain you're doing it wrong.
http://www.phdesigns.co.uk/sleeping-bag-cover-proofed
Which PHD bag? I'd be surprised if anything is lighter. I like sleeping bags and in a bivvi it works well. In a tent I do often unzip my sleeping bag and use it as a quilt. But that wouldn't really work in a bivvi.