I had a question the other day from a friend about building code and attic ventilation. I sent him a particularly long explanation that I thought might be useful to others. So here it is:
IRC 806.2 says that 1/150th of the attic is the required ventilation…. except that in Climate zone 6, 7, or 8 ….the rest doesn’t matter.
We are climate zone 2 and considered moist/warm-humid.
IRC 806.1 says to ventilate each enclosed attic & rafter bay.
We’ll ignore conditioned attics (IRC 806.5) because we are talking about ventilated attics. A different set of rules apply to attics inside the thermal envelope and there are some footnotes for cathedral ceilings…which you have a partial.
Practically speaking to meet 806.2 and 806.1 you need to divide the ventilation so that you will get air-flow. Gable vents are largely a poor design choice in coastal windstorm areas and have largely fallen out of favor. That leaves us with eave inlet vents and either ridge vents or box vents (powered or unpowered). Each vent type has a “net free area” rating.
You need to start with the square footage of the “attic” and then divide that by 300 to figure out how much ventilation you need. Then you divide that in half to figure out how much needs to be at the top and how much at the bottom.
Sidenote: Code is like the Betty Crocker cookbook. It’s the minimum standard. Cooking what’s in it shouldn’t kill you, but it probably won’t win you a prize. Ventilation is your friend in moist humid climates. hint – you can have more, but not less.
Once you know the numbers you need then you choose the products that provide enough net free ventilation area.
Here’s an example for a house that is 1200 s/f, single story. Attic is the same size as the floor. We need 4 square feet of ventilation. 2 at the top, 2 at the bottom.
We’re going to use Hardie Soffit for this example. Per the manufacturer instructions, each linear foot of soffit product provides 5 square inches of net free area. I need 57.6 linear feet at 5 inches per LF to provide 288 square inches of ventilation area. I shouldn’t have any issues with this on a 30×40 house. I can’t make all the eaves intake though or I’ll create 700 square inches of intake which is 4.86 s/f.
For the exhaust we’ll use ridge vents. I’m using Cobra by GAF because it’s handy. AMI you get 18 square inches per lf. I need 16 LF for design, but if I have more I can increase the ridge vents up to 38.89 LF. Depending on the roof design I can probably exceed code with my ventilation.
Now, I hate ridge vents because they are prone to damage by walking on them and animals…… so let’s look at turtle vents. Each one is 60 square inches. Quick math says 5 of them would provide 300 square inches of ventilation. 12 would meet 700 and let me max out the ventilation on this attic in theory.
The baffles provide a way for air to get from the eaves to the attic. I can’t find any free area calculations…but they stop insulation from clogging the gap between the roof and wall top plate. There is some chatter about code changes to top plate insulation, but I don’t do enough new construction stuff to have spent the time digging into that.
Net net is that it’s cheaper to exhaust hot air out of the attic for free than it is to run the AC to move the heat out of the house. The ventilation needs to be designed and installed according to the manufacturer installation instructions. It’s very common for ventilation mistakes to be made where builders combine turtle and ridge vents. That’s prohibited by the ridge vent installation instructions unless a PE wants to over-ride that with their E&O coverage. If the PE is willing you can do anything their E&O will cover.
