Sleeping bag for tarp

I would look at getting a water resistant bivy bag (6-7 oz) for use with your tarp. They can add a decent amount of warmth to a sleeping system especially in windy conditions. I have used my quilt in my Tarptent tent a few times and wasn't as warm as I hoped. However, I have been very pleased with the quilt/bivy combo under my tarp.

Spinntwinn is pretty big, so a bivy may not be needed. I don't use a bivy with it, but I do use one with a poncho tarp, more for water protection. You can pitch it low to the ground. I would spend the extra ounces on more ground insulation.

It sort'a depends. The SpinTwin looks good but, there can be a bit of spray inside. Pitched low it will be fine. Condensation will be slightly better. But, I think you will still get somewhat damp after a day of rain. Make sure your bag has a good shell. (I believe it does.)
Nikwax Downwash followed by Down Proof will give the bag it's maximum water resistance.
Not sure of your size, but an additonal layer of clothing worn inside the bag will help with warmth. I passed it up because the size at the shoulders was a bit small. I often use a down sweater for 25-35F to supliment warmth. Anyway, the 7×10 is a minimum size for a tarp without some sort of bivy if you expect any amount of rain. In good weather(light rain) you can go a lot smaller. But you pay for the weight of a bivy. Kind'of a wash between a larger tarp and no bivy and a tarp with a bivy, depending on exactly what you buy, of course.

Seems like so far you guys are lukewarm on the water-resistant lining on bags in favor of either bivvy or a bigger pad. Im certainly open to the bivvy, but am worried about condensation again. Im not even talking about rainy nights, I must be a giant mouth-breather or something because no matter how much I tried the tricks with the tarptent it was always raning in the tent coming morning. Assuming im like a giant humidity factory, would the bivvy be generally wet? It also seems like quite the weight hit. I'm not that worried about bugs (here…in the comfort of my living room :) )

I'm 5-10 185 if it matters, so I dont need those long bags or anything.

thanks!

Having owned a number of bags with "waterproof breathable" shells over the years, I am a big fan of the most breathable bag fabric possible and using a bivy if you need more protection.

Previous shell fabrics include: Gortex, Gortex Dryloft, Epic, Pertex Endurance, and several proprietary fabrics from Sierra Designs etc. The problem I experienced was moisture buildup within the bag over time due to the dew point occurring within the bag. I have not tried eVent as a shell cover, but I would still prefer a very breathable shell and then an eVent bivy if I thought I needed the protection eVent offers.

I don't use a bivy very often but find that Pertex works well for me. Based on the results of Momentum,(not Momentum 50) that was tested for water resistance by Richard Nisley in the cuben fiber thread, it looks as good as some silnylon and is purported to be very breathable and is available for sale either as fabric for MYOG or in a completed bivy by a number of cottage gear makers.

I've always been surprised how a tarp holds in my body heat. It is noticably warmer under the tarp than outside. It may be slightly cooler than inside a tent, but still warmer than the outdoors.

Yeah, tarps are warmer than just sleeping under the stars, cowboy style.
Last year, I did well through the spring and summer with a 40F bag and a somewhat larger tarp than the SpinTwin. It rained for 8 days straight one time and I was glad I had a larger tarp (10×10.)
The pertex shell was not a problem even camping near lakes or rivers every night. The bag got damp, but not enough to effect the warmth that much. Condensation was never as large under the tarp as it is in the Stephensons(even with 2 side windows open.) So, unless you pick a poor piece of ground, a NightLite or NeoAir is really all you need under you…maybe a piece of tyvek under the neoair.