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Community and Q&A

Insulating Plaster-and-Lathe Walls Without Cavities

Framo | Posted in GBA Pro Help on

Hi there. Looking for some feedback and assistance please.

I have an old 1740’s home that has no wall cavities. I am wanting to insulate from the inside by framing out the wall over top of existing plaster/lath  with 2×4. Do not want to go 2×6 to avoid shrinking the room size too much. My question is can I insulate directly on top of plaster and would I need a vapor barrier or possibly need to remove plaster altogether ?(which I would prefer not to) I am wanting to use Mineral wool batts to insulate due to its sound damping qualities etc. I just want to know if I should be concerned about the plaster still being present and if moisture etc could be an issue going this route.

I have removed some clapboard from the outside of the house and directly behind it I believe it is a tar paper?? It is flexible and black . Unsure if this is permeable but it was on the house before we came. Behind the paper are 1×6 ish vertical boards that run from the sill to the attic. Then on top of that the lathes and then plaster. House has a full basement, 1st floor and an attic that is vented and fully insulated with blow in cellulose and stays nice and dry. We keep humidity in basement to 50% but other wise it is a dry basement. We run heat pumps/wood stove in winter so humidity is typically low. Plus we vent out the bathroom and vent while cooking etc to control moisture. We are in zone 6, Connecticut shoreline.

Would  really appreciate any feed back on this.

Thanks Dan


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  1. GBA Editor
    Kiley Jacques | | #1

    I look forward to what Dana Dorsett might have to say, as he has answered a few questions pertaining to insulating plaster-and-lathe walls: Adding insulation to plaster walls – worried about moisture; Insulating an old house with lath and plaster walls from the outside.

  2. 1869farmhouse | | #2

    I’d cover the plaster with 1” polyiso and seal the gaps with great stuff before framing out the interior with the slightest of air gaps. This will give you another r6 and act as a vapor/air barrier. If you’re concerned about the extra 1”, you could use 2x3s and “safe n’ sound” mineral wool for fit. The r value you’re giving up with the 1” mineral wool would be more than made up by the polyiso.

    Plaster and lathe is vapor permeable, as is tar paper.

  3. Expert Member
    Dana Dorsett | | #3

    > "We are in zone 6, Connecticut shoreline."

    The CT shoreline is the warm edge of IECC zone 5, soon to be the cool edge of IECC zone 4 as climate change marches on.

    I need a better picture of the wall assembly & stackup. As described I'm only coming up with :

    outdoor air | clapboards | tarpaper | 1" vertical planks | lath | plaster | paint (any wallpaper) | indoor air.

    That doesn't seem very structural- what's holding it up in a high wind? I suspect there is some post & beam type structural elements(?) with the vertical planking as in-fill between the beams?

    How deep are the roof overhangs, and how tall is this house?

  4. Framo | | #4

    Hey guys. Appreciate your feedback.

    Austin - Thanks for your input. Certainly like the idea of what you are proposing. I will also let Dana weigh as well.

    Dana - Thanks for as well for your input. Here is some more info - Essentially we have 1400 square foot of living space. Half the house is 2015 modern construction with vaulted ceiling that is tied into the older 1700's half of the house. The 1700's half of the house i believe is balloon frame construction. This section of the house is split into 3 rooms of almost even size. As mentioned it sits upon a full basement which has an insulated ceiling. Unsure if foundation on new or old part of house are insulated or not externally. The ground sills are about 6x6 that the house sits upon and the same size beams run vertically on each corner of the house and within the walls at certain points but not sure of this spacing. The second floor or living space is sitting on another set of these beams. Then in the attic the same is happening but with many more cross beams to support ceiling. On the outside of this 6x6 framing they have 2x6 running vertically from basement sill to the top plate in the attic. Yes, the structure is tied together post and beam style. The there is Tar paper on top of 2x6 and then clapboard. Internally the plaster in one room is covered with wall paper and in the other rooms the plaster is painted. Within the last year or so double glazed windows have also been installed.

    Basement is very water tight but not air tight. We dehumidify to 50% We also vent out bathroom and while cooking to control additional humidity . We have recently spent a bit of time air sealing around doors, windows, attic hatch door (this is insulated) and so forth.

    The overhangs for roof only exist on one long side of the house approx. 35 feet and is about 6 inch of overhang . Other walls do not have over hang at least on the old part of the house - only gutters or flashing. This is all on the OLD part of the house. Newer part has modern construction with soffits running full length of overhang etc.

    Highest point of roof in 1700's side of house is approx. from basement to highest point 30 feet

    Attic has no floor and has R50 blow in cellulose. Newer part of the house which is connected to old and is open to each other in attic has around at least R49 as well - Pink batts

    Ultimately, we want to save energy, enjoy out warmth longer and avoid any potential moisture issues with insulating from the inside by adding a cavity wall.

    Thanks for your time and feedback everyone.



    1. 1869farmhouse | | #5

      Hi Dan,

      I’m definitely not going to have the depth that Dana will in my response, but I was under the impression that your wall assembly was only a couple inches deep with the boards arranged more like:

      ————————— instead of the more typical:

      | | | | | | | | | |.

      Balloon framing is typical the latter making the wall assembly at least 4-6” in depth with a continuous channel inside the wall from crawlspace to attic. I’ve seen the former, and Dana is right that it’s typically the older style post and beam construction.

      It’s also possible it’s more of a plank/frame construction style like this?:

  5. Framo | | #6

    Thanks Austin. I certainly don't need too much depth just some assurance about my approach. Yes, I agree construction seems to be more like plank/frame. Clapboard, Tar paper, vertical 2x6, lathe and then plaster. All on the outside of 6x6 beams. So yes, walls are pretty thin. I am going to go with your idea of the foam board, mineral wood and then new sheet rock.

    Thanks for your help on this.



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