To Vent Or Not To Vent
Seattle, Climate Zone 4-C
Re-building a roof on a 1950’s garage with concrete foundation and walls. My half will be a workshop and tool storage space, only occasionally conditioned in the winter by electric heat. My neighbor’s half will remain unfinished.
Initial plan was to insulate the floor and walls of my space and leave the rafters and underside of zip sheathing exposed. Air seal well and perhaps add a small dehumidifier to the space.
My roofer strongly recommended ventilating the standing seam roof, which doesn’t seem like a bad idea, especially given my neighbor’s half of the garage will have no vapor protection on the existing walls and conc. floor and moving some air across the underside of the zip could be beneficial.
See attached drawing for the current ventilation plan for my space. I’ll be starting in on the fascia this week and roofing next week, so this is my last chance to determine which path to take. A few questions:
-I’m constrained by what I’ve already built. So while I recognize there are better ways to detail air-intake at the eaves, the best I’m going to get is two (2) 2″ dia. holes per 16″ o.c. joist bay. I know this is not ideal (you want more intake than exhaust venting), but is it a deal-killer, especially given the low slope? My hope is that leaving the entire depth of the rafter bay clear for ventilation would at least help.
-Are there any other issues regarding air-sealing the underside of the joists that I’m not thinking about? This has been my first opportunity as a builder to implement some of the strategies I’ve read on here, but the fact that the space is only infrequently conditioned has complicated my ability to parse out the different issues.
Ultimately, to vent or not to vent is the question. What do you all think? Thanks in advance for you time and thanks to those who weighed in on a prior post regarding this project.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part