View Full Version : Sleeping Pad Question
I am considering getting a Big Agnes Air Core pad to go inside the pad pocket on my Big Agnes sleeping bag but I'm not sure if it would provide enough insulation for the colder months on the trail.
My question is whether or not a mylar (reflective) emergency blanket under the pad/bag would reflect my body heat back into the air core sleeping pad to keep me warm enough. Or will I have to go with a different option to be safe.
There are two type (well, three, okay three) types of heat loss. Conductive, Radiative and Convective. Really conductive and convective are the same. Conductive is when you are touching something and it forms a path to something colder. The heat will conductively be moved from something warm to something cold. Convective heat loss is through the air, but in reality this is also conductive, as it is the air that "touches" you. You heat up the air, it moves away from you, and new cold air moves in that you have to eat up. The final way to lose heat is radiative. This is in the form of IR radiation. This is how you feel the heat from the sun, since there is no mode for conductive heat to travel through space.
Metal is only good to prevent radiative heat loss. The way a space blanket works is that it traps the air so you get no convective heat loss, and as long as it isn't touching you, it reflects your own heat back at you so there is no radiative heat loss (metal is a very good reflector in IR wavelengths).
HOWEVER if you are touch a lot of the blanket, you lose the advantage of the metal. In fact, it could even be worse, since metal is a very good conductor of heat (for the same reasons that it is a good reflector - funny how that all works) You will heat up the blanket through conduction, and then the air on the outside of the blanket transport the heat away via convection. The only isolation you will get from the blanket is in its ability to not conduct heat from the inside of the blanket to the outside via thermal resistance (vacuum being the best resistor, air the next best).
What this means is that you might think that space blanket under you is an insolator, but the reality is in that situation (full contact) it is actually a very good heat sink!
The best insolator would be a vacuum between you and the ground to prevent conductive heat loss. Since that isn't really feasible, the next best thing is trapped air. Closed cell foam should work best, as the air is trapped in its own air pocket (hence the closed cell). Is is a poor conductor, and so you will warm up the first few pockets close to you, but each layer deeper you go, the less you will warm it up. The foam can also conduct the heat (although I have no idea how well) and so you want to most air pockets possible. Of course too many, and the mattress will go flat under you, and you are screwed. The more airpockets = less foam = less of a path to the heat sink below you (The ground). The next best think is the thermarest, which traps the air in it, but the air inside can still move around. The more that the air can move, the more convective heat loss (You heat up the air close to you, the ground cools the air close to it. THe hot air mixes with the cold air, and you have to reheat the air close to you).
Well, I'm sure I made that clear as mud...
gravityman ...clear as a bell...
I use a closed -cell foam pad.a 3/4 length ridgerest... if it's getting that cold I curl up like the dog that I am... so the extra length and wieght would do me no good... same reason that I don't like mummy bags is because i like to sleep in a fetal to semi fetal position..so the 3/4 length suits me..
I USED to use a lightweight thin plastic sheet, until I had lost mine once and being in a hurry to head for the hills, cut a small section of 'emergency blanket' .(mine is blue on one side, silver on the other)
I was amazed at how WARM it was! I really noticed a significant difference in my sleeping comfort in my tent (I often go when its coldest out)
One of the greatest things was, when i stayed at a shelter one night- it really helped to stop those nasty drafts from sneaking up under me!!
Its small enough that I can easily whip it out in a windstorm without too much wrestling during a lunchbreak, and large enough that I really sleep comfy!
I think my piece is maybe 28x54"? doesn't weigh much either.
'Don't think I've ever weighed this... it earned its way into my pack whatever it is !
Happy and WARM hiking to you all!
Thanks for the replies folks.
Gravityman: You said that the next best thing to a vacuum is trapped air for insulation. If I have an air mattress (Big Agnes Air Core pad) wouldn't that fall into the trapped air category? Does the air move within the mattress when I move around during sleep? Does the trapped air need some sort of insulator like the foam to keep it from absorbing the cold air from the ground? It seems like a closed cell foam pad with a thin air mattress attached to the top of it would make a helluva comfy, light and warm pad. I don't know, there's probably a problem with it somehow.
Perkolady: How did you use the blanket? Did you just lay on top of it with your pad and sleeping bag?
smokeymtnsteve: I like to sleep on my side in fetal position too but a 3/4 length pad doesn't work well for me. I found out the hard way with ankle and knee sore spots when I woke up after an overnighter.
In an air mattress, the air moves when you turn over and also convective currents can be set up in the pad because the air spaces are so big. The foam in Therma-rests minimizes air movements and so you have more insulation. The lighter die-cut foams in the Therma-rest ultralights, guidelights, and LE allow more air movements than the more solid foams in the standard Therma-rests. And these air spaces are still not as efficient as closed cell foam (1/2 inch of closed cell foam is approx. equivalent to 1 1/2 iches of Therma-rest). An air-core pad on top of closed-cell foam almost makes sense, but it will still allow your heat to escape from the sides of the air core pad. You might want to try it.
Get a big cushy mushy Therm-A-Rest, and an 800fill Down Blanket. It's like sleeping at home for a nominal increase in weight. Get a tapered blanket with a footbox to save weight. The fact is, down underneath you doesn't do much (Do to it's crushed height), and the weight you spend on down and fabric beneath you could be better spent on a thicker pad.
