R-value is the industry shorthand for how well a material resists heat flow: the higher the R-value, the better the insulating performance. Measured in units of thermal resistance per inch (often reported as R-value per inch and as total R-value for a given thickness), it’s the principal metric used to compare insulation materials and to determine how much insulation a building assembly needs to meet energy codes and comfort goals. Understanding R-values helps homeowners, builders, and designers choose the right product for walls, attics, floors and foundations so heating and cooling systems work less, energy bills fall, and indoor comfort improves.
Common insulation materials span a wide range of R-values, construction types and costs. Typical R-values per inch are roughly: fiberglass batts (about R-2.2 to R-3.8), cellulose loose-fill (around R-3.2 to R-3.8), mineral wool (R-3.0 to R-3.3), open-cell spray polyurethane foam (R-3.5 to R-3.7), closed-cell spray foam (R-6 to R-7), and rigid foam boards such as polyisocyanurate (R-6 to R-6.5), extruded polystyrene XPS (about R-5), and expanded polystyrene EPS (about R-3.6–R-4.0). Natural fiber insulations—like sheep’s wool and cotton—tend to fall in the R-3 per inch range. Reflective radiant barriers aren’t rated the same way; their benefit depends on orientation and air spaces rather than a simple per-inch R-value.
R-values listed in product literature are laboratory measurements and can be affected in the field by many factors: how the insulation is installed (gaps, compression, or voids reduce performance), thermal bridging through studs or framing members, settling in loose-fill materials, moisture intrusion, and natural aging. The useful comparison is often the total assembled R-value of a wall, roof or floor system, not just the R-per-inch of an isolated material. Climate matters too: colder climates require higher R-values in building envelopes, while milder regions can meet efficiency goals with lower ratings but may prioritize air sealing and moisture control instead.
This article will walk through typical R-value ranges for the most common insulations, where each material performs best, trade-offs in cost and environmental impact, and practical tips for accurate R-value calculations and proper installation. Whether you’re planning a new build, upgrading an attic, or comparing retrofit options, knowing the real-world meaning of R-value will help you make choices that balance energy savings, budget and long-term durability.
R-value per inch for common insulation materials (fiberglass, cellulose, mineral wool, spray foam, rigid foam)
R-value per inch is a measure of how much thermal resistance a material provides for each inch of thickness; higher numbers mean better resistance to heat flow. Typical per-inch R-values vary by material and by product form (batts, loose-fill, board, spray). As general guidance: fiberglass batts commonly range from about R-2.9 to R-3.8 per inch (many standard batts sit near R-3.1–3.4/in); cellulose loose-fill or dense-packed cellulose is commonly in the R-3.2–3.8 per inch range (typical ~R-3.5–3.7/in); mineral wool (rock or slag wool) is often around R-3.0–3.7 per inch depending on density; spray polyurethane foam differs by type — open-cell spray foam is roughly R-3.5 per inch while closed-cell spray foam is much higher, typically R-6.0–6.8 per inch; rigid foam boards vary by chemistry — expanded polystyrene (EPS) is around R-3.6–4.2 per inch, extruded polystyrene (XPS) around R-4.5–5.0/in, and polyisocyanurate (polyiso) commonly rated near R-5.6–6.5 per inch (though polyiso’s effective R-value can drop at low outdoor temperatures).
Those per-inch numbers are useful for comparing materials, but they are not fixed constants. R-values depend on density, manufacturing, and test conditions; for example, higher-density fiberglass or mineral-wool products can push toward the upper end of their ranges, while loose-fill versions may sit lower. Temperature sensitivity also matters: polyiso’s R-value is often measured at moderate temperatures and can decline in cold climates, and closed-cell foam’s performance can be affected by aging and trapped blowing-agent diffusion. Installation quality changes effective performance too — compression of batts, gaps, misfit around framing, settling of loose-fill cellulose, or poor coverage from spray foam will lower the real-world thermal resistance compared with lab-rated R/inch.
