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Vented vs. Unvented Roofs

Vented Roof

There are traditionally four main reasons for venting a roof:

  • Removing moisture from roof cavities, structural members, sheathing and insulation.

  • Controlling ice damming by keeping the roof cold.

  • Enhancing roofing material life span by reducing sheathing temperature.

  • Reducing cooling loads and increased occupant comfort during the cooling season.

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:

  • Roof leaks (which may be caused by ice dams), flashing problems, roofing failure, and wind-driven rain and snow.

  • An air leak in the building envelope which transports water vapor into the roof cavity.

  • From inside the building via water vapor diffusing through the interior sheathing.

  • From inside the building through holes in the vapor retarder that allow water vapor suspended in the air to bypass the vapor retarder.

There are three major strategies for controlling moisture in roof assemblies:

  • Controlling moisture entry.

  • Controlling moisture accumulation.

  • Removing moisture that does accumulate by drying.

 

 

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.

  • Air must be able enter through the soffit vent and exit through a ridge or gable end vent.

      • 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.

  • The exterior air must be dry enough to absorb moisture as it moves through the roof assembly.

      • 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.

  • The moisture must be evaporated and removed faster than it accumulates

      • 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.

“Historically roof assemblies have not contained enough insulation. So, heat loss from the conditioned space has warmed the attic area. Cold air entering this area is warmed up, which increases its ability to absorb moisture. So in this scenario the ambient air can remove the moisture.”

A vented roof system can only work properly and remove the moisture from a roof assembly if:

  • The building envelope is tight enough to prevent warm, moist air from escaping from the building interior into the roof assembly.

  • The roof assembly has enough insulation and it is properly installed.

  • There is enough exit ventilation space to match the amount of soffit ventilation inlet. Inlet and outlet areas must also be evenly distributed to provide uniform cooling.

  • The outside air is able to absorb moisture equal to or faster than is it accumulated.

Un-vented Roofs

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:

  • The building must have a tight building envelope, which includes a properly installed air barriers and vapor retarders.

  • The different parts of a building must be properly pressurized to negate the stack effect. The stack effect occurs when the top portion of a building is under positive pressure, which causes warm moist air to try and escape through holes in the building envelope. The positive pressure at the top portion of a building causes the bottom portion to be under negative pressure, which causes air from outside to enter through holes in the building envelope.

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,

“Un-vented roof assemblies performed well, showing no excess moisture accumulation, while the only vented roof assemblies that preformed equally well had good vapor barriers and good workmanship.”

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,

“Researchers have compared the shingle temperature of both vented and un-vented roof systems. It has been shown that ventilation has a great effect on the attic air temperature, but much less on shingle temperature. The exterior surface of the shingle is practically unaffected by the presence or absence of ventilation in the attic.”

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.

References

Bynum, Richard, 2001. Insulation Handbook, McGraw-Hill, New York, NY

Lstiburek, Joseph and John Carmody, 1993. Moisture Control Handbook, Van Nostrand Reinhold, New York, NY

Lstiburek, Joseph, 1998. Builders Guide: Cold Climates, Building Science Corporation, Westford, MA

Rose, Bill, 2002. “Rethinking Attic Ventilation Requirements”, 2002. Energy Design Update, Aspen Law & Business, New York, NY, July

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