The Big agnes is a good option which takes a liken to this concept (which I looked at as well). But I like more moving around room, and when it's 50 out, you'll be opening, closing, opening, closing that bag all night. Your going on the AT, not to Antarctica (Unless you sleep on Mt. Washington in January or something). You can use your clothing to supplement the blanket as well. If it's real cold put all your clothes on. If it's nice, sleep in your base layer/main clothing.
I was going to make a 4" loft blanket (effective to 0*F) 5' wide by 6.5' long, and realized I was getting carried away. I'm now going for a 2.5" High tapered blanket with a footbox.
If you have a sleeping bag now, and would like to try what a blanket is like, unzip the side all the way, and drape it over your body like a blanket, sticking your feet in the "footbox". You'll have alot of extra unneeded width, but it's a good way to experience it. Now imagine the pad beneath you growing an inch thicker, as that extra width of the bag comes off, as well as the hood (use your balaclava or hat while sleeping). There will also be no zipper to break or rip the fabric (saving yet more weight). It also packs up smaller too.
I'm making my own. You can order them from Nunatak if your not that motivated. Less than a pound and a half for a roomy 20* blanket is awesome. If you alotted say 2.5lbs for a bag, and 1.5lbs for a pad, you now have 2.5lbs for a pad! Or lug a couple cans of your favorite beer/soda with you, or enjoy the lightweight savings! (I incidently chose to spend the weight on my pad and then some).
This is what the physics pretty much add up to. And remember, a good nights sleep is worth infinitely more than eight extra ounces on your pack.
Ok Rant over.
I've tinkered with the idea of making a blanket for the warmer months because I've tried unzipping my old sleeping bag down to the foot pocket area and found it to be pretty comfortable on warmer nights. I move around a lot when I sleep though and ended up losing the "blanket" throughout the night. This wouldn't work too well for me when it's cold.
Thanks for the reply.
i made a 2 inch thick primalft quilt cut to 84 in. by 42 in. and used it successfully a 18 deg. F, but my upper arms and elbows got a bit cold as i tucked it under at those points and it compressed the loft there. that was wearing medium capilines, wool balaclava and lite fleece pullover. pad was t-rest camprest(big 3 lbs.) my next quilt is going to be cut 10 inches wider and tapered below the hips to try to warm up the arms without adding too much more weight. i realize what works for one seldom works for another but i thought i'd pass this along just for comment and consideration. by the way i'm built rather large, could have been called waterbuffalo but the name is already taken.:D
Slabfoot...thanks for the post
Always Ready? USCG Vet?
yeah, quit high school and enlisted in 1969, been retired from the guard for 13 years now.
Slabfoot you speak my laguage. I also have a Therm-a-Rest Camp Rest, and am making my "Quilt" with similar dimensions.
Cant wait. Ordering my materials this week! What type of fabric did you use?
for the quilt i used a 1.1 oz. ripstop i bought from OWF. dark blue on bottom and a powder blue ontop. pretty much followed plans from the lightweight backpackers make it yourself site.
as far as the big thermarest, i really tried to do the little lite blue closed cell pad but i couldnt sleep very well at all. i just never was able to find ray jardines "connection" or enough "duff" to get comfortable.
Amen. I'm pretty damn sure this 3.5lb anchor is now permanently welded to my hull. But I'll save weight in the top area with a lightweight quilt. My anticipated base weight excluding food/water/fuel/hiking poles/and clothes I'm wearing is still under 15 pounds including the monster Camp Rest.
I could drop into the coveted sub 12 pound range, but I can't seem to sleep well on 1/2" blue foam. Maybe when I find the magical connection I can shed my anchor.
i know rock an several other will read this and think "why dont thes guys just try a hammock? "
I think at 2lb, my new tent is lighter than a hammock.
BTW, this is my new tent. It also has nine 0.5oz 6 3/8" titanium v-stakes. With stuff sack & stakes it's about 2lb.
If I'm tenting, I usually use the blanket as my groundcloth.
During the day, i may use it for somethin to sit on, or just for a 'surface' for laying stuff out on that i may be sorting thru....
When at a shelter, or tarping, I use it to lay my sleeping pad on ...
Recently , I tried it in my new hammock and it REALLY made it warmer in there (although, it did 'travel' some, lol)
hope this helps.
Best of hiking to ya!
BUT yer gonna have to get below 15 oz to beat out the hammocks.
I didn't know hammocks were under a pound. I thought with the neccessary tarp overhead, and lines they were 2lb+. Who sells a 15oz hammock setup?
hennessy has a racer model thats 15 oz.
Well what is the average weight of a hammock that most people use? I'm sure there are super-light versions. Give a tarp a bug screen and its a tent that weighs 8oz. What does the average hammock weigh that most people use?
i don't have a hammock but from what i've gathered they aversge 2 to 2 3/4 lbs. due to being very comfy and off the ground they provide a real weight advantage in the pad and groundsheet department. from what i've read they get rave reviews for 3-season use but pose some challenge keeping your butt warm in the winter. hopefully someone with experience will post some accurate info for you.
most hammocks actually weigh a lot but the hennessy ones use lighter materials,no zippers and so on. the normal ul a-sym is 28 oz. the clark ones run about 4 lbs I think and rest are even more.
in the summer you can leave the padding home which also saves a lot of weight.
down to about freezing a closed cell pad will keep you warm but so far I have not heard of any sure-fire sytem that works below that. I am working on a new underside design but it will be a bit before its ready.personnally I am of the opinion that this is the route most likely to win.