Surface Parameters

Surface Type

Surface Area (sq ft)

Assembly R-Value

Indoor Temp (°F)

Outdoor Design Temp (°F)

Heating Degree Days (HDD)

Zone 1–2: <2,000 · Zone 3: 3,000 · Zone 4: 5,000 · Zone 5–7: 6,000–12,000

Gas Price ($/therm, for cost estimate)

Heat Loss Results

Heat Loss Rate

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BTU/hr through this surface

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U-Value

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ΔT (°F)

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Annual MBtu

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Annual Cost

Upgrade Comparison

ⓘ Formula: Q = U × A × ΔT where U = 1/R. Annual loss: U × A × HDD × 24.

Heat Loss at Current vs. Upgraded Insulation

Scenario	R-Value	U-Value	BTU/hr	Annual MBtu	Annual Cost	Savings

Upgrade Parameters

Insulation Upgrade Cost ($)

Upgrade Target R-Value

Energy Price Inflation (%/yr)

ROI Analysis

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Simple Payback

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Annual Savings

Year	Cumulative Savings	Net ROI

Wall Assembly R-Values

Assembly	R-Value	U-Value	Notes
Single-stud 2x4 fiberglass batt	R-11 to R-13	0.077–0.091	Pre-1980 standard
2x4 stud + R-13 + 1" foam	R-18 to R-20	0.050–0.056	Code minimum (many states)
2x6 stud + R-21 fiberglass	R-20 to R-21	0.048–0.050	Better practice
2x6 + R-21 + 2" foam	R-30 to R-32	0.031–0.033	High-performance
Double-stud 12" dense-pack	R-40+	< 0.025	Passive House typical

Ceiling/Attic R-Values

Assembly	R-Value	Notes
6" fiberglass batt	R-19	Pre-1990 standard
10" fiberglass batt	R-30	Code minimum (warm zones)
12" cellulose blown-in	R-38 to R-40	Recommended (most zones)
16" cellulose blown-in	R-49 to R-52	Code min (cold zones)
20"+ blown-in cellulose	R-60+	High-performance / zone 6–7

Window U-Values

Window Type	R-Value	U-Value	SHGC
Single pane	R-0.9	1.10	0.86
Double pane (air gap)	R-2	0.50	0.70
Double pane Low-E	R-3.3	0.30	0.35
Triple pane Low-E	R-5 to R-8	0.13–0.20	0.20–0.25
Dynamic (electrochromic)	R-4 to R-6	0.17–0.25	Variable

Infiltration & Thermal Bridging

Factor	Effect	Mitigation
Thermal bridging (studs)	Reduces wall R by 15–25%	Continuous exterior foam
Air leakage (ACH 0.35)	25–40% of heat loss	Air sealing + blower door
Duct leakage (15% typical)	Adds 10–30% to HVAC load	Mastic sealant, Manual D
Thermal mass	Reduces peak loads 10–20%	Concrete, brick, PCM
Ground contact (slab)	Lower ΔT (use 0.5× ΔT)	Underslab insulation

How to Use This Calculator

Select the surface type — wall, ceiling/roof, floor, window, or door. R-value presets update automatically.

Enter the surface area — measure the square footage of the specific assembly you want to analyze.

Choose the R-value — select from the preset options or enter a custom value. Higher R = lower heat loss.

Set temperature conditions — indoor setpoint (typically 70°F) and outdoor design temperature for your climate.

Review results — the calculator shows peak BTU/hr loss, annual energy use, and potential savings from insulation upgrades.

Glossary

R-Value

Thermal resistance — how well a material resists heat flow. Higher R = better insulation. R-13 is common for 2x4 wall cavities; R-38 for attics in cold climates.

U-Value

Thermal transmittance = 1/R. The rate of heat transfer per unit area per degree of temperature difference. Used for windows and whole assemblies.

ΔT (Delta-T)

Temperature difference between indoor and outdoor. A higher ΔT means more heat escapes per hour. Design conditions use the 99% heating dry-bulb temperature.

HDD (Heating Degree Days)

Annual cumulative degree-days below 65°F. Used to estimate seasonal energy use. Zone 4 (Chicago) ≈ 5,000 HDD; Fairbanks AK ≈ 14,000 HDD.

Thermal Bridging

Heat bypass through framing members (studs, joists) that have lower R-values than insulation. Continuous exterior insulation eliminates thermal bridging.

Simple Payback

Upgrade cost divided by annual energy savings. A 5-year payback is generally considered good for insulation. Factor in IRA tax credits to reduce effective cost.

Frequently Asked Questions

What is the difference between R-value and U-value?

R-value is thermal resistance — how well a material insulates. U-value is thermal transmittance (1/R) — the rate of heat flow. Insulation is rated in R-values; windows and building codes use U-values. To convert: a window with U-0.30 has R-3.3. Lower U-value = better performance.

Does this calculator account for thermal bridging through studs?

No — this is a simplified conduction-only calculator using the assembly's nominal R-value. In real walls, wood studs (R-1.25 per inch vs R-3.7 for fiberglass) reduce the effective R-value of a 2x4 wall by about 15–20%. For high-accuracy analysis, use a whole-wall R-value that accounts for framing fraction.

Why is the annual cost estimate based on therms?

Natural gas is billed in therms (100,000 BTU). The annual heat loss in BTU is divided by 100,000 to get therms, then multiplied by your gas price. For other fuel types, the cost will differ based on fuel BTU content and fuel price per unit. Use the Furnace Sizing calculator for multi-fuel cost comparisons.

What R-value should my attic have?

DOE recommends R-49 to R-60 for most cold climates (zones 5–7) and R-38 for mixed climates (zone 4). Many pre-1990 attics have only R-11 to R-19. Adding blown-in insulation to R-49 typically pays back in 3–7 years and is one of the highest-ROI energy upgrades available.

How do I find my outdoor design temperature?

The heating design temperature is the 99% dry-bulb temperature from ASHRAE data — meaning outdoor temperature is at or above this value 99% of the time. For Chicago: +4°F; Minneapolis: -12°F; Boston: 6°F; Atlanta: 19°F. Use the HVAC Load Calculator for ASHRAE design temperatures by climate zone.

How to Use This Calculator

Core Formulas

Glossary

Frequently Asked Questions

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