Great question — answering as the UphillTech team:
A very useful (and not expensive) way to adjust a sleeping bag's insulation is a liner. This also reduces the need to wash the sleeping bag, prolonging its life. With access to even a basic sewing machine or patience for hand-sewing, liners are easy enough to make , if it's easier to get the fabric than a ready-made liner. There are several types: The cheapest liners are thin polycotton; even they add a little warmth (perhaps a couple of degrees) - or allow you to open the foot vent without feeling a full draught, if you're a little warm. Pure cotton liners are comfortable, a little warmer when dry, no good if they get damp. They're more meant for hostel use. Silk liners are supposed to be...
Full answer by Chris H: https://outdoors.stackexchange.com/a/25814 (CC BY-SA 4.0).
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