Insulation & Building Science Glossary

ACH50

Air changes per hour at 50 pascals.

Measure of a building's air leakage rate, captured during a blower-door test. The blower depressurises the building to 50 pascals and reports how many times the indoor air volume turns over per hour. ENERGY STAR's savings model uses 17.4 to 18.2 ACH50 for a typical 1970 to 1989 home, so older homes often measure well above 10 ACH50. Lower is better.^[1]

ASTM C518

Standardised R-value test method.

Steady-state thermal-resistance test at 75°F mean temperature using a heat-flow meter. The R-value printed on every insulation bag and product spec comes from this test or the alternative guarded-hot-plate method ASTM C177; both are current methods accepted under the FTC R-Value Rule (16 CFR 460.5), which requires testing at a 75°F mean temperature. Real-world R-value can differ when temperature, moisture, or installation quality drift from lab conditions.^[2][3][4]

Blowing agent

The gas that creates cells in foam insulation.

Determines a foam's R-value per inch, environmental impact, and long-term performance. Closed-cell spray foam and rigid-foam boards rely on blowing agents trapped in cells. Modern HFO (hydrofluoro-olefin) blowing agents have low global warming potential; older HFC blowing agents have a high global warming potential (for example HFC-134a at about 1,430) and are being phased out.^[5]

Spray foam insulation guide →

Cavity insulation

Insulation installed between framing members.

Goes inside stud bays, joist bays, or rafter bays. Standard fiberglass batts, blown-in cellulose in walls, and spray foam between studs are all cavity insulation. Limited to the depth of the framing (3.5" in 2×4, 5.5" in 2×6). Compare to continuous insulation, which goes outside the framing and is uninterrupted by studs.^[6][7]

Climate zone (IECC)

IECC zones 1 (very hot) through 8 (subarctic).

The 2021 IECC divides the US into 8 climate zones (zone 1 hottest through zone 8 coldest, including Alaska), drawn from ASHRAE Standard 169 climate data; the 2021 IECC incorporates the ASHRAE 169-2013 changes per PNNL. Each zone has minimum R-value requirements per Table R402.1.3. Marine subzones (3C, 4C) apply to coastal Pacific Northwest and Northern California with a milder, wetter climate and reduced drying potential.^[8][9]

Climate Zone Map →

ci (continuous insulation)

Insulation uninterrupted by framing.

Rigid foam or mineral-wool board applied over the exterior of the framing (or over interior framing) so heat must pass through the insulation rather than around it through wood studs. Reduces thermal bridging and improves whole-wall R-value. "R-13+5ci" means R-13 cavity insulation plus R-5 of continuous insulation applied across the framing.^[9][10]

Closed-cell spray foam

Dense spray foam with sealed gas-filled cells.

R-6.0 to R-7.0 per inch (R-value per inch varies with thickness, as the FTC R-Value Rule requires manufacturers to disclose). Acts as insulation, air barrier, and vapor barrier in one application. Adds structural rigidity. 2× to 3× more expensive than open-cell. Best for rim joists, basement walls, crawl spaces, and limited-depth cavities where R-value per inch matters.^{[11][12][13][14][15][16][17][18][19][20]}

Open-cell vs closed-cell comparison →

Conditioned space

Indoor area kept at heated/cooled temperature.

Living rooms, bedrooms, finished basements - any space served by HVAC. Unconditioned spaces include vented attics, vented crawl spaces, and detached garages. The thermal envelope is the boundary between conditioned and unconditioned space; insulation goes on this boundary.

Dew point

Temperature at which water vapor condenses.

Critical in wall and roof assemblies because if the dew point falls inside the insulation cavity, moisture condenses on framing or sheathing and can cause rot. Vapor retarders and air barriers are placed strategically to keep the dew point in a non-damaging location for the climate.

Crawl space encapsulation

Sealing crawl space and bringing it inside the envelope.

Seal vents, install heavy-duty vapor barrier on dirt floor and up walls, insulate crawl space walls (closed-cell foam or rigid foam). The crawl space becomes part of the conditioned envelope - eliminates moisture problems, makes ducts in the crawl perform better, costs $3,000-$15,000 depending on size.

Crawl space insulation guide →

FSI / SDI

Flame Spread Index / Smoke Developed Index.

