My 1955 home has a flat roof with 1x8 sheathing. It's time to replace the roof, and I am considering replacing blown-in attic insulation with batts, at the same time.

The attic is only 20" between living space ceiling drywall and roof sheathing. I'm trying to figure out two things:

1) If I should have the insulation replaced from below by creating attic access points in the drywall, or have it replaced from above by removing/replacing the 1x8 sheathing.

2) If there is any reason to replace the 1x8 boards with 4x8 osb/plywood sheets.

1 Answer 1


If it were my house, I would leave the blown in insulation in there and remove enough sheathing to blow in more, say maybe 16" total or at least dd enough to get it to code.

I would think 1 or 2 boards about every 6-8' apart will allow good coverage and depending on the roofing going back down, just add a layer of plywood over the existing sheathing since it is a strong base there to build on.

If the space is a continuous 20" deep over the whole attic, only place a maximum of 18" for the hot air to travel over the insulation to the vents

  • My reasoning for blown-in -> batt is because of attic work I'd like done. Remove/replace knob & tube, install ceiling fan bracing, convert to hard-wired smoke detectors, add a skylight, etc. Granted one could argue that once the work is done, its done, but it just seems like it might never be done and batts are so much cleaner when you need to do attic work. 20" space includes 2x6 ceiling joists 16OC + 2x6 roof rafters 32OC, so about 8-10" of air space between support boards. Current blown-in is about 3" deep.
    – dabi
    Commented Aug 15, 2019 at 4:24
  • If you have knob and tube wiring, your house is a lot older than 1955, knob and tube was used widely between 1890 and 1940. If you are gong to get into that rework, then pull the ceilings down to get the wring in place properly, along with potentially other things like HVAC and plumbing vents. Boy it will be messy, but it will beat trying to pull blown insulation out of small access holes, which will be messy itself. besides difficult to navigate the insulation to the holes.
    – Jack
    Commented Aug 15, 2019 at 5:10

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