As I mentioned I've been trying my Phreeranger as a single skinned tarp tent, in fact I slept in it again last night and to be honest I'm wondering if I really need a double skinned tent. There was some condensation on the inside of the fly this morning but again no drips. I was using a bivvy bag anyway so a few drips or moisture knocked off due to heavy rain/wind wouldn't have been a problem. There are advantages to using a double skin tent though, the oft quoted one is that they reduce condensation but I'm not convinced that this is the case as I've often found the inside of the fly to be wet when using a tent, what the double skin provides though is a barrier between yourself and the fly but with the Phreeranger and most Tarp Tents there's so much space that there's little chance of touching the fly anway. The other 2 advantages of an inner are heat/draft reduction and bug proofing, the fist may not be too important if you're using a bivvy bag as it protects from drafts and should upgrade the performance of the sleeping bag, I would however prefer to have some protection from insects and I don't believe a bivvy bag even if it has a bug net offers quite the same protection/ease of use.
I've been looking at some of Henry Shires 'Tarptents' and they solve the bug issue by having sewn in netting and groundsheet. The only issue remaining is loss of heat but I wonder if the bivvy bag overcomes this. My bivvy bag is an old heavy Goretex one weighing in at 596g which is heavier than my inner tent but I don't think I need the level of protection that my bag provides so something smaller and lighter would be fine assuming it was breathable enough, perhaps even something like the PHD Drishell cover which only weighs 163g. If using a bivvy bag allowed me to use a sleeping bag with a lower rating then some of the weight of the bivvy bag is offset against the weight saved by using a lighter sleeping bag.
I like the idea of the 'Tarptent' Rainbow/Double Rainbow, the Rainbow including poles/groundsheet/bug net and 6 pegs weighs 965g, add the PHD sleeping bag cover for a total weight of 1128g, the larger Double Rainbow weighs 1135g, or 1298g including the bag cover but offers a massive amount of space, 2250mm L x 1220mm w x 1100mm H with 600mm wide (at the apex) porches on both sides.
I can't see any other disadvantages but if anyone can think of something that I've overlooked please add your comments.