Plan clean demolition containment with confidence today. Size negative air units from volume and ACH. Reduce dust spread, protect occupants, and meet project requirements.
Use room dimensions, target air changes, and equipment airflow to estimate how many negative air units are needed for containment ventilation.
| Length (ft) | Width (ft) | Height (ft) | ACH | Unit Airflow (CFM) | Safety | Leakage | Volume (ft³) | Adjusted Airflow (CFM) | Units Needed |
|---|---|---|---|---|---|---|---|---|---|
| 20 | 15 | 10 | 6 | 2000 | 1.20 | 1.00 | 3000 | 360 | 1 |
Negative air units help keep dust and contaminants inside a containment by exhausting air through filtration and directing flow inward at openings. Correct unit counts reduce migration risk to adjacent areas, improve worker comfort, and support cleaner handoff to finishing trades during controlled demolition and renovation. Sizing also helps plan power needs and equipment placement.
The first step is enclosure volume (V = L × W × H). In imperial units, baseline airflow is Q = (V × ACH) ÷ 60. A 20 × 15 × 10 ft zone is 3,000 ft³. At 6 ACH, baseline airflow is 300 CFM before allowances. In metric, Q = V × ACH using m³/h.
ACH is a planning target that links activity intensity to ventilation. Many teams start between 6 and 12 ACH, increasing for higher dust loads, frequent debris movement, or stricter cleanliness expectations. For sensitive areas, extra ACH can be paired with better sealing to stabilize conditions. Reassess after the first shift based on observed dust and traffic.
Rated airflow can drop as filters load, bends add losses, and long ducts raise resistance. The safety factor covers performance losses, while the leakage factor covers door traffic, imperfect sealing, and temporary openings. For the 300 CFM baseline example, using 1.20 safety and 1.00 leakage gives 360 CFM adjusted demand.
Divide adjusted airflow by unit capacity and round up. With a 2,000 CFM unit, 360 CFM requires one unit; with a 250 CFM unit, it requires two. Splitting airflow across multiple units can add redundancy and simplify maintenance. After setup, confirm directional airflow at entry points and log filter changes, readings, and notes so daily checks remain consistent.
It estimates how many negative air units you need to meet a target ventilation rate based on enclosure volume, desired ACH, and the airflow rating of each unit, with optional safety and leakage adjustments.
Use a planning ACH that matches task intensity. Light work may use 6 ACH, while heavy dust generation can justify 10 to 12 ACH. When unsure, start higher and refine after stable site observations.
Rated airflow can drop with filter loading and duct resistance. Leakage and door openings increase air demand. These factors help avoid under-sizing and improve reliability during peak activity.
No. Negative pressure depends on enclosure tightness, make-up air pathways, and exhaust location. Airflow sizing supports ventilation targets, while pressure stability requires good sealing, controlled openings, and consistent exhaust performance.
Multiple units can provide redundancy and flexible staging. If one unit fails, others can maintain partial ventilation. Multiple exhaust points can also help balance airflow, depending on containment geometry and duct routing.
Select Metric to use meters and m³/h. Enter unit airflow in m³/h, not per minute. The calculator uses Q = Volume × ACH for metric, matching the hourly basis of ACH.
Capture dimensions, ACH target, unit ratings, factors used, calculated unit count, filter configuration, and site notes. Exporting the CSV or PDF helps keep a consistent record for supervisors and quality documentation.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.