How can I improve the insulation and air sealing of my walk-in attic?
I recently decided to tackle the air sealing and insulating of my attic. The house was built in the 40s and I’m in climate zone 5 (NJ). It is a walk-in attic with a combination of vermiculite and fiberglass batt insulation on the floor and what looks like mineral wool covered (mostly) with plywood on the wall separating the finished portion of the attic with the unconditioned. In addition, there’s a “knee wall” (I may be using the wrong term here) along a portion of the attic which divides the unconditioned side from the cathedral ceiling in the living room. I’ve done a lot of research on the best approach for all of this but I could still use help in some areas.
I’m going to use blown-in cellulose on the attic floor after performing air sealing. Ideally I’ll be adding 2×10’s on top of the original joists to allow for additional depth, attempting to get to around R-48, while retaining the walk-in functionality. Orienting these new joists perpendicular to the existing 2×6’s would reduce thermal bridging but could create structural problems (existing max span is 12’ at 16” OC). As an alternative, I may do the cellulose to fill the current joist bays this year (which will still be a huge improvement) and sort out the raised joists in the spring. Thoughts?
The “knee wall” currently has the standard fiberglass batts which are falling off. I plan to install new batts in the bays which are 3.5” deep and then cover the whole thing with rigid polyiso. Does this sound reasonable? Any moisture concerns using faced batt/board insulation? My plan would be the same for the entryway wall as well. Would the R-20 wood frame wall requirement be an appropriate target for both of these cases?
Also, I do have soffit vents and a ridge vent, but most of them would not appear to have a path to the attic due to various obstructions (chimney, skylights). I’m going to use Accuvent baffles to keep the functioning ones clear. I suppose it doesn’t hurt, but how much ventilation is “worth it” given that most of the pathways do not appear to be open? I’ve read the suggestion that a ventilated attic should be REALLY vented.
Or should I give up on all of this, pull the current insulation and just insulate the attic roof? There isn’t any ductwork in there to consider.
Edit: I was just realizing that converting to a conditioned attic would be challenging, because the house and garage attics are currently joined, which I guess would mean creating an insulated wall between the two.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part