Building a sustainable home aspiring to “net zero” performance, a home that produces as much energy as it consumes each year, begins with a meticulous plan for tightening and fortifying the exterior shell. To increase comfort and reduce energy load, the building’s envelope must be sealed via a comprehensive air-sealing and insulation strategy that encompasses not only walls, windows, and roof, but also the home’s foundation. Up to 20% of heat loss in a typical cold-climate home can be attributed to the basement walls and floor.

How Much Slab Insulation?

A poured concrete slab has low thermal performance with R-values ranging from 0.1 to 0.2 per inch. In addition to poor resistance to conductive heat flow, warm conditioned air can escape through the slab during heating months, predominantly at the peripheries where the floor meets the foundation walls. These combined energy losses create chilly basements and add to the energy demands of a home. Furthermore, temperature differentials between the slab and ambient air can cause condensation that could lead to mold problems.

Yet, despite these performance issues, many jurisdictions do not require slab insulation per code. And while there appears to be consensus among high-performance builders that slab insulation is necessary, at least in zones 4-8, the specifications vary greatly. According to builder, author, and editor Martin Holladay, the amount of slab insulation depends on four factors: (1) climate zone, (2) depth of slab at grade, (3) function of the space, and (4) whether the slab has hydronic heat. Based on these factors, Holladay recommends slab insulation values from zero to R-25.

The U.S. Department of Energy Zero Energy Ready Home (ZERH) program Version 2 requires the IECC 2021 minimum R-10 to R-15 continuous insulation at least 4 feet around the periphery of basement slabs in Zones 4-8. ZERH slab edge requirements follow Energy Star 3.2 , a minimum R-5 vertical insulation for all areas where the slab is 12 inches or less below grade. To achieve passive house standards, the slab would need to be insulated to at least R-30.

Queen of Zero Specs

At the Queen of Zero ZERH project, JLC’s 2024 Case Study, we chose to exceed the slab insulation requirements per code and net zero certification. The local weather of this Zone 4 property can vary from frigid cold winters to sticky hot summers. The property slopes from front to back so that the rear foundation is above grade. The basement will also be a high traffic area, as the homeowner intends to use half of the 1,200sf footprint as his music band practice space, which means that in addition to providing human comfort, the space needs to maintain optimal humidity levels for the protection of the owners’ instruments.

Beneath the entire slab, we installed 2 inches of R-10 Foamular NGX rigid XPS insulation, a certified Greenguard Gold and LEED-compliant product, over 4 inches of a crushed-gravel capillary break. The gravel had to be compacted and smoothed precisely so that the Foamular sheets lay completely flat. We also installed R-10 rigid XPS at slab edges and on the interior of all foundation walls. Seams of the insulation were sealed with vapor-impermeable Stego Tape. The total upgrade cost of our slab insulation was about $5,500 in labor and materials.

For additional protection from moisture and radon gas, we used a 6-mil polyethylene sheeting, lapped 6 to 12 inches, as per the ZERH specifications. The concrete slab was poured 4 inches thick, reinforced with welded-wire fabric (WWF).

Slab Insulation Options

JLC contributors have documented several variations of slab insulation for high-performance building. For example, contractor Wade Paquin advocates for closed cell spray foam as an easier and more stable alternative to rigid foam under the slab. Custom builder Jim Woffler and architect Steve Baczek have perfected an above-slab application with a continuous layer of 4-inch EPS rigid insulation sandwiched between the structural slab and finished floor. Baczek and builder Steve Demetrick explain that for optimal performance of a tight home, the R-value of the foundation should be proportional to the insulation values for the walls and roof. They also make a cogent argument that the concrete slab can be eliminated in lieu of a properly-detailed floating OSB floor.

Across the literature, experts emphasize the importance of continuous coverage—in particular, where horizontal and vertical insulating materials meet along the periphery. Similar to all aspects of air sealing and insulation in sustainable home-building, the ultimate performance depends upon a flawless installation without gaps or cracks from the ground up. As I walk the site inspecting the foundation, I look for any voids and silently repeat the mantra for high-performance builders: “devil's in the details, devil's in the details, devil's in the details, my friend.”

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