Xjyuk

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 one: what would be the (approximate) T-comf for such solo sleeping?..

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 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).

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.

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 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.