Vented vs. Unvented Roofs
There are traditionally four main reasons for venting a roof:
The two most important reasons for venting a roof in cold climates are removing moisture and controlling ice dams. When considering the purpose of removing moisture from a roof assembly it is important to remember that moisture can enter a roof assembly in several ways:
There are three major strategies for controlling moisture in roof assemblies:
Roof ventilation employs the third strategy. The assumption is made that it is nearly impossible to control moisture entry into a roof assembly, so removing it serves to prevent moisture accumulation. For roof ventilation to be successful a few conditions must be met.
Proper air movement from soffit vent to ridge vent is not always possible. In snow country, a poorly designed ridge vent could become covered with snow, which would allow air into the roof assembly, but not allow it to escape.
Moisture could actually be brought into a roof assembly. The ability of air to absorb moisture depends on the amount of water that is already in the air or its relative humidity. If the relative humidity is at 100%, the air cannot absorb more moisture. The colder the air the less moisture it can hold.
If one or more of the above conditions are present, moisture could accumulate faster then it can be removed by the ventilation. This would make the ventilation systems in effective and the building would still develop moisture related problems.
In the Moisture Control Handbook, Joe Lstiburek explains why venting a roof may have worked better in the past.
A vented roof system can only work properly and remove the moisture from a roof assembly if:
Un-vented roofs operate on the principle that moisture entering the roof assembly can be controlled, and that there is no need for venting to control moisture accumulation. In order for an un-vented roof assembly to function properly, the following conditions must be in place:
In recent years building scientist have been conducting test and re-thinking the accepted theory on roof ventilation. The Journal of Light Construction featured an article on the topic, in which Ralph Britton from the Housing and Home Finance Agency is quoted as saying,
One of the major concerns of shingle manufactures is that un-vented roofs will increase the temperature of the roof sheathing and shingles, which would lower their life span.
Bill Rose of the University of Illinois states in an unpublished report that,
Another traditional reason for venting a roof assembly is to reduce cooling load and increase occupant comfort. Tests have been conducted to see if ventilation did indeed help to keep a building cooler. The National Bureau of Standards published an article “Summer Attic and Whole House Ventilation” (Dutt and Harrji, 1979), in which the authors stated that “with recommended levels of insulation, the attic air temperature had little effect on the cooling load.” They also observed an increase in cooling costs because venting the roof caused pressure differences within the building envelope. These pressure differences caused cool air from the interior of the house to be sucked up into the attic and out via the ventilation system.
Foam and the Roof Assembly
Foam insulation can play an important role in both vented and un-vented roof assemblies. In a vented roof, foam can seal the openings and serve as an air barrier and a vapor retarder in addition to its insulating function. In an un-vented roof, foam can serve as the air barrier and vapor retarder and prevent moisture from getting in to the roof cavities. Preventing moisture is essential for having a properly functioning un-vented roof.
Rose, Bill, 2002. “Rethinking Attic Ventilation Requirements”, 2002. Energy Design Update, Aspen Law & Business, New York, NY, July