You wake up because you feel a cold drop of water hit your cheek, or you hear a gurgling sound coming from the tubing before you even open your eyes. Sometimes the water pools enough that it splashes when you move, and it can be startling enough to jolt you fully awake. Beyond the annoyance, it can leave your pillow damp and make you dread turning the humidifier back on the next night, even though you added it specifically to feel more comfortable.

Why this happens

This is called rainout, and it happens when warm, humidified air from your machine travels through tubing that’s cooler than the air itself. As the air cools inside the tube, it can no longer hold as much moisture, so some of that moisture condenses back into liquid water, the same way a cold drink sweats on a warm day. This is worse in winter or in a cold bedroom, worse with humidity settings turned up high, and worse with standard, non-heated tubing, since there’s nothing keeping the air at a consistent temperature from the humidifier all the way to your mask.

Steps to troubleshoot

1. Switch to heated tubing if you don’t already have it, since this is the most effective fix by keeping air temperature consistent the entire length of the tube instead of letting it cool and condense
2. If you already have heated tubing but still see rainout, check that the tube temperature is set appropriately, since a mismatch between humidity and tube heat can still allow condensation
3. Lower your humidity setting by one level at a time if rainout persists, since more moisture in the air raises the risk of condensation once it cools
4. Position your CPAP machine below the level of your head and mask, so gravity pulls any condensation that does form back toward the machine instead of down the tube toward your face
5. Keep your bedroom slightly warmer, or insulate the tubing with a hose cover, to reduce the temperature gap between the humidified air and the room
6. Empty and dry your water chamber each morning, and let tubing fully air dry after cleaning, so you’re not starting the night with residual moisture already in the system

Who to talk to

Your respiratory therapist can help you fine-tune your humidity and tube temperature settings together, since these work as a pair rather than independently. Your DME can supply heated tubing or a tube cover if you’re currently on standard tubing. Your sleep doctor doesn’t usually need to be involved unless rainout is disrupting your sleep enough to affect your nightly use, in which case it’s worth mentioning at your next follow-up.

Evidence

Rainout isn’t just an annoyance, bench testing on bi-level and CPAP devices has shown that tubing condensate can meaningfully disrupt the pressure actually reaching the patient: one study found that once humidity output crossed a certain threshold, inspiratory pressure dropped by 2 to 15 cm H2O and breath triggering was delayed, and that heating the tubing eliminated this effect while insulating the tubing or raising room temperature only helped marginally.¹ A related bench study on CPAP delivery found that water condensation caused pressure to swing well below the set level during inhalation and above it during exhalation, underscoring that rainout is a real deviation in delivered therapy, not just a comfort issue.²

References

1. Ryan S, Doherty LS, Rock C, McNicholas WT, Boyle P. Effect of tubing condensate on non-invasive positive pressure ventilators tested under simulated clinical conditions. Sleep Breath. 2010;14(4):323-328.