ASHRAE 62.2 Morsels – 1

Cover of ASHRAE 62.2-2016

As a fifteen-year member of the ASHRAE 62.2 committee, I am often asked about the more difficult or contentious details of the 62.2 Standard. These questions range from how to measure dwelling height, should the basement be included in the floor area, or how to determine the airflow rate of balanced ventilation. This is the first of two articles including a collection of these questions from users of the 62.2 Standard and the RED Calc 62.2 tools, along with my best answers.

How is Dwelling Height Measured?

The ASHRAE 62.2 Standard defines dwelling height as the “vertical distance between the lowest and the highest above-grade points within the pressure boundary1 in feet or meters.2 In the 62.2 Standard, this value is represented by “H”. The diagram below from the ASHRAE 62.2-2016 User’s Manual3 illustrates the dwelling height from the lowest to the highest above-grade points.

Figure 1 - Illustration of dwelling height for ASHRAE 62.2 Standard (Source: ASHRAE 62.2-2016 User’s Manual)

The dwelling height is only needed if you wish to determine the effective infiltration rate; this is because the infiltration rate is increased by greater dwelling height. When using the RED ASHRAE 62.2 tools, if you turn the infiltration credit feature off, the dwelling height input disappears because it is not needed.

If you are sizing the dwelling-unit ventilation for a horizontally attached4 multifamily building, such as a row or garden apartment, the dwelling height is determined exactly the same way as for a single-family detached dwelling.

How Do I Calculate the Floor Area of a Basement?

Whether using the RED Calc tools or another calculation aid to find the dwelling-unit ventilation airflow rate, questions often arise regarding the basement floor area; should it be included or not, and if so, what parts of a basement should be included?

Before I answer, keep in mind that including the basement in your floor area will increase the amount of ventilation. The basic equation for dwelling-unit ventilation is

     Qtot = 0.03 Afloor +7.5(Nbr +1)        (equation 1)

Qtot = total ventilation required from mechanical ventilation and infiltration, cfm
Afloor = dwelling unit floor area, ft2
Nbr = number of bedrooms (not to be less than 1)

This equation makes it clear that as the floor area increases, the total ventilation, Qtot, also increases.

The ASHRAE 62.2 committee members have discussed how to define floor area for years. The issue of measuring floor area is usually clear for dwellings without basements, but when basements are involved, the definition becomes murkier.  The ASHRAE 62.2-2016 Standard, the most commonly used version, defines floor area as: “all above- and below-grand finished areas as defined in ANSI Standard Z765.”5 The definition in the 62.2-2022 Standard includes the above with the added phrase: “. . . and unfinished below-grade occupiable spaces.” An “occupiable space” is defined as a space within the pressure boundary and intended for human activities.

I know this remains a bit murky, but the committee has made progress. My interpretation of the floor area definition for basements is this: if the basement is within the pressure envelope (most are), include all of its area in your square footage calculation, except for the basement area housing the central heating/cooling equipment.

How Do I Determine a Balanced Ventilation Airflow Rate?

Balanced ventilation is usually a Heat Recovery Ventilator (HRV) or an Energy Recovery Ventilator (ERV). Both are designed and intended to exhaust and supply air at the same rate, say 50 cubic feet per minute (cfm) of fresh air supplied to the dwelling and 50 cfm of stale air extracted from the building, simultaneously. This 50 cfm supply and 50 cfm exhaust do NOT add up to 100 cfm, but represent an effective 50 cfm ventilation rate. This is really quite similar to a 50 cfm exhaust-only fan; 50 cfm, of stale air is expelled through the fan and 50 cfm of fresh outdoor air enters (is supplied to) the dwelling through cracks and holes, rather than through a dedicated duct, as it is with an HRV or ERV.

It is important to keep in mind that the supply-exhaust airflow balance of an HRV/ERV can be significantly upset by a dirty filter on either the supply or exhaust sides of the unit. This imbalance can reduce the effectiveness of the ventilation system. Any filters should be checked regularly and cleaned or replaced as needed.

Is it the Number of Occupants, or Bedrooms Plus One?

When the ventilation system for a new dwelling is designed and ready to build, the floor plan is the only indication of the number of occupants that will live in the house. The rule in the 62.2 Standard is to assume two occupants for the master bedroom and one for each additional bedroom. In other words, to determine the number of occupants, count the number of bedrooms and add one, so a three-bedroom house is assumed to have four occupants. The minimum number of occupants for any dwelling is two.

