Though most building codes require a minimum of 1 sq. ft. of ventilation per every 300 sq. ft., the 300:1 rule, as it is commonly known, has no basis in building science. In fact, a paper titled, "The History of Attic Ventilation, Regulation and Research" found no concrete scientific basis for the 300:1 rule, which was first put in motion in 1942 by writers of the Federal Housing Agency. The rule, according to Rose, was arbitrarily enacted with no research or field-evidence to support it.
Since the 1940's, home construction methods have changed dramatically. Then, roof-decking consisted of board (plank) sheathing, a rather "breathable" material. Today, we employ continuous sheets of plywood or oriented strand board (OSB), which creates a relatively air-tight barrier to the outside. Additionally, homes built in the early to mid-20th century contained little, if any, attic insulation, whereas today, we build tight, well-insulated homes.
This change in building materials and construction techniques creates an entirely different dynamic in terms of the way a home behaves under certain temperature and/or moisture conditions.
When an attic is uninsulated, the space becomes semi-conditioned, as it receives heat that rises and escapes from the conditioned areas below. This rising heat also brings with it moisture, in the form of vapor, the main culprit in roof failures. In a semi-conditioned environment, this moisture tends to stay in vapor state, since the dew-point is never reached, which means condensation can't occur.
In a well-insulated attic, the space becomes completely unconditioned. Little heat reaches the attic, making the attic a much colder space. And since few builders to a good job of air-sealing the attic plane (the ceiling of the rooms below), moisture-vapor still reaches the attic space. Except in this scenario, the temperature of the attic DOES reach dew-point, causing the vapor to condense into liquid water, which ultimately kills the roof.
The point to all of this?
Homeowners in cold climates sometimes experience damaged, decayed roof systems caused by condensing moisture. And when they run into this problem, the first people the often call is a roofer or general contractor. And with almost 100-percent certainty, the first recommendation they make is -- add attic ventilation.
Why is this bad?
Attic ventilation inherently increases the "stack-effect of a home, increasing air and moisture-movement into the attic. More moisture means more problems. Furthermore, there's no scientific evidence that attic ventilation does what it's purported to do, which is allow the roof to air-out and dry sufficiently to head-off moisture-damage. It's merely an assumption that we've all gone along with, without question.
If the roofer or GC is worth their salt, they'll recommend not more attic ventilation, but a combination of air-sealing and MECHANICAL ventilation. Sealing the attic plane keeps moisture out of the attic altogether. Mechanical ventilation, such as a properly sized bathroom, removes excess water-vapor from the entire home.
Cathedral ceilings, where there's no attic at all, receives similar treatment. The ceiling itself should be very well-sealed to prevent moist air from getting inside. If recessed lights are installed in the ceiling, use only sealed canisters. If using a finish other than sheetrock or plywood, such as tongue and groove panels, a continuous moisture-barrier should be installed between the finished ceiling and the rafters.
According to building science expert Joseph Lstiburek, one extra level of defense to protect cathedral ceilings (in cold climates only) is to install a layer of rigid-foam insulation on the outside of the roof-decking. This keeps the underside of the decking warm and inhibits condensation.
The nice thing about the one-two punch of air-sealing and mechanical ventilation, is that it's also better for energy bills, while reducing uncomfortable drafts.
So if you have the great misfortune of a failed roof, don't call the roofer until you've first hired an insulator to seal the home, and an HVAC technician to check and/or upgrade the adequacy of your mechanical ventilation.
