Air Leak Rate Calculator

Air Leak Rate Calculator

Understanding Air Leakage Rate

Air leakage, often referred to as infiltration or exfiltration, is the unintended movement of air into or out of a building. This occurs through cracks, gaps, and openings in the building envelope – the barrier between the conditioned interior and the unconditioned exterior. Common sources include around windows and doors, electrical outlets, plumbing penetrations, attic hatches, and wall cavities.

Understanding and quantifying air leakage is crucial for several reasons:

• Energy Efficiency: Uncontrolled air leaks allow conditioned air to escape and unconditioned air to enter, significantly increasing heating and cooling loads. This leads to higher energy bills and reduced comfort.
• Indoor Air Quality (IAQ): Air leaks can draw in outdoor pollutants, dust, and moisture, negatively impacting indoor air quality and potentially leading to health issues. They can also allow unconditioned, humid air to enter wall cavities, promoting mold growth.
• Moisture Control: Air movement carries moisture. Leaks can transport moisture into building assemblies, leading to condensation, mold, and material degradation.
• Building Durability: Persistent moisture issues from air leakage can compromise the structural integrity of a building over time.

How this Calculator Works: This calculator estimates the air leak rate based on a common method used in building diagnostics, often related to blower door testing.

The core idea is to pressurize or depressurize the building using a fan (like in a blower door test) and measure how quickly the pressure difference dissipates. A faster pressure drop indicates higher air leakage.

The calculation involves determining the initial pressure difference and the time it takes for that pressure to reduce by half. From these values, along with the building's volume, we can estimate the air exchange rate (ACH), which is a standard metric for air leakage.

Formulas Used:
The calculation is an approximation, often based on empirical relationships derived from blower door test data. A simplified approach to relate pressure decay to air change rate (ACH) involves the following:
1. Calculate the leakage area (an equivalent hole size) or directly relate pressure decay to air flow.
2. Estimate the Air Change Rate per Hour (ACH). A common approximation for the air change rate at a specific pressure difference (e.g., 50 Pascals) can be derived.
This calculator uses a simplified model that relates the time constant of pressure decay to ACH, considering the building volume.
Note: Precise calculations often involve more complex fluid dynamics equations and specific fan flow rates during testing. This tool provides an estimate based on pressure decay time.

