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Heat Loss Calculator (U-Value + Delta T)

Calculate steady-state building envelope heat loss across multiple structural assemblies using U-value transmittance coefficients, exposed surface areas, and design winter temperature delta.

Heat Loss Inputs

Temperature Delta

Structural Assemblies

Assembly Matrix

Assembly 1

Equiv: R-15.4

Assembly 2

Equiv: R-2.2

Assembly 3

Equiv: R-38.5

Live Results

Total Enclosure Heat Loss

14,472.0 BTU/hr

Envelope Component Breakdown

  • Exterior Framed Walls5,850.0 BTU/hr
  • Double-Pane Windows6,750.0 BTU/hr
  • Insulated Ceiling/Attic1,872.0 BTU/hr

Envelope density: 4.9 BTU/sqft at 60.0°F ΔT

High-Performance Air Boundary — Enclosure density registers below 10 BTU/sqft. Exceptional insulation limits building energy drain. Safely optimized for modern air-source hydronic heating units.

How to Use This Heat Loss Calculator

  1. Set design indoor and outdoor winter temperatures. Enter the target indoor setpoint (default 70°F) and the design outdoor winter temperature (default 10°F). The calculator derives ΔT as the positive difference between these values.
  2. Review or customize structural assembly rows. Three baseline assemblies are pre-loaded: exterior framed walls (U-0.065), double-pane windows (U-0.450), and insulated ceiling/attic (U-0.026). Edit labels, U-values, and exposed areas or add/remove rows as needed.
  3. Use the equivalent R-value helper under each U-value field. Each assembly row displays an equivalent R-value (R = 1/U) beneath the U-value input for quick trade reference when comparing insulation profiles.
  4. Review total enclosure heat loss and per-assembly breakdown. The Live Results panel shows total building heat loss in BTU/hr, a component-by-component breakdown for each named assembly, and the envelope density metric in BTU/sqft.
  5. Evaluate the envelope density compliance badge. RED indicates high-loss assemblies above 20 BTU/sqft, YELLOW reflects conventional mid-2000s performance between 10–20 BTU/sqft, and GREEN signals high-performance thermal boundaries below 10 BTU/sqft.

Formulas & Example

Steady-state transmission heat loss sums independent assembly contributions using the core Manual J envelope equation. U-value and R-value relate inversely: R ≈ 1/U.

ΔT = max(0, T_indoor − T_outdoor)

Assembly Heat Loss (BTU/hr) = U × Area (sq ft) × ΔT

Total Enclosure Heat Loss = Σ (Uᵢ × Aᵢ × ΔT)

Envelope Density (BTU/sqft) = Total Heat Loss ÷ Total Exposed Area

Performance Thresholds:
  > 20.0 BTU/sqft  → RED   (High-Loss / Uninsulated)
  10.0–20.0        → YELLOW (Conventional Standard Build)
  ≤ 10.0           → GREEN  (High-Performance Thermal Boundary)

Worked Example

Default assemblies at 70°F indoor, 10°F outdoor (ΔT = 60°F):

Walls:   0.065 × 1,500 × 60 = 5,850.0 BTU/hr
Windows: 0.450 ×   250 × 60 = 6,750.0 BTU/hr
Ceiling: 0.026 × 1,200 × 60 = 1,872.0 BTU/hr
─────────────────────────────────────────────
Total:                        14,472.0 BTU/hr
Total Area: 2,950 sq ft → 4.9 BTU/sqft → GREEN badge

Add slab edges, doors, or foundation walls with the assembly row builder to model complete building envelopes. Lower U-values (higher R-values) directly reduce per-assembly heat loss at a given ΔT.

Frequently Asked Questions

What formula does the heat loss calculator use?▾
Steady-state transmission heat loss follows Q = U × A × ΔT, where U is the thermal transmittance (BTU/hr·ft²·°F), A is exposed surface area in square feet, and ΔT is the design temperature difference (indoor minus outdoor, floored at zero). Total building heat loss is the sum of all assembly contributions.
How is design temperature difference (ΔT) calculated?▾
ΔT equals the design indoor temperature minus the design outdoor winter temperature, with a minimum of zero. For example, 70°F indoor and 10°F outdoor yields a 60°F ΔT applied uniformly across all assemblies.
What U-values are used in the default assembly baselines?▾
Defaults include exterior framed walls at U-0.065 (≈ R-15), double-pane windows at U-0.450 (≈ R-2.2), and insulated ceiling/attic at U-0.026 (≈ R-38). These represent common mid-code residential construction profiles and can be edited for your specific envelope.
What do the RED, YELLOW, and GREEN envelope density badges mean?▾
The badge evaluates total heat loss divided by total exposed area (BTU/sqft). Above 20 BTU/sqft is RED (high thermal bridging risk), 10–20 BTU/sqft is YELLOW (conventional standard build), and below 10 BTU/sqft is GREEN (high-performance thermal boundary).
Can I add custom building assemblies like doors or slab edges?▾
Yes. Click '+ Add Assembly Row' to insert additional surfaces, assign a custom label, enter the U-value (step 0.001), and set the exposed area in square feet. Each row contributes independently to the total enclosure heat loss.

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