Air infiltration is typically THE MOST SENSITIVE assumption in all of Manual J. Small changes in air infiltration assumptions have a large impact on:
Sensible heating
Sensible cooling
Latent cooling
We offer two primary methods for calculating infiltration loads
The ACCA Manual J "tightness" method
Blower Door
The ACCA Manual J "tightness" method
This is the default method (Method: Tightness) for a project. You can select your assumption about the Envelope Tightness based on your observations in the room.
Set your "Tightness" assumption
For each option, we display guidance text to help you make more informed decisions:
Tight:
Would measure less than 2 ACH50 via blower door test
Only seen in high-performance homes
Semi-Tight:
~3 ACH50 via blower door test
Common for homes built since 2010 or homes with recent professional air-sealing improvements
Average:
~5 ACH50 via blower door test
Common for homes built 1980 - 2010s or pre-1950s home that has had recent professional air sealing improvements
Semi-Loose:
~7 ACH50 via blower door test
Common for homes built 1950 - 80s
Loose:
Greater than 10 ACH50 via blower door test
Common for homes built pre-1950
Review Infiltration Area
The tightness calculations are also very sensitive to the Infiltration Area (see more details in Table 5A below of how underlying assumptions change with Area).
Infiltration Area is automatically set to equal the Scanned Living Area of the home. Assuming you have scanned all conditioned rooms in the building, the scanned area is the most accurate representation of the heated/cooled floor area of the home. The value in the Infiltration Area is what we use for performing infiltration calculations.
We display the Estimated Living Area as a gut check comparison against what you have scanned. The Estimated Living Area comes from real estate databases, which are often not accurate but are good as a rough estimate for actual conditioned space.
Exception: When to edit the Infiltration Area
Unable to scan a small closet or other very small space: Manually edit the Infiltration Area to add a few square feet of the areas that you were unable to scan
Source: ACCA Manual J 8, v2.5, p. 390, Table 5A. Note that Air Changes per Hour in the table are in ACH4 (ACH Natural) units. Note how the infiltration rates change based on the input floor area of the home.
Fireplaces
Make sure you indicate how many active fireplaces (i.e. the flue has not been permanently blocked) there are in the home. Wood stoves or an open hearth are both considered a "fireplace" by Manual J.
Blower Door Method
Using real data is always preferred and more accurate than guessing how tight a home is. If you have access to Blower Door test results, use the the Blower Door method. We offer two data options:
Blower Door (ELA4)
Blower Door (CFM50)
Blower Door (ELA4)
Enter the Effective Leakage Area.
NOTE: Do NOT guess at this value, only use numbers from a properly calibrated blower door test result.
Effective leakage area @ 4pa (ELA4) is the size (sq in) of an engineered opening that would produce the same amount of leakage as the cracks and penetrations in the building envelope at natural pressure differential (i.e. 4 pascals).
Select a Shielding Class
Blower Door (CFM50)
Enter the Leakage CFM50 value (leakage rate in cubic feet per minute @ 50 pascals).
NOTE: Do NOT guess at this value, only use numbers from a properly calibrated blower door test result.
Select a Shielding Class
