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Calcimator

Thermal Envelope Calculator

R-value and continuous insulation from climate zone.

About this calculator

Composite Wall R-value and U-value respond only to Cavity R-value, Continuous Insulation R-value, Framing Fraction, and Framing R-value -- the four inputs that build up the wall assembly's insulation performance -- and never to Heating Degree-Days, Wall Area, or Energy Cost per BTU, which only scale the downstream heat-loss and cost figures. Heating Degree-Days is a pure multiplier on Annual Heat Loss and Annual Energy Cost: at zero HDD (no heating season) Annual Energy Cost drops to $0 regardless of how well-insulated the wall is, while 15,000 HDD (a cold climate) pushes it to about $1,335/year at this calculator's other defaults, with Composite R-value staying fixed at 16.18 throughout.

Cavity R-value has the largest swing on the composite R-value itself: at 0 (no cavity insulation) Composite R-value is only about 7.1 and Meets ASHRAE 90.1 flips to 0 (fails); pushing Cavity R-value to 40 alone raises it to about 25.4 and flips Meets ASHRAE to 1 (passes) even with Continuous Insulation left at its default. Meets ASHRAE 90.1 compares Composite U-value against a fixed 0.064 threshold this calculator uses for its Zone 4 comparison -- it's a pass/fail flag, not a graduated score.

Inputs

sq ft
%
°F

Results

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
  1. Enter Total Wall Area in sq ft — opaque wall only, excluding windows and doors.
  2. Set Cavity Insulation R-Value (e.g., R-13 for 2×4 walls) and Continuous Insulation R-Value for exterior foam or mineral wool.
  3. Enter Framing Fraction (%) — typically 20–25% — and Framing R-Value to account for thermal bridging.
  4. Set Design ΔT (°F) and Heating Degree-Days for your location.
  5. 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

Cavity insulation R-valueComposite wall R-valueComposite U-value
6.512.30.08
9.7514.370.07
2019.390.05
3323.690.04

What each input means

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).

What each result means

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 wood-framed wall max), 0 otherwise.

How this is calculated

Worked example, using the default values

  1. 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
  2. Calculate Composite wall R-value
    Composite wall R-value = 1 / compositeU
    16.18 = 16.18
  3. Calculate Composite U-value
    Composite U-value = (1 - ff) * uInsulation + ff * uFraming
    0.0618 = 0.0618
  4. Calculate Design heat loss
    Design heat loss = compositeU * wallArea * deltaT
    4946 = 4946
  5. Calculate Annual heat loss
    Annual heat loss = annualHeatLossBTU / 1000000
    14.84 = 14.84

Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Does a colder climate (higher Heating Degree-Days) change the wall's R-value or U-value?

No. Composite R-value and Composite U-value are set entirely by Cavity R-value, Continuous Insulation R-value, Framing Fraction, and Framing R-value -- Heating Degree-Days only scales Annual Heat Loss and Annual Energy Cost downstream. At zero HDD, Annual Energy Cost drops to $0 even though the wall's R-value and U-value stay exactly the same.

How much does Cavity R-value alone change whether the wall passes the ASHRAE check?

Substantially. At this calculator's other defaults, Cavity R-value of 0 gives a Composite R-value of about 7.1 and fails Meets ASHRAE 90.1 (flips to 0), while raising Cavity R-value to 40 alone lifts Composite R-value to about 25.4 and passes (flips to 1) -- without touching Continuous Insulation at all.

Does setting Heating Degree-Days to 0 mean the annual energy cost also goes to 0?

Yes -- Annual Energy Cost is directly proportional to Heating Degree-Days, so a 0 HDD input (no heating season) always produces exactly $0, regardless of how poor the wall assembly's R-value is. At 15,000 HDD, the same wall costs about $1,335/year at this calculator's other defaults.

What does the Meets ASHRAE 90.1 flag actually check?

It compares Composite U-value against a fixed 0.064 threshold built into this calculator for its Zone 4 comparison, returning 1 if your assembly's U-value is at or below that number and 0 if not -- a simple pass/fail flag, not a graduated compliance score or a substitute for a project-specific code review.

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