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??)
- 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')
- 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?
- 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