GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Is it OK to use spray foam on the back of a plaster wall and then fill the stud bays with fiberglass?

uQxwtmnHZu | Posted in Energy Efficiency and Durability on

First, a little background: I am a plumbing, heating, and cooling contractor in Wilmington, DE. I own a circa 1874 balloon-framed house.

I am in the planning stages of my renovations. The wood lath plaster walls are in great shape, so I do not want to tear them out. As I’m sure you all have deduced, my house has no insulation except in the attic.

Because of the balloon framing, the walls have no sill plate, not top plate and no fire blocks. You can shine a light from the basement and see all the way to the attic, 20+’ up. I measured the R value of my walls to be about R-4, but the drafting in them negates that I think.

The siding on the house was originally 5/4 German lap siding that was covered over in the forties with tar paper and cedar shakes. There is no sheathing, and best I can tell no metal strapping to support the frame. The wiring is going to be replaced, so there will be no worries about knob and tube.

OK, my thought is to do a modified flash-and-batt insulation, since filling the 6.5” deep stud bays on approximately 5,000 sq. ft. of wall space is going to be too cost prohibitive.

Here is my plan: remove the siding, add blocking in the walls at the sill, top plate and at least between floors, add blocking to floor joists, and add a 1” layer of spray foam to the back of the plaster, the plates and the blocking. After that I would fill the voids with R-19 fiberglass batts, add sheathing, Tyvek, 1” foam board and fiber cement siding. Obviously every thing would taped and sealed along the way.

I realize that because of the construction of my house I would need to work in sections while doing this at least until the sheathing is on so I maintain structural integrity.

I’m not sure this would work and I’m worried about moisture in the walls. I think it will work since the vapor will be stopped at the wall surface but I’ve never seen this done so I’m unsure. Also I have been stripping the walls down to bare plaster and painting with vapor barrier primer as I work on each room.

I have talked to an insulation contractor I know he was unsure if it would work but he doesn’t really know spray foam insulation they are just getting started in it. What do you think? Will this work or am I going to have moisture and rot issues if I do this?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You are proposing to build what I call a foam sandwich: rigid foam on the exterior, then fiberglass batts, then closed-cell spray foam on the interior side.

    I usually advise people to avoid building a foam sandwich if possible. If you want to use foam -- whether you choose closed-cell spray foam or one of the many varieties of rigid foam -- it's best to put it on only one side of your wall, not both. That way the wall assembly can always dry in at least on direction in case it ever gets wet. You want the wall assembly to be able to dry to the exterior or to the interior if possible.

    If you decide to put your foam on the exterior of your wall, then you should skip the spray foam. You have to be sure that the exterior rigid foam is thick enough for your climate, since thin foam is dangerous. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    If you decide to install spray foam against your plaster, then you shouldn't install any exterior rigid foam. If you really want more insulation on the outside of your wall, you could consider installing a layer of mineral wool on the exterior side of your wall. For more information on this option, see Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |