What Insulation Does (and What It Cannot Do)
Insulation reduces conductive heat flow through solid surfaces.[4] A material's thermal resistance R-value characterizes how strongly it resists that heat flow: higher R-value = less heat per square foot per degree Fahrenheit per hour. Doubling R-value cuts conductive heat loss in half through that surface.
The physical mechanism behind R-value is straightforward. All insulation materials work by trapping still air (or a less-conductive gas like argon or krypton) in small cells or pockets. Still air has very low thermal conductivity (~0.025 W/m·K) compared to the solid materials it displaces. The R-value of any insulation is proportional to its thickness and inversely proportional to the conductivity of the trapped-air-plus-material composite.
The implications for selection. Higher density materials (closed-cell spray foam, rigid foam, mineral wool) have smaller air pockets and higher R per inch. Lower density materials (fiberglass batts, open-cell spray foam, blown cellulose) have larger air pockets but more total air volume — they achieve high overall R-value through thickness rather than per-inch density. The right choice depends on space available, moisture conditions, and budget.
R-Value by Insulation Material
| Material | R per inch | Density (pcf) | Installed cost per sq ft | Best application |
|---|---|---|---|---|
| Fiberglass batt (R-13 cavity) | 3.1–3.4 | 0.5–1.0 | $0.50–$1.00 | New construction wall cavities, attic between joists |
| Fiberglass batt (R-15 cavity) | 3.7 | 0.8–1.3 | $0.70–$1.20 | New construction; higher density compressed into 3.5" cavity |
| Blown fiberglass | 2.2–2.7 | 0.5–0.8 | $1.00–$1.80 | Attic floor over open joists; cheaper than batts at high R |
| Blown cellulose | 3.2–3.8 | 1.5–2.5 | $1.50–$2.50 | Attic floor (most common attic retrofit); dense-pack walls |
| Open-cell spray foam | 3.5–3.8 | 0.5–0.8 | $1.50–$3.00 | Wall cavities, rim joists, attic flat below roof deck |
| Closed-cell spray foam | 6.0–7.0 | 1.8–2.5 | $3.00–$6.00 | Limited-thickness cavities, foundations, moisture-prone areas |
| Mineral wool batt | 3.0–3.7 | 2.0–4.0 | $0.80–$1.50 | Fire-rated walls, sound insulation, exterior continuous insulation |
| Mineral wool board (rigid) | 4.0–4.2 | 6.0–8.0 | $2.00–$3.50 | Exterior continuous insulation, foundation walls |
| Expanded polystyrene (EPS) | 3.6–4.2 | 1.0–2.0 | $0.60–$1.20 | Under slab, foundation exterior, some roof applications |
| Extruded polystyrene (XPS) | 5.0 | 1.5–2.5 | $1.20–$2.50 | Below-grade foundations, ground-contact applications |
| Polyiso (PIR) | 5.7–6.5 (room temp) | 2.0–3.0 | $1.50–$3.00 | Roof boards, exterior wall sheathing in warm/mixed climates |
| Polyiso (cold) | 4.5–5.0 (at 25°F) | 2.0–3.0 | Same as above | Performance derates at low temps; less ideal for cold climates exterior |
The cost-per-R-value-per-square-foot ranking changes the cheapest-by-thickness ranking. Per dollar of installed insulation, blown cellulose is typically the leader (cellulose at R-3.5/inch × 14 inches = R-49 attic at $2.00/sqft = $0.041 per R-value-square-foot).
Fiberglass batt is similar in cost-per-R-square-foot but is less convenient in retrofit applications. Closed-cell spray foam, while having the highest R per inch, is roughly 3-4× the cost per R-value-square-foot of cellulose or open-cell foam.[2]
The right material depends on application, not on which has the highest R per inch in isolation. For attic floors with unlimited depth, cellulose wins on cost. For 2x4 wall cavities where 3.5 inches is the maximum thickness, closed-cell foam wins because of the highest R-per-inch. For exterior continuous insulation under siding, rigid foam (polyiso or mineral wool board) wins because of structural rigidity and moisture tolerance.
DOE Recommended R-Values by Climate Zone and Location
The attic R-value targets that drive most residential insulation decisions are summarized below. Lower end of each range corresponds to existing-home retrofit recommendations; upper end corresponds to new construction targets and ENERGY STAR Northern Climate specifications.
| Climate zone | Attic | Wall (cavity + continuous) | Floor over unconditioned | Foundation wall |
|---|---|---|---|---|
| 1 (Miami FL, Honolulu HI) | R-30 to R-49 | R-13 to R-15 | R-13 | R-0 to R-5 |
| 2 (Houston TX, Phoenix AZ) | R-30 to R-60 | R-13 to R-15 | R-13 | R-0 to R-5 |
| 3 (Atlanta GA, San Diego CA) | R-30 to R-60 | R-13 to R-20 | R-19 to R-25 | R-5 to R-13 |
| 4 (Kansas City MO, San Francisco CA) | R-38 to R-60 | R-13 to R-20 + R-5 c.i. | R-25 to R-30 | R-10 to R-13 |
| 5 (Chicago IL, Denver CO, Boston MA) | R-38 to R-60 | R-13 to R-21 + R-5 c.i. | R-25 to R-30 | R-15 |
| 6 (Minneapolis MN, Burlington VT) | R-49 to R-60 | R-20 + R-5 c.i. | R-25 to R-30 | R-15 |
| 7 (Duluth MN, International Falls MN) | R-49 to R-60 | R-20 + R-7.5 c.i. | R-30 to R-38 | R-15 to R-19 |
| 8 (Fairbanks AK, Anchorage AK) | R-60 | R-30+ assembly | R-30 to R-38 | R-19+ |
The DOE recommendations are practical retrofit targets, not regulatory requirements.[1] New construction must meet IECC 2021 minimums (which match the lower end of the DOE ranges in most zones).[3] The recommendations are intentionally conservative — meeting them puts a house in the upper third of US envelope performance for its climate.
The continuous insulation ("c.i.") notation matters in zones 4-7. A wall labeled "R-13 + R-5 c.i." means R-13 cavity insulation between the studs plus R-5 of continuous foam board on the outside of the sheathing, before the siding. The continuous layer interrupts the thermal bridge through the studs (covered next section) — without continuous insulation, the effective whole-wall R is much lower than the cavity R.
Air Sealing Versus Insulation: Which First?
The conventional advice "seal before you insulate" is correct, but the reasoning matters because it determines the order and magnitude of work.
Insulation slows conductive heat transfer through a surface; it has minimal effect on air leakage. If air bypasses the insulation through gaps, holes, or penetrations, the insulation's R-value barely affects the bypassed flow. A typical 12 ACH50 leaky house with R-49 attic insulation loses roughly the same amount of energy as the same house with R-19 attic insulation, because air leakage dominates the loss budget at that tightness.[5]
The major air leakage locations in a typical residential building are well documented and prioritized in DOE guidance:
- Attic floor penetrations: bath fan housings, recessed lights, plumbing chases, electrical wires, attic hatch — these typically leak 30-50% of total envelope air loss.
- Rim joist (basement ceiling perimeter): the junction between basement walls and first-floor framing — typically 10-20% of total leakage.
- Sill plate at foundation: gap between wall framing bottom plate and foundation top — typically 5-15%.
- Windows and exterior doors: weatherstripping failures, frame-to-rough-opening gaps — typically 5-10%.
- Plumbing and electrical penetrations to exterior: where pipes and wires exit the envelope — typically 3-8%.
- Dropped ceilings and soffits: especially over showers — typically 2-5%.
Addressing the top three accounts for 75% of typical leakage and is achievable in 1-2 days of contractor labor for $500-$1,500 in a typical 2,000 sq ft house. The remaining items add another 1-2 days for $300-$800. The total $800-$2,300 of air sealing work typically reduces ACH50 from 10-12 to 4-6 and saves 15-25% on heating and cooling load.[5]
Thermal Bridging: The Reason Effective Whole-Wall R is Less Than Cavity R
Insulation only resists heat where the insulation is. In a wood-frame wall, the wood studs themselves are thermal bridges — paths of higher conductivity through the otherwise-insulated assembly.
