Rate of heat flow through a material or assembly

U-value measures how much heat passes through a building component per unit area, per degree temperature difference. Measured in W/(m²·K) or BTU/(h·ft²·°F). Lower U-value = better insulation. Windows, walls, and roofs all have U-value ratings.

Example: Window with U=0.30 W/(m²·K) loses 30 watts per square meter for every 1°C temperature difference. U=0.20 is 33% better insulation.

Material's ability to resist heat flow

R-value is the reciprocal of U-value (R = 1/U). Higher R-value = better insulation. Measured in m²·K/W (SI) or ft²·°F·h/BTU (US). Building codes specify minimum R-values for walls, ceilings, and floors based on climate zones.

Example: R-19 fiberglass batt provides 19 ft²·°F·h/BTU resistance. R-38 in attic is twice as effective as R-19.

Material property: how well it conducts heat

Thermal conductivity (λ or k) is an intrinsic material property measured in W/(m·K). Low k-value = good insulator (foam, fiberglass). High k-value = good conductor (copper, aluminum). Used to calculate R-value: R = thickness / k.

Example: Fiberglass k=0.04 W/(m·K), steel k=50 W/(m·K). Steel conducts heat 1250× faster than fiberglass!

Higher numbers = better insulation

R-value is intuitive: R-30 is better than R-15. Used in North America for insulation products. Values add in series: layers stack. Common in residential construction, building codes, and product labeling.

• Units: ft²·°F·h/BTU (US) or m²·K/W (SI)
• Range: R-3 (single-pane window) to R-60 (attic insulation)
• Wall example: R-13 cavity + R-5 foam = R-18 total
• Rule of thumb: R-value per inch varies by material (R-3.5/in for fiberglass)
• Typical targets: R-13 to R-21 walls, R-38 to R-60 ceilings
• Marketing: Products advertised by R-value ('R-19 batts')

Lower numbers = better insulation

U-value is counter-intuitive: U-0.20 is better than U-0.40. Used globally, especially for windows and whole-building calculations. Doesn't add simply—requires reciprocal math. Common in commercial construction and energy codes.

• Units: W/(m²·K) or BTU/(h·ft²·°F)
• Range: U-0.10 (triple-pane window) to U-5.0 (single-pane window)
• Window example: U-0.30 is high-performance, U-0.20 is passive house
• Calculation: Heat loss = U × Area × ΔT
• Typical targets: U-0.30 windows, U-0.20 walls (commercial)
• Standards: ASHRAE, IECC use U-values for energy modeling

Heat flow through solid materials

Heat transfers through direct contact between molecules. Metals conduct heat rapidly, while insulation materials resist. Governed by Fourier's Law: q = k·A·ΔT/d. Dominant in walls, roofs, floors.

• Metal studs creating thermal bridges (25% heat loss increase)
• Hot pan handle conducting heat from stove
• Heat flowing through wall from warm interior to cold exterior
• Insulation reducing conductive heat transfer

Heat transfer via fluid/air movement

Heat moves with air or liquid flow. Natural convection (warm air rises) and forced convection (fans, wind). Air leaks cause major heat loss. Air sealing stops convection; insulation stops conduction.

• Drafts through gaps and cracks (infiltration/exfiltration)
• Warm air escaping through attic (stack effect)
• Forced air heating/cooling distribution
• Wind increasing heat loss through walls

Heat transfer via electromagnetic waves

All objects emit thermal radiation. Hot objects radiate more. Doesn't require contact or air. Radiant barriers (reflective foil) block 90%+ of radiant heat. Major factor in attics and windows.

• Sunlight heating through windows (solar gain)
• Radiant barrier in attic reflecting heat
• Low-E window coatings reducing radiant heat
• Infrared heat from hot roof radiating to attic floor

Homeowners and builders use R-values and U-values daily:

• Insulation selection: R-19 vs R-21 wall batts cost/benefit
• Window replacement: U-0.30 triple-pane vs U-0.50 double-pane
• Energy audits: thermal imaging finds R-value gaps
• Code compliance: meeting local R-value minimums
• Retrofit planning: adding R-30 to R-19 attic (58% reduction in heat loss)
• Utility rebates: many require R-38 minimum for incentives

U-values determine heating and cooling loads:

• Heat loss calculation: Q = U × A × ΔT (Manual J)
• Equipment sizing: better insulation = smaller HVAC unit needed
• Energy modeling: BEopt, EnergyPlus use U-values
• Duct insulation: R-6 minimum in unconditioned spaces
• Payback analysis: insulation upgrade ROI calculations
• Comfort: lower U-values reduce cold wall/window effect

Large buildings require precise thermal calculations:

• ASHRAE 90.1 compliance: prescriptive U-value tables
• LEED certification: exceeding code by 10-40%
• Curtain wall systems: U-0.25 to U-0.30 assemblies
• Cold storage: R-30 to R-40 walls, R-50 ceilings
• Energy cost analysis: $100K+ annual savings from better envelope
• Thermal bridging: analyzing steel connections with FEA

Ultra-efficient buildings push thermal performance limits:

• Windows: U-0.14 to U-0.18 (triple-pane, krypton-filled)
• Walls: R-40 to R-60 (12+ inches foam or dense-pack cellulose)
• Foundation: R-20 to R-30 continuous exterior insulation
• Airtightness: 0.6 ACH50 or lower (99% reduction vs standard)
• Heat recovery ventilator: 90%+ efficiency
• Total: 80-90% heating/cooling reduction vs code minimum

• Air sealing first: $500 investment, 20% energy savings (better ROI than insulation)
• Attic insulation: R-19 to R-38 pays back in 3-5 years
• Window replacement: U-0.30 windows reduce heat loss by 40% vs U-0.50
• Basement insulation: R-10 saves 10-15% heating costs
• Door replacement: insulated steel door (U-0.15) vs hollow wood (U-0.50)

• Infrared camera: reveals missing insulation and air leaks
• Blower door test: quantifies air leakage (ACH50 metric)
• Touch test: cold walls/ceilings indicate low R-value
• Ice dams: sign of inadequate attic insulation (heat melts snow)
• Condensation: indicates thermal bridging or air leakage

• Cold climates: maximize R-value, minimize U-value (insulation priority)
• Hot climates: radiant barriers in attic, low-E windows block solar gain
• Mixed climates: balance insulation with shading and ventilation
• Humid climates: vapor barriers on warm side, prevent condensation
• Dry climates: focus on air sealing (bigger impact than humid regions)

• Best ROI: Air sealing (20:1), attic insulation (5:1), duct sealing (4:1)
• Moderate ROI: Wall insulation (3:1), basement insulation (3:1)
• Long-term: Window replacement (2:1 over 15-20 years)
• Consider: utility rebates can improve ROI by 20-50%
• Payback: Simple payback = cost / annual savings

Igloos maintain 40-60°F inside when it's -40°F outside using only compressed snow (R-1 per inch). The dome shape minimizes surface area, and a small entrance tunnel blocks wind. Snow's air pockets provide insulation—proof that trapped air is the secret to all insulation.

Space Shuttle thermal tiles had such low thermal conductivity (k=0.05) that they could be 2000°F on one side and touchable on the other. Made of 90% air-filled silica, they're the ultimate insulation material—R-50+ per inch at high temperatures.

Pre-1940s homes often have zero wall insulation—just wood siding, studs, and plaster (total R-4). Adding R-13 to R-19 insulation reduces heat loss by 70-80%. Many old homes lose more heat through walls than through poorly insulated attics.

Ice has k=2.2 W/(m·K), glass is k=1.0. But air (k=0.026) trapped in ice crystals makes snow/ice a decent insulator. Paradoxically, wet snow on roofs is better insulation (R-1.5/inch) than solid ice (R-0.5/inch) due to air pockets.

Fiberglass batt rated R-19 (5.5 inches) compressed to 3.5 inches loses 45% of its R-value (becomes R-10). The air pockets—not the fibers—provide insulation. Never compress insulation; if it doesn't fit, use higher-density material.

Aerogel is 99.8% air and holds 15 Guinness Records for insulation. At R-10 per inch (vs R-3.5 for fiberglass), it's NASA's go-to insulator. But cost ($20-40/sq ft) limits it to specialized applications like mars rovers and ultra-thin insulation blankets.

R-value measures resistance to heat flow (higher = better insulation). U-value measures heat transmission rate (lower = better insulation). They're mathematical reciprocals: U = 1/R. Example: R-20 insulation = U-0.05. Use R-value for insulation products, U-value for windows and whole-assembly calculations.

Yes, but with diminishing returns. Going from R-0 to R-19 cuts heat loss 95%. R-19 to R-38 cuts another 50%. R-38 to R-57 cuts only 33%. First, air seal (bigger impact than insulation). Then add insulation where R-value is lowest (usually attic). Check for compressed or wet insulation—replacement beats adding more.

Convention and complexity. Windows have multiple heat transfer mechanisms (conduction through glass, radiation, convection in air gaps) making U-value more practical for overall performance rating. Walls are simpler—mostly conduction—so R-value is intuitive. Both metrics work for either; it's just industry preference.

Absolutely! R-value resists heat flow in both directions. In summer, R-30 attic insulation keeps heat OUT as effectively as it keeps heat IN during winter. Hot climates benefit from high R-value + radiant barriers + light-colored roofs. Focus on attic (R-38 minimum) and west-facing walls.

Air sealing first, then insulation. Air leaks can bypass insulation entirely, reducing R-30 to R-10 effective. Studies show air sealing provides 2-3× ROI vs insulation alone. Seal first (caulk, weatherstripping, foam), then insulate. Together they reduce energy use 30-50%.

Divide 1 by R-value: U = 1/R. Example: R-20 wall = 1/20 = U-0.05 or 0.28 W/(m²·K). Reverse: R = 1/U. Example: U-0.30 window = 1/0.30 = R-3.3. Note: units matter! US R-values need conversion factors for SI U-values (multiply by 5.678 to get W/(m²·K)).

Steel is 1250× more conductive than insulation. Metal studs create thermal bridges—direct conductive paths through the wall assembly. A wall with R-19 cavity insulation and steel studs achieves only R-7 effective (64% reduction!). Solution: continuous insulation (foam board) over studs, or wood framing + exterior foam.

Depends on climate zone (1-8) and building component. Example: Zone 5 (Chicago) requires R-20 walls, R-49 ceiling, R-10 basement. Zone 3 (Atlanta) requires R-13 walls, R-30 ceiling. Check local building code or IECC tables. Many jurisdictions now require R-20+ walls and R-40+ attics even in moderate climates.