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Duct Tightness Testing
Case for Better Duct Sealing

Passing the total duct leakage test for DET Verification leads to a false sense of energy saving for a new house.  An existing house and a new house may have the same duct leakage as a percentage of floor area, but actual duct leakage for the new house may be twice as much!

Duct Leakage Based on Floor Area. You may have noticed something peculiar about how percentage duct leakage is determined.  Floor area, rather than air handler blower motor fan flow, is used. 

The floor area standard is used for simplicity and because it works well with air-leaky houses.  Using floor area with air tight and well insulated building is false security.

Tight Homes Use Smaller Heating & Cooling Systems. When the building code is complied with, including passing the blower door and duct tests, one of the primary benefits is much smaller HVAC equipment and ductwork. 

Therefore, a ton of air conditioning effectively covers a larger area.  A 1000 sqft/ton is normal.  The typical range for existing houses is 500-700 sqft/ton.

Case for Much Less Duct Leakage

For this discussion to make the case, assume existing houses are 500 sqft/ton and new houses are 1000 sqft/ton: the new house covers twice as much area.  Let’s use the same house of 2000 sqft, one built to older building codes and the other to the current one.


Existing House

New House

Floor Area (sqft)



Air Hander (tons)



Air Handler Airflow (cfm)



Duct Test Airflow (cfm)



Total Duct Leakage



- Using Floor Area



- Using Air Hander Airflow




From the results, it’s clear the effect of duct leakage in a new house is twice what it is for an existing one.

To put this in perspective, 100 cfm is typically allocated to smaller bedrooms.  If the duct leakage is lost to outside, enough air for 2 bedrooms is lost!  What’s worse is that airflow is not as strong as it needs to be to mix air in all the other rooms.

Beware: Don't be satisfied with merely passing the duct leakage test criteria with a new house!  Home InSight is already being consulted for addressing air flow problems in new houses.  Total duct leakage rates of less than 3% are routinely possible.  Fight for it!


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