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Energy-Smart Details

Insulation Retrofit for an Existing Concrete Slab and 2×4 Walls

Rather than scrapping what's there, enhance and detail it to improve performance

Energy retrofits are always challenging. They are a delicate balance between what to keep, what to eliminate, and what to add. That’s what we were facing on this recent project. The existing structure had 2×4 walls 16 in. o.c. and a turn-down concrete slab foundation. We stripped the building down to the studs by removing the exterior fiber-board sheathing and interior gypsum board. With naked 2×4 walls and an exposed slab, we started the retrofit. 

The first step was to insulate the slab, which was positioned significantly above exterior grade. We spec’d 2-in. polyisocyanurate rigid insulation. Because the existing slab is coupled to the ground temperature, which is most likely cooler than the desired conditioned temperature of the living space, there is a constant thermal drive for the room to attempt to warm the ground below. The polyiso provides resistance to that thermal drive. 

The continuous nature of the polyiso insulation nearly eliminates thermal bridging across the floor assembly. (Note that the level of insulation here is appropriate for a moderate climate. For colder climates, 4 to 5 in. of rigid foam would be advisable.) On top of the 2 in. of rigid insulation, we added two layers of 3/4-in. Advantech floor sheathing. The second layer was laid perpendicular to the first layer with the joints offset by 2 ft., and the two layers were glued and screwed together to form a 1-1/2-in. “floating floor” assembly. (The Advantech allows for any type of finished flooring—hardwood, tile, carpet, etc.)

On the walls, we added new 7/16-in. Zip sheathing to the exterior framing. All the joints were cleaned, taped, and rolled. The Zip is the primary air barrier. Outboard of that we installed 2-in. foil-faced polyisocyanurate rigid insulation to which we applied 1×3 wood furring strips in order to space the siding away from the insulation, thereby creating a drainage and ventilation plane. The 3/4-in.-thick ventilated cavity that the furring strips create not only allows for exceptional drainage but also enhanced drying potential. 

The 2×4 stud cavities were filled with dense-packed cellulose insulation (R-13.3). The combination of cellulose with R-13 rigid insulation yields an exterior wall with an R-value of 26.3. It is worth noting that while the rigid insulation and cellulose insulation have roughly the same R-value, the rigid insulation is a far better enhancement to the assembly because it not only insulates the wall cavities but also the opaque area of the wall frame. The cellulose insulates only at the cavity. (It is assumed that the cellulose occupies about 65% of the wall. The wall framing and plates occupy about 15% of the opaque area of the wall. The benefit of the rigid insulation on the outside is that it occupies 80% of the wall.) The wall was finished with cement board siding installed over the wood furring strips. 

Two areas to pay close attention to are the bottom edge of the rigid insulation and the bottom of the rainscreen cavities. To deal with the exposed edge of rigid insulation, we applied a 2x continuous treated wood block. At the bottom of the rainscreen, we installed a mesh screen capable of providing adequate drainage downward and ventilation upward.

Whenever remodeling a house, we aim to improve its performance. In this case, we were able to provide thermal continuity by insulating a previously uninsulated slab assembly. We also nearly doubled the R-value of the walls. With this detailing we have enhanced the structure’s long-term durability, comfort, and energy efficiency. 

_________________________________________________________________________

Alexandra Baczek is an associate at Steven Baczek Architect. She is a graduate with a Master of Architecture from Roger Williams University. Illustration by the author.

12 Comments

  1. James Howison | | #1

    Any thoughts on the trade off between adding the insulation over the slab vs continuing it down the edge of the slab?

    1. Alexandra_Baczek | | #3

      Continuing it down the edge of the slab doesn't solve for heat loss to the ground.

  2. Charlie Sullivan | | #2

    This looks like a good solution all around and I really appreciate the level of detail provided. I'm curious about two things.

    One question is the 2x4 blocking at the base of the drywall. It seems like that would add some minor thermal bridging at a corner that is a bit of a thermal weak point anyway, and I'm not sure what function it serves. Is it to anchor netting for blowing in cellulose? Or to facilitate detailing the drywall as a secondary air barrier?

    My other question is that I think some people avoid polyiso in direct contact with a slab for fear of moisture wicking into it. Is that more of a concern in a basement and less of a concern with a slab that is at or above grade? Or is it a case-by-case decision based on whether it seems like there have been moisture problems in the past?

    1. Alexandra_Baczek | | #4

      The blocking at the base of the drywall is solely there because the builder wanted it for base board attachment.

      As for the polyiso, it is a case by case, measured against water risk

  3. Expert Member
    Malcolm Taylor | | #5

    Alexandra,

    A question recently came up ab0ut rain-screens in the comment section of your last blog. Do you worry much about venting the top of rain-screen cavities?

  4. Thomas Pippin | | #6

    I just worked out a similar detail yesterday for existing detached garage to be converted to an ADU. Existing curb & 2x4 wall assembly with stucco to remain. I was able to get the floor assembly up enough for floor finish to cover the curb.
    Because the slab slopes to previous garage openings, I've included a self leveling compound as well as vapor barrier over concrete prior to XPS foam insulation.
    Interested in any feedback.

    1. Charlie Sullivan | | #7

      Your detail has a lot more thermal bridging than at the corner than the one in the article, through concrete which is much more conductive than wood. The results will be cold baseboards, increased heating energy consumption, and possible mold problems in those corners. Raising the floor more, similar to what's in the article would be the best solution, but if you can't do that, some exterior insulation would help.

      Also, conventional Foamular XPS has huge climate impact, way beyond the other materials that people worry about when they get into worrying about embodied carbon. If you must use XPS, Owens-Corning now has an "NGX" version with only 10% of the impact of their conventional one and that can be special ordered anywhere in North America. Graphite-loaded EPS or polyiso are even better, but no matter what, don't specify conventional XPS.

      1. Thomas Pippin | | #12

        Hey Charlie, thanks for the input. Just saw this now.... Agree with all you've said. Thanks!

  5. Charles Campbell | | #8

    "(The Advantech allows for any type of finished flooring—hardwood, tile, carpet, etc.)" It's been years since I checked, but the Tile Council of America used to only approve plywood subfloors. Is OSB now approved?

  6. tslagle21 | | #9

    I’m retrofitting an existing slab that looks to be overall in good shape. Should I worry about using a self leveler or is that not a concern?

  7. Charles Campbell | | #10

    "To deal with the exposed edge of rigid insulation, we applied a 2x continuous treated wood block." Why can't the edge be exposed?

    1. Expert Member
      Malcolm Taylor | | #11

      Charles,

      For both mechanical protection against damage, and to stop pests gaining access.

      It also provides backing for the vent at the base, and a level surface to start the exterior insulation and rain-screen furring from.

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