How do i calculate how many rectangular soffit vents i need for my house?

Question

if you could give me an example to that would be great..

Answer 1

You have to know how much air flow the vent will allow.  That will require a bit of research on your part.  Plus if you have box, ridge, or gable vents you need to know how much they vent.   What you need is a balance between soffit vents and vents up high, ie ridge, box, or gable vents.  In general, it is better to start with the amount of ventilation you need up high and then match the soffit to that.  A general rule of thumb, you need 1 sqft of ventilation for every 150 sqft of attic space.  And then you would match the soffit ventilation to that.  A typical gable roof, of course, has one ridge and two soffits, so you can divide the amount of the ridge vents between the two soffits.  If you assume the ridge ventilation is done right, then take the sqft of the attic space divide by 150 (so say 1800 sqft of attic space divide by 150, you get 12 sqft), then if you know how much air flow each soffit vents allows divide the 12 sqft by the that number to get the number of vents to get.

But if you are reverse engineering, you may need to just match what you have as ridge vent you have to how much you are going to vent.  A little research on how much ventilation you have at the ridge is needed.  If you are going to err, err on the side of overventing the soffit.  If you overvent the ridge and undervent the soffit you won't get the airflow from the soffit to the ridge.

And you shouldn't use a combination of gable vents and ridge vents.  What can happen, the ridge can pull airflow from the gable and not the soffit.  In fact, during rain the ridge can pull rain in from the gable.  If you have this combination I'd suggest you board off the gable vents from the inside, and the gable vent will be decorative, then figure the amount of soffit vents based off the ridge vent only.

You are probably going to do some research to figure out what how much ventilation you have know.  No matter what you come up with, if you don't have any type of soffit ventilation, then any vents would be an improvement over none.

Answer 2

On average, when framing a standard ceiling type home, we'll typically use pattern of one vent then two solid blocks one vent two solid, etc. Vaulted ceilings,with no attic space above may require a vent in every bay.