I wouldn't think that the R-value for a trip like what you have in mind would be critical.

1. The major advantage of a Neo air is its small volume (roughly a soda can) when deflated. I have a small Neo Xlite that I use as a torso-length pad on hiking trips.
2. Another benefit is that you can carry an inflatable inside your panniers. It means that your pad will be dry even in you ride or break camp in a downpour (a closed cell will usually be exposed to the elements)

3. There is a very significant price penalty if you choose an inflatable.
4. You may want to carry a patch to repair unlikely but possible punctures.
5. You'll spend a couple of minutes every day inflating/deflating

6. I was worried about the bag-of-chips sound that comes form the neo-air if you toss in your sleeping bag. I either got used to it or it actually gets fainter over time. Not an issue for me

As far as I am concerned, the critical variable is volume. I prefer the neo-lite.

On that point I really agree, my last tour I brought an air mattress instead of pad because the air mattress was more compact when rolled up. It weighed a bit more than the pad I use for backpacking but the volume was more important to me because I wanted it inside a pannier.

I'm really starting to think about the benefits of not having to store our sleeping pads on the top of the rear rack which will leave room for other things... Thanks for the thoughts all, will probably pick up a neo air unless the sales staff at MEC can convince me otherwise.

Another question - does anyone have experience with neo airs or similar and hammocks? I've got a Hennessy and have not found an ideal pad or under layer of some sort for it yet.

That is a big reason why I made the switch to air mattresses, more compact.

heck, even 25 years ago there were thermorests that were not as thick, and could be folded in half before rolling up.Mmine was a full length lightweight one, still the self inflating ones, and my wife had a 3/4 length one, even more compact. I always put in in a pannier along with my sleeping bag., kept only tent on rack.
They were pretty slim, so not great at all for sleeping on your side, this is what I like so much about the neoair type ones, I can sleep on my side fairly comfortably, and depending on how much air you put it, you can set the firmness of it, for me it works great.

I have the NeoAir xLite and it has been fine for the occasional sub freezing night. I have been OK for a pretty cold night (18 F), but it wasn't winter so the ground wasn't all that cold despite the night time air temp. For real winter camping you probably want something warmer unless you supplement it with a thin foam pad in the winter. I think that I'd use mime with a z-lite pad if I wanted to winter camp.

It is quieter than my older original model NeoAir, but neither ever bothered me with the noise. When I camped with others they said it didn't bother them, but I don't move much while I am sleeping.

Never had a problem being cold with a 1" Thermarest even on ski tours in the Sierra or 1/2" Insolite on climbing trips.

The bag and your metabolic rate make the difference. I've been cold when either of those were insufficient, but never from a pad.

One trick is carry about a 16"-18" square of Insolite to put under your feet if you are using a 3/4 length pad.

It also doubles as a dry, warm place to sit or a pad under the stove to keep it from melting out a hole, if you can't find a suitable rock to put it on.

I used to use, and still have, a square section of blue closed foam cut out from an old blue sleeping pad, that I woiuld throw into my knapsack when going out cross country skiing where we would stop for lunch or whatever. It weighed nothing but you wouldnt get a cold or wet bum sitting on the snow. I actually had cut it out originally as a back protector for when I used to go karting at these indoor karting places, and the seats would hurt my back, so I took that and it helped a lot with not getting a bruised back from sliding around against the hard seat in corners. Fun as heck, but pretty expensive.

This is a Thermarest Prolite 4. I believe it has an R factor of 3.2. It is good for winter conditions and is the same pad we carry for bike tours.

http://i783.photobucket.com/albums/y..._Camping-2.jpg

There is a simple formula that will answer your question. U x A x Delta T = btu/hr of heat transfer.
U: U-factor (the inverse if R-value)
A: area
Delta T: temperature difference between the inside of your sleeping bag and the ground temperature.

I thought you said simple formula??