Building Heat Loss Calculator

Q = U·A·ΔT for each building envelope component — walls, windows, doors, roof — where U is the assembly's overall heat transfer coefficient, A is area, and ΔT is the design temperature difference between indoors and outdoors. Total envelope heat loss is simply the sum across every component, which is what the Detailed mode component breakdown does after subtracting window and door openings from the gross wall area.

R-value (thermal resistance) and U-value (thermal transmittance) are reciprocals: U = 1/R. Insulation is conventionally rated by R-value (higher is better), while heat loss calculations conventionally use U-value (lower is better) because U-values combine additively for parallel heat-flow paths, and multiply directly by area and ΔT in the Q=UAΔT equation. A wall assembly's overall U-value accounts for every layer in the wall (interior air film, drywall, insulation, sheathing, cladding, exterior air film) combined in series.

Heat loss calculations use a winter design temperature — typically the ASHRAE annual heating design dry-bulb condition at 99% or 99.6% occurrence, published for thousands of locations in the ASHRAE Handbook of Fundamentals — not the coldest temperature ever recorded, and not an average. Using the true record-cold minimum would oversize equipment for the vast majority of the heating season; using an average would leave the system undersized on genuinely cold days. This calculator does not include location-specific design temperature data — enter the correct value for your project's actual location and code jurisdiction.

Q_slab = F·P·ΔT, where F is the slab's perimeter heat-loss factor (W/m·K or BTU/hr·ft·°F) and P is the exposed slab perimeter — not area. This reflects a genuinely different physical reality from wall or roof loss: heat loss through a slab-on-grade floor concentrates heavily at the perimeter edge, where the slab is closest to outdoor conditions, while the interior of a large slab loses comparatively little heat straight down into stable-temperature soil. ASHRAE 90.1 Appendix A includes slab-on-grade F-factor tables for its own envelope calculations. Use the F-factor required by the method or jurisdiction governing your project; do not assume coefficients from different procedures are interchangeable.

Q_inf = ρ·c_p·V̇·ΔT, commonly simplified to Q(W) ≈ 0.335·ACH·Volume(m³)·ΔT(K) in SI units, or Q(BTU/hr) = 1.08·CFM·ΔT(°F) in US units — both derived from air's density and specific heat. Air changes per hour (ACH) here is a user-supplied natural infiltration rate used only for screening. A blower-door result such as ACH50 is not the same quantity and should not be entered directly; converting measured leakage to design infiltration requires an accepted infiltration method and weather/exposure assumptions.

The Simple mode intentionally trades component-level precision for speed: it applies one blended effective U-value (based on a general insulation-quality tier) across a shoebox-model envelope area (estimated from floor area and ceiling height, assuming a roughly square footprint) rather than requiring separate wall, window, roof and door inputs. This gives a fast order-of-magnitude estimate suitable for early planning or a sanity check — the Detailed mode provides a more transparent component screening calculation, but final equipment sizing should use the complete procedure required for the project.

The Simple mode uses a single blended U-value and an estimated (not measured) envelope area based on a square-footprint assumption, while the Detailed mode uses component areas and individual assembly U-values — real buildings are rarely perfectly square, and window/wall/roof U-values differ substantially from each other, so the two methods will generally agree in order of magnitude but not exactly. This gap is expected and is exactly why the Simple mode is positioned as a fast estimate, not a design-grade calculation or Manual J result.

For a slab-on-grade floor, heat doesn't escape uniformly downward through the whole slab — it concentrates heavily at the edges, where the slab is close to outdoor conditions, while the center of a large slab sits over soil that stays relatively stable in temperature year-round. The F-factor method (F × perimeter × ΔT) reflects this physical reality directly, which is why a long, narrow building with the same floor area as a square one will show meaningfully more slab heat loss — it has more perimeter for the same area.

Use a natural infiltration rate appropriate to the project and the calculation method you have adopted. The Simple-mode ACH choices are illustrative screening presets only. Do not enter ACH50 from a blower-door test directly as natural ACH; converting leakage-test data to design infiltration requires a recognized procedure that accounts for building and exposure conditions.

It gives a screening estimate of winter heat loss, which can be an input to preliminary sizing — but real equipment selection also needs to account for equipment derating at low outdoor temperatures (especially for heat pumps), duct losses if applicable, a reasonable safety margin, and the specific heating equipment's actual output curve, none of which this calculator includes. Treat the result here as the building's demand, not a final equipment capacity.

Because this is specifically a winter heat loss calculation, which by convention and design intent looks at the worst-case (design) condition — a cold night with no sun and minimal internal gains — to ensure the heating system has adequate capacity even under the least favorable conditions. Solar gain and internal loads (people, lighting, equipment) matter enormously for cooling load calculations, which follow a different, generally more complex methodology since gains and losses interact with the building's thermal mass over time rather than existing in a single steady-state condition.