Ductwork Sealing - Mastic and Aeroseal How-To

Why this matters

Duct leakage in U.S. residential HVAC averages 20-30% - meaning 20-30% of conditioned air leaks into attic or crawl space before reaching the room. Customers pay full energy cost for partial comfort. Sealing the ducts is one of the highest-ROI HVAC services, with ticket size + a customer savings story. Two methods dominate: hand-applied mastic + Aeroseal aerosol sealing. This is the working procedure for both.

Why duct leakage matters

For a HVAC bill with 25% leakage: savings annually after sealing.

Plus IAQ: ducts in attics pull dust, fiberglass, vermin droppings, + summer humidity into the supply stream.

Diagnostic: Duct Blaster test

Quantitative measurement of duct leakage:

• Equipment: Retrotec or Energy Conservatory Duct Blaster ()
• Procedure: seal supply + return openings; pressurize ductwork; measure CFM at 25 Pascals
• Result: CFM25 - leakage rate at standard test pressure
• Target: < 4% of nominal system airflow (e.g., 16 CFM25 for a 400 CFM system) for code compliance

Reports differentiate:

• Total duct leakage (all directions)
• Leakage to outside (most important - leakage to conditioned space is less wasteful)

A duct blaster test is the before + after measurement that proves the value of sealing work. Charge for the test alone; some companies include in the sealing project.

Method 1: Hand-applied mastic + foil tape

For visible, accessible duct seams.

Tools

• Duct mastic (gallon container, water-based, UL 181 listed: Hardcast 351 or RCD #6)
• 4" putty knife or paint brush for application
• Fiber mesh tape (UL 181) for gaps > 1/4"
• Foil tape (UL 181-rated) for small joints
• Drop cloths
• Latex gloves
• Headlamp + flashlight

Procedure

Step 1: Survey + plan

1. Trace all duct runs in unconditioned space (attic, basement, crawl)
2. Identify visible joints, branch takeoffs, register boots
3. Photograph baseline condition
4. Estimate linear feet + joint count

Step 2: Identify leak sources

Focus on:

• Trunk-to-branch takeoff joints (often 8x worse than straight seams)
• Register boots (where flex meets the metal boot through the ceiling)
• Return air grille connections
• Plenum-to-air-handler connection
• Filter slot at air handler (huge leak path)

Step 3: Pre-clean

Wipe duct surface around joint clean. Mastic doesn't bond to dust + dirt.

Step 4: Apply mastic

1. Brush or putty knife
2. 1/16" thick layer minimum, 1/8" preferred
3. Spread 2" each side of the joint
4. For gaps > 1/4": embed fiber mesh tape in first layer, then second layer of mastic over

Mastic cures 4-24 hours; remains flexible for 20+ years.

Step 5: Cure + retest

• Cure 24 hours minimum before re-pressurizing
• Run duct blaster again; document improvement
• Typically 50-70% reduction in leakage with quality hand sealing

Method 2: Aeroseal

Patented aerosol sealing - the company is essentially required for residential branded use.

How it works

1. Plug all registers + returns
2. Connect Aeroseal machine to supply plenum
3. Machine pressurizes ductwork + atomizes sealant aerosol into the air stream
4. Aerosol particles deposit on edges of leaks, building up to seal
5. Computer measures real-time leakage reduction
6. Process runs 60-90 minutes typically
7. Achieves 80-95% leakage reduction

When Aeroseal wins over hand sealing

• Ducts inaccessible (interior walls, finished basements, complex routes)
• Many small leaks vs few large ones
• Customer wants documented result (Aeroseal generates a printed report)
• Time-constrained (faster than hand sealing on a complex system)

Aeroseal certification + equipment

• Per-job consumables:
• Per-job revenue:
• ROI strong; not all markets have local Aeroseal contractor + opportunity exists

Procedure (Aeroseal-certified contractor)

Step 1: Pre-test

• Duct blaster baseline measurement
• Verify system can be isolated

Step 2: Mask system

• Plug supply registers
• Plug returns
• Cover indoor coil to prevent sealant build-up
• Tape filter slot
• Connect Aeroseal machine to supply plenum

Step 3: Run process

• Software runs sealing cycle
• Real-time leakage measurement
• Customer + tech monitor decline curve

Step 4: Verify + finish

• Final duct blaster test
• Remove masking
• Print report
• Customer signature

Common pitfalls (both methods)

• Missing big leaks: register boots + plenum connections often more important than long-run seams
• Air handler not sealed: cabinet leakage matters too
• Filter slot not sealed: huge leak path
• Wrong sealant in damp location: foil tape on damp duct fails
• Aeroseal without coil masking: sealant deposits on evap coil; airflow reduced

Code requirements

IECC + most state codes require duct sealing to meet leakage thresholds:

• New construction: < 4 CFM25 per 100 sq ft of conditioned floor area (total leakage)
• Retrofit: code-trigger varies; many jurisdictions require certification on alteration work

A duct blaster test certifies compliance for inspection.

Acceptance criteria

• Total leakage reduced 50%+ (hand sealing) or 80%+ (Aeroseal)
• Post-test report
• Photos of completed work
• Customer signed work order + report

Customer math

For a typical 2,500 sq ft home with 25% duct leakage:

• Estimated savings from sealing to 8% leakage:
• Payback: 9-12 years on energy alone

Add comfort + IAQ benefits + property value: customer math is solid even on payback alone.

Customer talking points

The conversation that sells:

1. "Most homes leak 20-30% of their conditioned air into the attic + walls."
2. "We measure your specific leakage with a duct blaster test - you'll see the number."
3. "Sealing reduces that to under 10%."
4. "You save on energy + your bedrooms get more even temperatures."
5. "It's also an IAQ improvement: ducts no longer pull dust + humidity from the attic."

Show the duct blaster reading + report. Numbers close.

References

• ACCA Manual D Residential Duct Systems
• ASHRAE Handbook (Systems + Equipment)
• IECC R403.3 Duct Sealing + Insulation
• ENERGY STAR Home Performance with Energy Star
• Aeroseal contractor training materials
• Manuall internal: MERV Ratings and Air Filtration Reference