weston basic duct sealing doe - weatherization. weston...zone like a ventilated attic or crawl space...

Basic Duct Sealing

– Lil Weston, Rappahannock Area Agency on Aging –Fredericksburg, Virginia

By attending this session, participants will:

• Identify health and safety issues involved with sealing duct systems;

• Gain a fundamental understanding of the importance of duct sealing;

• Identify different types of duct systems and design (flex, metal, duct board, floor panning, etc.); and

• