Thermal Envelope Calculator

R-value and continuous insulation from climate zone.

Wall area (sq ft)

Cavity insulation R-value

Continuous insulation R-value

Framing fraction (%)

Framing R-value

Design ΔT (°F)

Heating degree-days (HDD)

Energy cost ($/BTU)

Tab to move between fields

Composite wall R-value

16.18

Composite U-value

0.06

Design heat loss (BTU/hr)4,946

Annual heat loss (MMBTU)14.84

Annual energy cost ($)$445.11

Meets ASHRAE 90.1 (1=yes)1

How to Use This Calculator

Enter Total Wall Area in sq ft — opaque wall only, excluding windows and doors.
Set Cavity Insulation R-Value (e.g., R-13 for 2×4 walls) and Continuous Insulation R-Value for exterior foam or mineral wool.
Enter Framing Fraction (%) — typically 20–25% — and Framing R-Value to account for thermal bridging.
Set Design ΔT (°F) and Heating Degree-Days for your location.
Review Composite Wall R-Value, Composite U-Value, Annual Heat Loss (MMBTU), and Annual Energy Cost to evaluate insulation upgrades.

How the result changes with Cavity insulation R-value

Wall area (sq ft): Total opaque wall area (exclude windows/doors).
Cavity insulation R-value: R-value of batt or blown insulation in the stud cavity (e.g., R-13 for 2×4, R-19 for 2×6).
Continuous insulation R-value: R-value of exterior continuous insulation (XPS, polyiso, mineral wool).
Framing fraction (%): Percentage of wall area that is studs/framing (typically 20-25%).
Framing R-value: R-value of wood framing (wood ≈ R-1.25/in, so 3.5" stud = R-4.375).
Design ΔT (°F): Temperature difference between indoor and outdoor design conditions.
Heating degree-days (HDD): Annual heating degree-days for your location (base 65°F).
Energy cost ($/BTU): Cost per BTU of heating energy (~$0.00003 for gas, ~$0.00009 for electric).

Composite wall R-value: Effective whole-wall R-value including thermal bridging.
Composite U-value: Effective U-factor (BTU/hr·ft²·°F), lower is better.
Design heat loss (BTU/hr): Peak heat loss through walls at design temperature difference.
Annual heat loss (MMBTU): Estimated annual heating energy lost through walls.
Annual energy cost ($): Estimated annual heating cost attributable to wall heat loss.
Meets ASHRAE 90.1 (1=yes): 1 if composite U ≤ 0.064 (Zone 4 mass wall max), 0 otherwise.

Identify Input Parameters
4 parameters
Wall area (sq ft) = 2000, Cavity insulation R-value = 13, Continuous insulation R-value = 5, Framing fraction (%) = 25 = 8 input(s) provided
Calculate Composite wall R-value
Composite wall R-value = 1 / compositeU
16.18 = 16.18
Calculate Composite U-value
Composite U-value = (1 - ff) * uInsulation + ff * uFraming
0.0618 = 0.0618
Calculate Design heat loss
Design heat loss = compositeU * wallArea * deltaT
4946 = 4946
Calculate Annual heat loss
Annual heat loss = annualHeatLossBTU / 1000000
14.84 = 14.84