It seems that I don't need to install a ridge vent. The old passive boxes --I have two or three of them with pretty large soffit vents-- seem to do the job; It is 85 degrees here this morning, feels like 94 outside, and it is quite tolerable to be in the attic. The old roof with two layers is 40 years old, so I guess the lack of a ridge vent has not hurt the life of the roof... I am going to use an OC Duration shingle and in the instructions it speaks specifically of installation of the ridge vent. so if i do not install a ridge vent will that invalidate my warranty?
This is in contrast with the instructions for valleys where they talk about using their weatherblock system or some other insulation. I am going to use a piece of membrane covered with a laminate or metalized paper covered by the shingle. The instructions leave open the possibility for this. The instructions do not say you must use a ridge vent but seem to assume it. Is it necessary to put in the ridge vent?
Ridge vent is arguably the best method for roofing ventilation, but it is not the only method that provides proper ventilation.
If your using a manufacturer certified roofing contractor and paying for one of the extended manufacturer warranties than the ridge vent may very well be a must, ask your roofer.
Shingles have changed a lot in 40 years, the average shingle sold 40 years ago was equal in quality to the life time shingles of today, thus ventilation plays a much more important part in the life span of shingles today.