Short answer
Concrete is a poor insulator. A solid poured concrete wall has an R-value well under R-1 for typical thicknesses, and a figure of roughly R-0.1 or less per inch is commonly cited. Hollow concrete block (CMU) walls come in around R-1 to R-2 depending on thickness and fill. To get a comfortable, code-compliant wall you generally add insulation, or build with a system that includes it, such as insulated concrete forms (ICF).
Exact values depend on density, thickness, moisture content and the source you consult, so treat the numbers below as approximate.
R-value of common concrete walls
| Wall type | Approximate R-value |
|---|---|
| Poured concrete, 8 inches | Under R-1 |
| Concrete block, 8 inch hollow | Roughly R-1 |
| Concrete block, 12 inch hollow | Roughly R-1 to R-1.5 |
| ICF wall (concrete between foam layers) | Often advertised around R-20 or higher, depending on the product |
| Concrete wall plus a few inches of rigid foam | Adds the foam’s R-value to the wall |
Manufacturer figures, especially for ICF, are sometimes quoted for the foam only or as a “whole-wall” value, so read the fine print. A whole-wall value accounts for thermal bridging and other details that a single-material number does not.
Why concrete insulates poorly but still stores heat
R-value measures resistance to heat flow. Concrete conducts heat easily, so it scores low. What concrete does well is thermal mass: it absorbs heat and releases it slowly. That can help smooth out temperature swings in some designs, but it does not replace insulation, especially in cold or hot climates where heat flows through the wall all day.
Also note that R-value alone does not describe air leakage. A wall with a good R-value can still lose a lot of energy through gaps, so air sealing matters as well.
Should you insulate concrete walls?
Often, yes, particularly for conditioned living space:
- Basements and foundation walls that are part of a finished or heated area are commonly insulated. Requirements vary with your climate zone and local code.
- Garages and unheated storage usually do not need it.
- Above-grade concrete or block walls of a heated building lose heat quickly when uninsulated.
Insulating also reduces the chance of cold, clammy walls where warm indoor air can condense. That condensation is a moisture risk you want to avoid, which leads to the next point.
Fix moisture first
Do not cover a wet wall. Before adding any insulation:
- Look for active leaks, damp patches, efflorescence (white mineral deposits) or mold on the concrete; see water leaking through a foundation or basement.
- Fix grading, gutters and downspouts that send water toward the foundation.
- Have cracks assessed. Significant, widening or horizontal foundation cracks can indicate a structural problem, so call a structural engineer or a qualified foundation professional rather than covering them up.
Insulation placed over a persistently wet wall can hide the damage and hold moisture against the framing and finishes.
Ways to insulate a concrete wall
Rigid foam board against the wall. Common materials are expanded polystyrene (EPS, roughly R-4 per inch), extruded polystyrene (XPS, roughly R-5 per inch) and polyisocyanurate (often higher per inch, but performance can drop in cold conditions). Seal the seams, and cover foam on the interior side with an approved finish such as drywall, because exposed foam plastics are a fire concern. Building codes require a protective covering, so check your local requirements.
Framed wall with mineral wool batts. Build a stud wall slightly in front of the concrete and fill it with mineral wool, which is not combustible and tolerates some moisture better than paper-faced materials. If you are choosing batts for the stud bays, our guide to faced vs unfaced insulation explains the vapor retarder question. Many installers keep a small gap and use pressure-treated lumber for the bottom plate that touches the slab. Some building guidance prefers foam against the concrete first, then framing, so the batts do not touch a cold, possibly damp surface. Follow your local code.
Exterior insulation. Foam or mineral wool boards on the outside of the foundation keep the concrete warm and protect it. This is often done during new construction or when the foundation is already excavated.
ICF construction. For new buildings, ICF blocks cast the concrete between foam layers. This is a design decision for a builder, not a DIY retrofit.
How much R-value do you need? Does more help?
The recommended R-value for a wall depends on your climate zone, the wall’s location and your local energy code. Colder zones call for more. Your building department, local energy office or the energy code itself will list the number for your area. Rather than guess, look it up.
More insulation does help up to a point, but returns diminish. The first few inches stop most of the loss, and each additional inch saves less. Air sealing and fixing weak spots (rim joists, windows, gaps) often matter as much as adding more thickness. Also remember the whole-wall effect: studs, fasteners and gaps reduce performance below the label on the batt.
How thick should mineral wool be?
Mineral wool batts are sold by thickness for standard framing. A batt sized for a 2x4 wall is about 3.5 inches thick and a 2x6-size batt about 5.5 inches, with R-values typically in the neighborhood of R-15 and R-23 respectively, though products vary. Check the package. If you are building a wall in front of concrete, the cavity depth you frame sets the thickness. Fill the cavity without compressing the batt.
Mistakes to avoid
- Insulating over moisture problems.
- Leaving exposed foam with no code-approved covering.
- Using paper-faced batts directly against damp masonry without checking local guidance.
- Assuming the label equals the wall’s performance. Gaps, compression and air leaks lower it.
- Drilling or fastening into concrete without checking for rebar, utilities or conduit.
When to call a professional
Call a licensed professional if you find significant or growing cracks, bowing walls, active water entry, or signs of mold. An insulation or energy auditor can estimate the right R-value for your home. Electrical work needed during a basement finish should be done or checked by a licensed electrician. If you suspect that old materials in the area contain asbestos, such as old insulation, tiles or coatings, do not disturb them and have them tested first.
FAQ
What is the recommended R-value for exterior wall insulation? It varies by climate zone and local code. Look up your local energy code or ask your building department.
Does adding more insulation help? Yes, but with diminishing returns. Air sealing and moisture control are also important.
How thick should ROCKWOOL insulation be? It depends on the framing depth. Common batts are about 3.5 inches for 2x4 walls and about 5.5 inches for 2x6 walls.
Should I insulate concrete walls? In heated spaces, usually yes, once moisture issues are fixed. Check what your local code requires.