For homes with ducted heating and cooling systems, the best place to locate the duct system from an HVAC performance standpoint is within the conditioned space of the home, either in dropped ceilings, or between floors, or in a sealed and insulated basement, crawlspace, or attic. If the ducts must be located within a vented attic, one option for protecting the ducts and helping to minimize heat transfer between ducts and the unconditioned attic is to lay the ducts on the attic floor and cover them with the same blown insulation that is used to cover the attic floor. The ductwork should be installed so that it is in direct contact with the ceiling and/or truss lower cords. Metal, flex, or fiber board ducts can be used. The ducts should be covered with R-8 or higher duct insulation and the duct insulation should include a vapor barrier cover. All duct connections should be mechanically fastened and mastic-sealed. The ducts should be tested for total duct leakage and any leaks should be sealed. Once these preparations are made, the ducts can be buried in loose-fill insulation (typically blown fiberglass or cellulose) at the same time that the attic floor is being insulated. The insulation levels should meet or exceed the code-required R value for attic insulation. Figure 1 shows a duct that is partially, fully, or deeply buried in blown attic insulation.

If using this technique in the humid or marine climates, the ducts must be encapsulated with spray foam before installing the blown insulation. See Encapsulated Ducts and Ducts Buried in Attic Insulation & Encapsulated for more information.

Buried ducts are laid on the floor of a vented attic then covered with spray foam and blown attic floor insulation

View Image Download

Source

Courtesy Of

Steven Winter Associates, Inc.

The effective R-value of a buried duct installation depends on the size of the ducts, the R-value of the duct insulation, and the depth of the loose-fill insulation. Table 1 shows effective R-values for 8-inch round ducts at three attic insulation levels and three duct insulation levels (R-4.2, R-6, and R-8).

Typically, the duct design will consist of one or more main supply trunk(s) and perpendicular duct branches serving each of the ceiling registers. If the trunk is perpendicular to the truss bottom chords, then the duct branches can be parallel and rest directly on ceiling. If the main truck is placed parallel to the ceiling supports, then the branches will need to run perpendicular to and rest on top of the truss chords. Either configuration will work, giving the designer the flexibility to select whichever method works best for a particular circumstance. In every case, a compact, low-profile layout should be a primary goal.

How to Install Buried Ducts After Ceiling is in Place

1. Install ceiling gypsum board prior to installing buried ducts (Figure 2).

Ceiling gypsum board has been installed

View Image Download

Source

Reference

Measure Guideline: Reducing Thermal Losses and Gains With Buried and Encapsulated Ducts in Hot-Humid Climates

Author(s)

Mantha Pallavi,

Zoeller William,

Shapiro Carl

Organization(s)

Consortium for Advanced Residential Buildings,

CARB,

Steven Winter Associates,

SWA

Description

Document covering the technical aspects of buried and insulated ducts (BEDs), as well as the advantages, disadvantages, and risks of BEDs compared to alternative strategies.

1. Install ductwork across lower truss cords or rafters or resting on the ceiling drywall. Install insulated ducts that have a minimum of R-8 duct insulation and in integral vapor barrier. Install the ductwork in accordance with a low-profile compact duct design (Figure 3).

   

2. Properly fasten and seal ducts at all connections. To attach flex duct, pull back outer liner, fasten inner liner over collar with tool-tightened tension tie, mastic seal the connection. Pull insulation and outer liner over the joint and seal to attached duct or boot with mastic or foil tape. The outer liner should not be attached with a tie as shown here but should be connected with mastic or foil tape to avoid compressing the insulation.

The inner liner of the flex duct is fastened to the collar with a tension tie, the connection is sealed with mastic, then the outer layer is pulled over and sealed with mastic or foil tape (Steven Winter Associates 2013).

View Image Download

Source

Courtesy Of

Steven Winter Associates, Inc.

1. Test total duct leakage with a duct blaster to ensure that the ducts have been adequately sealed (total duct leakage < 3 cfm25 per 100 ft² of conditioned space) (Figure 5).

   

2. Air-seal the ceiling plane penetrations, including sealing the duct register boots to the gypsum board ceiling using canned spray foam (Figure 6).

Duct boots and other ceiling penetrations are air-sealed using spray foam

View Image Download

Source

Courtesy Of

Steven Winter Associates, Inc.

1. Install loose-fill insulation to the specified depth, and verify that the ducts are covered to the level desired (Figure 7).

   

   strong>How to Install Buried Ducts before the Ceiling Is in Place

   1.  

      1. Install ductwork across lower truss cords or rafters. Install insulated ducts that have a minimum of R-8 duct insulation and an integral vapor barrier. Install the ductwork in accordance with a low-profile compact duct design. Where ducts are running parallel to ceiling framing, provide temporary blocking to hang ducts approximately 2 inches above the ceiling plane to allow for the thickness of the spray foam. (Figure 8).

         

      2. Connect, seal, and test ducts as described above in Steps 3 and 4.
      3. Install ceiling gypsum board after installing ducts.
      4. Air seal ceiling plane and and install attic floor insulation as described in Steps 5 and 6 above.