Publication Index:

Factory Visits, Data Collection, and Data Description

Figure 1. Duct blaster used to test air tightness of duct systems during factory visits.

Factory Visits

Data and findings presented here were gathered during 39 factory visits to 24 factories of six HUD Code home manufacturers interested in improving the energy efficiency of their homes. Researchers conducted tests on 101 houses representing 190 floors .

During an initial factory visit, BAIHP researchers typically meet with factory managers for an introduction to Building America and the systems engineering approach to building better houses. Factory managers explain their objectives, challenges they are facing (call backs, reoccurring problem, etc), and conduct a factory tour for BAIHP researchers. During the tour, researchers observe assembly techniques and identify areas of potential improvement.

Figure 2. BAIHP Researcher Neil Moyer sets up blower door for testing the air tightness of a manufactured house.

Researchers test completed duct systems (Fig. 1) in the factory and in finished houses (Fig. 2), if available, to assess initial duct tightness. This creates a benchmark for gauging progress as managers implement duct sealing and assembly recommendations.

After the factory visit, BAIHP researchers provide the factory managers with a Trip Report detailing the findings of the visit, including test results, and making recommendations for improvements. Recommendations may cover heating and cooling equipment efficiency and installation, marriage line details, insulation installation, infiltration and moisture control strategies, window specifications, and duct system air tightening.

Duct System Air Tightness

Duct tightening is among the most commonly recommended improvements. It improves the indoor environment, durability, energy efficiency, and comfort of the home simultaneously. (Appendix B, Duct Leakage and House Pressure Concepts). The EPA Energy Star Program for Manufactured Homes also requires that duct leakage to the outside be reduced to below 3%, 5%, or 7% of the conditioned floor area (Qn_out) depending on the package of energy features selected (MHRA, 2001.)

Studies in new and existing site built homes have documented that duct leakage can be reduced to a Qn_out of less than 5% by sealing the joints with a combination of fiberglass mesh and mastic.

A compilation of findings from field studies around the country shows average savings from air tight duct construction in new and existing homes to be 15% cooling energy savings and 20% heating energy savings (Compilation of findings in Cummings, et al, ‘91 and ‘93, Davis ’91, Evans, et al, ‘96, and Manclark, et al ‘96.) Field repairs in these studies were usually made using UL181 listed tape and/or mesh and mastic.

Duct Sealing

Mastic is an elastomeric material specifically made for permanently sealing the fabricated joints and seams in heating, cooling, and ventilating ducts and thermal insulation. BAIHP recommends the water based formulae for installer safety and easier cleanup. Mastic is supplied in either buckets or tubes for application with a brush, trowel, or caulk gun. Some preparations include a fiber reinforcement component. The elastomeric properties of mastic allows it to expand and contract as the dimensions of the duct system change during each cycle of heating or cooling.

Mastic should be UL181 listed for the type of duct material/insulation being sealed.
When sealing holes larger than ¼” a fiberglass reinforcing membrane (mesh) is used to cover the hole and form a bed for applying the mastic (RCD, 2003.) UL181 listed tape may be used in lieu of the mesh. Generally, this size hole does not occur in the factories after cutting quality and component alignment has been improved.

The longevity of mastic yields a performance advantage over tape. Whereas taped systems may perform well initially, they may become leakier over time if the adhesive fails due to material movement at the joints surface and/or temperature differences and changes. Mastic, on the other hand, tolerates the temperatures differences between inside and outside the duct as well as the frequent temperature changes over the life of the system.

These savings and performance advantages are achieved at relatively low first cost (see Economics of Duct Tightening for Manufactured Housing, p. 19) compared to other energy improvements such as equipment efficiency and window upgrades.

After manufacturers have implemented BAIHP recommendations, researchers may return to the factory for reassessment. Depending on the success of implementation, additional recommendations and reassessment are sometimes needed.

Duct System Recommendations

Duct system recommendations discussed in this report include:

• Set duct tightness target of Qn equal to or less than 6% total and 3% to outside. Note that the ultimate real goal is the latter target, 3% to outside. Conventional wisdom estimates leakage-to-outside to be 50% of the total leakage. Thus the 6% total target serves as a surrogate for leakage-to-out when only leakage to outside can not be measured, such as during production or before set up is completed.
• Achieve duct tightness by properly applying tapes and sealing joints with mastic and, when needed, fiberglass mesh
• Accurately cut holes for duct connections
• Fully bend all tabs on collar and boot connections
• Trim and tighten zip ties on flex duct with a strapping tool
• Provide return air pathways from bedrooms to main living areas

Data Collection and Duct Tightness Goals

BAIHP duct system testing follows building science standard test procedures using a calibrated fan (duct blaster) to depressurize the duct system to a specific pressure, in this case 25 pascals (pa). The amount of air in cubic feet per minute (cfm) needed to achieve this pressure is determined, yielding a measure of total system leakage at a standard test pressure. This measure is referred to as CFM25_total.

A further test can be performed after the house is completed by using another calibrated fan to simultaneously depressurize the house and the duct system to the same pressure. This eliminates air flow between the two, yielding a meaure of system leakage to the outside at the same standard test pressure. All duct leakages measured during this test involves air from outside the conditioned space. This measure is referred to as CFM25_out. A low leakage to the outside is generally the ultimate objective since leakage to the house is considered much less detrimental to air quality, durability, comfort, and energy efficiency. To have “substantially leak free ductwork”, BAIHP recommends keeping system leakage to the outside below a CFM25_out measurement equal to 3% of the conditioned floor area (Qn_out).

However, the CFM25_out test can not be performed until the house is nearly finished. Researchers and factory staff need to assess duct system tightness during production, when the duct system is still accessible for repair if needed. Based on the assumption that only a portion of the total leakage will be lost to the outside, duct tightness goals for production are set in terms of total leakage, CFM25_total, typically 6% of conditioned floor area (Qn_total) as illustrated in the following example.

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