Sleeping Bags: How Layering Affects Ratings- Question

[Hugs With Trees]

First- I know sleeping bag temperature ratings aren't famous for being accurate, but I've found (generally) to add about 5 degrees sleeping fully clothed or 15 degrees naked and I'll have a ballpark figure as to my comfort.
Where I live, a zero degree is considered "standard" among avid campers who aren't afraid of month long trips or winter or weather in general. My zero degree... well, different story involving mice and a snake, suffice it to say I currently don't have a zero degree bag... So I'm left with a neg 20 degree and a +40 degree sleeping bag. The neg 20 can handle anything, except for being carried comfortably. The 40 hasn't failed me down to about 50 degrees when I remained dressed, haven't tried any colder. I do like that it stuffs to about the size of a NERF football; I originally got it for cycling trips.
So my question after that long-winded explanation- I still need a zero degree sleeping bag that I can carry AND will keep me warm. But I'd like to make it a layer system and incorporate the 40 as a lining inside of a new one. Does anyone know how that works? As in, to achieve the equivalent of a zero degree, would I need something as low as a ten, or would I want something as high as a 35 lined with the 40? (keep in mind, packing efficiently and as lightly as possible is part of the goal, along with having 3 versatile options out of the 2 bags for different times of the year)
Thanks in advance for any advice you guys can give me- I'm searching for my new bag right now and have found a few I like and think will work, but it would certainly help to have a 2nd (and 3rd & 4th!) opinion as to what I really need that will do what I want it to do.

[terasec]

Re: Sleeping Bags: How Layering Affects Ratings- Question

Instead of using the 40 degree bag as a liner how about a liner for the 40 degree bag?
If you search sleeping bag liners will find some that claim to add 25 degrees of insulation
Not down to your 0 degree's but close
Maybe double up on liners?

[Mike]

Re: Sleeping Bags: How Layering Affects Ratings- Question

What you are describing is the sleep system I use as my standard in temps below zero. Instead of using one super heavy duty sleeping bag, I use two smaller bags. I select two bags that seem to make sense depending on the temps I expect. So, if temps like minus forty Faranheit are expected, I choose one thin winter poly fill and my best BIG down bag. I like poly on the inside because it is not as subject to warmth loss due to moisture AND it dries faster than down when I hang it outside.

I don't pay attention to the ratings on bags. They are too subjective and too influenced by the marketing department to provide any degree of accuracy to the user.

Just mix and match your bags in a way that makes sense to you. if you plan to camp in extreme cold, you really have to know your gear well. If you are lucky enough to live in temperatures similar to where you camp, you can test your gear at home by sleeping outside. ha ha. You know, I have tried that several times and every time, after a few hours sleeping on the back porch, I think about my beautiful wife sleeping inside and realize how crazy it is for me to be sleeping alone on the porch, so I go back inside and snuggle with her. So, a lot of my testing gets done in the field anyway.

Whether adding bags to bags or wearing clothes, it is critical that you have plenty of free air space inside your bag. If the bag is tight and you cannot move easily when inside, then you are likely to be cold. The old Boy Scout advice was to sleep without clothes as it was said to be warmer than sleeping with clothes. Well, they left out the details. Of course, sleeping in clothes can be warmer than without as long as you have room to move around freely inside your bag and inside your clothes. Tight clothes and tight bag is a trip to coldville. For this reason, I always buy winter bags oversized - the largest size I can so I can slip another sleeping bag inside of it.

One thing I use in common no matter what bags is my sleeping pad. If you have a very good insulating sleeping pad, you can make mistakes on the bag choice and still be OK.

[Logtec]

Re: Sleeping Bags: How Layering Affects Ratings- Question

I have a north face: summer or light sleeping bag(it has 3 dif temp ratings depending how you use it; inside out /up down...), at the lowest it's rated to 10 I can't remember what it is called at the moment, but it's sold as a liner or summer sleeping bag. It works great in the summer months, In the winter I use this SB with one of those thin sleeping bag liners, that apparently adds +10 of warmth to your sleeping bag.
I've used these two as my sleeping system for winter backpacking, the lowest I've used it for was -15. I wasn't warm, but I wasn't cold either...
( Anything colder then that I use a warmed bag, with the north face as a liner.)
Anyway both SB and the liner can be rolled tight down to about 5" round and 8" long.

[genewiseman]

Re: Sleeping Bags: How Layering Affects Ratings- Question

I have a 0 degree bag and it is good down to about 40 degrees for me without clothes on but I am from a fairly warm climate. I usually just take warm long johns and regulate the heat that way. I layer up depending on how cold it is supposed to get. The temp. rating of the bag really depends on what you are used to. I am sure an Eskimo would be burning up in my bag at 0 degrees but an Okie like me is freezing at 30 degrees if I don't layer up. Long johns are the lightest way that I have found to be prepared for whatever the weather throws at me.

[wiiawiwb]

Re: Sleeping Bags: How Layering Affects Ratings- Question

A few thoughts come mind. First, you can layer sleeping bags and achieve a net result lower than the lowest bag. Here is the equation I've always heard of:

x -(70 - y)/2 = z

x = first bag (higher rated/lower degree)
y = second bag (lower rated/higher degree)
z = rating of doubled bags

Second, we all know down loses some of its ability when wet. If you have a down bag and add another down bag on top of it the moisture from you body will go through the first bag and pentrate the top bag. eventually, when the moisture reaches the outer layer of the top bag it will condense or freeze depending upon the temperature. That argues against having a top bag of down.

I think a better system is the have a down bag with a synthetic quilt on top. When the moisture eventually reaches the top quilt the synthetic fill is less affected by moisture than the down would be. Enlightened Equipment makes fantastic quilts.

[wiiawiwb]

Re: Sleeping Bags: How Layering Affects Ratings- Question

To answer the OP question:

You would need to get a 15F bag to use with your 40F bag.

15F - (70F-40)/2 = z

15F - 30F/2 = z

15F - 15F = z

z = 0F

Most companies are notorious for selling a "35F" bag that does not keep you warm at 35F. Caveat emptor...let the buyer beware.

There are a few companies who do accurately rate their bags. Western Mountaineering and Feathered Friends are two. If you get one of their bags, you can rest assured the temperature rating is accurate.

Using a bivy sack around your sleeping bag will also enable you lower the bag temperature rating potential.

I've never used a bag inner liner but have read from many backpackers they do not lower the bag temperature rating consistent with the claims of companies who sell them.

[tplife]

Re: Sleeping Bags: How Layering Affects Ratings- Question

We found Marmot bags to have real world temp tatings.