WorldTourer's advice to always use a liner and avoid washing is good.
However, I would hate to throw out a still useable bag, so I have washed my 1978 vintage 3 season down bag once. The washing part is easy with the appropriate soap product, it was a long time ago so I have forgotten the brand. Drying is more difficult.
An example of up to date instructions for washing and drying a down bag here:
https://www.mountain-equipment.com/pages/caring-for-down-sleeping-bags I didn't notice any appreciable difference after washing the down bag, which for a long time was my only sleeping bag. I used it with a lightweight cotton liner until I got a thermal liner.
About 15 years ago I got a lightweight synthetic bag, which is better for me for summer use as it is cooler. I now have two sleeping bags, a quilt and a thermal liner:
- LightLine 3-season down bag. 880g and the bulkiest. For cool weather camping.
- Lightweight synthetic bag. 520g and less bulky. For warmer weather camping and indoors if not too hot.
- Gramexpert synthetic quilt. 352g, packs very small. When it is too hot for the synthetic bag. Also good if sleeping on a camping mattress inside a building.
- Sea To Summit Thermolite Reactor inner liner. 250g, packs very small. This is warmer than a cotton liner and easier to wash/dry, in really hot weather, this is often enough on its own, but I always have at least the quilt in case I need it.
Nowadays I always take the Thermolite liner. Depending on expected conditions I add one of the sleeping bags and/or the quilt. The quilt can be used without a sleeping bag, or as an extra layer over one of the sleeping bags. I haven't yet needed it over the down bag.
Liner, quilt and synthetic bag is about the same weight and overall bulk as liner and down bag, but is more versatile so long as it doesn't get really cold.
