In the illustrated embodiment, the moisture reservoir comprises a tube of gas-permeable, hygroscopic material. The exhalation flow path is into a blind axial bore of a spool like member having a series of transverse notches or cuts at the top and bottom which provide interspaces leading from the bore to the tube which covers the notches at their periphery. The notches leave integral fins or lamellae with large surface area exposed to the in-flow of dry respiration gas radially through the tube wall and into the interspaces which provide further reservoirs for moisture and warmth.