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

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.

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