Attic Insulation and Air Baffles Questions

Question

I'm a young homeowner who has been renovating my attic to turn it into a 2nd floor. I've gotten great advice from reading lots of posts in this sub, and now I've come for help. I'm getting ready to insulate along the roof and have a few questions, but first, the important details:

1. Home was built in the Northeast US in 1920
2. So the rafter spacing is not uniform, varying from 16" on center to 24" on center
3. Most of the rafters are 2x6, although the dormers are 2x4 rafters
4. I do not have a ridge vent, so I plan on having a small "attic space" above the flat ceiling
5. I plan to have the plastic baffles up against the roof and the insulation up against the baffles

With all that in mind, my questions are:

1. can I still use R-19 on the 2x6 boards with the baffles (and R-13 on the 2x4s)?
2. does faced vs unfaced make a difference?
3. for the sections larger the 16" apart, do I just cut a 2nd piece long ways and bud the 2 strips of insulation next to each other?

Answer 1

isoboard is 5.7 R per inch, fiberglass is 2.9 per inch. Given you have little space and it is quite difficult to upgrade later, I'd get isoboard and fill the space remaining after you fit your baffles. you buy it in 4'x8' sheets and just cut it to size. You can spray foam any seams. I'd use tuck taped poly before drywall to have a good air seal.

I'd also drill 2" circular holes in the blocking between your rafter tails on the exterior to increase airflow up the baffles. They sell little circle plastic vent caps for these holes. I agree that your 1920 house is going to be quite leaky but you want the airflow to come up the low point of the roof sheathing and travel along the underside of that sheathing to ideally a ridge vent (in your case probably the 2 roof vents). If the air comes from somewhere else then you can short circuit the ventilation flow which is meant to cool the sheathing and help your shingles last longer.

I suspect 2 outflow vents is not enough for the size of your roof. Assuming an 800 sq ft size for your top floor you require 384 sq inches of ventilation. The typical square roof vent is 50-60 sq inches so you'd need 7. 2 is not enough. Installing a ridge vent can be done after your roof is finished. I'd do it now so you can see if they screw up and you have leaks before you finish your top floor.

https://www.owenscorning.com/en-us/roofing/components/vent-calculator