Washing a sleeping bag is easy, as long as you take a little care not to damage the filling.
Down filled sleeping bags rely on the natural oils of the down to help give them loft, so it’s important not to use chemicals which can strip these oils.
Hand-washing sleeping bags is the best plan…
You will need to use a bathtub filled with warm water to a depth of about 2 inches. Add some soap flakes or Nikwax Down Wash to the water and place the sleeping bag in the mix. Let it soak in for 10 minutes, gently stirring and moving things around occassionally. Then rinse the bag several times, until the water runs clear. For any stubborn stains and oily areas around the hood, rub a slightly stronger soap solution in with a soft brush.
It’s best to air-dry a down sleeping bag, occassionally fluffing it up with your hands to help lift the loft. You can (with some bags) use a tumble dryer and a couple of tennis balls, but be warned that I’ve killed a down bag doing this.
Be sure not to store your down sleeping bag until it’s absolutely dry.
Washing a synthetic sleeping bag to get rid of your body oils and dirt will help improve its insulation.
Use mild soap flakes or Nikwax Tech Wash in a clean washing machine (clean out any residue of washing powder or fabric conditioner) or hand-wash it in a bath. If you use a washing machine I recommend setting it to Wool Wash so the cycle isn’t too violent. You should be able to use a spin-dry cycle without problem.
Dry the bag according the its label. Some bags allow you to use a tumble drier, some don’t.
Be sure not to store your sleeping bag until it’s dry.