HVAC & Filtration Protocol

Air Systems Cannot Save a Wet Wall

This is the foundational thesis of the Good Air Standard, and it applies most directly to HVAC design. The building science industry has spent decades developing increasingly sophisticated air handling systems. Many of those systems are excellent. None of them can fix a wall assembly that traps moisture between layers of impermeable material, feeds mold with organic sheathing, and provides no drying path.

Air quality starts with the building envelope. Ventilation, filtration, and humidity control are the reinforcement — critical reinforcement, but reinforcement. If you are reading this page because you want to install better air systems in a home that has moisture problems, start with Moisture Control first.

Balanced Mechanical Ventilation: ERV or HRV

The Good Air Standard requires balanced mechanical ventilation in every certified home. This means controlled fresh air in and stale air out, through a heat recovery system that transfers energy between the two airstreams without mixing them.

ERV (Energy Recovery Ventilator)

Transfers both heat and moisture between incoming and outgoing airstreams. Recommended for hot-humid and mixed-humid climates where incoming air carries significant moisture that would otherwise increase indoor humidity and increase cooling loads. The moisture transfer reduces the dehumidification burden on the HVAC system.

HRV (Heat Recovery Ventilator)

Transfers heat only, not moisture. Recommended for cold-dry and cold climates where winter indoor air tends toward low humidity. The HRV brings in fresh dry air, which in winter is desirable — it does not add moisture to already-dry indoor air. In humid climates, an HRV can introduce excessive moisture.

Sizing and Commissioning

ERV/HRV sizing follows ASHRAE 62.2 for residential ventilation rates: continuous whole-house mechanical ventilation calculated from floor area and occupant count. The system must be properly commissioned — not just installed. Commissioning includes airflow measurement at every supply and return register, static pressure verification, and balancing to ensure the home operates at neutral or very slight negative pressure relative to outdoors (to prevent moisture-laden air from being pushed into wall cavities).

Filtration: MERV-13 Minimum

The Good Air Standard requires MERV-13 minimum filtration on the primary HVAC system. MERV-13 captures particles between 0.3–1.0 microns at approximately 50% efficiency and particles between 1.0–3.0 microns at approximately 85% efficiency. This includes most mold spores, fine dust, pollen, and many bacteria.

Why Not HEPA?

HEPA filtration (equivalent to approximately MERV-17+) captures 99.97% of particles at 0.3 microns. It is more effective than MERV-13 at particle capture. It is also substantially more restrictive to airflow, requires larger filter housings or dedicated standalone units, increases energy consumption, and requires more frequent filter changes. For whole-house HVAC integration, MERV-13 provides the best balance of filtration effectiveness, airflow compatibility, energy efficiency, and maintenance practicality.

For heightened-sensitivity occupants who require HEPA-level filtration, the standard recommends standalone HEPA units in occupied rooms (bedroom, living space) in addition to MERV-13 on the central system. This layered approach provides HEPA performance where occupants spend the most time without forcing the central system to overcome HEPA-level pressure drop.

Activated Carbon for VOC Reduction

Particle filters (MERV, HEPA) do not capture gaseous contaminants — volatile organic compounds pass through them. For homes where VOC reduction is a priority beyond material selection, activated carbon filtration can be added as a supplemental stage. Carbon media adsorbs gaseous molecules and is particularly effective for formaldehyde, toluene, and other common residential VOCs.

Carbon media has finite capacity and must be replaced on a defined schedule (typically every 3–6 months depending on loading). Saturated carbon media can re-emit captured compounds. This is a maintenance-dependent technology, not a set-and-forget solution. The Clean Air Construction Protocol uses portable carbon scrubbers during finishing phases specifically because the carbon media is then removed from the building after use.

Humidity Control: 30–50% RH

Indoor relative humidity between 30% and 50% is the target range for the Good Air Standard. Below 30%, mucous membranes dry out and respiratory irritation increases. Above 50%, conditions begin to favor mold growth on surfaces and within wall cavities.

Dehumidification

In hot-humid and mixed-humid climates, mechanical dehumidification is typically required beyond what the air conditioning system provides. Dedicated whole-house dehumidifiers integrated into the HVAC system maintain consistent humidity independent of cooling demand. This is especially important during shoulder seasons when the home needs dehumidification but not cooling.

Where practical, the dehumidifier's wet coil and condensate drainage should be located outside the primary clean-air environment, or isolated in a dedicated, inspectable mechanical zone — the same placement logic covered in full on HVAC Equipment Placement & Failure-Tolerant Design.

