 Patients will undergo three consecutive nocturnal polysomnographs. Patients will have a baseline therapy of NIMV without humidification for 1 week. They will then undergo a nocturnal polysomnograph on the last evening of the baseline therapy. Patients will then be randomly assigned to use HH or HME for 1 week and undergo a nocturnal polysomnography on the last evening . The patients will then be assigned to the second method of humidification for 1 week and undergo a third nocturnal polysomnograph on the last evening. The whole evaluation will be performed with the same ventilator ideally the patients device using the patients mask and with the usual ventilator setting. 