1920s small house remodel – insulation for a vented roof with 2×4 rafters!?! – GreenBuildingAdvisor

I am getting close to starting a remodel of my house and this will be the first of probably many posts where I am looking for advice and/or validation of how I plan to execute certain details. Here’s some context to begin with:

Located in Bellingham, WA – Climate Zone 4C – The existing house is a 28’x28’ single gable box built in ~1920 that hasn’t had much done to it since then. I plan on adding about 150sqft to the west side of the house to enlarge the kitchen and laundry/mech room. The existing exterior walls and roof that I plan to leave are 2×4 studs and 2×4 rafters.

My main question for this post is if I will be fine to leave the roof as-is, or if I should tear it all off and replace it with something that will have a better insulation value at the top plate locations? Attached is a look at how I plan to detail the existing house at this location. Is there anything you’d change if you were going to leave the existing 2×4 rafters? I’ve seen and read where people will use spray foam at the area right over the top plates but I am generally wary of spray foam and would like to stay away from it, if possible.