So, there are some unpowered options, none of which are as efficient individually as a powered humidifier, but may work in tandom: 1) Plants - they release the

So, there are some unpowered options, none of which are as efficient individually as a powered humidifier, but may work in tandom: 1) Plants - they release the water you put into them into the air, as part of how they pull water up from the ground to the leaves. 2) Open Water - Aquariums, fountains, etc. 3) Humidifying balls - basically rough ceramic balls which sucks water up into them, and the water naturally evaporates. Slow, though, and if your water has any impurities they will accumulate on the surface. 4) Humans - we breathe out a lot of water, and if your house is well enough sealed, you'll actually have to de-humidify, even if the air outside is dry. Costly, though.