Why am I freezing?

Mark, space blankets work well as ground cloths. They are lighter than most other ground cloths and will give you a little more heat. Pretty much a win win. A little noisy though.
They work better taped to the top of your tarp or shelter than straight on top of you.

Other than that, space blankets only become amazing when you combine them with a fire. One taped to your tarp, one pitched on the other side as a reflector, with a fire in the middle, you just created an oven.

I have two comments from my personal experience:

First, my AMK Heatsheet Emergency Bivy at 3.5oz saved my butt last year. I was using an REI Nooksack synthetic bag rated to 35 degrees ( more like 45!). My last night of a trip the temperature dropped below freezing with winds up to 25ish. My TT Contrail wasn't exactly blocking the wind. Being too cold to sleep I decided to try one last option before packing out at 1am. Being the last night I just slid my pad and bag into the bivy and crawled in. Within minutes I was warm. I became so warm during the night that venting became necessary. In the morning there was heavy condensation on my bag, but the shell caught it all. Not bad for 3.5oz!

Secondly, I was caught in a 20 degree night in my Katabatic Palisade ( rated 30 degrees) My only headwear was a smartwool thin cap and a thin fleece beanie. I found through trail and error that I was much warmer having my WM Flight jacket wrapped around my head than draped over my chest area.

Jerry

Mark, I have 4 Montbell bags and two of them are the newer UL Super Stretch down bags. I really like these bags. They are light, compress well and keep me warm. I'm 6' 220 so I use the "long" bags. I don't have that claustrophobic feeling in the Montbell bags. I sleep on my side with one knee pulled up. No problem with room. I also have the Neo Air pad and it is a summer pad. I do have a ccf pad to use under it if conditions dictate but the better choice for me is to use my exped insulated pad if it's really cold. I also wear an OR windstopper Peruvian hat when it's cold. I can't remember the last time I got out of a tent or hammock to take a leak during the night. I have a bladder the size of a walnut so a pee bottle has been standard equipment for a long time. Screw the lid on tight and use it as a hot "water" bottle. I do have some extra thick Woolrich socks and down booties if it gets to that. I have never used the down booties in the bag. They are just for around camp and will only be making the trip if it is really cold or if I'm canoe camping.

Mark, I use a neo-air and Montbell ss-ul #3 bag with it. I found this to be very comfortable at philmont in 2010. I plan on using this setup again this summer. Our arrival date in 2010 was june 23d and the lowest temp we saw was 36 degrees one night.
I slept in a short sleeve shirt and shorts with a possum down stocking cap. Most nights were closer to 42 to 45 degrees. I think that a #1 15 degree bag would be to warm for me at least. Hope this helps. I am 6'- 205lbs. dave

I went on another overnight backpacking trip this weekend. The overnight temperature was between 36-40.

Here is what I changed from last time:

I used a footprint under my Quarterdome T2 tent for an extra barrier between the tent and the ground. (don't know that this helps, but what the heck)

I took along a blue ccf pad from Wal-Mart and put it on top of my Neo Air.

I ate a full Mountain House package meal 2 hours before going to sleep.

I did jumping jacks before going to sleep.

I ate a package of Peanut M&Ms right before sleeping

I put on a heavier pair of wool socks

I folded my quilt lengthwise and shook it to help the down settle to the very middle so it would be warmest right above me.

I tied my quilt under me everywhere there were tie-offs and also closed down the neck opening as tight as comfortable.

These changes kept me toasty through the night. At 7AM when I woke up I wasn't warm, but I wasn't cold either… just comfortable. I was still wearing my Stoic Merino 200 long tops and bottoms and my Dome Perignon Hat. I had another fleece balaclava that I had on to begin with, but I was too hot in that.

I feel pretty strongly that the foam pad on top of the Neo Air combined with closing off the quilt around me made the biggest difference as many of you suggested. I've ordered a 1/8" ccf pad from Gossamer Gear, and I'm curious to see how it compares to the $7 pad from Wal-Mart.

I really DON'T want to have to carry a bulky pad at Philmont. Hopefully the 1/8" pad will be enough AND fit inside my ULA Circuit pack without taking up too much room. I'll also keep the package of Trail Mix in the PhilFood lunch packs for eating right before bed.