Is humidification needed for non-invasive positive pressure ventilation (NIPPV) and high flow nasal cannula (HFNC)?

This is somewhat controversial. 

For short periods of time (hours), NIPPV may be tolerated without humidity, but for longer periods of time (days) humidity is essential, especially at high FIO2 (100% oxygen from a tank or other high pressure source is ~anhydrous). Leaks with NIPPV (around a mask seal) create issues with humidification and is one reason HMEs don’t work well with NIPPV. 

For HFNC, most people find >6LPM NC intolerable without humidification, but with humidification most people don’t notice flow until it exceeds 15LPM and will tolerate >40LPM. With HFNC (and nasal CPAP) gas flow is unidirectional (in the nose and out the mouth) and thus there is no possibility for reclaiming moisture from the exhaled breath. Without heat and humidification most patients will find this intolerable.