| Cavity insulation | Sheathing + continuous | Whole-wall effective R | Bridging loss |
|---|---|---|---|
| R-13 fiberglass batt | ½" plywood + ½" gypsum (no foam) | R-9.2 | -29% |
| R-15 fiberglass high-density | ½" plywood + ½" gypsum (no foam) | R-10.5 | -30% |
| R-13 + R-5 continuous foam | ½" plywood + R-5 polyiso + ½" gypsum | R-15.7 | -13% |
| R-19 fiberglass batt (2x6 wall) | ½" plywood + ½" gypsum (no foam) | R-13.7 | -28% |
| R-21 high-density (2x6 wall) | ½" plywood + ½" gypsum (no foam) | R-15.1 | -28% |
| R-21 + R-7.5 continuous (2x6 wall) | ½" plywood + R-7.5 polyiso + ½" gypsum | R-22.6 | -22% |
| Closed-cell foam (2x4 cavity) | ½" plywood + ½" gypsum (no foam) | R-13.1 | -32% |
| Double stud wall, R-39 cavity | ½" plywood + ½" gypsum | R-32.5 | -17% |
The 13-32% bridging penalty matters because it determines what wall assembly actually performs.[7] A "R-21 wall" with no continuous insulation has whole-wall R closer to 15. The same wall with R-7.5 continuous foam shifts the whole-wall R to roughly 23 — the continuous layer interrupts the bridge through the studs, dramatically raising effective performance.
Modern high-performance assemblies (PHIUS, Passive House, Zero Energy Ready) typically rely on continuous insulation or double-stud construction to reduce bridging penalties to 10-15%. Standard IECC 2021 construction in cold climates (zones 5-8) requires continuous insulation precisely for this reason — without it, the effective whole-wall R falls below code targets even when the cavity insulation alone meets nominal R-value.
Steel-stud framing has roughly 5× the thermal-bridging penalty of wood-stud framing because steel conducts heat about 300× faster than wood. Commercial buildings using steel framing therefore rely heavily on continuous insulation; residential steel-framed houses are rare and require continuous insulation to compete with wood-framed equivalents.
Installation Quality: Why Catalog R Often Does Not Reach the Field
The R-value printed on an insulation product label is measured in a laboratory under standardized test conditions: uniform density, no compression, no gaps, no air leakage around the sample. Real installation routinely fails to achieve those conditions.
Common installation failures that reduce actual installed R-value:
Fiberglass batts compressed in undersized cavities. A R-19 batt compressed into a 3.5-inch 2x4 cavity (the batt is 6.25 inches thick) loses roughly 30-40% of its R-value. The catalog number is for the batt at full loft, not at compressed depth.
Voids around obstructions. A batt installed around a junction box, plumbing pipe, or electrical wire leaves small air spaces unless carefully cut and stuffed. Even small voids (5-10% of the wall area) can drop whole-wall R by 20-30% because the air bypasses through the void.
Sloppy attic blown-in. Blown insulation that fails to reach corners and edges leaves uninsulated zones near the eaves and along ridge beams. Field measurements often find R-19 to R-30 in the open attic floor but R-3 to R-7 along the eaves where the depth has thinned.
Wind washing at eaves. Soffit-vent airflow can blow loose insulation away from the attic perimeter, reducing depth and disturbing settled cellulose. Baffles (vent channels) installed to direct soffit air over the insulation prevent the loss but are often skipped in older installations.
Settling over time. Blown cellulose settles 10-20% over the first 5-10 years; blown fiberglass settles 5-10%. The catalog R is the installed R, not the long-term R. Specifying 14 inches of cellulose at install gets roughly 12 inches after settling — still R-42, but lower than the R-49 specified.
The cumulative effect: actual installed R in a typical residential wall or attic is often 70-85% of nameplate. High-quality installation (with care for compression, voids, and edge effects) recovers most of the loss; sloppy installation produces field performance that is dramatically below catalog claims. The cheapest envelope-improvement investment a homeowner can make is often paying for a careful installation rather than a higher-spec material.[2]
Where to Insulate: Attic, Wall, Floor, Foundation
The four envelope locations have different return profiles, costs, and accessibility.
Attic. The most cost-effective insulation target in most US homes. Attic floors are typically accessible, blown insulation goes in quickly, and the building physics favors attic insulation because (1) heat rises, (2) attic temperatures swing more than indoor temperatures, and (3) attic insulation work can typically be done without disrupting living space. Most existing-home retrofits start in the attic.
