I am planning to buy a sleeping bag that is rated -40, because not only for the increased warmth, but also for the increased comfort - better comfort = better sleep. People sleep better in loftier beds. I had a Big Agnes Mountain Grouse rated -9C, and it was way too cold for my EU winters. I do random travels and I never know in what climate will I end up, and I am a permanent camper, so I cannot carry or own several bags, I don't have a car. Will I be fine during summer in a -40 bag, or I must choose a layered system instead for my scenario? I want to know if people are sweating in a -40 bag in summer, but they most likely are. Now sleeping bags cannot really be layered right? I already bought the warmest liner, but that with say a -15 bag will be still too cold for say Siberia, and the -15 alone too warm forthe summer. So if you wanted a full year system that you permanently carry, how would you solve it so that you wouldn't be too hot int he summer and not too cold in the winter, in the 10 to -40 Celsius range? If you think about that people sleep in thick down duvets inside where there is 20 C at night, I shouldn't be worried sleeping in a -40 bag at night even in summer for being too hot, right?
I would recommend a Down Nylon blanket and a compression sack. A cold rated sleeping bag is going to be as poor a fit in the summer, as your current one is in the winter.
If you can’t stash and swap your summer and winter gear (which I would advocate for as the best solution. There are various sets of gear that are best for winter vs temperate camping) and you are not likely to encounter -40 C conditions, then I would recommend adding a blanket similar to the following: Nylon Blanket example.
The advantage is, you can pull this blanket inside of your exist sleeping bag. These blankets work by trapping air within the down, which acts as an insulator within the sleeping bag. The outer sleeping bag will also create an insulation against airflow pulling away the trapped heat.
With a compression sack like the following, you will minimize space in your pack significantly. Compression Sack example
The only other thing to consider is heat loss into the ground. Especially in the winter when the ground has been cooled significantly. Air sleeping pads, foam sleeping pads, or layering material to create your sleep area help to minimize this problem. They also provide an improved comfort. I really enjoy my air sleeping pad. Sleeping pad example
If you anticipate encountering -40 C conditions, it’s best to get appropriately rated gear. This is for safety as these more extreme colds can be life endangering when not properly equipped.
These items are not my suggested purchases, and are provided for reference only.
I am someone who wants plenty of blankets in winter and two in summer, and who has a good 4 season sleeping bag. Under normal summer conditions I do not use the zipper but the velcro strips to close it.
But in very hot circumstances that sleeping bag is still too hot and I have to use it as a mattress.
I would advise you not to buy a four seasons bag unless you really need it.
For a full year system, I would go with the -40 degree bag and some sort of liner/blanket thing. To certain extent, you barely need any sort of bag in the summer. This would be where the liner comes in, just use it as a blanket and the sleeping bag as an extra pad. I have used my -40 bag in a heated tent, and it isn't too bad. I don't zip it up all the way which helps.
I do find that that temp ratings are also misleading (at least for me). I would go colder than you expect to experience just to have some extra comfort. So the -40 bag and a liner would be the way to go even if you only expect to see -15.
Sleeping on the bag compresses it and possibly(probably) damages it. Any technology. And that's without the possibility of any stick/thorn/rock poking a hole in the water resistant shell.
Unzipping at the feet end is essential. But not sufficient. Sweat dehydrates and has other effects.
Also washing is difficult, compared to using a cotton or silk liner.
Look for layered. But observe the weight and volume.