When choosing insulation, consider R-value per inch together with other properties and practical constraints. If space is limited (thin cavities, retrofit work), materials with a higher R/in — closed-cell foam or polyiso boards — let you meet target R-values with less thickness, but they cost more and have different vapor/moisture and fire-performance trade-offs. Fiberglass, cellulose, and mineral wool are lower-cost options that perform well when installed correctly and combined with good air sealing; spray foam can combine insulation and air barrier benefits, so its effective whole-assembly performance may exceed what R/inch alone suggests. For accurate design and code compliance always use manufacturer data for the exact product you plan to install and account for assembly details (framing fraction, continuous insulation, air sealing, moisture control) that influence actual in-place R-value.
Typical R-value ranges by product type (batts, loose-fill, spray foam, foam board)
Batts: manufactured blanket products (fiberglass, mineral wool) typically provide roughly R‑3.0 to R‑4.3 per inch depending on density and product. Standard fiberglass batts are commonly in the R‑2.9 to R‑3.8 per inch range, while higher‑density fiberglass and some specialty batts can reach about R‑3.7–4.3 per inch. Mineral‑wool batts are usually similar to or slightly higher than standard fiberglass in thermal resistance, often in the R‑3.0–3.3 per inch neighborhood. When choosing batts, consider that their rated R‑value assumes proper fit with no compression, gaps, or voids in the framing cavity.
Loose‑fill: loose or blown materials (cellulose, loose fiberglass, loose mineral wool) show wider variation. Loose‑fill cellulose typically delivers about R‑3.2 to R‑3.8 per inch (dense‑packed cellulose tends toward the higher end). Loose fiberglass tends to be lower, roughly R‑2.2–2.7 per inch, because of lower density and settling potential. Loose mineral wool falls between loose fiberglass and batts, commonly around R‑2.8–3.3 per inch. Loose‑fill works well for irregular cavities and attic coverage but settling, voids, and installation density strongly affect the actual in‑place R‑value.
Spray foam and foam board: spray foam and rigid foam boards give the highest R per inch. Open‑cell spray foam typically has about R‑3.5 to R‑3.7 per inch; closed‑cell spray foam typically ranges from about R‑6.0 to R‑7.0 per inch (most common products are ~R‑6–6.5/in). Rigid foam boards vary by material: polyisocyanurate (polyiso) commonly rates around R‑5.6–6.5 per inch under moderate temperatures (performance can decline at lower temps), extruded polystyrene (XPS) is around R‑4.5–5.0 per inch, and expanded polystyrene (EPS) is roughly R‑3.6–4.2 per inch. Foam boards are often used for continuous exterior insulation or spot applications where higher R per inch or moisture resistance is needed.
What are the R‑value ratings for common insulation materials? Typical per‑inch and installed examples: fiberglass batt ≈ R‑2.9–3.8/in (3.5″ 2×4 fill ≈ R‑10 to R‑13; 5.5″ 2×6 fill ≈ R‑16 to R‑21). Cellulose (dense‑packed loose) ≈ R‑3.2–3.8/in (3.5″ ≈ R‑11–13; 5.5″ ≈ R‑18–21). Mineral wool ≈ R‑3.0–3.3/in (similar installed numbers to fiberglass or slightly higher). Open‑cell spray foam ≈ R‑3.5–3.7/in (5.5″ ≈ R‑19–20); closed‑cell spray foam ≈ R‑6–7/in (2″ closed‑cell board ≈ R‑12–14). Polyiso board ≈ R‑5.6–6.5/in (1″ ≈ R‑6 typical), XPS ≈ R‑4.5–5/in (1″ ≈ R‑5), EPS ≈ R‑3.6–4.2/in (1″ ≈ R‑3.6–4). Actual installed R depends on thickness used, product density, and real‑world conditions; manufacturers’ labelled R is a lab value and can vary with temperature and installation quality.
