Exhaust Shaft Sealing: Why Vertical Exhaust Risers Leak and How Aerosol Sealing Fixes Them

Key Takeaways

1. What an Exhaust Shaft Does

In most mid-rise and high-rise apartment buildings and hotels, bathrooms and kitchens are stacked floor over floor. Instead of giving each one its own fan and wall vent, designers often run a vertical shaft, also called a riser, through the stack. Each unit connects to the shaft through a short branch duct and a grille. A fan at the top pulls air up and out.

The system is designed so each grille pulls a set airflow, measured in CFM (cubic feet per minute). That only works if the shaft is reasonably airtight. Every leak gives the fan an easier path to pull air from, and that air comes from somewhere other than the units.

2. Why Exhaust Shafts Leak

Shafts are long, mostly hidden and built by several trades over many floors. A NYSERDA study by Steven Winter Associates found significant leakage at a few predictable points, and found that sealing quality depended heavily on contractor training. Typical leak sources include:

• The roof curb. Where the shaft meets the fan curb on the roof.
• Joints along the shaft. Seams between duct sections on every floor.
• Branch and grille connections. Where the branch duct meets the grille in each unit.
• Shaft construction. In some buildings the masonry or gypsum board enclosure itself forms part of the air path, and every seam and penetration can leak.
• Age and later work. Renovations, new penetrations and settling open gaps over time.

Learn more about why ducts leak.

3. Symptoms of a Leaky Shaft

The NYSERDA study found that in the buildings examined, over 50% of the air pulled by roof exhaust fans was drawn from building cavities through leaky ducts. You’ll see that as:

• Bathroom fans that barely pull, often on lower floors
• Cooking odors or smoke moving between units
• Lingering moisture and mildew in bathrooms
• Roof fans running at high speed with weak results at the grilles
• Measured exhaust that doesn’t meet the design airflow

In a 2016 HPAC Engineering article written by Aeroseal about a hotel with 23-story ventilation shafts, bottom floors received 5 CFM or less of exhaust while top floors got the designed 40 CFM. Shaft leakage tends to hit the floors farthest from the fan hardest.

4. Why Leakage Affects Ventilation and Air Quality

Exhaust ventilation removes moisture, odors and pollutants from kitchens and bathrooms. The NYSERDA study notes that under-ventilation in parts of a building can harm indoor air quality, while over-ventilation elsewhere wastes energy. It also notes that when exhaust fans run on timers, shafts can become passive paths for moving contaminants between units.

Stack effect adds to the problem. Warm air rises in tall buildings, so pressures in a shaft change from floor to floor and season to season. A leaky shaft is much harder to balance under those changing conditions. Sealing the shaft gives balancing dampers and fans a stable system to work with. See air quality.

5. How Shaft Leakage Is Measured

Leakage testing on a shaft works much like testing a home duct system, on a larger scale.

The fan is removed or isolated, every grille on the shaft is temporarily blocked and a calibrated fan pressurizes the shaft to a reference pressure. The airflow needed to hold that pressure equals the leakage. Aeroseal reports its measurements conform to Method D of ASTM E1554, and that pre- and post-seal leakage is always reported at the same reference pressure. The NYSERDA study used 50 Pascals as its reference for exhaust shafts. Airflow at each grille can also be measured with a flow hood before and after. See how to read a duct leakage test.

6. How Inside-Out Aerosol Sealing Works on a Shaft

Aeroseal pressurizes the shaft and injects a fine mist of sealant particles. The particles stay suspended in the moving air until they reach a leak, where they stick to the edges and build up until the gap closes. Aeroseal reports leaks up to 5/8 inch across are sealed this way without coating the duct interior. Larger openings are patched by hand first.

Our Typical Exhaust Shaft Sealing Process

1. Assess. Review drawings, inspect the roof fan and curb, and sample grilles in units.
2. Prep. Remove grilles, temporarily block openings and repair large gaps by hand.
3. Baseline test. Measure shaft leakage at a reference pressure.
4. Seal. Inject sealant while a computer tracks leakage dropping in real time.
5. Final test. Measure leakage again at the same pressure.
6. Restore and report. Reinstall grilles, reconnect the fan and document before and after results.

Aeroseal reports multifamily sealing can be done with tenants in place. See how it works.

7. Manual Sealing vs Aerosol Sealing vs Rebuilding a Shaft

Most shaft projects combine hand repair at accessible points with aerosol sealing for the hidden leaks. Compare approaches on Aeroseal vs manual sealing vs replacement.

8. What Aerosol Sealing Does Not Fix

Sealing is one part of a working exhaust system. It doesn’t replace worn fans, fix stuck or missing balancing dampers, or rebalance grilles. It also doesn’t address commercial kitchen grease exhaust ducts, which are a separate, code-regulated system. If we find these issues during an assessment, we’ll tell you which trade to involve.

9. Results Reported by Aeroseal and Researchers

Aeroseal reports 80–92% leakage reduction per sealed section, about 90% on average, with post-seal leakage typically 1–3% of design airflow. In the hotel article above, Aeroseal reports average shaft leakage fell from 397 to 62 CFM. In an Aeroseal case study of an occupied 138-unit apartment building, total exhaust leakage fell 89%, alongside fan replacement and curb repairs.

These are Aeroseal’s and researchers’ results, not MaxSeal projects. Every building differs, and results vary.