6.5 Mechanical Ventilation: ASHRAE 62.2, HRVs, ERVs & Michigan Energy Code Duct Insulation

Mechanical Ventilation: ASHRAE 62.2, HRVs, ERVs & Michigan Energy Code Duct Insulation

Modern building codes require tightly sealed building envelopes to drastically curb residential and commercial energy waste. However, an airtight structure without dedicated fresh air exchange traps occupant-generated moisture, volatile organic compounds (VOCs), formaldehyde, carbon dioxide, and radon. To safeguard indoor air quality and structural integrity, modern codes pair airtight construction with mandatory mechanical ventilation and strict duct thermal insulation standards. In Michigan, these regulations are codified under the Michigan Residential Code (MRC Chapter 11 / Section N1101), the Michigan Energy Code (Part 10a Rules, adopting ASHRAE 90.1 / IECC), and ASHRAE Standard 62.2.

---

ASHRAE Standard 62.2 Whole-House Ventilation Mandates

ASHRAE Standard 62.2 (Ventilation and Acceptable Indoor Air Quality in Residential Buildings) is the national benchmark for residential ventilation, adopted by reference into the Michigan Residential Code. The standard establishes minimum continuous whole-building ventilation rates designed to dilute indoor atmospheric contaminants.

Continuous Whole-House Ventilation Formula

Under ASHRAE 62.2 and MRC Section N1101, the minimum continuous mechanical ventilation airflow rate is determined by the floor area and the number of bedrooms:

$\text{CFM} = 0.03 \times A_{\text{floor}} + 7.5 \times (N_{\text{bedrooms}} + 1)$

Where:

• CFM = Continuous mechanical ventilation airflow rate in cubic feet per minute.
• A_floor = Conditioned floor area of the dwelling in square feet (sq ft).
• N_bedrooms = Number of bedrooms (dwelling unit cannot be considered to have fewer than 1 bedroom).
• (N_bedrooms + 1) = Assumed baseline occupant load of the home.

Calculation Walkthrough

Calculate the required continuous whole-house ventilation rate for a 2,800 sq ft home with 4 bedrooms:

1. Area component: 0.03 ×2,800 sq ft = 84 CFM
2. Occupancy component: 7.5 ×(4 + 1) = 7.5 ×5 = 37.5 CFM
3. Total continuous rate: 84 + 37.5 = 121.5 CFM (rounded up to 122 CFM).

Intermittent Operation Multiplier

If the ventilation system operates intermittently rather than continuously, the mechanical fan must move higher CFM during its run cycle to deliver the equivalent daily air volume. The required intermittent airflow (Q_fan) is calculated using the fractional runtime (f):

$Q_{\text{fan}} = \frac{\text{CFM}}{f}$

For example, if the 122 CFM requirement is satisfied by a fan running 20 minutes every hour (f = 20 / 60 = 0.333):

$Q_{\text{fan}} = \frac{122}{0.333} \approx 366\text{ CFM}$

Local Exhaust Requirements

In addition to whole-house continuous ventilation, ASHRAE 62.2 / MRC mandates source-specific local exhaust to remove moisture and cooking pollutants directly from bathrooms and kitchens:

• Bathrooms: Intermittent exhaust fan rated at minimum 50 CFM vented directly outdoors, or a continuous exhaust fan running at 20 CFM.
• Kitchens: Intermittent range hood rated at minimum 100 CFM vented directly outdoors, or a continuous exhaust fan running at 25 CFM (or 5 air changes per hour based on kitchen volume).

---

Mechanical Ventilation System Configurations

Mechanical ventilation systems are categorized into three primary architectures, each having distinct pressure impacts on the building envelope:

  [ EXHAUST-ONLY ]               [ SUPPLY-ONLY ]                 [ BALANCED (HRV/ERV) ]
     OUTDOORS                       OUTDOORS                            OUTDOORS
        ^                              |                                ^      |
        |                              v                                |      v
[Exhaust Fan CFM]              [Supply Fan CFM]                 [Exhaust]      [Supply]
        |                              |                                |      |
   HOUSE INTERIOR                 HOUSE INTERIOR                     [ ENTHALPY CORE ]
(Negative Pressure:           (Positive Pressure:                   (Neutral Pressure:
 Infiltration through          Exfiltration into                   Balanced Mass Flow)
 Envelope Flaws)                Wall Cavities)

1. Exhaust-Only Systems:
   • Operation: Typically a continuous-duty bathroom exhaust fan running at low speed, constantly expelling indoor air outdoors.
   • Pressure Dynamic: Creates negative indoor pressure, drawing replacement air into the home through cracks, window seals, and wall penetrations (uncontrolled infiltration).
   • Concerns: Can draw moist air into exterior wall cavities, pull soil gases (radon) through basement foundations, and cause backdrafting of Category I atmospheric gas appliances (natural draft water heaters or furnaces).
2. Supply-Only Systems:
   • Operation: A motorized damper and fan introduce outdoor air directly into the return air plenum of the central furnace or air handler (Central Fan Integrated Ventilation - CFIV).
   • Pressure Dynamic: Creates positive indoor pressure, forcing indoor air outward through the building envelope (exfiltration).
   • Concerns: During freezing Michigan winters, positive pressure drives warm, moisture-laden indoor air outward into cold exterior wall and attic cavities, where it hits cold exterior sheathing, condenses into liquid water, and triggers structural rot and mold.
3. Balanced Systems:
   • Operation: Uses two dedicated fans—one supplying fresh outdoor air and one exhausting stale indoor air at identical volumetric rates.
   • Pressure Dynamic: Maintains neutral pressure across the building envelope, avoiding backdrafting hazards and wall cavity condensation.
   • Standard Equipment: Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs).