Choosing among these is a balance of desired R per inch, cost, moisture and air‑control needs, and assembly details. High R per inch (closed‑cell spray foam, polyiso) is useful where space is limited or where continuous exterior insulation is desired; batts and cellulose are cost‑effective for full‑depth cavity fill when installed correctly. Remember that whole‑assembly performance is also affected by air leaks, thermal bridging through framing, compression of insulation, and moisture — so pick materials and an installation approach that address those practical factors as well as nominal R‑value.
Installation and real-world factors affecting R-value (compression, gaps, settling, moisture)
Installation quality and on-the-job realities are as important as the labeled R-value of an insulation product. Compression of batt or blanket materials (for example stuffing a 3½‑inch R‑13 fiberglass batt into a 2×6 cavity) reduces the air pockets that provide thermal resistance and can cut effective R-value significantly. Gaps, voids and misfitted pieces create convective bypasses and thermal short-circuits—small uninsulated channels around pipes, wiring, junctions or at the top and bottom of cavities can erode whole-assembly performance far more than the nominal per‑inch R difference between products. Loose‑fill materials can settle over time; installers must account for settled density and depth, because a settled attic of cellulose or fiberglass can end up well under the intended R-value if initial depth was insufficient.
Different materials respond differently to moisture, temperature and aging, and those responses change their real-world R. Fibrous insulations (fiberglass, cellulose, mineral wool) rely on trapped air and will lose insulating effectiveness if wet, compressed, or disturbed; cellulose is hygroscopic and can absorb water that both reduces R and raises the risk of mold or compaction unless properly detailed with vapor control and drainage. Spray foams have the advantage of air‑sealing while insulating—open‑cell spray foam has a lower R per inch (roughly R‑3.5 per inch) but resists convective bypass better than loose fibrous fills; closed‑cell spray foam has a much higher nominal R (about R‑6 per inch) and resists water intrusion, but is more expensive and can be sensitive to application quality and temperature. Rigid boards (polyiso, XPS, EPS) hold their rated R well if kept dry and continuous; note that some rigid products (polyiso) lose R at low temperatures, so climate and assembly details affect real performance.
For planning and specifying insulation, use both the product’s per‑inch R-values and realistic adjustments for installation and assembly effects. Typical nominal per‑inch ranges you can expect: fiberglass batts ~R‑2.9 to R‑3.8/in; cellulose loose‑fill ~R‑3.2 to R‑3.8/in; mineral/rock wool ~R‑3.0 to R‑3.3/in; open‑cell spray foam ~R‑3.5–3.7/in; closed‑cell spray foam ~R‑5.5–R‑6.5/in; polyisocyanurate board ~R‑5.6–R‑6.5/in (but declines at low temps); XPS ~R‑5/in; EPS ~R‑3.6–R‑4/in. To get the expected in‑place R, insist on proper cavity fill without compression, continuous insulation to limit thermal bridging, correct blow densities and depths for loose fills, good air‑sealing details, and moisture management; when in doubt, plan for some performance loss (commonly 10–40% depending on the issue) and use whole‑assembly calculations rather than just batt R-values.
Building code and climate-zone R-value requirements and recommendations
Building codes set the minimum insulation (R‑value) requirements for different assemblies (attics/ceilings, walls, floors, slabs, and basement walls) based on climate zones. Most codes use climate-zone maps to scale requirements: colder zones require higher R‑values to limit heat loss, while milder zones require less. Compliance can be prescriptive (meet the tabled R‑values for each assembly) or performance-based (demonstrate equivalent or better whole‑building energy performance), and many jurisdictions publish both paths. Because codes are minimums, they are intended to ensure safety and basic energy efficiency but not necessarily to deliver the lowest operating costs or best comfort for long-term performance.
