Duct Leakage in a Conditioned Crawlspace
A contractor is scoping work on a 1,800 sq ft single-story home with a sealed and conditioned crawlspace. The duct system is in the crawlspace and is insulated with R-6 wrap. A duct blaster test measures 180 CFM25 total duct leakage, and the blower door measures 3,200 CFM50. The contractor also measures 90 CFM25 of leakage to outdoors using a pressure pan or duct leakage to outside test. The homeowner complains of high energy bills and dust in the house. The contractor is considering encapsulating the crawlspace further and sealing ducts. The budget is limited, and the contractor wants to prioritize the duct sealing versus the crawlspace encapsulation.
1. Calculate the duct leakage as a percentage of system airflow and as a fraction of the blower door result, and interpret whether the leakage is primarily to conditioned or unconditioned space.
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Assuming a typical 3-ton system at 1,200 CFM, 180 CFM25 total duct leakage is 15% of system airflow. That is high - many programs target 10% or less, and 15% represents meaningful lost capacity and energy.
Of that 180 CFM25, 90 CFM25 is leakage to outside - half the total. The remaining 90 CFM25 leaks into the conditioned crawlspace, which is inside the pressure boundary. So half the leakage is effectively a distribution loss (supply air dumped into the crawlspace, return air pulled from it) and half is a true energy loss to outdoors.
Relative to the blower door: 180/3,200 ≈ 5.6% of the envelope leakage is duct leakage, and the leakage-to-outside is about 2.8%. Because the crawlspace is conditioned, the leakage to the crawlspace is not an envelope loss but still degrades delivery and can pull crawlspace air (and dust) into the living space on the return side - consistent with the dust complaint.
The interpretation: duct sealing is worthwhile, but the crawlspace being inside the boundary changes the calculus - sealing ducts to the crawlspace improves distribution and dust, while sealing ducts to outdoors saves energy.
2. Given the limited budget, should the contractor prioritize duct sealing or further crawlspace encapsulation? Justify with building science reasoning.
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Prioritize duct sealing, specifically sealing the 90 CFM25 of leakage to outdoors and the return-side leaks that draw crawlspace air into the house.
Reasoning: The crawlspace is already sealed and conditioned, so the major encapsulation benefit - bringing the crawlspace inside the pressure and thermal boundary - has largely been captured. Further encapsulation has diminishing returns. Duct leakage, by contrast, is a direct distribution efficiency and indoor air quality problem: 15% total leakage means the system is losing capacity, and return leaks pull dust and crawlspace air into the living space, explaining the dust complaint.
Sealing ducts to outdoors saves energy directly. Sealing return leaks improves IAQ. Both are lower cost than additional encapsulation and address the homeowner's stated symptoms.
Caveat: the contractor should verify the crawlspace is truly sealed and dry before relying on it as a conditioned buffer. If the crawlspace has moisture problems, encapsulation or drainage may need to precede duct work. But absent moisture issues, duct sealing is the higher-value next step.
3. After duct sealing, the contractor wants to verify success. What tests and metrics should be used, and what targets are reasonable?
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Re-run the duct blaster total leakage test and the duct leakage to outside test under the same conditions (same fan speed, same house pressure setup). Compare pre- and post-values: 180 CFM25 total and 90 CFM25 to outside.
Reasonable targets: reduce total duct leakage to 10% of system airflow or less (roughly 120 CFM25 for a 1,200 CFM system), and reduce leakage to outside substantially - ideally by half or more. Many weatherization programs target leakage to outside at or below 5% of system airflow.
Also verify: (1) system airflow after sealing - sealing can reduce leakage but should not reduce delivered airflow; measure static pressure and confirm the fan curve is in range; (2) room-by-room pressure balance and register airflow, since sealing return leaks can change distribution; (3) the dust complaint - inspect the return plenum and filter for crawlspace debris and confirm the return is sealed; (4) a blower door re-test if envelope work was also done, to keep the pressure boundary documentation current.
Document before/after numbers and include them in the client report so the homeowner can see the improvement.