Upgrading Multifamily Buildings

Title 24 Requirement
ASHRAE 62.2-2007

No distinction is made betweeen ventilation rates for multifamily and single-family housing. To meet Title 24 ventilation requirements, determine multifamily whole-building ventilation rates by using the methods in Step 3.

ASHRAE 62.2-2010

Addendum j: For multifamily units, use the Table Method or the Formula Method above to calculate the whole-building ventilation rate. Section 8.4 in Addendum j requires airsealing measures for common walls, ceiling, and floors to prevent the transfer of air between units.

Best Practice
Recommendation

Currently, following ASHRAE 62.2-2010, Addendum j recommendations is best practice for providing whole-building ventilation in multifamily units.

Considerations When Upgrading Ventilation in Multifamily Buildings

Apartment and condominium buildings have issues related to air leakage from other units and fire code compliance that may limit ventilation options. Air leaks between neighboring units can transfer odors and smoke. Also, a strong fan in one unit can overpower a weaker fan next door. Often the best strategy is to add ventilation to all the units to reduce cross-leakage problems. But even then, balancing air flows in all the units can be difficult. Outside expertise may be required.

While analyzing and understanding a large multifamily complex may be more challenging than assessing a single-family home, a contractor can enjoy economies of scale once a plan is developed to upgrade ventilation equipment. Repeated installations in multiple units allow refinement of technique. Equipment can often be ordered in large enough quantities to warrant discounts from suppliers.

For multifamily buildings, here are some questions that need to be considered when assessing a project for ventilation improvements:

  • Does stack effect cause air to be drawn in through the lower floors and leak out the upper units?
  • Can cigarette smoke or other contaminants be drawn up from lower units or across from adjacent units?
  • Does the Fire Code limit the penetrations that can be made in the interior or exterior of the units?
  • Are there pickup grilles in the unit that move air into central stacks with large rooftop fans? Is the total discharged at the roof much larger than the sum of the pickups? If so, large leaks in the stacks may be drawing air from unknown spaces.
  • Who pays the power bill—the owner or the occupant?
  • Which direction is the framing going if you need to run a duct?

Increased Whole-Building Ventilation Rate for Multifamily Units

Research has shown that multifamily units have less infiltration of outside air than comparably-built detached homes because apartment and condominium units have less exterior wall area. Air leakage through common walls and ceilings brings in air from the neighbors instead of outside. So to provide adequate whole-building ventilation, multifamily units often need higher levels of mechanical ventilation to compensate for less infiltration.

How much additional mechanical ventilation do multifamily units need? The most up-to-date version of ASHRAE Standard 62.2 includes higher required airflows for whole-building ventilation, plus airsealing requirements between units in multifamily buildings. Section 8 in Addendum j to ASHRAE 62.2-2010 covers multifamily units. The Addenda are available online at no cost on the ASHRAE website.

Below are Addendum j's Table Method and Formula Method for determining required whole-building ventilation rates for multifamily units.

Table Method

ASHRAE Standard 62.2 Table 8.2.1a
Multifamily Ventilation Air Requirements in cfm
Floor Area (sq ft) Number of Bedrooms
0 - 1 2 3 4 5
< 500 30 38 45 53 60
501 - 1000 45 53 60 68 75
1001 - 1500 60 68 75 83 90
1501 - 2000 75 83 90 98 105
1001 - 2500 90 98 105 113 120
2501 - 3000 105 113 120 128 135
3001 - 3500 120 128 135 143 150
3501 - 4000 135 143 150 158 165
4001 - 4500 150 158 165 173 180
4501 - 5000 165 173 180 188 195

Formula Method

whole-building continuous ventilation rate for multifamily units in cfm =

3 x floor area


100

+ (number of bedrooms + 1) x 7.5

Optional Calculation

The Optional Calculation cannot be used to reduce the whole-building ventilation rate, since it is very difficult to differentiate the leakage of outdoor air from leakage from adjoining units through common walls, ceilings, and floors.