For best results, designers and builders commonly recommend exceeding the code minimums in several situations: colder climates, buildings with high heating/cooling loads, or where a long payback is acceptable. Continuous exterior insulation is often recommended to reduce thermal bridging through framing and achieve a higher effective assembly R‑value without enlarging cavity thickness. Equally important are air sealing, moisture control, and correct installation—compressed batts, gaps, settling, and wet insulation all reduce effective R‑value. In retrofit projects, pay attention to existing assemblies (vented vs. unvented attics, conditioned basements, etc.) and consider combining cavity insulation with continuous exterior foam or dense‑pack methods to meet both code and performance goals.
What are the R‑value ratings for common insulation materials? Approximate R‑values per inch of thickness are: fiberglass batt insulation ~ R‑3.0 to R‑3.4/in (loose‑fill fiberglass is lower, ~R‑2.2–2.7/in); cellulose loose‑fill ~ R‑3.2 to R‑3.8/in; mineral wool (rock or slag wool) ~ R‑3.0 to R‑3.3/in; open‑cell spray polyurethane foam ~ R‑3.5 to R‑3.7/in; closed‑cell spray foam ~ R‑6.0 to R‑6.8/in; rigid foam boards vary by type—EPS ~ R‑3.6–4.2/in, XPS ~ R‑5.0/in, and polyisocyanurate (polyiso) ~ R‑5.6–6.5/in (performance depends on temperature and product). Remember these are nominal values: actual installed performance depends on density, temperature, quality of installation, thickness, and the impact of thermal bridging and moisture. Always verify manufacturer data and the applicable local code tables when specifying insulation.
Whole-assembly R-value and thermal bridging (framing, continuous insulation)
Whole-assembly R-value is the effective thermal resistance of an entire building element (wall, roof, floor) that accounts for every parallel heat flow path — framed studs, insulated cavities, sheathing, fasteners, and penetrations — not just the nominal R-value of the cavity insulation. Thermal bridging occurs where higher-conductivity components (wood or steel studs, metal fasteners, window frames, etc.) bypass the insulation and provide a lower-resistance path for heat. Because heat follows the path of least resistance, even relatively small fractions of framing or other bridges can substantially lower the effective R-value of the whole assembly compared with the R-value indicated on a batt or foam board alone.
You can quantify whole-assembly performance with a parallel-path calculation: compute U (heat transfer coefficient) for each distinct area fraction as U_i = 1/R_i, multiply by that area fraction, sum to get the assembly U, and invert to get assembly R (R_total = 1 / sum(f_i/R_i)). For example, a wall with 80% cavity insulated at R-19 and 20% wood studs at an effective R-6 gives U = 0.8*(1/19) + 0.2*(1/6) ≈ 0.0754, so R_total ≈ 13.3 — a big drop from R-19. Adding continuous exterior insulation dramatically reduces bridging because the continuous layer is in series with both the stud and cavity paths. Using a 1″ continuous layer of R-6 outside raises the stud path to ~R-12 and the cavity path to ~R-25, yielding R_total ≈ 20.6 in the example — closer to the cavity value and much better overall performance.
Typical R-values per inch for common insulation materials (approximate, product and density dependent) are: fiberglass batts ~ R-3.0 to R-3.7/in, loose-fill cellulose ~ R-3.2 to R-3.8/in, mineral wool (rock wool) batts ~ R-3.0 to R-3.7/in, open-cell spray polyurethane foam ~ R-3.5 to R-3.8/in, closed-cell spray foam ~ R-6.0 to R-6.8/in, polyisocyanurate foam board ~ R-6.0 to R-6.5/in (can vary with temperature), extruded polystyrene (XPS) ~ R-4.5 to R-5.0/in, and expanded polystyrene (EPS) ~ R-3.6 to R-4.2/in. When designing or specifying insulation, use those material R-values in the whole-assembly calculations, allow for installation effects (compression, gaps, settling, moisture), and prioritize continuous exterior insulation or thermal breaks where thermal bridging would otherwise degrade the assembly performance.