How to Add an Air Duct to a Room step by step instructions. Frame-in and mount the new vent boot, install the rigid duct board start collar and connect the flexible duct branch line to the duct board trunk. This project is continued from How to Add a Room Air Duct – Part 1.

Frame-in the Ceiling Vent Boot

The vent hole has been cut in the drywall ceiling and work now moves to the attic. The insulation is brushed away from the drywall ceiling cutout for the new vent boot and air register:

Install a Room Air Vent: Drywall Ceiling Cutout - Attic View

Install a Room Air Vent: Drywall Ceiling Cutout – Attic View

Here’s the same view from inside the bedroom.

Drywall Ceiling: HVAC Register Cutout - Bedroom View

Drywall Ceiling: HVAC Register Cutout – Bedroom View

A 2×4 frame is made to mount the vent boot and register box. I used 3 inch wood screws to fasten the frame to the joists. Notice the vent boot at the left side of the photo to check the fit with the wood box frame.

Air Vent Boot Box Frame between Attic Joists

Air Vent Boot Box Frame between Attic Joists

Connect the Vent Boot to the Flexible Air Duct

The insulation jacked it pulled back to reveal the flexible duct inner duct. The inner duct is then slipped over the vent boot:

Room Air Vent: Flexible Duct Inner Core and Vent Boot

Room Air Vent: Flexible Duct Inner Core and Vent Boot

The flex duct liner is secured to register boot with metal foil HVAC tape for an air-tight seal. (Do not use the cloth “duct tape” because it will not last!) The HVAC metal foil tape makes an air tight seal between the flex duct and vent boot:

Air Duct Installation: Flexible Duct Taped to the Vent Boot

Air Duct Installation: Flexible Duct Taped to the Vent Boot

The flexible duct outer insulation jacket is pulled over the vent boot and mechanically fastened with a large nylon zip tie:

Air Duct Installation: Flexible Duct Zip Tied to the Vent Boot

Air Duct Installation: Flexible Duct Zip Tied to the Vent Boot

The register box is fastened to the wood box frame with 1/2 inch self-tapping sheet metal screws. The screws are fastened from the inside:

HVAC Vent Boot Mounted in Ceiling

HVAC Vent Boot Mounted in Ceiling

Seal the Duct Boot and Drywall

The new energy efficient building codes requires the gap between the metal vent boot and the drywall be sealed with caulk (see diagram on page 14) to prevent air leaks. Caulking is easiest option because it can be applied from inside the room in a clean and precise bead.

Vent boot sealing wasn’t required by the building code when my home was built. I’ll wait until another day to seal all the vent boots while working inside the attic.

Install the Duct Board Start Collar

The flexible duct branch line will be connected to the rigid duct board trunk with a flex duct start collar (also called a “take-off”). Always connect the branch duct to a trunk duct for the best air flow. An outline is traced in the rigid trunk duct and cut out with a utility knife. The start collar and take-off hole in the duct board trunk:

Flexible Duct Branch Line: Start Collar and Hole Cut in the Duct Board Trunk

Flexible Duct Branch Line: Start Collar and Hole Cut in the Duct Board Trunk

The start collar is placed into the rigid duct board trunk. The start collar mounting tabs (or fingers) are inside the rigid duct board trunk. The fingers are bent outward against the duct board trunk to hold the starting collar:

Flexible Duct Start Collar Installed in the Duct Board Trunk

Flexible Duct Start Collar Installed in the Duct Board Trunk

The start collar fingers are bend outward, then the start collar is sealed to the rigid duct board with metal foil HVAC tape. Mastic wasn’t required when my home was built, however sealing with mastic and fiberglass mesh reinforcing tape is required by the new building code.

Starting Collar Sealed with Metal Foil Tape to the Duct Board Trunk

Starting Collar Sealed with Metal Foil Tape to the Duct Board Trunk

Connect the Flexible Duct to the Start Collar

The flexible duct inner liner is pulled over the start collar and fastened with two layers of metal foil tape as was done at the vent boot.

HVAC Flex Duct Branch Line: Duct Liner Pulled over the Start Collar

HVAC Flex Duct Branch Line: Duct Liner Pulled over the Start Collar

The outer insulation jacket is next pulled over the start collar and secured with a nylon zip tie. Notice the gentle bend in the flex duct – it is very important to avoid kinks and pinches in the flexible duct that will restrict the air flow:

New HVAC Branch Line Flexible Duct

New HVAC Branch Line Flexible Duct

Tip: The flex duct branch line and start collar can be installed in the side of the duct board trunk to avoid that extra bend by coming out the top of the trunk if you have sufficient room and access.

Update:

The compressor was failing on my 17 year old central AC system. The AC contractor recommend replacing the rigid duct board distribution boxes (above photo) with round sheet metal duct. The HVAC technician is mounting the saddle take-offs for the flex duct branch lines here:

Central Air Conditioner Installation - Round Sheet Metal Trunk Saddle Take Offs for Branch Ducts

Central Air Conditioner Installation – Round Sheet Metal Trunk Saddle Take Offs for Branch Ducts

The new flexible duct branch line into the attic above the bedroom is shown in the following photo. The flex duct should be covered with more blown insulation to reduce heating and cooling losses in the attic. The attic is the hottest and coldest part of the house and overall the worst place to run ductwork. I plan to upgrade my attic insulation later.

HVAC Flexible Duct Branch Line Installation

HVAC Flexible Duct Branch Line Installation

The new ceiling vent register is attached with two screws included with the unit:

Flex Duct Branch Line: Ceiling Air Register

Flex Duct Branch Line: Ceiling Air Register

I turned on the furnace and checked the air flow from the new register. I was pleased to have a strong air flow that matched the other ceiling register – which is expected since both branch lines are attached to the same central duct. My daughter’s room is now warm and comfortable.

Attic Fiberglass Batt Insulation

The blown-in white insulation did a poor job of covering the attic above the bedroom. Later I installed the Solatube Skylight and added a new layer of unfaced Owens Corning R-30. Unfaced insulation without a vapor barrier is required because it’s laid on top of the blown insulation and needs to “breath” to prevent condensation. R-30 insulation is quite thick as you can see by the section in the center of the photo. This made a noticeable improvement in the year-round comfort of the bedroom.

R-30 Fiberglass Insulation Batts

R-30 Fiberglass Insulation Batts

Take care,
Bob Jackson

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