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Executive Summary

Over the past 10 years, researchers at the Florida Solar Energy Center (FSEC) have worked with the Manufactured Housing industry under the auspices of the U.S. Department of Energy (DOE) funded Energy Efficient Industrialized Housing Program and the Building America (BA) Program (www.buildingamerica.gov). FSEC serves as the prime contractor for DOE’s fifth Building America Team: the Building America Industrialized Housing Partnership (BAIHP) which can be found online at: www.baihp.org.

Data and findings presented here were gathered between 1996 and 2003 during 39 factory visits at 24 factories of six HUD Code home manufacturers interested in improving the energy efficiency their homes. Factory observations typically showed that building a tighter duct system was the most cost effective way to improve the product’s energy efficiency.

BAIHP and others recommend keeping duct system leakage to the outside (CFM25_out) equal to 3% of the conditioned floor area, termed Qn_out. However, most homes seen in a factory setting cannot be sealed well enough to perform a CFM25_out test. Results of many field tests suggest that CFM25_out will be roughly 50% of total leakage (CFM25_total). Thus, to achieve a Qn_out of less than 3%, manufacturers should strive for a CFM25_total of less than 6% of the conditioned area (Qn_total).

Researchers measured total duct leakage and/or duct leakage to the outside in 101 houses representing 190 floors (single wide equals one floor, double wide equals two floors, etc.). Ducts systems observed in these tests were installed either in the attic (ceiling systems) or in the belly (floor systems). Researchers tested 132 floors with mastic sealed duct systems and 58 floors with taped duct systems.

Of the 190 floors tested by BAIHP, the results break down thus: For mastic sealed systems (n=132), average Qn_total=5.1% (n=124) with 85 systems achieving the Qn_total = 6% target (68%). Average Qn_out=2.4% (n=86) with 73 systems reaching the Qn_out = 3% goal (85%). For taped systems (n=58), average Qn_total=8.2% (n=56) with 19 systems reaching the Qn_total = 6% target (34%). Average Qn_out=5.7% (n=30), more than double the mastic average, with 5 systems reaching the Qn_out = 3% goal (17%).

The results show that, while it is possible to achieve the BAIHP Qn goals by using tape to seal duct work, it is far easier to meet the goal using mastic. What isn’t illustrated by the results is the longevity of a mastic sealed system. The adhesive in tape can’t stand up to the surface temperature differences and changes or the material movement at the joints and often fails. Mastic provide a much more durable seal.

Typical factory visits consist of meeting with key personnel at the factory, factory observations, and air tightness testing of duct systems and house shells. A comprehensive trip report is generated reporting observations and test results, and pointing out opportunities for improvement. This is shared with factory personnel, both corporate and locally. Often, a factory is revisited to verify results or assist in the implementation of the recommendations.

• The most commonly encountered challenges observed in the factories include:
• Leaky supply and return plenums
• Misalignment of components.
• Free-hand cutting of holes in duct board and sheet metal.
• Insufficient connection area at joints.
• Mastic applied to dirty (sawdust) surfaces.
• Insufficient mastic coverage.
• Mastic applied to some joints and not others.
• Loose strapping on flex duct connections.
• Incomplete tabbing of fittings.
• Improperly applied tape

• Set duct tightness target Qn equal to or less than 6% total and 3% to outside.
• Achieve duct tightness by properly applying tapes and sealing joints with mastic
• Accurately cut holes for duct connections
• Fully bend all tabs on collar and boot connections
• Trim and tighten zip ties with a strapping tool
• Provide return air pathways from bedrooms to main living areas

Summary of BAIHP Approach to Achieving Tight Ducts in Manufactured Housing:

• Set goal with factory management of achieving Qn_out<=3% using Qn_total<=6% as a surrogate measurement while houses are in production.
• Evaluate current practice by testing a random sample of units
• Report Qn_total and Qn_out findings; make recommendations for reaching goals
• Assist with implementation and problem solving as needed
• Evaluate results and make further recommendations until goal is met
• Assist with development of quality control procedures to ensure continued success

Finally, duct tightness goals can be achieved with minimal added cost. Reported costs range from $4 to $8. These costs include in-plant quality control procedures critical to meeting duct tightness goals.

Achieving duct tightness goals provides benefits to multiple stakeholders. Improving duct tightness diminishes uncontrolled air (and moisture) flow, including infiltration of outside air, loss of conditioned air from supply ducts, and introduction of outside air into the mechanical system. Uncontrolled air flow is an invisible and damaging force that can affect the durability of houses, efficiency and life of mechanical equipment, and sometimes occupant health. With improved duct tightness, manufacturers enjoy reduced service claims and higher customer satisfaction, while homeowners pay lower utility bills, breathe cleaner air, and have reduced home maintenance.

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