Air Changes Per Hour (ACH) Impact on Cooling

What Are Air Changes Per Hour (ACH)?

ACH measures how many times per hour your home's entire air volume is replaced by outdoor air through leaks, cracks, and gaps. An ACH of 0.5 means half your indoor air leaks out and is replaced by outdoor air every hour.

Why it matters for cooling: Every cubic foot of hot, humid outdoor air that infiltrates must be cooled and dehumidified. A leaky 2,000 sq ft home with 1.2 ACH brings in 16,000 cubic feet of 95°F outdoor air per hour—requiring 4,000-6,000 BTU/hr just to condition infiltration air.

Our BTU calculator includes infiltration in load calculations. This guide helps you understand and reduce ACH for lower cooling requirements.

How ACH Affects Cooling Load

Infiltration Load Formula

BTU/hr = (Volume × ACH × Temp Difference × 0.018) + (Volume × ACH × Humidity Factor)

Sensible Heat (Temperature):

BTU/hr = Home Volume (cu ft) × ACH × (Outdoor Temp - Indoor Temp) × 0.018

Latent Heat (Humidity):

BTU/hr = Home Volume × ACH × (Outdoor Humidity - Indoor Humidity) × 0.68

Example: 2,000 Sq Ft Home, 8 Ft Ceilings

• Volume: 2,000 sq ft × 8 ft = 16,000 cu ft
• ACH: 1.2 (typical leaky home)
• Outdoor conditions: 95°F, 65% humidity
• Indoor target: 75°F, 50% humidity

Sensible Load:

16,000 × 1.2 × (95 - 75) × 0.018 = 6,912 BTU/hr

Latent Load (simplified):

~2,500 BTU/hr (humidity difference contribution)

Total Infiltration Load:

~9,400 BTU/hr just from air leaks

After Air Sealing to 0.5 ACH:

16,000 × 0.5 × 20 × 0.018 + ~1,000 = 3,900 BTU/hr

Savings: 9,400 - 3,900 = 5,500 BTU/hr reduction (58% less infiltration load)

For this home's total load of 28,000 BTU, reducing infiltration by 5,500 BTU allows downsizing from 2.5 tons to 2 tons—saving $800-1,200 on equipment plus $150-250/year in operating costs.

Measuring Your Home's ACH

Method 1: Blower Door Test (Most Accurate)

• What it does: Pressurizes home to 50 Pascals, measures airflow needed to maintain pressure
• Results: ACH50 (air changes per hour at 50 Pa pressure) and CFM50 (cubic feet per minute leakage)
• Cost: $300-500 as standalone test, $150-250 as part of energy audit
• Convert ACH50 to natural ACH: ACH natural ≈ ACH50 ÷ 20 (typical divide factor)

Example Blower Door Results:

• Test result: ACH50 = 12 (12 air changes per hour at 50 Pa)
• Natural ACH: 12 ÷ 20 = 0.6 ACH under normal conditions
• Rating: Tight home (0.5-0.7 ACH is good for non-Passive House)

Method 2: Estimation by Home Age/Type

Adjust upward if your home has many windows, fireplace, attic access, or visible cracks. Adjust downward if recently air-sealed, spray-foamed, or Energy Star certified.

Common Air Leakage Points

Top 10 Leak Sources (Ordered by Impact)

1. Attic Access & Pull-Down Stairs (15-25% of leakage)

• Problem: Attic hatch gaps, unsealed stairs
• Fix: Weatherstrip hatch, install insulated cover ($50-150)
• DIY difficulty: Easy

2. Rim Joists (10-20% of leakage)

• Problem: Where floor meets foundation, often uninsulated
• Fix: Spray foam or rigid foam + caulk ($200-600 DIY, $800-1,500 pro)
• DIY difficulty: Moderate

3. Windows & Doors (10-20% of leakage)

• Problem: Gaps around frames, worn weatherstripping
• Fix: Caulk exterior gaps, replace weatherstripping ($50-200 DIY)
• DIY difficulty: Easy

4. Electrical Outlets & Switches (8-12% of leakage)

• Problem: Holes in drywall around boxes leak to wall cavities
• Fix: Foam gaskets behind cover plates ($20 for 50-pack)
• DIY difficulty: Very easy

5. Plumbing Penetrations (8-15% of leakage)

• Problem: Gaps around pipes entering walls/floors
• Fix: Expanding foam or caulk ($30-100 DIY)
• DIY difficulty: Easy

6. Recessed Lighting (5-12% of leakage)

• Problem: Non-IC rated cans vent directly to attic
• Fix: Replace with IC-rated or install airtight covers ($15-30 per light)
• DIY difficulty: Moderate (electrical work)

7. Fireplace Dampers (5-10% of leakage)

• Problem: Damper doesn't seal completely
• Fix: Chimney balloon ($50-80) or top-sealing damper ($150-400 installed)
• DIY difficulty: Easy (balloon), moderate (damper)

