Wool survival blanket!

Im new to backpacking and i am trying to get all my gear ready for my first winter trip. I live and hike mostly in the southeast. I was wandering if these 100% wool survival blankets that i have seen at the army surpluse store are actually as warm as they claim to be. i was going to use this with a 20 degree sleeping bag becuase i cant really afford to buy a nice bag right now. if any of you have any experience with these blankets let me have your input. you guys would know better than i would.

Most of the "wool survival blankets" I've seen in surplus stores are

1) heavy
2) usually about 50% wool.

aka, generally kind of crappy.

While they may actually be warm, If you already have a 20 degree bag, when are you trying to go out? You can probably do better heading into a second hand clothing store and finding yourself some fleece and/or cashemere sweaters to wear in the bag at night. alternatively find second cheap barrel bag at wal-mart and put your existing bag inside it. It'll be lighter and you'll probably be warmer.

I carry a horse-blanket-style wool blanket with me... in the trunk of my car. It makes sense there. I suspect that a synthetic-fill sleeping bag that is lower-rated than 20 would make more sense if you plan to hike in a very low temp season of the year.

Remember, the main function of insulation is to create dead air space near your body that your body heat can linger within. The colder it is, the deeper a layer of dead air you need surrounding you to stay warm. A wool blanket may not be the best way to enlarge that space.

Also, you will benefit a lot from good R-values under you, to insulate you from the ground. A dense closed cell foam pad is the most efficient way to keep the ground (or snow) from sucking heat out of you from below.

However, if a wool blanket is what you think you can afford and you are willing to carry it, then it may make sense for you.

Not to knock them, but let me put it this way... I grew up in a woodstove-heated house in Alaska, and some mornings it would be in the lower 40s inside the house. A wool blanket wouldn't keep me warm at night back then. You might stay warmer by wearing most of your hiking clothes inside your 20 degree bag. One of those polarfleece bag liners might also work just as well if not better.

I used to keep a wool blanket in the car, though, because I wouldn't have to worry about one losing its fluff if left in a stuffsack forever.

My dad taught me how to happily camp using a wool Marine Corps blanket and some matches. It was bliss! Of course here in Texas, you don't need much blanket, in spring and summer. A down quilt, however, is much lighter and packs much smaller.
I do still use wool 'disaster' blankets (from Campmor) when car camping. They work well in hammocks and on cots. I typically use two, one for my back. I like wool and those blankets are indestructible, plus they feel warmer than the same weight in fleece. They work well as ground cover, on grass. They feel like stiff felt until you wash/dry them a few times and they soften up.

I bring a wool blanket instead of a blue foam pad on day hikes.
It is heavier, but more versatile, and about the same volume.

How cold are the temps where you are going? I use a bag rated to -5C (+23F) plus an overbag, which really just a lightweight bag, in temps down to about +15. I toss my parka over the bag if I wake up cold at night. This is in a winter tent. I sleep on two pads-a Ridgerest and a Thermo-rest that insulate me from the snow.

If the wool blanket is heavy, it will tend to compress your bag and lessen the insulating effect of the airspace in it. I would wear a base layer (long synthetic underwear-not made from cotton) plus a fleece jacket and pants, heavy socks and a fleece beanie or balaclava in place of the wool blanket. If your bag has a hood, that will help as well. You could even wear a light pair of synthetic or wool gloves. I don't usually wear the fleece or the gloves while sleeping, but do wear a beanie sometimes.

You want something cheap, probably avaliable in a military surplus store and lighter than a wool blanket? Go look for a poncho liner. If all you're using it for is to line you're sleeping bag that'll be just as affective as an old green wool military blanket. I've used poncho liners numerous times to make things warmer when I was in the Army. They're always nice when you've got to sleep out in -30 F weather.

I suggest you go to the fabric store right now(On sale) and get two six foot lengths of fleece and then sew them together. Lighter than wool blankets, warmer than them and dries quickly if they get wet. Total cost was about $15. Packs about the same...which isn't great, but definatly lighter.
I take this with my cheap 20 degree sleeping bag and when combined with my self inflating Therm-a-rest, I'm quite comfy into the mid teens.

Oh and another thing, I lay the fleece blanket over the top of my sleeping bag, not inside or under.