function calculateAirLeakRate() { var volume = parseFloat(document.getElementById("volume").value); var pressureDifference = parseFloat(document.getElementById("pressureDifference").value); var time = parseFloat(document.getElementById("time").value); var resultDiv = document.getElementById("result"); var leakRateDisplay = document.getElementById("leakRate"); var leakRateACHDisplay = document.getElementById("leakRateACH"); // Clear previous results leakRateDisplay.textContent = ""; leakRateACHDisplay.textContent = ""; if (isNaN(volume) || isNaN(pressureDifference) || isNaN(time) || volume <= 0 || pressureDifference <= 0 || time ln(0.5) = -k * t_half => k = -ln(0.5) / t_half. // Then Q = k * V * P. If we want ACH at a standard pressure (e.g., 50 Pa), we need to extrapolate. // ACH = (Q_at_50Pa / V) * 3600. // Q_at_50Pa = k * V * 50 = (-ln(0.5) / t_half) * V * 50. // ACH = ((-ln(0.5) / t_half) * V * 50 / V) * 3600 = (-ln(0.5) / t_half) * 50 * 3600. // This simplified formula only depends on t_half and is often normalized to ACH50. // However, the user provides VOLUME and PRESSURE DIFFERENCE which are likely intended to be used. // Let's use a more direct estimation from pressure difference and time to half-pressure, // and then scale it to ACH50 using the provided volume. // This requires assuming a flow exponent. Let's assume n=0.65 as a typical value for building leakage. // Q = C * P^0.65 // dP/dt = – (C * P^0.65) / V_eff (where V_eff is effective volume, roughly V) // This differential equation is hard to solve analytically for time to half-pressure. // Alternative: Use a standard blower door test formula as a basis, but simplify inputs. // A common metric is ACH50 (Air Changes per Hour at 50 Pascals). // The flow rate (Q) at 50 Pa is often related to the leakage area. // Q_50 = C * (50 Pa)^n. For typical buildings, n is around 0.6 to 0.65. // Time to 50% pressure drop from P0 is roughly t_half = V / (Q_average * constant) // Let's use a simplified approach often seen in basic calculators that relates time to decay and volume to ACH. // This is highly approximate. // Formula often seen for approximate ACH based on volume and decay time: // Let's assume the provided pressure difference is the STARTING pressure for the decay. // The time given is for it to drop by 50%. // A very common simplification relates the decay rate constant 'k' to volume and flow. // If we assume Q = flow rate, then dP/dt = -Q/V. // If Q itself depends on P, it's more complex. // Let's assume Q is related to pressure difference in a way that a specific time 't' for a pressure drop // implies a certain leakage rate when scaled by volume. // A common simplified relationship for ACH when knowing the time to drop pressure by half (t_half): // ACH = (k * V * P_test) / V_unit_volume // Where k is related to the decay rate. // If we consider the time constant of decay tau = V/Q, and Q is related to P. // For P(t) = P0 * exp(-t/tau), t_half = tau * ln(2). // So tau = t_half / ln(2). // Q = V / tau = V * ln(2) / t_half. // This Q is the AVERAGE flow rate during the decay. // To get ACH50, we need to estimate flow at 50Pa. // Q_50 = Q_avg * (50 / P_avg)^n, where P_avg is some average pressure during decay. This gets complex. // Let's use a heuristic: Higher pressure difference, shorter time, and smaller volume all imply higher leakage. // A simplified formula that captures this: // Leakage Index = (Volume / Time) * (Pressure Difference Factor) // We need to convert this to ACH. // Let's stick to a common approximation for pressure decay: // If we know the time (t) it takes for pressure to drop by HALF (from P_initial to P_initial/2), // the time constant (tau) is approximately t / ln(2). // The average flow rate (Q_avg) driving this decay is V / tau = V * ln(2) / t. // This Q_avg is the flow rate that would maintain the average pressure during the decay. // To estimate ACH50, we need to extrapolate the flow rate to 50 Pa. // Assuming a flow exponent n (e.g., 0.65): Q_50 = Q_avg * (50 / P_initial)^n // Then ACH50 = (Q_50 / V) * 3600. // ACH50 = ( (V * ln(2) / t) * (50 / P_initial)^n / V ) * 3600 // ACH50 = ( ln(2) / t ) * (50 / P_initial)^n * 3600 // Let's use n = 0.65 as a common exponent. var n = 0.65; // Flow exponent var ln2 = Math.log(2); // Approximately 0.693 var secondsPerHour = 3600; var estimatedACH50 = (ln2 / time) * Math.pow((50 / pressureDifference), n) * secondsPerHour; // The direct leak rate can be expressed as the total leakage area, or as flow per volume. // Let's report the ACH50 as the primary metric. // We can also report an "Equivalent Leakage Area" if we assume Q = C * P^n and use the flow at 50Pa. // Q_50 = C * (50)^n. We found Q_50 = ACH50 * V / 3600. // C = Q_50 / (50)^n = (ACH50 * V / 3600) / (50^n). This C is related to leakage area. // Leakage Area (cm^2) = C * (100)^2 / ( (2 * rho * 1000)^0.5 * 3600 ) where rho air ~ 1.2 kg/m^3. This is too complex. // Let's provide a simpler "Leak Rate" metric in CFM (Cubic Feet per Minute) at 50 Pa, // which is a very common blower door output. // We need to convert our estimated Q_50 (in m^3/s) to CFM. // 1 m^3/s = 2118.88 CFM. var Q_50_m3_per_sec = estimatedACH50 * volume / secondsPerHour; var Q_50_CFM = Q_50_m3_per_sec * 2118.88; // Display results resultDiv.style.display = "block"; leakRateDisplay.textContent = "Estimated Air Leakage Rate at 50 Pa: " + Q_50_CFM.toFixed(2) + " CFM"; leakRateACHDisplay.textContent = "Estimated Air Changes per Hour at 50 Pa (ACH50): " + estimatedACH50.toFixed(2) + " ACH"; }