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Install wood framing cross pieces in the attic rafter bays on each side of the duct chase

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Install bottom layer of rigid insulation

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Insulation does not cover boot and is not sealed

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Moisture Performance of Unvented Attics with Vapor Diffusion Ports and Buried Ducts in Hot, Humid Climates

Reference

Report on the use of vapor diffusion ports to reduce moisture issues with buried ductwork in unvented attics in hot-humid climates.

Prepare chase with adhesive for bottom insulation

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Seal seams in fiber board ducts with out-clinching staples, UL-181A-approved tape, and mastic

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Insulation does not cover boot and is not sealed

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Insulation does not cover boot and is not sealed

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No insulation installed in cavity and not air sealed

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Wrong: Ducts are not laid across the lower truss cords or ceiling, but are hung from the rafters by straps. As a result, ductwork is not buried

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Second layer of rigid insulation is adhered with foam

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Two pieces of flex duct are spliced together with a metal sleeve, nylon draw bands, mastic, metal tape, and more mastic

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Cut fiber board with a red V-groove tool and a gray shiplap tool to create mitered corners and a shiplapped edge for duct sections

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Boot has been covered with insulation and sealed with mastic

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Inadequate amount of insulation installed with compression, misalignment, and voids

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Right - The rigid metal HVAC ducting is installed between the floor joists rather than in an unconditioned attic or crawl space to minimize heat loss.

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Right – R-25 of open-cell spray foam lines this new home’s attic ceiling, to protect HVAC ducts from heat and cold.

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Seal bottom layer of rigid insulation with adhesive, tape and nails

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Wrong – Fibrous insulation does not air seal

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A second layer of rigid insulation is installed over the 2 in. by 4 in. retaining strip

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Seal all joints and seams in the metal ductwork with mastic before installing insulation

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Right- This flue pipe has been air sealed with sheet metal and fire-rated caulk and an insulation dam has been constructed to keep insulation from touching the hot flue pipe

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Form an insulation dam of heat-resistant insulation with at least 3 inches of clearance around a flue pipe in the attic after air sealing around the pipe with sheet metal and fire-rated caulk or canned foam

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A box was constructed of rigid foam and sheathing tape to insulate this return plenum in the attic.

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Wrong: This exhaust fan duct is too long and terminates in the attic instead of going through the roof and it is improperly supported; it needs straps all along its length to prevent sagging.

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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).

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Cavity used for return is not insulated and is not air sealed, which will pull in air from outside

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Wrong: open wall cavity connected to this return air plenum is allowing cellulose attic insulation to be pulled into the furnace

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Monitoring of Unvented Roofs with Fibrous Insulation, Diffusion Vents, and Interior Vapor Control in a Cold Climate

Reference

Report detailing a three-winter study on unvented roof assemblies in a cold climate, assessing insulation material, ridge diffusion vent presence, vapor control membranes, relative humidity, and interstitial airflow. Results indicate that utilizing air-permeable fibrous fill insulations in unvented...

Monitoring of Unvented Roofs with Fibrous Insulation, Diffusion Vents, and Interior Vapor Control in a Cold Climate

Reference

Report detailing a three-winter study on unvented roof assemblies in a cold climate, assessing insulation material, ridge diffusion vent presence, vapor control membranes, relative humidity, and interstitial airflow; results indicate that utilizing air-permeable fibrous fill insulations in unvented...