Walls. The second-largest envelope element by area, but the hardest to upgrade in existing homes because the cavity is enclosed. Dense-pack cellulose can be added via small holes drilled through siding or drywall, raising R-value 30-40% in many cases. Full upgrade requires either tearing off siding (to add exterior continuous insulation) or tearing off drywall (to add cavity insulation), both of which are expensive. Wall upgrades are usually done during other renovations rather than as standalone projects.
Floor over unconditioned space. Common in homes with unheated crawlspaces or basements. Adding R-19 to R-30 of batt or rigid foam insulation between joists is straightforward if the floor is accessible. Critical detail: insulation must be supported in place permanently (mesh stretched across joists or rigid foam mechanically fastened); just stuffing batts up between joists fails because the batts fall down over time.
Foundation walls and slab. Increasingly important in new construction, less commonly retrofitted. A basement wall with no insulation loses 8-15% of total home heating load to the surrounding soil at deep-winter conditions; adding R-10 to R-15 of foam to the basement wall cuts that to 2-4%. Slabs benefit most from perimeter insulation (the slab-to-foundation edge accounts for most heat loss); under-slab insulation matters mostly for slabs above heated soil rather than below frostline.
When Insulation Pays Back (and When It Does Not)
The financial return on insulation upgrades depends on climate, baseline insulation, fuel cost, and incentive availability.
| Upgrade | Climate | Cost | Annual savings | Payback before incentives | Payback after 25C credit |
|---|---|---|---|---|---|
| Add R-30 over R-19 attic | Zone 5-6 (cold) | $1,500–$2,500 | $150–$250 | 8–12 years | 5–8 years |
| Add R-30 over R-19 attic | Zone 3-4 (mixed) | $1,500–$2,500 | $80–$150 | 12–20 years | 7–13 years |
| Add R-30 over R-19 attic | Zone 1-2 (hot) | $1,500–$2,500 | $70–$120 | 14–25 years | 9–17 years |
| Air seal whole house (12→5 ACH50) | All climates | $800–$2,000 | $150–$400 | 3–7 years | 2–5 years |
| Dense-pack walls (R-7 to R-13) | Zone 5-7 | $5,000–$10,000 | $200–$400 | 15–25 years | 10–17 years |
| Foundation wall R-15 | Zone 5-7 | $2,500–$5,000 | $100–$200 | 15–30 years | 10–20 years |
| Window replacement (single→triple) | Zone 5-7 | $10,000–$25,000 | $200–$500 | 25–50 years | 20–35 years |
The 25C tax credit shifts every payback by roughly 30% — meaningful but not transformative. The bigger lever is climate: cold climates produce 2-3× faster payback than hot climates on the same upgrade because heating-fuel costs are higher per delivered BTU than electricity-cooling costs at most fuel-price ratios. Insulation in cold climates almost always pays back; in hot humid climates the math is closer.
Insulation upgrades make most economic sense when bundled with other work that opens up the envelope:
- Re-siding a house? Add R-5 to R-7.5 continuous foam under the new siding.
- Replacing the roof? Add R-30 to R-49 of rigid foam above the roof deck.
- Finishing a basement? Insulate the foundation walls before drywall.
Each of these scenarios reduces the marginal cost of the insulation work to a fraction of the standalone project cost, and the payback improves dramatically.
What This Sub-Hub Covers
Articles
- Attic R-value reference — detailed coverage of attic insulation: DOE recommendations, materials, depth measurement, air-sealing primacy
Planned articles
- Wall insulation methods (planned) — batt vs dense-pack vs continuous, retrofit strategies
- Basement and foundation insulation (planned) — exterior vs interior insulation, moisture management
- Floor over unconditioned space insulation (planned) — batt vs rigid foam, support methods
Related load and equipment topics
- Building science fundamentals — the parent hub for envelope thermal performance
- Window U-factor reference — the fenestration counterpart
- HERS Index — whole-home performance score
- Manual J load calculation — uses R-values as primary inputs
Calculators
- Attic R-value calculator — multi-layer R-value computation with zone-specific upgrade recommendations
- Manual J load calculator — full envelope load math that takes insulation R-values as inputs