---

Heat Recovery Ventilators (HRVs) vs. Energy Recovery Ventilators (ERVs)

Both HRVs and ERVs deliver balanced mechanical ventilation while reclaiming thermal energy that would otherwise be lost through exhaust air.

Technology Comparison

• Heat Recovery Ventilator (HRV):
  • Utilizes a sensible-only heat exchanger core (typically aluminum or polymer plates).
  • The incoming cold fresh airstream and outgoing warm stale airstream pass through alternating channels separated by the plates without physically mixing.
  • Sensible Heat Transfer: In winter, warm exhaust air transfers its thermal heat across the plates to pre-heat incoming cold outdoor air with sensible efficiencies typically between 65% and 85%.
  • Moisture Transfer: Transmits no moisture. Moisture in the exhaust air condenses inside the core channels and is drained away via a condensate trap.
• Energy Recovery Ventilator (ERV):
  • Utilizes an enthalpy core fabricated from a permeable membrane coated with microscopic desiccant materials.
  • Total Energy Transfer: Transfers both sensible heat and latent heat (moisture vapor) between airstreams.
  • In winter, warm, humid exhaust air transfers both heat and water vapor to the dry, cold incoming outdoor air.
  • In summer, hot, humid outdoor air transfers both heat and humidity to the cool, dry outgoing exhaust air, reducing air conditioning load.

Climate Suitability for Michigan (Climate Zones 5A & 6A)

Michigan spans IECC Climate Zone 5A (southern and central Lower Peninsula) and Climate Zone 6A (northern Lower Peninsula and Upper Peninsula), characterized by severe, freezing winters and warm, moderately humid summers.

Contractor Rule of Thumb: In airtight Michigan homes during sub-zero winter temperatures, an HRV can over-dry the living space (dropping indoor RH below 20%, causing respiratory irritation and hardwood floor shrinkage). An ERV is preferred because it keeps moisture inside the home while pre-warming the fresh air.

---

Michigan Energy Code Duct Insulation & Sealing Rules

The Michigan Energy Code (Part 10a of the Construction Code Rules) sets mandatory thermal performance and sealing standards for all duct systems to prevent conduction losses and air leakage.

Mandatory Duct Insulation R-Values

When ductwork is routed through unconditioned spaces (spaces outside the thermal building envelope), severe temperature differentials cause massive energy loss and surface condensation. Michigan Energy Code mandates:

• Supply Ducts in Unconditioned Attics: Minimum thermal resistance of R-8.
• Supply Ducts in Other Unconditioned Spaces: Minimum R-6 in ventilated crawlspaces, unconditioned basements, and exterior wall cavities.
• Return Ducts in Unconditioned Attics: Minimum R-6.
• Return Ducts in Other Unconditioned Spaces: Minimum R-4.2.
• Ducts Located Within Conditioned Space: No insulation required, though vapor retarders are recommended if ducts convey chilled air through humid utility chases.
• Exterior Ducts: Ducts routed completely outdoors on rooftops require R-8 insulation with a weather-resistant, UV-resistant protective membrane jacket.

Mandatory Duct Sealing & UL 181 Standards

Air leakage through duct seams and transverse joints can waste 20% to 40% of conditioned air. The Michigan Mechanical Code (Section 603.9) and Energy Code enforce strict sealing standards:

1. Approved Sealants:
   • Mastic and Fiberglass Mesh: Water-based or solvent-based elastomeric mastic applied with an embedded woven fiberglass mesh tape. Mastic must be applied to a minimum wet thickness of 20 mils (the thickness of a nickel).
   • UL 181A-P & 181A-M: Pressure-sensitive tape (P) or mastic (M) tested and listed for rigid fiberglass duct board.
   • UL 181B-FX & 181B-M: Pressure-sensitive tape (FX) or mastic (M) tested and listed for flexible air ducts.
2. Prohibited Materials: Standard cloth-backed, rubber-adhesive duct tape (commonly sold as "duck tape") is strictly prohibited by code for sealing ductwork. Under repeated thermal expansion and contraction cycles, the rubber adhesive oxidizes, dries out, and delaminates within months.

Duct Leakage Testing (Duct Blaster Test)

Under the Michigan Energy Code, when any portion of the duct system is located outside the building's thermal envelope (in attics, crawlspaces, or unconditioned garages), field duct leakage testing is mandatory prior to final inspection.

• Testing Instrument: A calibrated fan and digital micromanometer assembly (Duct Blaster) connected to the air handler or main return opening, with all supply registers and return grilles temporarily sealed with adhesive film.
• Test Pressure: The duct system is pressurized or depressurized to 25 Pascals (0.10 in. w.g.).
• Maximum Leakage Threshold: Total duct leakage must not exceed 4.0 CFM per 100 square feet of conditioned floor area at 25 Pa:

$Q_{25} \le 0.04 \times A_{\text{floor}}$

• Rough-in Test Option: If tested at rough-in before drywall installation with the air handler cabinet installed, leakage cannot exceed 4.0 CFM per 100 sq ft. If tested at rough-in without the air handler installed, leakage is restricted to 3.0 CFM per 100 sq ft.

$\text{For a 2,500 sq ft home: } Q_{25, \text{max}} = \frac{2500}{100} \times 4.0 = 100\text{ CFM at 25 Pa}$