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Community and Q&A

Insulating a wall that is half plaster/half drywall?

Al Ciu | Posted in General Questions on

We live in a home that was built around 1920 near Chicago, IL. Our plot is narrow and the front portion of the home is the original house, with a very large addition on the back from the previous owners (2007). The staircase in the old house was sloping heavily to the right. We had a structural engineer come take a look and he attributed it to natural settling and didn’t see any signs of continuing movement and didn’t make any recommendations aside from sistering one joist. I’m not a master carpenter, but I have built a staircase or two in my life. We have another staircase to access the upstairs so I decided to take on the project of tearing out and rebuilding the stairs. After removing the stair case and landing, I’ve discovered a number of problems. The previous homeowners did some rehabbing on the old part of the home, but all the work they did was pretty shoddy. Above the stair stringers, they removed the plaster from the lath and installed sheetrock. Belong the stair stringers, which is the headroom for the stairs to the unfinished basement, the plaster was left alone and paneled over (although I think the panels were there prior to the previous residents). I removed the panels to find the plaster in very poor condition. It came off in large chunks and was unsalvageable. I removed the rest of it and I also found that some of the lath was dry rotted or split. I removed some of those sections and found something VERY strange, in my opinion. I found wallpaper of different patterns attached to the studs, with the patterns facing into the cavity. The whole wall seems to be wallpapered this way beneath the lath from what I can see. Is this some kind of vintage attempt at a vapor barrier? After removing the crumbling paper, I found what was left of some cellulose insulation that has all accumulated at the bottom. However, I’m not interested in removing the sheetrock that’s already been installed over the existing lath. I also see that the house still has it’s original cladding, but the previous homeowners put hardieboard cement siding on top of it. I’m not sure how this affects the needs for vapor/moisture barriers. The wall that would need to be insulated is huge, roughly two and a half stories.

So, in case you couldn’t follow all those questions:

– What’s the best way to go about insulating a wall that is part bare, part lath and drywall?
– Why was wallpaper used where a modern vapor barrier would be?
– Does the fact that the previous homeowners put cement board siding over the original wood siding make a difference in how to insulate?

Thanks for any suggestions for this mess I’ve gotten myself into.

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  1. Al Ciu | | #1

    I only have this photo for the time being:

  2. GBA Editor
    Martin Holladay | | #2

    When the original builders installed the lath, I'm guessing that they installed the wallpaper (which was probably stained or degraded wallpaper available for free) either as an air barrier or to limit the amount of plaster that squeezed through the lath gaps.

    Your stud bays can be insulated with dense-packed cellulose. Insulation contractors who are familiar with cellulose do it all the time. For more information on this type of retrofit insulation work, see How to Install Cellulose Insulation.

  3. Al Ciu | | #3

    Hi Martin,

    Thanks for your response. That's interesting about the wallpaper, thanks for solving that one for me.

    I'm not going to have the opportunity to have someone install cellulose until probably the spring. After looking again, I think I'm better off removing all the lath I can. It's not in good condition, most of the nails are popped, and it's not level or plumb, so installing drywall on top will be a challenge. I'm wondering if I could nail in some 2x4 pieces horizontally at the points where the lath underneath the drywall will meet the area that I'd like to open up. Once I do that, could I install some fiberglass batts in those cavities and then drywall over (probably sistering the studs and pushing those 2x4s out slightly to accommodate for the additional lath width at the top)? This would at least allow me to insulate around that exterior basement door and part of the window. Or would this be a bad idea if I'm planning cellulose later? Thanks again for your help.

  4. GBA Editor
    Martin Holladay | | #4

    Your plan to insulate the stud bays that are accessible from the interior will work fine. As you suggested, you'll need some horizontal blocking between the studs to separate the areas that will be insulated with fiberglass batts from the areas that will be insulated with cellulose.

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