Sensible and latent infiltration loads by the air-change method, corrected for altitude.
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Building Volume
Infiltration & Conditions
Infiltration Flow
Infiltration CFM
—
ft³/min
Altitude Factor
—
relative to sea level
Cooling Season Infiltration
Sensible Cooling
—
BTU/h
Latent Cooling
—
BTU/h
Total Cooling
—
BTU/h
Cooling Tons
—
tons
Heating Season Infiltration
Sensible Heating
—
BTU/h
Heating MBH
—
MBH (× 1,000 BTU/h)
Infiltration Rate Reference
ACH
Construction Type
Typical Use
0.05–0.10
Very tight — blower door ≤ 1 ACH50
Passive house, high-perf new construction
0.10–0.25
Tight — energy code compliant new commercial
Most new commercial buildings post-2010
0.25–0.50
Average — some air sealing, older construction
Pre-2000 commercial, mid-range residential
0.50–1.00
Leaky — minimal sealing
Older warehouse, industrial, poorly sealed
1.0–2.0
Very leaky — open dock doors, large gaps
Uninsulated storage, loading docks
Reducing ACH from 0.50 to 0.25 cuts infiltration loads in half. Air sealing is often the highest-ROI energy measure in existing buildings. A blower door test precisely measures envelope leakage.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This infiltration load calculator estimates the heating and cooling loads from air leaking through a building envelope. Enter the building volume (or floor area and ceiling height), an infiltration rate in air changes per hour, site elevation, and the indoor and outdoor design conditions; the tool returns infiltration airflow, the altitude factor, and sensible, latent, and total loads for both seasons.
Infiltration is the uncontrolled leakage that adds to designed ventilation, and it can be a meaningful share of heating load in leaky buildings. Quantifying it shows the payoff from air sealing and feeds the envelope portion of a full load calculation.
Formula & Method
Quantity
Equation
Infiltration CFM
CFM = ACH × Volume (ft³) ÷ 60
Altitude factor
(1 − 6.8754×10⁻⁶ · elev)⁵⋅²⁵⁵⁹
Sensible load
Q‑sens = 1.08 · alt × CFM × ΔT
Latent load (cooling)
Q‑lat = 0.68 · alt × CFM × ΔW (gr/lb)
The air change method converts an assumed leakage rate into airflow, then applies the standard-air constants 1.08 for sensible heat (0.075 lb/ft³ × 60 min/h × 0.24 Btu/lb·°F) and 0.68 for latent heat (0.075 × 60 × 1,076 Btu/lb ÷ 7,000 gr/lb, with ΔW in grains per pound), each scaled by the barometric pressure ratio for elevation from ASHRAE Fundamentals Ch. 1. (Fundamentals Ch. 18 writes the sensible constant as 1.10, using moist-air cp = 0.244; that is about 2% higher.) Cooling sums sensible and latent; heating uses sensible only, since wintertime moisture addition is generally beneficial rather than a load. A design temperature or humidity difference at or below zero gives no load for that component.
Frequently Asked Questions
How is infiltration airflow estimated from ACH?
The air change method converts an assumed infiltration rate in air changes per hour into airflow: infiltration CFM equals ACH times building volume divided by 60. For example, 0.25 ACH in a 20,000 cubic foot building is about 83 CFM. Infiltration ACH values are much lower than ventilation ACH because they represent unintended envelope leakage, not designed supply air.
How do I calculate sensible and latent infiltration load?
Sensible load is 1.08 times infiltration CFM times the indoor-outdoor temperature difference. Latent load is 0.68 times infiltration CFM times the humidity-ratio difference in grains per pound. The 1.08 and 0.68 are sea-level airside constants that this tool scales by the altitude factor. Cooling combines sensible and latent; heating typically counts only sensible, since added winter moisture is usually beneficial.
What infiltration rate should I assume?
It depends on construction tightness. Very tight, blower-door-tested new buildings can be 0.05 to 0.10 ACH, energy-code-compliant new commercial around 0.10 to 0.25, average or older construction 0.25 to 0.50, and leaky buildings 0.50 to 1.0 or more. Halving the rate roughly halves the infiltration load, so air sealing is often a high-return measure. The reference table on this page lists typical ranges.
Why does the calculator apply an altitude correction?
Air density falls with elevation, so each cubic foot per minute carries less heat and moisture. The tool multiplies the sensible and latent factors by the barometric pressure ratio (1 minus 6.8754e-6 times elevation) raised to the 5.2559 power. At several thousand feet this noticeably lowers the calculated loads, and ignoring it would overstate the infiltration load at altitude.
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.