Insulation, Radiant and Vapor Barriers - Any validity to this?

I've been researching methods of insulating a garage roof if I keep open rafters. I don't know what to think of this advice from a manufacturer called Nu Wool. They cite scientific studies that say it is best NOT to vent roofs with insulation between the rafters (cathedral ceilings) and not to use a vapor barrier. They acknowledge this is contrary to most industry practice. They claim that moisture will escape the building through the roof and that this actually is better than vents in the ridges, etc. and are never adequate and especially better than vents used in conjunction with vapor barriers. They even question the use of vapor barriers in walls is more of an accepted myth in most cases than scientifically a necessity.

Does any of this make sense or is it hocus pocus?

Thanks for any input.

I am always skeptical of anyone whose product flies in the face of current thought. While occasionally this is due to innovation and we will then adjust common practice to take advantage, most of the time it's a bad idea. I would not do this until someone knowledgable in this area and whom you trust says it's good stuff.

I will have to disagree with this product claims. First we have to look at the primary purpose of attic ventilation. Roofing materials are designed to keep moisture out of the house. As such, it will not only prohibit moisture from entering the house from the outside, it will also prohibit moisture generated inside the house from leaving. Therefore the primary purpose of attic ventilation is to bypass the low vapor permeability of roofing materials in order to avoid moisture accumulation inside the attic space.

Vapor barriers are only used with insulation for a variety of reasons. However, one of the most compelling factors for their application is pressure induced moisture flow. A common rule with equilibrium is high to low. For example, an object of higher humidity, temperature or pressure will give some of their humidity, temperature or pressure to objects of lower humidity, temperature or pressure and not vice-versa. If one air mass is at 70 degrees Fahrenheit and the other is at 32 degrees, the air mass at 70 degrees give heat energy to the other air mass at 32 degrees. It will continue giving heat energy to the lower temperature air mass until the temperature of both air masses are equal. The higher temperature air mass will lose temperature and the lower temperature air mass will increase in temperature.

Whenever you heat air it expands. Since the house cannot get bigger as the air expands when heated inside the house during the winter, it creates an area of high pressure inside the house. Though the amount of pressure is so slight (2 to 3 Pascals) humans cannot sense it, it is significant enough to influence moisture flow. Where objects inside the house that have a lower humidity gives humidity to objects of higher humidity inside your walls and ceiling. Hence pressure induced moisture flow is an exception to the rule High to Low. This condition dramatically increase the probability that a moisture problem will occur inside your walls and attic during the winter. The proper application of vapor barriers to these components dramatically reduce the probability of a moisture occurring inside your walls and attic.

Though is is an oversimplification of the process, what was not discusses was Psychrometrics. which is also a compelling factor for the use of vapor barriers. This describes how moisture behaves inside objects at different temperatures (RH%).

You bring up a number of issues.
If your roof is part of the garage envelope,ie, no drywall, you have a dilemma here. Check your building code first before doing anything.

The only way to insulate is blown wet cellulose which will stick to the roof. i am not sure if it is allowed under building codes.

You could add drywall on the bottom of the rafters and insulate, but you will need ventilation in the eves. You could also blow in cellulose and dense pack it in lieu of ventilation.
Many northern states promote dense packing of insulation.

Any way, if you use fiberglass, you need the vapor barrier and air space between the insulation to the roof.
Mike

Only way that I know where you will not have to ventilate or have a vapor barrier is with closed cell spray foam.

You havn't indicated if you are talking about a detached garage or one that is part of a house. I did a detatched garage like you wished to do. I wanted to use the space above the truss chord for storage and I was heating the building with a wood stove. I used styrofoam inserts that were 3 inches thick cut to fit between the rafters., they were held in place by wooden slats. It worked well except that when the wind blew hard, they squeeked some as the garage moved a bit. I also cut pieces to fill in the gable ends. An attached garage is completely, different as building codes are far more stringent for ouccipied dewellings and Insurance co have different rules too. I never did get around to insulating or drywalling the walls etc. but that old wood stove could keep er warm in -30 weather, here in Canada.