I am also a mouth breather because I have a badly deviated septum and congestion problems. I thought I would never be able to sleep with a nasal mask.
However after about 3+ months I noticed that I don't sleep with my mouth open when I sleep on my right side or right cheek smashed into the pillows. My mouth opens when I sleep on my left side, which is only about 20% of the night. I thought if I got a chin strap, I might be able to get a smaller mask. After almost 5 months of cpap therapy I went to my DME and asked to try a Mirage Activa LT. They didn't have one in my size but they had a Mirage Micro that fit my face, or more specifically my nose quite well. I bought it with a chin strap and took it home to try immediately.
The chin strap needed the help of wide elastic band safety pinned on to stop it from sliding off but the the nasal mask worked quite well for the nap. And there were no leaks. No leaks!!! For those of us with full face masks and who wiggle often throughout the night, leaks are hard to avoid.
I slept for 2 nights with the nasal mask and had no problems. The third night my husband asked me a question as I was getting ready for bed. The nasal mask was already on my face and the feeling of air gushing out of my mouth when I talked was so uncomfortable, I ripped the mask off my head. I never had that feeling with the full face mask. Granted, my husband couldn't understand anything I said to him when I wore the full face mask but that's not important here. A night or two later I started coughing from a dry throat. Wearing the nasal mask made me aware of every cough because the air would escape out of my mouth and it was a very unpleasant feeling. Once again, I never had that feeling when I coughed with the full face mask.
Later that night I was coughing here and there but the feeling was so uncomfortable I took my mask off my nose and put it on my forehead. I'm sure my forehead appreciated the blowing air while I slept for 2 additional hours. This was the first time in almost 5 months that I slept without my cpap therapy and also the first time I wanted to and needed to take my mask off while sleeping.
Now I sleep with both masks but not at the same time!!! If I can't breathe well before going to bed or if I have a dry throat I'll use the full face mask. If I can breathe well before going to bed and haven't coughed at all, I'll reach for the nasal mask. I no longer use the chin strap because I can't tolerate the feeling of the air coming out of my mouth. If I'm on my "bad side" I'll put my fist under my chin to keep it shut until I roll back over to my good side.
Until I had this experience, I never understood why people would take their masks off in the middle of the night. Now I know.