Fire-safety ratings for insulation, measured with ASTM E84 (noncombustibility is measured with ASTM E136). Class A per IBC 2021 Sec. 803.1.2 is a flame spread index of 0 to 25 and a smoke-developed index of 0 to 450 (Class B 26 to 75; Class C 76 to 200). The "SDI ≤50" value often quoted for unfaced blankets is a separate product-standard limit from ASTM C665 (per NAIMA), not the Class A threshold. Most fiberglass and mineral wool meet Class A. Foam plastic insulation must be separated from the living space by an approved thermal barrier such as ½-in. gypsum wallboard unless the product's evaluation report allows otherwise (IBC 2021 Sec. 2603.4; residential rule in IRC R316.4); check the product's ICC-ES report and local code.^{[21][22][23][24][25]}

HDD / CDD

Heating Degree Days / Cooling Degree Days.

How heating and cooling demand are quantified. HDD measures how cold the climate is across the year (cumulative degrees below the 65°F base per day); CDD measures heat. The IECC uses HDD and CDD to define climate zones - zone 1 has lots of CDD and few HDD; zone 8 is the opposite.^[26]

IECC

International Energy Conservation Code.

The model energy code published by the International Code Council. US jurisdictions adopt the IECC (sometimes with state amendments) as the legal energy code for new construction. The 2021 IECC bumped attic R-values to R-49 in zones 2-3 and R-60 in zones 4-8; the 2024 IECC reverses this change back to R-38 in zones 2-3 and R-49 in zones 4-8.^[27][9]

IECC requirements by ZIP →

Ice dam

Ridge of ice that forms at the eave in winter.

Caused by heat escaping through the attic, melting snow on the roof above the conditioned space, and the meltwater refreezing at the cold eave. Ice dams back up under shingles and leak into walls. Prevented by adequate attic insulation (especially over the exterior wall top plates), air sealing, and proper attic ventilation.

Attic insulation guide →

K-value (thermal conductivity)

How readily a material conducts heat.

Inverse of R-value. K-value is per-inch, in BTU·in/(hr·ft²·°F). A material with K-value of 0.25 has R-value per inch of 4.0. R-value is the more common metric for insulation in the US; K-value is used more often in commercial/scientific contexts.^[28]

Marine subzone (3C, 4C)

Coastal Pacific Northwest and Bay Area zones.

A IECC subdivision applied to climate zones 3 and 4 in coastal areas with mild, wet, low-diurnal-range climates: Seattle, Portland, San Francisco. In the 2021 IECC Table R402.1.3 (unchanged in 2024), 3C tracks the zone 3 requirements, but Marine 4 (4C) follows the zone 5 requirements: floor R-30; basement and crawl walls 15ci or 19 or 13&5ci; no SHGC requirement in the Marine zone. The milder, wetter climate gives these zones reduced drying potential.^[9]

Open-cell spray foam

Lighter spray foam with interconnected cells.

R-3.5 to R-3.8 per inch (R-value per inch varies with thickness, as the FTC R-Value Rule requires manufacturers to disclose). Vapor permeable (semi). Cheaper than closed-cell. Acts as an air barrier but not a vapor barrier. Best for interior walls, attic roof decks (in conditioned attics), and cathedral ceilings where you want some drying capability through the foam.^{[11][16][29][30][31]}

Open-cell vs closed-cell comparison →

Perm rating

How easily water vapor passes through a material.

Measured in perms (US) or ng/(Pa·s·m²) (SI). Under the IRC (Sec. R702.7), Class I vapor retarders (poly sheet) are 0.1 perm or less - essentially impermeable. Class II (kraft facing) is greater than 0.1 up to 1.0 perm. Class III (latex paint, "smart" retarders) is greater than 1.0 up to 10 perm. Higher-permeability materials let vapor pass; impermeable materials block it.^[32][33]

R-value

Thermal resistance - higher is better insulation.

Measured in h·ft²·°F/BTU in the US. Higher R-value = more resistance to heat flow. R-values are additive along one path: R-13 cavity + R-5 continuous + air films 0.85 + ½" gypsum 0.45 + ½" sheathing ≈0.6 ≈ R-19.9 center-of-cavity (before framing). That is the center-of-cavity sum, not the whole-wall R-value, which is lower once studs are counted. The label R-value is tested at 75°F mean temperature; real-world performance varies.^{[4][34][35][28][36][37][6][38]}

Full R-value explainer →

RESNET grade (I, II, III)

Insulation installation quality grade.

