What R-value does a poured concrete wall have?
Poured concrete has approximately R-0.08 per inch. A standard 8-inch poured concrete basement wall provides only R-0.64 total — far below code minimums in most zones. Concrete requires added insulation to meet any IECC climate zone minimum.
What is ICF and what R-value does it provide?
Insulated Concrete Forms (ICF) are stay-in-place foam blocks filled with reinforced concrete. The foam stays on both sides of the wall permanently. Standard ICF walls from brands like Fox Blocks, Nudura, and BuildBlock provide R-22 to R-24. Premium systems like the Nudura XR35 reach R-35 to R-48.
What is dimple board and does it add R-value?
Dimple board (also called dimple mat or drainage membrane) is a studded HDPE plastic sheet installed against foundation walls. It creates a drainage plane and capillary break — keeping water away from insulation. Its thermal value is minimal (R-0.1 to R-0.3) but it protects your insulation's R-value from moisture degradation.
Can you insulate a river rock or stone foundation?
Yes, but it requires interior insulation only — stone and rubble walls cannot have exterior foam applied without excavation. Closed-cell spray foam applied directly to the interior face of a stone wall (2–3 inches) provides both air sealing and R-13 to R-20. It also stabilizes loose mortar.
Should basement insulation go on the inside or outside?
Both work, and both are code-compliant. Exterior insulation (XPS rigid foam outside the foundation) keeps the concrete wall warm, reducing condensation risk. Interior insulation (spray foam or rigid foam inside) is cheaper and easier but must be protected from fire with drywall. Interior is more common for retrofits.
What is the best insulation for a CMU block wall?
Closed-cell spray foam applied directly to the block surface is the most effective — it air-seals between blocks and provides R-6.5 per inch. Alternatively, 2 inches of XPS rigid foam board on the interior achieves R-10, which meets code in Zones 3 and 4. Core-filling the blocks with perlite or EPS adds R-8 to R-10 but doesn't address air leakage.
Does ICF meet building code without additional insulation?
Yes — standard ICF at R-22 to R-24 meets or exceeds basement wall code minimums in all 8 IECC climate zones (maximum required is R-15 for Zones 5–8). ICF also provides a continuous insulation layer with no thermal bridging, unlike stud-framed assemblies.
What R-value do I need for my basement walls?
Zones 1–2: R-0 (no requirement). Zone 3: R-5. Zone 4: R-10. Zones 5–8: R-15. Use the calculator above with "Basement" selected to check your complete assembly — including concrete wall, insulation, and drywall — against the IECC minimum for your climate zone.