Home Ventilation & Air Exchange Guide

Why Ventilation Matters

Modern homes are sealed tight for energy efficiency, but that traps CO2, VOCs, moisture, and cooking particulates indoors. Controlled ventilation brings in fresh air intentionally — exchanging stale air at a rate that maintains quality without blowing your energy budget.

Understanding Air Changes Per Hour

Air changes per hour (ACH) measures how many times a space’s air volume is fully replaced in one hour. Building codes recommend a 0.35 ACH minimum for homes — roughly one-third of your air replaced each hour.

For a 1,500 sq ft home with 8-foot ceilings (12,000 cu ft of air), that means about 70 CFM of fresh air.

Types of Ventilation

Exhaust-Only Ventilation

Bathroom fans and range hoods pull stale air out. Fresh air enters passively through gaps and cracks, creating slight negative pressure.

Pros: Inexpensive. Uses equipment you already own.

Cons: No control over where makeup air comes from — it may be drawn through the garage, attic, or crawl space. Negative pressure can cause backdrafting in gas appliances.

Best for: Mild climates and older, leakier homes.

Supply-Only Ventilation

A fan or duct brings filtered outdoor air in. Stale air exits passively through leaks and exhaust points, creating slight positive pressure.

Pros: You control and filter incoming air.

Cons: In humid climates, positive pressure forces moist air into wall cavities, causing condensation and mold.

Best for: Dry climates.

Balanced Ventilation (HRV and ERV)

HRVs and ERVs simultaneously exhaust stale air and bring in fresh air through a heat exchanger, recovering 70-85% of the energy.

HRV vs ERV: HRVs transfer only heat — best for cold, dry climates where you want to shed indoor moisture. ERVs transfer heat and moisture — best for hot, humid climates where you want to keep humid air out.

Pros: Energy efficient, balanced pressure, controlled air quality.

Cons: $1,500-$3,000 installed. Requires ductwork and filter maintenance.

Best for: Tight, energy-efficient homes in extreme climates.

Practical Steps for Any Home

Run Exhaust Fans Properly

• Run the bathroom fan for 20 minutes after every shower, not just during.
• Run the range hood every time you use the stove, especially gas burners (they produce NO2 and CO).
• Verify fans actually vent outside. Some older homes exhaust into the attic.

Strategic Window Opening

• Open windows on opposite sides of the home for 10-15 minutes for cross-ventilation — the fastest way to flush indoor air.
• Even in winter, 10 minutes will not meaningfully affect your heating bill. Thermal mass in walls and furniture recovers quickly.
• Skip this on high-pollen days if allergies are a concern.

Monitor CO2 as a Ventilation Indicator

A portable CO2 monitor ($50-$100) is the best way to gauge ventilation. Outdoor air sits around 420 ppm. Well-ventilated indoor spaces stay below 800 ppm. If your bedroom reads above 1,000 ppm in the morning, you need more ventilation.

Maintain Your HVAC System

• Replace furnace filters every 1-3 months.
• Have ductwork inspected every few years. Leaky ducts dump conditioned air into the attic or pull contaminated air into living areas.
• Keep all vents unblocked. Do not close vents in unused rooms — it creates pressure imbalances that increase air leakage.