SMUD HPP infiltration and ventilation standards

In order to align with revised Title 24 standards, the SMUD Home Performance Program will be adopting ASHRAE’s mechanical ventilation standards, which are described in  ANSI/ASHRAE 62.2-2010 addendum n (click to view or download PDF). An adaptation of these ASHRAE standards replaces the previous program requirement of achieving infiltration 0.45 ACHn or less in each home.

In the BPI standards, mechanical ventilation is recommended when the total required home ventilation minus the home infiltration is greater than zero. The formulas for calculating total required home ventilation and home infiltration (in cfm) are available in addendum n.

How will I know my ventilation standard and infiltration target?

The new SMUD JRT will automatically calculate the minimum infiltration (in CFM50) permitted without continuous mechanical ventilation. If the home’s infiltration is below this number (either before or after the retrofit), mechanical ventilation is recommended. This replaces the previous mechanical ventilation threshold of 0.245 ACHn.

The JRT will also calculate a maximum allowable infiltration (in CFM50), above which air sealing must be performed in order for the home to qualify for a SMUD HPP rebate. (Unlike the mechanical ventilation standard, this is a requirement, rather than a recommendation.) This upper limit is 130% of the minimum infiltration permitted without ventilation.

If you are not able to seal a house to 130% of the minimum infiltration threshold or tighter, you are expected to provide a written description of why this target could not be achieved, and the home may be selected for field verification.

This diagram illustrates the infiltration target and ventilation standard:

In order for the ventilation standard and infiltration target to be calculated, enter the following data into the JobInfo tab of the JRT:

  • Total conditioned floor area
  • Number of bedrooms
  • Number of floors
  • Average ceiling height

You can also enter the pre-retrofit, proposed, and post-retrofit infiltration (in CFM50) and the JRT will alert you if additional infiltration is needed or if mechanical ventilation is recommended. Green cells indicate no additional action is required or recommended, while pink cells indicate more sealing is needed or
mechanical ventilation is suggested. For example:

How is the standard calculated?

Behind the scenes, the JRT determines the minimum infiltration (in CFM50) permitted without mechanical ventilation using this formula:

CFM50 = [1051.2 * (0.03Afl + 7.5 * (Nbr + 1))] / [55 * wsf * (H / 8.2)0.4]

Where Afl is the total conditioned floor area, Nbr is the number of bedrooms, and wsf is a geographic-specific “weather and shielding factor” developed by ASHRAE. (For the Sacramento area, the wsf is 0.51.) H is the vertical distance between from the lowest floor to the highest ceiling in the home. To account for the space between floors, H is calculated as:

H = #Floors * (AvgCeilingHt + 0.2 * (#Floors – 1))

Mechanical ventilation is recommended in this home. Now what?

If mechanical ventilation is recommended based on the calculations described above, BPI guidelines dictate that the contractor must recommend mechanical ventilation to the homeowner. The homeowner can always decline mechanical ventilation. There is no lower limit at which mechanical ventilation is mandatory.

How is the whole-building ventilation rate calculated?

Per ASHRAE 62.2-2010 addendum n, whole-building mechanical ventilation should equal:

Qfan = 0.01Afl + 7.5(Nbr + 1)

Where Afl is the total conditioned floor area and Nbr is the number of bedrooms. The result of this formula is in CFM.

What types of fans will contribute to my whole-building ventilation rate?

  • supply ventilation only,
  • exhaust ventilation only, or
  • a combination of supply and exhaust

NOTES

  • Per Title 24’s 2013 guidelines, operable windows do not contribute to a home’s total ventilation.
  • The ACHn calculated in the new JRT is based on updated ASHRAE formulas, and may not equal the ACHn calculation of previous version of the JRT, which was based on LBNL formulas.