Survival blanket as bag liner?

Wanda writes:

>> How much do those things actually help with warmth?

If forced to come up with a number, I would say 10 degrees. But, beware:

They are fragile.
They are noisy.
They function as a vapor barrier. This means that it will reduce moisture accumulation in your sleeping bag, potentially increasing the bag warmth on successive days by preserving loft in the bag. But, it also means that you can not wear any of your insulating clothing (other than a thin baselayer) inside the liner. This may make your sleep system colder and/or heavier overall if you would otherwise wear your insulated clothing in the bag.

fwiw, 10 degrees corresponds to about 1/4″ of additional bag loft. 2 ounces of extra down in your bag could easily achieve this.

Good Luck,

-Mike

any clothing worn inside a vapor barrier will probably be covered in condensation in the morning, if you are looking for VB, try clothing that can be worn under extra insulating layers.

for a breathable liner, try the sea to summit thermolite liner, it is available at REI.

I have one and it is quite warm, you can use it alone at 55+ degrees and extends your sleeping bag rating 10-15 degrees.

I’ve used this as a VBL and it worked great:

http://www.backpackinglight.com/cgi-bin/backpackinglight/amk_emergency_sleeping_bag.html

…but I’m curious about its expected lifespan. How many nights should I expect out of it?

I’d say it gave me a solid 10 degrees of warmth. It was somewhat clammy like all VBLs (no big deal).

-j

It is really hard to say how long these blankets will last, my dad used one as a ground cloth for 10 days under his tent in new mexico without any tears, then the next time he went out with a tarp in virginia and a new blanket and it ripped the first night out. even though it was all dirt ground, no rocks?

of course it will last longer used as a liner, but my dad always brings a spare just in case, they are only about 2oz each. You can tell he is not quite a superultralighter like me, I mean a spare, not in my pack! :-)

The classic mylar survival blanket CAN be used as a VBL, but its best use is as an emergency item, used to create a reflector of heat.

It makes a truely excellent waterproof/windproof/thermally reflective layer when used for making a survival shelter for instance. It can also be wrapped around you, as you sit “indian style” with a small tea light candle carefully positioned between your legs to trap and reflect heat from the flame.

As a tool for increasing a bags temperature when doing conventional backpacking, it is quite lacking. “Space blankets” were intended for use in space, where only radiative heat flow is a factor. On earth, other heat loss factors apply… like convection, conduction, and evaporation. Your sleeping bag is covering the “radiant” heat loss far more effecently than the reflective mylar. The thin material of a space blanket will have no effect on conductive heat loss, and so your left with convective and evaporative heat loss. wrapping yourself inside a space blanket inside your sleeping bag will stem the loss of heat by creating still, dead, air close to the body (convective), and by trapping warm moist air lost through evaporative loss. However, both the convective and evaporative loss can be delt with by VB clothing, or even a simple trash bag, so really… the space blanket is not helping you out all that much.

In my opinion, a survival blanket really only earns its keep when used for making shelters. In that role, it is excellent (when used properly), otherwise its actually sort of gimmicky.

The economy of the idea appeals to me, but my first thought was that a bivy sack would be the best way to get more out of your sleeping bag and end up with an item that will be handy for all your trips.

On the economy side, I still need to make and test a Tyvek bivy. One made of the same stuff as the Frog Toggs with a waterproof bottom would be interesting too. I would see both as useful with a poncho/tarp rather than a stand-alone shelter.

“The classic mylar survival blanket CAN be used as a VBL, but its best use is as an emergency item, used to create a reflector of heat.”

Agreed. I would only use the mylar survival >bag< as a VBL. It would be almost impossible to get a good seal/barrier with the blanket.