8. Ductwork Leaks (10-30% of air, not house leakage but huge waste)

• Problem: Ducts leak into attic/crawlspace
• Fix: Mastic sealing ($800-1,500 pro, $100-300 DIY accessible areas)
• DIY difficulty: Moderate

9. Baseboards & Crown Molding (3-8% of leakage)

• Problem: Gaps where trim meets drywall
• Fix: Caulk gaps ($30-80 DIY)
• DIY difficulty: Easy

10. Dryer Vent & Exhaust Fans (3-7% of leakage)

• Problem: Backdraft dampers don't seal when off
• Fix: Replace with tight-sealing models ($30-80 per vent)
• DIY difficulty: Moderate

Air Sealing ROI by Climate Zone

Hot Climates (Zones 1-2)

• Typical home: 1.2 ACH → sealed to 0.5 ACH
• Cooling load reduction: 4,000-7,000 BTU/hr
• Annual savings: $250-450 (cooling) + $100-200 (heating)
• Sealing cost: $800-2,000 (professional) or $200-500 (DIY)
• Payback: 2-5 years professional, <1 year DIY

Mixed Climates (Zones 3-4)

• Annual savings: $200-350 (cooling) + $200-400 (heating) = $400-750 total
• Payback: 2-4 years professional, <1 year DIY

Cold Climates (Zones 5+)

• Annual savings: $100-200 (cooling) + $400-800 (heating) = $500-1,000 total
• Payback: 1-3 years professional, <6 months DIY
• Note: Heating savings dominate in cold climates—air sealing is even more valuable

Check your zone: Climate Zone Guide

DIY Air Sealing Priorities

Professional Air Sealing

What Pros Do That DIY Can't

• Blower door testing: Before/after measurements prove savings
• Infrared cameras: Identify hidden leaks in walls, attics
• Foam spray equipment: Access to closed-cell foam for rim joists, band joists
• Attic air sealing: Seal top plates, electrical/plumbing penetrations in attic floor (hard to access)
• Dense-pack insulation: Fill wall cavities via small holes

Cost Breakdown

• Blower door test only: $300-500
• Basic air sealing (accessible areas): $800-1,500
• Comprehensive air sealing (includes attic): $1,500-3,000
• Air sealing + insulation upgrade: $3,000-6,000

Get quotes from 3 contractors using our bid comparison guide.

Ventilation Considerations

Don't Seal TOO Tight Without Ventilation

Homes below 0.35 ACH need mechanical ventilation (ERV or HRV) to:

• Prevent indoor air quality problems (CO2, VOCs, odors build up)
• Control moisture (humid air trapped indoors causes mold)
• Provide fresh air for occupants

Target range: 0.35-0.5 ACH is ideal—tight enough for energy savings, loose enough to avoid IAQ issues without mechanical ventilation.

ERV vs HRV

• ERV (Energy Recovery Ventilator): Transfers heat AND humidity between incoming/outgoing air. Best for hot-humid climates (prevents bringing in humid outdoor air).
• HRV (Heat Recovery Ventilator): Transfers heat only. Best for cold-dry climates.
• Cost: $1,500-3,500 installed
• Operating cost: $50-150/year electricity

Impact on AC Sizing

Example: 2,000 Sq Ft Home, Zone 3

Before Air Sealing (1.2 ACH):

• Total load calculation:
  • Envelope (walls/roof/floor): 12,000 BTU/hr
  • Windows: 6,000 BTU/hr
  • Infiltration (1.2 ACH): 9,000 BTU/hr
  • Internal gains: 3,000 BTU/hr
  • Total: 30,000 BTU/hr → 2.5 tons

After Air Sealing (0.5 ACH):

• New load calculation:
  • Envelope: 12,000 BTU/hr (unchanged)
  • Windows: 6,000 BTU/hr (unchanged)
  • Infiltration (0.5 ACH): 3,750 BTU/hr (58% reduction)
  • Internal gains: 3,000 BTU/hr (unchanged)
  • Total: 24,750 BTU/hr → 2 tons

Savings:

• Equipment: 2-ton ($5,000) vs 2.5-ton ($6,000) = $1,000 saved
• Annual operating: $320/yr vs $400/yr = $80/yr saved
• Air sealing cost: $1,200 (professional)
• Net investment: $1,200 - $1,000 equipment savings = $200
• Payback: $200 ÷ $80/yr = 2.5 years

Plus: Heating savings of $200-400/year → total payback under 1 year!

Calculate your specific load reduction: BTU Calculator with different ACH inputs.

Tools & Resources

• BTU Calculator: Model load changes with different ACH rates
• Insulation Guide: Combine air sealing with insulation upgrades
• Climate Zone Guide: Regional infiltration impacts and savings
• Humidity Control: How infiltration affects latent load