“Researchers have compared the shingle temperature of both vented and un-vented roof systems. It has been shown that although ventilation has some effect on the attic air temperature, it has much less effect on shingle temperature. The exterior surface of the shingle is practically unaffected by the presence or absence of ventilation in the attic.”
>> Unvented Attic and Cathedral Ceiling Construction
Excerpts from a report by Bill Rose of the University of Illinois in which he states the following:
>> History of the VENTED ROOF
Before the introduction of insulation, moisture was not a problem in roof spaces. Roofs were exposed to warm, humid interior air. This warm air raised the interior temperature of the roof space and decking materials. The roof itself, made of vapor-permeable, natural materials, allowed water vapor to pass through it to the outside without condensing on the interior surface.
With the introduction of insulated roof spaces (attic and cathedral ceilings), the temperature in attics was reduced and water vapor passing through the ceiling to the attic encountered decking materials that were now colder than before. Condensation resulted, causing moisture problems and in winter, a build-up of ice.
The solution was to install a vapor diffusion retarder (VDR) on the warm side of the insulation, and to ventilate attics to remove any water vapor that succeeded in passing through the VDR and the insulation.
Similarly with cathedral ceilings, designers and builders faced similar moisture problems. Their solution was to leave an air space between the roof deck and the insulation material. Vents at the soffits and ridges allowed outside air through the space. The function of the air space in a cathedral ceiling is exactly the same as the function performed by attic vents.