Air Leakage Rate Calculation

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Air Leakage Rate Calculator

Understanding Air Leakage Rate

Air leakage, often referred to as infiltration or exfiltration, is the uncontrolled movement of air into or out of a building through cracks, gaps, and holes in the building envelope. This phenomenon significantly impacts a building's energy efficiency, indoor air quality, and occupant comfort. Quantifying air leakage is crucial for diagnosing building performance issues and implementing effective energy conservation strategies.

Why Calculate Air Leakage Rate?

A higher air leakage rate means more conditioned (heated or cooled) air escapes the building, and unconditioned outside air enters. This leads to:

  • Increased Energy Consumption: HVAC systems work harder to maintain desired temperatures, resulting in higher heating and cooling bills.
  • Reduced Comfort: Drafts and uneven temperatures can make occupants uncomfortable.
  • Poor Indoor Air Quality: Unfiltered outside air can bring in pollutants, allergens, and moisture, potentially leading to mold growth and other health issues.
  • Moisture Problems: Air leakage can transport moisture into wall cavities, leading to condensation and material degradation.

How is Air Leakage Rate Calculated?

The most common method for measuring and calculating air leakage is using a blower door test. This test depressurizes or pressurizes the building to a specific pressure difference and measures the resulting airflow required to maintain that pressure. The air leakage rate is then typically expressed in two ways:

  • Air Changes per Hour (ACH): This indicates how many times the entire volume of air within the building is replaced by outside air in one hour at a specific pressure difference. A common target for residential buildings is 3 ACH50 (3 air changes per hour at a pressure difference of 50 Pascals).
  • Specific Leakage Rate (SLR): This normalizes the leakage to the building's envelope area.

The formula used in this calculator for Air Changes per Hour (ACH50) is:

ACH50 = (Total Leakage Flow Rate (CFM) * 60 minutes/hour) / Building Volume (cubic feet)

The blower door test directly provides the Total Leakage Flow Rate (CFM) at a measured Pressure Difference. The Blower Door Test Time is often a factor in the software used to record the CFM but is not directly used in this simplified ACH50 calculation formula itself, though it's a necessary input for the test's execution and data capture.

Interpreting the Results

The calculated ACH50 value helps assess the building's airtightness. Lower ACH50 values indicate a tighter building envelope and better energy performance. Building codes and energy efficiency programs often set maximum allowable ACH50 values for new construction and renovations.

function calculateAirLeakageRate() { var buildingVolume = parseFloat(document.getElementById("buildingVolume").value); var blowerDoorTestTime = parseFloat(document.getElementById("blowerDoorTestTime").value); // Not directly used in ACH50 formula but part of test context var pressureDifference = parseFloat(document.getElementById("pressureDifference").value); // Used to define the standard for ACH50 (e.g., 50 Pa) var leakageFlowRate = parseFloat(document.getElementById("leakageFlowRate").value); // Total CFM measured at the pressure difference var resultDiv = document.getElementById("result"); resultDiv.innerHTML = ""; // Clear previous results if (isNaN(buildingVolume) || buildingVolume <= 0) { resultDiv.innerHTML = "Please enter a valid Building Volume."; return; } if (isNaN(leakageFlowRate) || leakageFlowRate < 0) { resultDiv.innerHTML = "Please enter a valid Total Leakage Flow Rate (CFM)."; return; } if (isNaN(pressureDifference) || pressureDifference <= 0) { resultDiv.innerHTML = "Please enter a valid Pressure Difference (Pascals)."; return; } // blowerDoorTestTime is not strictly needed for the ACH50 formula itself, but is part of the test parameters. // We'll acknowledge its input but not use it directly in this simplified calculation. // Calculate ACH50 var ach50 = (leakageFlowRate * 60) / buildingVolume; resultDiv.innerHTML = "

Results:

"; resultDiv.innerHTML += "Air Changes per Hour at 50 Pascals (ACH50): " + ach50.toFixed(2) + " ACH50″; resultDiv.innerHTML += "Note: This calculation assumes the leakageFlowRate (CFM) was measured at the specified pressure difference of " + pressureDifference + " Pascals."; }

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