Adding two bags together?

[winland]

Any thoughts or testing on the "rating" you can get when you add two sleeping bags together? For example, if I'm in a 20 degree bag inside a 40 degree bag, am I as comfortable at 0 as I would be in a single zero degree bag? At 10 degrees?

If it matters, they are both down bags. The "outer" is a Montbell super stretch long and fits well (snug but not too snug) around the other one.

[JoshStover]

That Montbell bag is a 25 degree bag. Well if you have the same one I have it is... I would think you should be pretty comfortable but I could be wrong. I have never tried it myself.

[leaftye]

It's hard to say because it's snug. It's mostly about loft. If the snugness kills loft, it won't be as warm. That said, it may help make up for it by allowing fewer drafts. You may be able to calculate how warm the combo is if you can understand the material in this thread:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=9378

[BrianLe]

IMO it's tough enough to get a useable rating on just a single sleeping bag, never mind nesting them. To really get at "is my system warm enough?" you have to factor in your shelter (tent, tarp, bivy sack or whatever), how accurate the rating is on your sleeping bag (or bag plus overbag), what clothing you wear inside the bag and how well it's able to loft, your sleeping pad(s), the ground temperature vs. ambient air temperature. Factor in also dealing with moisture (in really cold conditions a vapor barrier), your personal metabolism .... it's just a realy challenge.

Beyond responses you get here, you might do a more general search; for example this thread on backpackinglight (http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=25690) addresses this general issue.

I guess the way I would approach your question would be to calculate the total loft of the two bags, including perhaps something for the dead air trapped between the bags. Look at what temp rating manufacturers give for the loft of just the single bags ...

That's just off the top of my head, however.

I personally would be more inclined to add more clothing (cocoon or thermawrap pants, for example, a down parka, down booties) and wear those inside your 20 degree bag so long as there's room inside the bag for such clothing to retain loft.

[brooklynkayak]

When layering bags you have to make sure that the outside bag doesn't compress the inner bag.
I'd guess you'd want the Montbell bag on the inside as the snug design will tend to compress an inner bag.

I layer two bags for winter, a smaller 40 degree bag on the inside and a roomier 20degree bag on the outside.
This allows both bags to fully loft.

I don't know how to rate this temp wise, but this combination feels twice as warm as the single 20 degree bag, The perception may be because of the reduced draft caused by having all those shell layers.

The military of many countries often use this system.
Stephenson's Warmlite uses a layered quit system that people swear by.

A layered system could be the way to go on a thu-hike. Use all layers during the colder days, Bounce the lighter bag during the shoulder seasons and bounce the heavier bag during the summer.

[LIhikers]

Try this thread, I think you'll find it helpful
http://www.whiteblaze.net/forum/showthread.php?t=43377&highlight=bag+inside

[The Old Fhart]

Sleeping bag ratings are a guide and may or may not agree with your basal metabolic rate. The best way to see if a particular sleeping bag, or combination of bags, will work for you is to try them in your back yard under the conditions you expect to use the bag.

A good starting point to determine the warmth of the bag is the loft , i.e., the number of inches of insulation above you. Here is a site (http://www.survivaltopics.com/survival/sleeping-bag-temperature-ratings/) that has a simple chart with this information. If the bags have about the same amount of uncompressed insulation on the bottom as on the top you can just lay the bag combination on a flat surface, measure the total height, and divide by 2. About 3" of loft (6" total height) will be about 0°F and 4.25" (8.5" total) will be about -30°F. YMMV.

Also "snug" is not good because it means insulation is being compressed and it is the trapped micro air pockets that provide the insulation, not the material used.

Keep in mind that most insulation on the bottom is going to be compressed by your body and will offer little warmth. In cold conditions it is very important to have thick (or multiple thin) pads under you. If you wake up in the morning and see you have melted the snow under where you slept, add more non-compressible insulation. An inflatable pad for the beach may be thick but the air inside isn't trapped in micro-pockets so there are circulating air currents that make these very inefficient for insulation. That's why Thermorest and other inflatable pads have open-cell foam inside to break up any air circulation. Keep in mind if you use an inflatable pad and it springs a leak you lose all insulation. I generally take both a inflatable and a closed-cell foam pad for safety.

[Grinder]

using the formula cited below, your combo would be equivalent to a 5 degree bag (20- (70-40)/2)=5 degrees.

I suppose the caveats about one bag squishing the loft out of the other could change the actual insulation value.

This subject came up on Hammock Forums and the formula came from (or was supported by) one of the principals of Jack or Better, makers of underquilts etc.

Interestingly, just for ****s and giggles I put the other bag on the outside and the equivalent rating computed to 15 degrees.

Veddy intereshtink

[Compass]

Put the bags together both ways and try to measure (in inches) which way gives the most loft with you in it. That would give you the best combination.

Then go to this chart to get an idea of effectiveness.
Sleeping Bag Temperature Ratings (http://www.survivaltopics.com/survival/sleeping-bag-temperature-ratings/)
Warmth of Sleeping Bags Loft Chart
TemperatureInches of loft30 degrees F1.8 inches102.302.8-103.2-203.7-304.2-405.2
I would predict the Stretch is going to literal squeeze the stuffing out of an inner bag.