supplementing a sleep system

You basically seem to have a lot of the relevant stuff nailed down: pad, clothing to wear while sleeping.

Do you use a tent? Tarp? I use a tarp (or nothing, when the weather is clear), and when I'm too cold, usually the first place I notice it is when the wind is blowing across my face. You could consider a wool hat, or a balaclava.

You can improve your warmth by sleeping at lower altitude, and by picking a campsite that's naturally sheltered from the wind (e.g., not the shore of a lake). A lot of people claim that valleys are cold because of katabatic winds.

I think bag liners can sometimes help a lot with warmth, but I've never used one, so maybe someone else could comment on this.

If you can find a place to sleep with a nice layer of pine duff, rather than packed dirt, I think you get better insulation that way.

If you've got an unlimited budget, I'd guess that a warmer bag would be more efficient in terms of weight than long underwear, etc. Some UL types like a quilt better than a bag, on the theory that the part of a bag that's under you is compressed, and therefore useless.

Campsite selection makes a HUGE difference. So does going to sleep with plenty of calories. Also, waste time getting cold in camp before climbing into your bag.

Adding another foam pad will definitely help, but you're adding a lot of bulk. Pad bulk is a problem I have. Both of us would probably do well to get Bender to make us a full length down air mattress. The weight may stay about the same, but bulk and comfort would be greatly improved.

The tough thing about boosting a bag rating is usually legs and feet, since it cooler temps you’re almost always bringing upper body insulation, but not generally lower body insulation unless it gets really cold.

If your down jacket weighs 19oz, and you’re going to be bringing that along anyways, that’d probably make up for a certain amount of the warmth lost between the Moonstone bag and the Phantom, and you’re saving 18oz in the bargain.

What are your midweight baselayers?

Looking at this chart to see how various clothing combinations keep one warm, in combination with this chart which explains the warmth rating of various garments is a good starting point for trying to figure out how much clothing you need to supplement a sleep system.

For instance. Unscientifically, I found that I was able to take my 32 degree rated down quilt (inside either a bivy sack or tent) to ~20 with a lightweight down jacket(Montbell UL Down Inner), a 5.5oz pair of 100wt Powerstretch fleece tights(purchased for Sierra Trading club for about $30, and using my BPL Featherweight mittens as sleep socks(heavier merino socks might also suffice.)

Wearing your waterproof/breathable rain gear inside your bag also boosts your warmth as a partial vapor barrier.

You already know to eat well before a cold night. Also, you can boil a quart of water and put it into your sleeping bag at least twenty minutes before you retire for the night. Of course, that assumes that you had the bag pre-fluffed, and that you slip the water bottle into some plastic bag, just to prevent a catastrophe in the event of leakage. I always have some plastic food bag for that purpose.

If it is cold air that you want to hide from, others have already commented. If it is cold ground that you want to hide from, then only a better pad will protect you. I manage to get all excess clothing to augment the sleeping pad.

I'm not sure if this fits your situation, but sleeping with other warm bodies in the same shelter will help enormously.

–B.G.–

A lot depends on how you sleep. Maybe you are like me, in that your legs rarely get cold, but your feet do. If so, then adding extra foot insulation would help. There are several ways to do that. One is to add a second pair of socks, but in a bigger size. These should fit easily over your existing socks (that you wear to bed). They also work well (in a pinch) if you need an extra pair. Another alternative is to get down booties. I remember a do-it-yourself synthetic version, but I forget where the link is. Essentially, you wrap some synthetic insulation around your toes and attach with some cord (no shell fabric). This is extremely light, for the warmth.

If your legs need warming, you might add puffy pants. These add way more warmth for long johns of the same weight. However, they do have a higher base end weight. If you just need an ounce or two of leg warmth, you can just trim long johns so that they cover the knees (your middle will be partly covered by your sweater and your shorts).

One of the keys, though, is to pay attention to head warmth. I ran into a similar situation, recently, and things rapidly improved when I put on my fleece hat, puffy jacket (with a nice hood) and tightened the hood on my sleeping bag. I had three layers of warmth for my head, and the rest of my body warmed up quickly.

If you have an inflatable, then consider adding a thin closed cell pad on top of it. You can get ones that are 1/8 inch think (e. g. Gossamer Gear sells them) and they add a surprising amount of warmth for very little weight and bulk.

Many thanks to all who responded. I appreciate all the input. I do sleep in a tent when I am backpacking with a hiking partner, which will indeed help with the warmth factor. I will look more closely at a GG thinlite pad and I'm looking seriously at some getting some GooseFeet or other down socks. Also great advice about campsite selection and not getting cold/hungry before bed!

Sleeping bag liner – I add in winter. S2S alleges 15 deg additional warmth but I'm skeptical.
Bivy – This will add both a wind block and capture some heat.
A warm hat – common sense
A warmer mat(s) or pad(s) – I use both a ridgerest deluxe and a thermorest 3 in the winter

I have used all of the above and slept warmly with a Golite UL20 quilt down below zero. I personally like the "layering" strategy because it lets me use the same quilt year around. Another possibility would be to add something like a UL-20 with your existing bag to take it really cold.