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My house was built in 1992. There are currently faced batts of fiberglass and a few inches of white blown-in insulation that is both scattered in some areas (mostly over the master bedroom from ceiling fan insulation and replacing the bathroom exhaust fan).

In my ideal world, I want to put down plywood for walking paths to areas where I need to do maintenance - bathroom exhaust fans and above ceiling electrical. Even better would be using some of the space for storage of holiday decorations and other low use stuff.

I am just outside Chicago, so nothing that could freeze would be in the space.

  1. Can I install plywood (probably 3/4 to be safe - I weigh around 250) over the existing insulation?

  2. I think (will need to verify) that this might compress the backed fiberglass. Will that cause any issues?

  3. I want to add more insulation when this is all done. I assume (possibly incorrectly!) that I can add insulation on top of the plywood I install. I have not decided if I want to use more faced batts or blown in, I am leaning toward batts as the blown in seems to VERY messy.

  4. I found a reference to loft flooring legs, but I don't see them available in the US, only the UK. Attic Dek flooring looks to be more expensive than what I want, but it also looks like it would not interfere with R value of insulation. I would just have to be VERY sure where it is when I use it for walking.

    I found 'Attic Decking Kit for a Storage Floor Above Deep Attic Insulation (8ft X 8ft Kit)', which looks like loft flooring legs, but is $200 for an 8X8 setup. Does anyone have experience with this? For storage it would be lighter boxes, under 50 pounds each for sure.

  5. Is there a better way to do this? The access routes would only be used on occasion, but the storage area would be used most of the year.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Michael Karas
    Dec 1 '20 at 5:10
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    What I did is attach 2x4s (or were they 2x6s?) edge-up at right angles to the bottom chords of the trusses, then put plywood over that.
    – Hot Licks
    Dec 3 '20 at 23:14
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Lay rigid insulation boards on the joists and top them with plywood.

The compressive strength of the insulation boards will be plenty for storing whatever you can carry up there. The load will be spread across the joists. It's unlikely that your ceiling will collapse under the weight of a few boxes but if you're planning to put anything heavy up there, you do need to consider the construction of the building in more detail.

Rigid insulation boards will insulate about four times better than the same thickness of fiberglass insulation which means you can use 2 or 3" thick boards. Lookup R-values if you're interested.

  1. Remove enough of the existing insulation to expose the top of the ceiling joists.
  2. Put down rigid foam boards on top of the joists (I'm familiar with Kingspan and Celotex in the UK).
  3. Tape the boards with foil tape to keep them together and stop draughts.
  4. Top the insulation boards with plywood or chipboard.
  5. Screw the plywood sheets together with short strips of wood to avoid them slipping apart from each other and leaving gaps or find some way of pinning them in place around the edges. (I didn't do this at home and they've shifted a bit).
  6. Use the fiberglass insulation that you removed to top up the insulation in the rest of the attic (where you didn't lay the rigid boards).

I've done this in our house in the central part of our attic. The problem we had was getting the boards (insulation and chipboard) through the small hatch but yours may be different. For the chipboard, I bought narrow tongue-and-groove "loft boards" designed for this purpose but for the insulation I had to slice it in half on one side and snap it back on itself to be able to get it into place; then I taped it back together.

I haven't yet needed to get to any of the electrics underneath. Some of the wires were rerouted around the edge and the bathroom fan is away under the eaves with hardly any headroom so still accessible under the mineral wool insulation.

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  • You can just screw the plywood to the joists right through the foam board insulation. A bit of a gap between the plywood isn't going to matter. And for a walking path as OP mentioned, you only need around a 3 foot wide area to provide a comfortable catwalk.
    – SteveSh
    Nov 30 '20 at 22:04
  • @SteveSh 3 foot wide area is huge; 1 foot (or 16 inches for spanning joist to joist) is plenty wide for the once-a-year use this will likely see, and much easier to fit in the cutout for the pull-down stairs. If it isn't going to be used for storing stuff like Christmas ornaments or whatever, but just walking seldomly for installation projects, some 2x10s are probably better and easier.
    – TylerH
    Nov 30 '20 at 22:15
  • @TyperH - Yeah, you're right, 3' is kind of wide. I think I made my walkway 24" wide. Need to get back up there and measure to be sure.
    – SteveSh
    Nov 30 '20 at 23:02
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Here is what I am going to do -

  1. Add insulation in the attic. Most likely remove the current mess and add new R-49 faced fiberglass.
  2. Make a few upside down Y saddles with 2X6s to support some plywood for storage flooring.
  3. Cut a board or two to leave in the attic as portable working surfaces so I have a place to either sit or kneel as needed.

I appreciate all of the assistance, and sorry for the discussion aspect of it. I would have used the foam board but the highest R value I can find is R10.

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