This occupant calculation is not as simple for many existing dwellings. What if you arrive at a house to conduct an energy audit and find an elderly couple living in a four-bedroom house? What do you do? Some would use two occupants, claiming they are sizing the dwelling-unit ventilation (DUV) for the current occupants. Others would use five occupants, stating the DUV should be sized for the house; the current elderly couple won’t live there forever.

I suggest you size the DUV for five occupants, after all, this is what the 62.2 Standard instructs users to do. However, with the controls, set the DUV to operate at a rate for two occupants (22.5 cfm lower) with the knowledge that it has the capacity to handle a “full house” when needed.

In order to operate this four-bedroom house for just two occupants, you must follow another requirement in 62.2. The Standard states in Section 4.1.3: “When approved by the authority having jurisdiction [AHJ], lower occupant densities may be used.” The challenge here is finding the AHJ; is it the code enforcement officer, your state weatherization monitor, a D.O.E. monitor, or someone else? Whatever you do, document the reason for your action and whom, as the AHJ, gave you approval.

Of course, the excess capacity you install for DUV is only helpful if in the future when a “full house” of occupants occurs, the DUV is boosted to the higher rate to comply with 62.2.

Another example that goes in the opposite direction; an apartment dwelling has two bedrooms and you find there are seven occupants living there. Install a DUV capacity for seven occupants and lower it later for a reduced occupant rate. AHJ approval is not needed for such cases. As with the previous example, clear instructions informing new occupants of the required ventilation changes are important; this can be a challenge because you are not likely to be around to inform the new occupants.

Does a Half-Bathroom Require an Exhaust Fan?

A half-bathroom, or toilet room, is a bathroom with a toilet, and sink (lavatory), but no bathtub, shower, or tub/shower combination. From the first version of the ASHRAE 62.2 Standard in 2003, a local exhaust fan has not been required in a half-bathroom.6 Of course, a half-bathroom can have odors, but without bathing facilities, it never has a high degree of moisture, so the 62.2 committee members have never thought local mechanical ventilation was necessary.

For existing dwellings, when the alternative compliance path of the 62.2 Standard is being used for local exhaust ventilation,7 any half-bathroom should be ignored because there is no requirement for it to have ventilation.

Why is the Weatherization Assistance Program (WAP) Sticking with ASHRAE 62.2-2016?

This is a morsel of a different character. It is not an explanation or clarification of the 62.2 Standard, but an educated guess for why the WAP has decided to allow the affiliated state programs to continue using the 2016 version of the 62.2 Standard, even though there are now two more recent versions. My assumption is that the WAP’s holding to the 2016 version is because of an interesting building-science-related feature added to the Standard in 2019: the recognition of the difference between the way balanced and unbalanced dwelling-unit ventilation (DUV) impact infiltration.8 First, what is this building science about?

The primary building science principle behind this is that the operation of unbalanced ventilation creates negative (exhaust ventilation) or positive (supply ventilation) pressure in the dwelling, whereas the operation of balanced ventilation does not create pressure. Any pressure in a dwelling reduces infiltration, whereas the neutral pressure of a balanced system does not create positive or negative. Keep this in mind.

The equations below are from the 62.2 Standard. Equation 2 shows that the total supply of fresh outdoor air in a dwelling, Qtot, is made up of the DUV, Qfan, and the infiltration, Qinf. A realistic example value for Qtot is 90 cfm for a 2000 ft2 house with three bedrooms (see equation 1, above, and the screenshots of the RED Calc Pro 62.2-2019 tool).

Equation 3 is a rearrangement of equation 2, showing that the value of the DUV, Qfan, is Qtot minus infiltration, Qinf. This is as far as the ASHRAE 62.2-2016 Standard went; it did not include equation 4 with Φ, the magic building science sauce (Φ is a Greek letter, pronounced “fee” or “fie” and is used as a mathematical symbol). ASHRAE 62.2-2019 does include equation 4.

