Calculate heat loss through walls using the Q = A × ΔT / R formula. Enter wall area, R-value, and temperature difference to determine BTU/h heat transfer rate.
Heat loss through walls is one of the largest energy expenses in buildings. Understanding the rate of heat transfer helps you evaluate insulation effectiveness, compare upgrade options, and estimate heating costs. The fundamental heat loss equation — Q = A × ΔT / R — calculates the rate of thermal energy transfer through a building assembly.
This heat loss calculator applies the steady-state heat transfer formula to your wall (or ceiling/floor) assembly. Enter the area, total R-value, and indoor-to-outdoor temperature difference to see how many BTU per hour flow through the assembly. You can compare different R-values side by side to quantify the energy savings from insulation upgrades.
The results help you understand where your building loses the most heat, prioritize insulation improvements, and estimate the impact on heating costs. Higher R-values dramatically reduce heat loss: doubling the R-value cuts heat loss in half.
Accurate calculation of this value helps construction professionals plan projects more effectively, reduce material waste, and ensure compliance with building codes and industry standards.
This calculator quantifies heat loss in BTU/h so you can convert insulation improvements into energy savings and dollar figures. It helps justify insulation investments by showing exactly how much energy each R-value increment saves. Having precise numbers at hand streamlines project planning discussions with clients, architects, and subcontractors, building trust and reducing costly misunderstandings on the job.
Q = A × ΔT / R Where: Q = heat flow (BTU/h) A = area (sq ft) ΔT = temperature difference (°F) R = total R-value (ft²·°F·h/BTU)
Result: 3,077 BTU/h
1,000 sq ft wall area × 40°F temperature difference ÷ R-13 = 3,077 BTU/h heat loss. If upgraded to R-21: 1,000 × 40 ÷ 21 = 1,905 BTU/h — saving 1,172 BTU/h (38% reduction).
Q = A × ΔT / R is the fundamental steady-state conduction heat loss equation. It assumes constant temperatures on both sides and works well for sizing heating systems and comparing insulation options. Real heat loss is more complex (solar gains, thermal mass, wind) but this equation provides excellent estimates.
Use this calculator to compare before and after: calculate heat loss with current R-value, then with the upgraded R-value. The difference is your energy savings in BTU/h. Multiply by heating hours per year to estimate annual savings.
A wall assembly includes multiple layers, each contributing R-value: interior air film, drywall, insulation, sheathing, WRB, siding, exterior air film. The total R-value is the sum of all layers. Thermal bridging through studs reduces the effective R-value by 10–20%.
The biggest energy savings come from insulating the weakest links. Attics and crawl spaces often have the lowest R-values and the highest ΔT exposure. Use this calculator on each building component to find where insulation upgrades provide the greatest return.
Use the "design ΔT" for your location: indoor setpoint (typically 70°F) minus the ASHRAE 99% heating design temperature. For example, if design temp is 10°F: ΔT = 70 − 10 = 60°F. This represents peak heating conditions.
Enter the total R-value of the entire wall assembly, not just the insulation. A typical wall: R-0.68 (air film) + R-0.45 (drywall) + R-13 (insulation) + R-0.6 (sheathing) + R-0.5 (siding) + R-0.17 (exterior air) = R-15.4 total.
BTU/h × heating hours/year = annual BTU. Divide by furnace efficiency (e.g., 0.95 for gas, 1.0 for electric). Multiply by fuel cost per BTU (natural gas ~$1.10/therm ÷ 100,000 BTU/therm = $0.000011/BTU).
Because the relationship is inverse: Q = A×ΔT/R. Doubling R exactly halves Q. This is why the first inches of insulation save the most energy (going from R-5 to R-10 saves 50%), while additional insulation has diminishing returns.
No, this is a simplified calculation. Wood studs have a lower R-value than cavity insulation, creating thermal bridges. The effective whole-wall R-value is typically 10–20% lower than the cavity insulation R-value alone.
This calculator only computes conduction (through-material) heat loss. Air leakage (infiltration) is a separate and often larger heat loss pathway. Addressing air sealing alongside insulation is critical for energy efficiency.
Calculate heat loss separately for each building component (walls, ceiling, floor, windows, doors) and sum them. Add air infiltration heat loss. The total is your building's peak heating load in BTU/h, used for furnace sizing.
The same formula applies for cooling season, but the direction is reversed (heat flows inward). The ΔT is outdoor temperature minus indoor setpoint. Higher R-values reduce both heating and cooling loads.