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 - 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 Ready Energy 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.