     Qtot = Qfan + Qinf                             (equation 2)

     Qfan = Qtot – Qinf                              (equation 3)

     Qfan = Qtot – (Φ x Qinf)                      (equation 4)

Qtot = total required ventilation rate from infiltration, Qinf, and mechanical ventilation, Qfan, cfm (L/s)
Qfan = required mechanical ventilation rate, cfm (L/s)9
Qinf = infiltration, cfm (L/s)
Φ = 1 for balanced mechanical ventilation systems, and Qinf/Qtot otherwise

Equation 4 is the same as equation 3 but with the addition of Qinf times Φ. I stated above that for balanced ventilation, infiltration, Qinf, is not impacted. Therefore, if you are sizing a balanced DUV system (an HRV or ERV) the value of Φ = 1. Multiplying Qinf by 1 does not change its value.

On the other hand, if you are installing an unbalanced DUV system, either exhaust-only or supply-only, Φ = Qinf/Qtot. This ratio of Qinf/Qtot is an estimate of the percentage unbalanced ventilation will reduce infiltration, Qinf, and it will always be less than 1

As an example, see the Figure 2 screenshot of the RED Calc Pro ASHRAE 62.2-2019 tool. Notice “Balanced ventilation” is being used in this example (highlighted in green) indicated by the toggle switch being set to YES, resulting in an “Infiltration credit, Qinf” (highlighted in blue) of 49 CFM and “Required mech vent, Qfan” (highlighted in yellow) of 41.2 CFM. In other words, for this house with balanced ventilation, the minimum DUV is just over 41 CFM.

For the same house, now let’s look at the DUV airflow required for unbalanced ventilation, say exhaust-only. The Figure 3 screenshot of the ASHRAE 62.2-2019 tool shows the “Balanced ventilation” (highlighted in green) toggle switch is set to NO, resulting in an “Infiltration credit, Qinf” (highlighted in blue) of 26 CFM and “Required mech vent, Qfan” (highlighted in yellow) or 63.6 CFM. In other words, for the same house with unbalanced ventilation, the minimum DUV is almost 64 CFM, an increase of about 23 CFM for exhaust-only DUV over balanced DUV.

Figure 2 - RED Calc Pro ASHRAE 62.2-2019 tool showing results for balanced DUV

The end result of this balanced/unbalanced DUV choice is a 30 to 50% higher airflow rate for unbalanced than for balanced DUV. As far as I know, this is the reason the WAP management is sticking with 62.2-2016; they know most installed WAP DUV is exhaust-only and fear that installed costs will increase because of this new 62.2-2019 feature recognizing the building science-related difference between balanced and unbalanced DUV.

Figure 3 - RED Calc Pro ASHRAE 62.2-2019 tool showing results for unbalanced DUV


[1] The ASHRAE 62.2 standard defines a pressure boundary as a “primary air enclosure boundary separating indoor and outdoor air.” For example, an attic or garage is outside of the primary air enclosure boundary, thus is not a part of it. The definition for pressure boundary in Section 3 of the 62.2-2016 standard goes on to state: “Exposed earth in a crawlspace or basement shall not be considered part of the pressure boundary.”

[2] ASHRAE 62.2-2016, page 5, equation 4.4.

[3] The ASHRAE 62.2-2016 User’s Manual is available from the ASHRAE bookstore as a PDF or printed edition. This document is filled with useful explanations and examples.

[4] The ASHRAE 62.2 Standard defines “dwelling unit, horizontally attached” as: “an attached dwelling unit that does not have a common floor/ceiling assembly with other dwelling units or spaces covered by ASHRAE Standard 62.1.” Another way of stating this is that the dwelling units have no other dwelling units below or above, but only to the sides.

[5] ANSI/NAHB Z765-2003, American National Standard for Single-Family Residential Buildings Square Footage—Method for Calculating. Washington, DC: National Home Builders Association.

[6] As a result of recent action by the ASHRAE 62.2 committee, 62.2-2025 will require a local 50 cfm (25 L/s) exhaust fan in each half-bathroom (toilet room) in new dwellings, but not in existing dwellings.

[7] Refer to Appendix A of ASHRAE 62.2-2016 for details of the alternative compliance path for existing dwellings.

8] This characteristic is sometimes called superposition. This mathematical term has to do with adding things together and finding they sometimes interact with one another, affecting the sum. In this case, the two things added are mechanical ventilation and infiltration.

[9] Note that where Qfan calculated for unbalanced ventilation is less than or equal to 15 cfm (7 L/s), a dwelling-unit ventilation system is not required (ASHRAE 62.2-2019, Exception to 4.1.2). This is sometimes referred to as the “de minimis”.

Rick Karg

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