Excuse me for not being clearer, I considered that it was plain we were discussing recessed fixtures. I'd expect, in addition, that since the words 'waterproof' and 'no ventilation' were being used, that IC rated fixtures were involved. That's a pretty safe assumption from what you said. They're required here since last November by energy code.
If the customer is experiencing repeated failures in IC fixtures, there is a chance that what is happening is not hot bulbs. In fact, it sounds as if a couple different bulbs have been tried without success. The other most likely failure mechanisms would then be corrosion in the socket/ switch, bad connection(s), or thermal overheat sensor(s). Any of these would present a light bulb with an arcing fault and 'certain' destruction.
IC-rated fixtures have been required to have thermal sensors in them since 1982. Among the possible causes of failure, sensors cannot be excluded. It is possible that the customer overheated the fixtures once. In which case, the sensor starts opening at 194 degrees and protects things. The next time it is heated it will open at, say, 160 degrees. After three or four activations we're in the 120 degree range. The contacts in those sensors are now pitted and hence the arcing fault I mentioned.
The other thing which happens with sensors is rough handling. To put it plainly, if they get hit they get damaged. They must be treated the same as light bulbs. The people who make them will not consider damage due to handling a warranty item.
Sorry for that, I hope this is clearer. Please re-write and re-post what I've tried to explain so that it makes more sense.
