Replies

1. GBA Editor
   Martin Holladay | May 29, 2012 04:55am | #1

   Richard,
   Let's imagine that your house has a gable roof, and that the length of the ridge is 50 ft. Therefore you have 100 feet (1,200 inches) of soffit. If your soffit vent is a 1/8-inch crack, you have 150 square inches of soffit vent. That's 1.04 square feet.

   Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area, so the amount of soffit ventilation that you have will only will only work if your attic is only 6 feet wide. That's unlikely. I think you need a wider crack.

2. 
   slugboy6000 | May 29, 2012 09:49am | #2

   I should've emphasized how small my place is. It's only 327 sq. ft.
   I've triple checked the math, I'm getting a small venting requirement for a small attic (I'll post the numbers below).

   I've been looking at perforated sheet metal, and insect screen. Depending on the NFVA of the material, the final soffit vent would need to be 1/4" to 1/2" wide.

   I just don't know what would perform better. A narrow continuous vent, or small prefab vents between each rafter tail.

   I am planning to have 2" of clearance above my insulation baffles.

   I know my place is rinky-dink, but I want to get the science right.

   Here's the math- - - - - - - - - - -

   Area of attic: 348 sq.ft. (16'9" x 20'9")

   Calculate NFVA required:
   348 sq. ft. ÷ 300 sq. ft.
   = 1.16 sq. ft. or 167 sq.in. of NFVA required

   60% for the soffit vents = 100 sq.in.
   40% for the ridge vents = 67 sq.in.

   Total soffit length: 576 lin.in. (48 lin. ft.)

   Soffit NFVA divided by total soffit length:
   100 sq.in. ÷ 576 lin.in. = 3/16 in. wide continuous unobstructed vent

3. GBA Editor
   Martin Holladay | May 29, 2012 10:04am | #3

   Richard,
   Either of your proposed approaches will work, as long as your local building inspector is satisfied. (However, it's hard to maintain a consistent 1/8-inch gap with ordinary building materials, and a 1/8-inch crack is easily clogged with dust, spider webs, floating pollen, etc.)

   Roof ventilation rules of thumb aren't particularly scientific, so I wouldn't worry too much about this aspect of your house. I'll quote from a blog I wrote on cathedral ceilings:

   "As building scientist Bill Rose has shown, code requirements for roof venting were never based on research or scientific principles. In a well documented JLC article on roof venting, Rose explained, “For the most part, the focus of codes, researchers, designers, and builders on roof ventilation is misplaced. Instead, the focus should be on building an airtight ceiling, which is far more important than roof ventilation in all climates and all seasons. ... Once this is accomplished, roof ventilation becomes pretty much a nonissue.”

   "For more information on roof venting, see two other articles by William Rose: “Early History of Attic Ventilation,” and “Issues Related to Venting of Attics and Cathedral Ceilings.”

   "Because of their unscientific origins, code requirements for venting roofs are often misunderstood. It's worth establishing a few basic facts:
   Roof ventilation cannot be used to lower indoor humidity levels.
   Builders should not encourage the migration of water vapor through a cathedral ceiling.
   During the summer, roof ventilation does not significantly lower the temperature of asphalt shingles or other types of roofing.
   While roof ventilation can lower the risk of ice damming, it's essential for builders to limit the flow of heat into roof ventilation channels by including one or more ceiling air barriers and by installing thick insulation, so that as little heat as possible escapes from the home.
   While roof ventilation can help dry out damp roof sheathing, it's essential to limit the flow of water vapor escaping from the home so that the roof sheathing never gets damp in the first place.
   In the absence of an airtight ceiling, roof ventilation can do more harm than good, since air movement in rafter bays can encourage indoor air to leak through ceiling cracks."

4. 
   slugboy6000 | May 29, 2012 11:54am | #4

   Thanks for the advice, and the links Martin. This is definitely going to help me.