Sleeping solo in a double sleeping bag

Question

I'm buying a double sleeping bag for travels with my girlfriend.
Parameters (of the bag, not the girlfriend): 1500g of goose down, 680 in³/oz (393 cm³/g), 90/10% down/feather, rated to -9...-15°C T-comf when two people are inside.

• Question one: Would I be comfortable sleeping in it alone or would the extra inner space sap my warmth? What if I hogtie the bag with a bungee/paracord?
• Question two: what would be the (approximate) T-comf for such solo sleeping?..

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NOTE RE: ZIPPABLE BAGS
Yes, I had had thought about them. However, there are a few cons:

1. They have to be Left/Right, i.e. matching. The company that I'm buying my bag from doesn't produce matching sleeping bags.
2. I've owned zippable bags and checked out some. All of them had one problem: the zip would stop at the shin level, basically creating two separate spaces for feet. That could be uncomfortable to sleep in (a double mummy-type bag would have a tapered but single feet space).
3. Two bags are more expensive than a single double bag, and we're talking down which already has a hefty price tag
4. Two bags (usually) weigh more than a single bag from the same model range

Answer 1

In my experience, most standard single sleeping bags, are built so you can zipper them together to make a double. They are designed to zipper to an identical/matched bag, so don't count on buying to different bags and have them work together.

Instead of buying a dedicated double bag, buy a matched pair that can be joined. Google sleeping bags that can be joined together or sleeping bags that can be zipped together

When you are sleeping alone, no problem.

When you are sleeping together no problem.

When you are in trouble and she makes you sleep alone, no problem.

Answer 2

The parameters of your girlfriend would matter depending on the ratio of 'space' occupying, but let's ignore that for now.

I would put a cord or something around it to minimize the space; air needs to be warmed up too. You should try to tie it in such a way, that you have on one side a 'triple' layer (half of the bag bent, so something like this

/--------------\    Top
|     You      |
| /------------/
| \------------\ 
\--------------/   Bottom

<-----Width---->

instead of the normal situation

/----------------------------\  Top
-       You           GF     |  GF = Girlfriend
\----------------------------/  Bottom

<------------Width----------->

If you have a thick (air) layer under you, you can keep the three layers of the sleeping bag on top of you, if you have a thin layer under you, since most cold is going through the ground, put the three layers of the sleeping bag underneath you (like in the picture).

Also you can fill the sleeping bag with clothes (they will be warm in the morning), water or other drink, and even your shoes (put them in a plastic bag).

Answer 3

If the bag is laid flat like it would be for two people, it will not be nearly as warm. Basically you are doubling the effective surface area in which to lose heat relative to a single person sleeping bag. If you fold the bag on top of you, you will be keeping the surface area about the same as a single person bag and effectively doubling the insulation. This will be very warm if the extra material stays on top of you.

Keeping the extra insulation on top of you is going to be difficult. Trying to tie yourself in the bag so that the multiple layers do not flop around seems like a bad idea. Putting the extra layers below you leads to them being compressed and losing most of their insulating properties.

Your best bet is to probably buy two bags with mate-able zippers. I tend to suggest that when men and women are buying a pair of sleeping bags to buy the woman a warmer (lower temperature rating) bag than the man. This tends to make the couple sleep more comfortably and gives a greater range of comfort when the bags are detached.

Answer 4

Zip-together bags would be the best solution but if you already have a double then just fold the sleeping bag in half and sleep in the top half with the other half underneath you. You get extra cushioning and extra insulation between you and the ground and your weight will keep the sleeping bag (mostly) folded.

Answer 5

It might be a little colder but I don't think it would be so much colder as people are thinking.

It's a bag, not a box or a balloon. The unused area should fall flat leaving little extra air-space inside for you to have to warm.

You will feel cool material if you turn in your sleep though. Depending on the weather that might even be a good thing.

Answer 6

I'll second the recommendations to buy bags that can zip together. We have the older North Face Cat's Meow, and they work well for my ultralight distance hikes, and for family camping.

While you can zip things down, remember that compressing loft is your enemy. Use a light touch.

Answer 7

Sleeping solo in a sleeping bag that's too big can be a miserable experience provided the night is cold enough to be marginal for the temperature rating.

My family still carries the vivid memories of a cool spring night (about 4 Degrees centigrade) when our then 11 year old daughter had forgotten her sleeping bag for a sail training camp. My husband decided to give her his own, and went to bed knowing it would be a long and chilly night for him. Little did he anticipate that our daughter wouldn't be much better, the vast and roomy sleeping bag felt like it just wouldn't warm up, and any movement just made things worse.

Unfortunately, neither of them communicated how uncomfortable they were, lying shivering in their respective tent compartments, trying not to stir in order not to wake the other (lying awake equally shivering through the night).

The following night I could join the trip and bring the missing sleeping bag along, the remaining nights both were comfortable again in their fitting equipment.

All sleeping bags involved were rated to -10 iirc.