How much vents do I need?

#1

My original roof lasted 26 years. It is 11 squares in size. Back then, they used felt underlayment which as you know is breathable. With the felt underlayment, they only had 4 turtle vents (that is Louver vents).

The question is: when I get my roof replaced now, they are going to install synthetic non breathable underlayment. In that case, will the four turtle / Louver vents provide enough attic ventilation ? Or, do you think that we will need to increase the amount of vents.

If you think that 4 louver (turtle) vents will not be enough, can you please suggest what type of vents and how many, if applicable, would be appropriate with the non breathable underlayment in the case of my roof ?

Should I add ridge vents to the turtle vents ? Or replace Turtle vents with ridge vent ? Or simply increase the number of turtle vents, and if so then how many ? What do I need to do to compensate for replacing the felt with synthetic non-repeatable underlayment ?

#2

I found this link to figure out how much vents are needed: https://www.owenscorning.com/roofing/components/vent-calculator

I don’t think it makes any difference whether you use breathable felt or non breathable synthetic underlayment because the shingles above the underlayment will cause a tight seal and will not allow any vapor to escape. You can refer to this paper from Owens Corning titled: “VAPOR PERMEABILITY PROVIDES
NO PERFORMANCE BENEFIT FOR
ROOFING UNDERLAYMENTS
IN VENTILATED ATTICS”

#3

You would NEVER mix ridge vents and turtle vents. Both draw air up through the soffit vents. Adding one to the other would be counter productive. I would think 4 turtle vents may be adequate for 11 squares. If you have a problem, check to make sure you have soffit vents and if you do, check to make sure blown insulation didn’t plug up those vents preventing air flow from the intake.

The felt choice is irrelevant with respect to your underlayment decision. You can go here to determine your ventilation requirements (number of turtle vents needed). Link to Lomanco Vent Calculator