Standardised installation grade, defined in ANSI/RESNET/ICC 301-2014 Addendum F-2018, Appendix A. Grade I = no more than 2% of the insulated area compressed below labelled thickness or with gaps or voids, and no compression deeper than ¾ in. Grade II = no more than 15% (none deeper than ¾ in.). An installation that fails Grade II is Grade III. Field tests show how much poor installation costs: ORNL (1998) hot-box tests of a 2×6, 24-in. o.c. wall with R-19 batts measured R-15.4 for a perfect install and R-13.2 (−14%) for a "worst typical" install with rounded batt shoulders, 2% voids, and inset stapling. A Johns-Manville study for the US Navy (1977) found a 4.2% uninsulated area cut an R-19 ceiling's thermal resistance by 34%. BRANZ hot-box tests found 16 mm edge gaps cut a wall system's R-value by 35%.^{[39][40][41][42][43][44]}

Settled density (for blown-in)

Final density after material settles.

Dry loose-fill cellulose settles about 10% from its initial installed depth, per manufacturer coverage charts (Applegate and Nu-Wool ≈10%, Cel-Pak 9.6 to 11.6%); stabilized cellulose settles only 0.7 to 5.3%. ORNL (1992) measured a 9% thickness loss and a 6 to 10% R-value loss. Manufacturers publish coverage charts in two columns: initial install depth and settled depth. Labelled R-values are rated at settled density (FTC 16 CFR 460.5), so installers install about 11% above the settled target for dry loose-fill (stabilized products only about 1 to 6% above). Blown fiberglass settles about 1% or less.^{[45][46][47][48][49][2][50][51][52][23][53]}

Stack effect

Buoyancy-driven air movement in buildings.

Warm interior air rises and escapes through the upper envelope (attic, can lights, top plates), pulling cold outside air in through the lower envelope (rim joists, crawl space). It is a major driver of heat loss in older homes. Air sealing - especially at the top plate and attic floor - addresses this.

How to air seal an attic →

Thermal bridging

Heat path through framing around insulation.

Wood framing conducts heat about 2.5-3.5× faster than the insulation between studs (softwood ≈R-1.25/in vs cavity insulation R-3.0 to 4.3/in). In a standard 2×6 wall framed 16" on center, framing is 23-25% of the wall area. This drops a cavity R-23 wall to about R-17.8 clear-wall (studs included). Continuous exterior insulation breaks the thermal bridge: adding R-5 of exterior foam recovers nearly all of the lost R-value.^{[54][55][37][19][38][36][6][56]}

Rigid foam insulation guide →

Type X drywall

Fire-rated drywall (5/8" with glass fibers).

Fire-rated gypsum board (typically 5/8" with glass fibers) used where codes require a rated barrier, such as over spray foam in occupied spaces, on garage separations, and in fire-rated wall assemblies. The glass fibers hold the gypsum together longer when exposed to fire. Where a project requires Type X, standard drywall is a different product; check the required thickness and rating against your local code.

U-value (U-factor)

Thermal conductance - inverse of R-value.

U = 1/R. A wall assembly with R-20 has U = 0.05. Lower U-value = better insulation (opposite of R-value). U-value is more common for windows and whole-assembly performance; R-value is more common for individual insulation materials.^[38][28]

Vapor barrier vs vapor retarder

How easily moisture passes through.

A vapor barrier (Class I, 0.1 perm or less) is essentially impermeable - poly sheet, closed-cell foam at sufficient thickness. A vapor retarder slows but doesn't stop vapor: Class II (greater than 0.1 up to 1.0 perm, e.g., kraft facing) or Class III (greater than 1.0 up to 10 perm, e.g., latex paint, smart retarders). These are the IRC definitions (Sec. R702.7). The right class depends on climate zone and assembly type.^[32][33]

Faced vs unfaced insulation →

Whole-wall R-value

Real assembly R-value including framing.

The effective R-value of a wall assembly accounting for thermal bridging through framing, air films, sheathing, drywall, and the cavity insulation. Always lower than the cavity insulation's labelled R-value. ORNL's Whole-Wall Thermal Performance Calculator paper documents the method; as a rough rule, a cavity-insulated wall assembly is 75-85% of its cavity R-value.^[57][58][59]