I have a Mountain Hardware Phantom sleeping bag with a DWR shell. I have been using it with a REI Ultralight Bivy and a 5'x5' MLD Dog Tarp. I am considering a lighter bivy and a bigger tarp, but am unsure about how the DWR MLD Superlight bivy (the bottom is silnylon) would work out.

Specifically I wonder abut the DWR top. Since the bag is already DWR will the DWR bivy make sense?

With my current setup condensation in the form of drops or ice crystals occurs but is easy to manage. It is either on the bag shell down near the feet or in the bivy down near the feet. Worst case in is on the DWR bag shell further up as well. It is pretty easy to shake it dry and/or wipe it off. The bivy seems to make it much warmer especially on windy nights. I have not found it to be a problem other than as a very minor annoyance.

Will the DWR bivy perform similarly with regard to condensation? My guess is that there would still be condensation in the same place but less of it. Is that likely to be the case?

I also figure that I would need a full coverage tarp with the DWR bivy rather than the 5'x5' one that covers only the upper half of my body with any reliability.

DWR bivy should have similar to less condensation than the WBP. Near freezing I imagine it would be very similar.

Yes you would want a full coverage tarp with the lighter bivy. The actual size will be highly dependant on the weather you expect and your comfort with cramped spaces - if you've used 5X5, I bet you'd be have with 5X8 or 6X9.

I'm curious, Pete, since I've considered a similar setup (5x5 MLD tarp + wpb bivy) -- other than the issue of weight, have you been happy with this combination? Also, please let me know if you decide to sell the 5x5 tarp. Feel free to send me a PM if you want to keep this thread on topic -- I was going to contact you by PM, but it looks like you aren't set up to receive messages.

It has been adequate for what I have used it for. That was mostly bicycle touring though. The only real drawback was that the bivy was kind of a hassle if it was hot and buggy. Retreating from mosquitoes to the bivy on a hot night was less than comfortable. Fortunately I have only had to deal with those conditions rarely.

If I expect a lot of biting insects I tend to take my tent. When I rode from San Diego to Sarasota I kind of wished I had sent the bivy and tarp home and had my tent mailed to me when I got to the Gulf Coast. Those Louisiana skeeters were brutal!

The tarp was pitched a variety of ways and also used to just pull over the top of the bivy and exposed gear when it unexpectedly rained. I don't think it would be much fun in a week of steady rain, but I have had a night of rain a few times and was OK. I will say that on a bike tour I did get a room sometimes when I expected really hard rain.

I actually have slept on top of the bivy and without the tarp quite a few times when the weather was nice. The open air and view of the stars is great. The bivy is very warm and comfy on a cold night though.