Duct and House Airtightness

We used a Duct Blaster (The Energy Conservatory) testing device to determine the relative leakage in the return and supply sides of the duct systems in both homes. In the Control home, the total CFM25 leakage of the duct system from outside the conditioned space: 122 CFM25. Given its 2,425 square feet of conditioned are, the duct leakage to outside is 0.05 cfm/ft2. This compares to the 0.03 cfm/ft2 proposed as a standard for utility new homes programs. In the PVRES home, the total CFM25 leakage of the duct system from outside the conditioned space was 50 CFM25 or about 0.021 cfm/ft2-- a low value. In summary, we found that the tightness of the duct system of the Control home was very average relative to most homes which we have tested. One limitation of the tests, however, is that with the air handler operating, all leaks are not the same. Ceiling penetrations close by the air handler can bring air from the attic -- air that is super heated in the Control home. The PVRES home with its interior duct system, on the other hand, had as low a leakage to the outside of the building of any that have been tested by FSEC. Moreover, any of the 50 cfm air that is unintentionally drawn from the attic is being taken from a space that typically gets no hotter than the outside ambient air temperature.

We also used the Minneapolis blower door to measure house tightness in both homes. For the Control home the total overall building tightness of the control house was 2025CFM50 or 6.3 ACH50 with a house ELA of 95.2 square inches. The overall tightness of the PVRES home was 1587 CFM50 or 4.9 ACH50 with a house ELA of 69.0 square inches. A technical discussion of blower door data and its uses can be found in an on-line publication available from LBNL. In both homes, we noted that much of the leakage to the outside appears to be from the 30 recessed lighting cans in the ceiling. For Central Florida's climate, a useful rule of thumb to determine the house natural air infiltration rate is to divide the house ACH at 50 pascals by 30. This indicates a typical natural air change rate of about 0.16 ACH in the PVRES home and 0.21 ACH in the Control.

Our test results indicate a fairly tight building envelope for both homes, with the PVRES home somewhat better. However, the leakage duct systems for the two homes show large differences, mainly due to the interior duct system in the PVRES site. Moreover, thermography showed that the leakage from the attic to the air handler at the Control site has a potentially disproportionate impact on cooling efficiency.