I have been doing duct leakage tests on single story homes that have all of the ductwork is in the attic. I am testing both total duct leakage and duct leakage to the outside. Quite often am seeing 60 to 80 CFM differences between the tests with the total duct leakage being higher. Is it normal to see a difference in these two numbers? I am wondering if there is anything I could be doing wrong in my set up, or is this a normal occurrence?
I suspect that what you are experiencing is an attic space that is not well vented to the outside and / or has large bypasses between the house and attic. In these instances, the blower door will pressurize the attic and reduce the duct to attic pressures resulting in lower duct leakage to outside numbers. If you run across this again, try doing the duct leakage to outside test by measuring the duct to attic pressure on channel A instead of the duct to house pressure and your total leakage and leakage to outside numbers should be the same.
Below is a copy of a section from Chapter 7 of our Duct Blaster manual that describes this situation.
7.1.a Building Pressure Measurements:
During the test, you will need to monitor the change in building pressure caused by the Blower Door system. Typically the Blower Door building pressure gauge will be setup to measure building pressure with reference to the outside (this is the typical set up for a Blower Door test).
However, if you are testing a duct system that is located primarily in one unconditioned zone (e.g. a single attic or single crawlspace), you have the option of setting up the building pressure gauge to measure building pressure with reference to that zone, rather than with reference to outside. The purpose of making this change is to ensure that the duct leaks located in that zone are subjected to the full test pressure.
For example, it is possible that a crawlspace containing most of the ductwork may be significantly pressurized by air being forced into that zone from the Blower Door fan (through air leaks between the building and the crawlspace). In this case, you may underestimate the duct leakage rate if you are measuring building pressure with respect to outside during your test because the leaks in the crawlspace ductwork will not be subjected to the full test pressure (i.e. they will be subjected to the test pressure minus the crawlspace pressurization caused by the Blower Door fan). Changing the reference tap on the Blower Door building pressure gauge from outside to the crawlspace would eliminate the underestimation problem in this building.