Humidification

In cold-dry climates, winter indoor humidity can drop below 20% without supplemental moisture. Whole-house humidifiers (steam or evaporative, integrated into the air handler) maintain minimum humidity. Flow-through evaporative humidifiers are preferred over reservoir types, which can harbor microbial growth if not maintained. Steam humidifiers are the lowest-risk option — no standing water, no wet media.

Monitoring

The Good Air Standard recommends continuous humidity monitoring with visible display in at least one occupied room. The occupant should know the current humidity level without checking an app or opening a panel. Many standalone hygrometers cost under $20 and provide this visibility. Integrated smart thermostats with humidity display also serve this function.

Duct Material Selection

Ductwork is often overlooked in healthy-home discussions, but duct material and installation quality directly affect long-term air quality.

Recommended

• Galvanized sheet metal: Lowest-risk duct material. Inorganic, non-porous, does not absorb contaminants, fully cleanable, inspectable. Higher material and installation cost than flex duct. The standard preferred option.
• Rigid fiberglass duct board (exterior-lined only): Acceptable when fiberglass is on the exterior of the duct, not interior. Interior fiberglass lining absorbs moisture, traps particles, and cannot be effectively cleaned.

Avoid for Sensitive Occupants

• Flex duct (excessive use): Short runs for final connections are acceptable. Long runs sag, create turbulence, restrict airflow, and are difficult to clean. Excessive flex duct is the most common duct installation problem in residential construction.
• Interior-lined fiberglass duct: The internal lining absorbs moisture and contaminants, cannot be fully cleaned, and degrades over time releasing fibers into the airstream. This is why the Clean Air Construction Protocol requires sealing all registers before finishing work — construction emissions absorbed into duct lining off-gas for months.

Fresh Air Intake Location

The location of the outdoor air intake for the ERV/HRV directly affects what enters the home. Requirements:

• Minimum 10 feet from any exhaust outlet (dryer, range hood, bathroom fan)
• Minimum 10 feet from parking areas, garages, or driveways
• Not directly above landscaping treated with pesticides or herbicides
• Screened to prevent insect and debris entry
• Accessible for inspection and filter maintenance

In practice, this means the fresh air intake should be on the cleanest side of the building, elevated above ground level, away from driveways and trash enclosures.

How It Can Fail

• Oversized HVAC systems: An oversized air conditioner cycles on and off rapidly ("short cycling"), removing sensible heat without running long enough to remove moisture. The result is a cool but humid home — exactly the conditions mold needs. Proper Manual J load calculation prevents this.
• ERV in wrong climate: An ERV in a cold-dry climate transfers moisture from outgoing stale air to incoming fresh air, potentially raising indoor humidity in a situation where the home is already adequately humidified. Climate-appropriate selection matters.
• Filter neglect: MERV-13 filters loaded beyond capacity restrict airflow, increase energy consumption, and reduce filtration effectiveness. Carbon media saturated beyond capacity re-emits captured compounds. Filter replacement schedules are maintenance commitments, not suggestions.
• Unbalanced ventilation: A home pressurized by excess supply air pushes moisture into wall cavities through every imperfection in the air barrier. A home under excessive negative pressure draws unconditioned air through the envelope. Balanced ventilation means measured and verified balance.
• Relying on air systems alone: The most sophisticated HVAC system in the world cannot extract moisture trapped inside a wall cavity with no drying path. It cannot remove formaldehyde continuously off-gassing from OSB sheathing. Air systems reinforce a good envelope. They do not replace one.

For Homeowners

The $79 Build Package includes HVAC specification guidance by build option and climate zone, including ERV vs. HRV selection, duct material recommendations, filter specification, and humidity control equipment categories. The package also includes the complete Clean Air Construction Protocol with equipment recommendations for managing emissions during finishing work.

Evidence Basis

Ventilation rates follow ASHRAE 62.2-2022 (Ventilation and Acceptable Indoor Air Quality in Residential Buildings). Filtration ratings per ASHRAE 52.2 (Method of Testing General Ventilation Air-Cleaning Devices). Humidity targets per ASHRAE 55 (Thermal Environmental Conditions for Human Occupancy) and EPA guidance on indoor moisture control. Dehumidification requirements per Building Science Corporation research on mixed-humid climate mechanical systems. Duct material guidance per SMACNA HVAC Duct Construction Standards and NADCA ACR (Assessment, Cleaning, and Restoration) standard for duct cleanability requirements.