Attic ventilation - intake:exhaust ratio vs total ventilation area?

Question

I have decided to go ahead and install a ridge vent in my house. The only current attic ventilation is soffit venting on one side of the house.

As part of this project, I am attempting to determine the appropriate ratio between intake and exhaust venting. From what I have read the things I am aiming for are:

• 1:150 ratio of vent area (in or out) : attic square feet - This is important so that there is sufficient ventilation for heat/moisture to escape
• 50:50 ratio of intake : exhaust (or if one is more, more intake than exhaust). - This is important so that your soffits remain "intakes" and ridge vent "exhaust" and not the other way around.

In doing my calculations I have found that even if all soffits are vented, and there is a matching amount of vent in the ridge vent, I have just enough venting to meet 1:150. For reasons outlined in the previous referenced question below (concern for snow coming in through soffits), I am considering limiting the amount of soffit venting on the side of house with prevailing wind. If I do that, then it becomes impossible to achieve the rule of thumb 1:150 (can you tell I am adverse to the risk of snow coming into my attic??)

Questions:

• If one of these "rules" must be sacrified, which seems like the better of the two? Less total ventilation, or more ridge ventilation than soffit ventilation?
• Is having a ridge vent for only a short portion of the ridge effective or normal? (say 18' out of 46')

For reference:

• 28' * 46' = 1288 sq ft attic floor
• 1:150 = 8.58 sq ft total ventilation required (1235 sq in)
• Soffit area: 100 sq ft (max), would like to minimize exposure of side with prevailing wind, so as low as 75 sq ft (note: soffit width = 15")
• Ridge length: 46'
• Ridge vent: 18 sq in net free area / linear foot
• Soffit vent: 6.4 sq in net free area / square foot
• Please see also my previous question: How to properly vent attic in windy/snowy areas to avoid snow infiltration?

Sample Calculation:

• Soffit: 75 sq ft * 6.4 sq in/sq ft = 480 sq in free area soffit
• Assume 40:60 split exhaust:intake, so 320 sq in required ridge vent
• Corresponds to only ~18 ft of ridge vent
• Total vent = 800 sq in; Required (by 1:150 rule) = 1235 sq in. Effective = 1:231

Answer 1

For the 50:50 rule, I don't see the ridge vent becoming an intake unless there is another second ridge vent. So given the choice, I'd ignore the 50:50 rule unless your ridge vent is not at a single height.

That said, in your specific situation, with a strong wind from one side and the soffits closed on that side, I think you're creating a situation where regardless of how big the ridge vent is, you could get a down draft from the upwind ridge vent to the downwind soffits. But more importantly, by closing some soffits, you are reducing the airflow under part of the roof. In areas of heavy snow, this airflow is critical to prevent heat from the home from melting snow from the upper part of the roof and having it refreeze on a lower part, forming an ice dam that would backup water into the roof (rot) and possibly into your home (leaks).

Because of the ice dam risk, I think you're better off going with shirlock's earlier suggestion to rely on the Styrofoam tunnels stapled to the rafters and adding some ridge vent filter to any soffits you are worried about. Then install a ridge vent of the proper size for your roof.

Answer 2

I have seen situations where ridge vent don't work so swell. 750 vents get covered with snow too. They aren't pretty but you get 4 times the venting of a 750 from one vent 200 nfa on a 12 inch and 300 on a 14 inch whirly bird and they don't get covered in snow and quit working like ridge vent or ,750's.you might only need one or two.keep ,at least ,10 feet apart.