Clearing the Air

Feb 28, 2025 | Dr. Energy | 0 comments

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Clearing the Air

Residential Ventilation – Part 1: Infiltration, Exhaust, Definitions, Codes, and Calculations

 Buildings Are Tighter than in the Past

Modern codes and construction practices create tighter homes, saving energy, enhancing comfort, and potentially increasing durability. These codes and practices minimize air infiltration. Current energy codes require specific air sealing and often mandate blower door testing to verify reduced leakage.

As building tightness increases, we must:

      • Ensure proper ventilation to maintain indoor air quality (IAQ).
      • Exhaust contaminants such as carbon dioxide, chemicals, smoke, moisture, and odors.
      • Provide make-up air for high-exhaust areas like kitchens and bathrooms as required by code.
      • Ensure gas appliances have compliant combustion air from outside.
      • Consider each of these factors in the mechanical design of the building.

Codes:
International Residential Code (IRC) and International Mechanical Code (IMC)

Consider the following summaries of relevant definitions in International Code Councils’ (ICC) set of codes currently adopted in Utah:

2021 IRC/IMC – VENTILATION. The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space. Natural ventilation is provided through leakage, opening windows, skylights, and doors. Mechanical ventilation is provided through Whole-House Ventilation Systems. See below.

2021 IRC/IMC – VENTILATION AIR. That portion of supply air that comes from the outside (outdoors), plus any re-circulated air that has been treated to maintain the desired quality of air within a designated space.

2021 IMC/IRC – WHOLE-HOUSE VENTILATION SYSTEM. An exhaust system, supply system, or combination thereof that is designed to mechanically exchange indoor air for outdoor air where operating continuously or through a programmed intermittent schedule to satisfy the whole-house ventilation rate.

2021 IMC/IRC – MAKE AIR: Outdoor air intended to replace air exhausted.

2021 IMC/IRC – LOCAL EXHAUST: An exhaust system that uses one or more fans to exhaust air from a specific room or rooms within a dwelling.

2021 IMC/IRC – COMBUSTION AIR: The air provided fuel-burning equipment including air for fuel combustion, draft hood dilution and ventilation of the equipment enclosure.

Air Movement into and out of a Building 

The process of moving air either from outside to inside or vice versa requires an accurate assessment of the volume of air being transferred. Each component must be considered within the context of an overall system to ensure it functions as intended. If 100CFM is removed from a building, 100 CFM must replace it.

Historically, building codes did not require air sealing for building envelopes. Excessive leakage and occasional door and window openings generally maintained indoor air quality. As energy codes began incorporating minimal air sealing, they also introduced ventilation requirements. This led to designs using natural ventilation to meet code compliance. However, assuming occupants will open windows when needed is unrealistic.

The substantial amount of feedback and questions regarding air sealing and ventilation underscores a prevalent lack of understanding among HVAC contractors, general contractors, design professionals, and inspectors regarding the building science and ventilation system code requirements. This knowledge gap frequently results in suboptimal building performance, inadequate IAQ, unsafe conditions, and moisture problems.

Residential Mechanical Ventilation

Without proper mechanical ventilation, Indoor Air Quality (IAQ) deteriorates, leading to mold, mildew, and condensation damage in wall, floor, and ceiling cavities.

We recognize the need for mechanical ventilation – but how much air is necessary and what is the appropriate method?

Presented here is a floor plan for a 1900 square foot, 4-bedroom, 3-bathroom, single-level home with an unfinished basement. This example floor plan has been provided courtesy of Ivory Homes. (Thank you!)

Referring to the 2021 IRC, we find the following pertinent information:

Section M1505 Mechanical Ventilation:

1505.4.1 – Design: The system may be supply, exhaust, or a combination of supply and exhaust. Supply ventilation may involve outside air being drawn into the HVAC system’s return air stream with balancing and back draft dampers. The positive pressure created by adding outside air will expel contaminated air through the thermal envelope, at a rate calculated with this formula:

Vent. Rate in CFM = (0.01x sq.ft.) + [7.5 x (# bedrooms + 1)]          

In this example:  = (0.01x 1900) + [7.5 x (4 + 1)] = 19 + 37.5 = 56.5 CFM

Rather than performing the calculation you can refer to a table in the IRC:

The calculated 56.5 CFM for 4 bedrooms and 1900 sq. ft. is accurate and within the table’s range. We will use 60 CFM as our design basis, which meets code if continuously operated. Continuous operation is required. For intermittent operation of at least 1 hour every 4 hours, the rate must be 240 CFM. This calculation applies to the main finished floor, and the same procedure applies to a finished basement.

We need to determine the method to achieve our goal. The primary options include:

      • Supply: A duct connecting to the outside and integrated with the return system. The furnace/air handler must operate continuously and include high-efficacy motors.
      • Exhaust Fan: Should be quiet, must be efficient, and run continuously. This is a straightforward option.
      • HRV or ERV: Often called balanced ventilation as there are two fans, one exhausting a specific cfm with a second supply fan, bringing in fresh air from outside at a rate matching the exhaust rate. These are effective but costly options. Will be the topic in the future.

We’ll choose an exhaust fan using a high-efficiency DC motor, like Panasonic or Broan, set at about 60 CFM and wired to run continuously. These fans are common in energy-efficient apartments and operate quietly while constantly exhausting air from one bathroom. However, they are more expensive than typical noisy bath fans.

Install a 60 CFM fan in the main bathroom to run continuously. Other bathrooms can use standard fans with switches. Alternatively, efficient fans totaling 60 CFM can be used in each bathroom, though this is more costly as continuous fans must be high efficacy. Running these fans creates negative pressure, drawing fresh air through the thermal envelope. The photo shows a high efficacy fan with a well-designed blower wheel and motor, unlike cheaper noisy fans.

If 60 CFM is continuously exhausted from the home as described, the home meets the ventilation requirements in the code and will be more durable. Part 2 of this discussion will address specific code requirements for effective whole-house ventilation, ensuring occupant safety, health, and welfare.

– Dr. Energy

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