If silk was a bad insulator, it would let heat through into the insulation. But cotton or silk, a liner will increase the warmth of your bag. It adds another layer to trap heat. If the liner was your only form of insulation, it would keep you only a couple of degrees warmer than the outside air. But when you put this inside your bag, it adds that few degrees more warmth, by trapping/ slowing down the loss of warmth your body produces.
Think about it this way. People are talking about letting your body heat into the insulation so it can keep you warm. Right. Because keeping that body heat closer to your body (in clothing or a liner) will make you colder...
Is it the sleeping bag or your body you want to keep warm?
Ideally, the outside of your sleeping bag shouldn't feel warm. If it did, it would mean that some heat was escaping from the insulation. You want as little heat as possible to permeate through to the outside of your bag. Therefore any extra insulative barrier on the inside will only aid this.
Thus, the liner, silk or otherwise, adds another pocket of dead air for your body to heat. So if you took the temperature inside the silk liner, it should be slightly warmer than the temperature between the liner and sleeping bag.
Think about it this way. People are talking about letting your body heat into the insulation so it can keep you warm. Right. Because keeping that body heat closer to your body (in clothing or a liner) will make you colder...
Is it the sleeping bag or your body you want to keep warm?
Ideally, the outside of your sleeping bag shouldn't feel warm. If it did, it would mean that some heat was escaping from the insulation. You want as little heat as possible to permeate through to the outside of your bag. Therefore any extra insulative barrier on the inside will only aid this.
Thus, the liner, silk or otherwise, adds another pocket of dead air for your body to heat. So if you took the temperature inside the silk liner, it should be slightly warmer than the temperature between the liner and sleeping bag.
