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Why is attic joist covered in OSB?

miracj | Posted in General Discussion on December 31, 2021 11:23am

I have a hip roof split level home built in 1986. I am currently upgrading my insulation in the attic.

Along 1 side of the house running with the joists, the last joist is covered with OSB the whole way, as shown in the picture to the left of the corner of the attic. On that side is a brick fireplace. I don’t think there is any insulation under that OSB.

So my question is why the OSB was there, and is it any risk cutting it out to put in insulation?

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Replies

  1. calvin | Dec 31, 2021 02:19pm | #1

    From the looks on this small screen, there’s a strip along the other wall about the size of the top plate and it appears the osb you mention is split on the joist (allowing you to further sheet the attic).

    Why? beats me

    Can you remove? I’d first drill a hole and fish around, they might have already insulated under there.

    When was this built?

    1. miracj | Dec 31, 2021 02:31pm | #2

      As I said, the house was built in 1986 and purchased new. I already know that it has little insulation, as I had to cut some holes in it to run network cable in the outside walls. The board is nailed down and a 2x4 is nailed above it along the walls. I can cut it with a jig saw or similar.

      My only thought was maybe the builders used it as a place to build the chimney from and to hold some of the materials and tools.

      But I guess the real gist of my question is whether this can be structural in any way?

  2. harryv | Dec 31, 2021 03:43pm | #3

    It’s possible it is some type of shear panel/bracing. Suggest drilling a couple of inspection holes.
    If you can see nails on both attachment sides, it’s probably safe to pull them, insulate and reinstall. If the nailing is a specific spacing, I would guess it’s a shear/bracing panel.

  3. User avater
    2sheps | Dec 31, 2021 09:12pm | #4

    It looks like because the rafters sit on top of the joists rather than on the top plate, this might have been done to stiffen the last joist, to keep it straight.
    If you are using blown in insulation, or even if not, how about making some holes with a holesaw and blowing it full?

    1. miracj | Jan 01, 2022 11:30am | #5

      Thanks for your ideas.

      I'm using Rockwool (Roxul) batts (R30) followed by another batt crossing the joists. I guess I could blow insulation in and had planned to put batts on top of it.

      Also, this OSB piece is only on this side of the house and not on any others. While I haven't inspected the joist for insulation yet, many years ago I discovered that the builder "forgot" to put insulation on the top floor's overhang, which I insulated. Made the rooms instantly warmer!

  4. User avater
    unclemike42 | Jan 01, 2022 01:07pm | #6

    I wonder about the sequence of floor joists topped by OSB then the plate on top, that supports rafter cutouts.

    I wonder if the OSB is on the last joist cavity because without it, that plate (2x4) would only be supported on one side by the last joist.

    If so, I see a couple options.

    1. remove the OSB, (cut at the inside edge of that plate)and install blocking to the next joist, to support the plate.

    2 cut enough of the OSB out to let you slip rockwool into the rest of the cavity, and install some blocking at the access point

    I would want to see what is under this, and the detail of any gaps to the wall below.

  5. User avater
    tfarwell | Jan 01, 2022 09:48pm | #7

    Do you have a soffit below? It could be to block insulation (blow in) from getting down there. Or is it an air return? Good luck.

    1. miracj | Jan 02, 2022 02:18am | #9

      No there is no soffit below the ceiling, nor is thearea in the attic used for a vent or return.

  6. User avater
    2sheps | Jan 01, 2022 11:06pm | #8

    I'm guessing the framing is something like this.

    1. miracj | Jan 02, 2022 02:19am | #10

      Yes, that is exactly right.

  7. bing0328 | Jan 04, 2022 12:02pm | #11

    No one has mentioned it but because the osb is covering the entire area between joists that area is dead air space and does have some insulative value. If you need more just put some fiberglass batts on top.

    1. miracj | Jan 09, 2022 01:07am | #12

      I tried putting batts on top and around that joist this evening with 14 degrees outside. There was no effect based upon looking at my ceiling with my FLIR One IR Camera.

      But I finally got to the area of the attic that had my hole cutout (as seen in my post further down) and took pictures. One side that went to a supporting beam about 2 ft away had no insulation. The other way going about 12 feet had nothing either.

  8. miracj | Jan 09, 2022 01:20am | #13

    I finally got to the part of the attic that had the hole and took pictures as attached. One way goes 2 ft before it runs into a supporting beam. The other way goes about 14 feet. In both cases, there was no insulation. I suspect the 12 feet beyond the support beam is no different. (IMHO, the builder should be shot)

    I tried putting batts over the problem area, and based upon my images with my FLIR One IR camera, with 14 degrees F outside, there was no effect.

    So Since I have a lot of loose fill "pink" insulation, I was thinking of blowing in that loose fill both ways.

    I was wondering if I could blow that in using a shop vac that has an exhaust to blow it down each way by putting pieces in front of the exhaust? Or do I need to rent a professional machine? Suggestions welcome.

    1. User avater
      unclemike42 | Jan 09, 2022 07:25am | #14

      If you don't want to rent a machine, you could use a 3 or 4 inch hole saw to make additional access holes a foot or so apart and hand-stuff the loose fill you have on hand. You have to be careful with this cavity, to avoid over-stuffing to the extent you push down on the ceiling below and make a crack or worse.

      From a machine manual: https://images.homedepot-static.com/catalog/pdfImages/c2/c23715f9-3e18-47b5-a71e-a16b3616007d.pdf
      If you are inexperienced with blowing sidewalls you may
      loosen or blow off sheetrock

      You could glue and screw an OSB cover over the new holes, but I would not worry about leaving them open.

      I do not think using a shop vac or leaf blower will do what you want it to

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