You want a "hot roof" with no ventilation. Look it up.

Full insulation is what makes sense. Don't be concerned about the "about the air not being able to move."

Dense packing a flat roof with cellulose or fiberglass with no other measures is a guarantee of damage and failure, according to Building Science Corp.

Dense packing works only if you air seal the underside of the deck with spf or add insulation to the top side of the deck.

Here is a link that might be useful: Don't be Dense

This post was edited by worthy on Thu, Oct 30, 14 at 19:42

worthy-
Good cautions about dense packing, but the OP is talking about using fiberglass in a roof that "has vents all around." I don't see a problem with that. I have had that situation in all of the homes I have owned in the last 40 years with no issues. In my experience, 5-1/2" of fiberglass batt insulation placed horizontally in a 5-1/2" cavity will sag and compress naturally a bit and leave enough of an air space for adequate moisture transport, as long as it's vented. However, if one decides to overstuff such a space with batt insulation, then there could be issues.

This post was edited by kudzu9 on Thu, Oct 30, 14 at 23:51

Same problem, according to Dr. Lstiburek. Likely, the ceiling is not tight and the ventilation is inadequate. According to Lstiburek, at least 6" of clear air is required above the insulation to provide proper ventilation.

Unless you can access the space, you can't be sure what's going on up there.

This post was edited by worthy on Fri, Oct 31, 14 at 19:28

It may also be a function of where one lives. I'm in a temperate area where it rarely freezes (although there is a lot of rain in winter), and it doesn't get much past the 70's most days in the summer. There are a lot of cathedral ceilings in my neck of the woods that are insulated with batts, and ventilation in each bay, and I have not heard of problems like this. And the building departments, which are fairly conservative, continue to approve this design. I'm not saying it can't be a problem. Just sharing my own experience.

It may also be a function of where one lives.

Yes! In those areas where the average temperature doesn't dip below 45ÃÂF. and there is no a/c in the summer, you can get away with a lot. So the OP and his vented attic could be home free.

With unvented attics, as Lstiburek points out in the document linked below,"[O]nly hot-dry climates do not require the control of condensing surface temperatures or the use of air impermeable insulation at the underside of the roof deck. All other regions require some form of control."

Living in a climate that goes from 22ÃÂ C. two days ago to slushy snow accumulation this evening, I tend to forget those cursed with comfort. (One half of my family is from Hawaii where climatewise life is very boring!)

Here is a link that might be useful: Understanding attic ventilation.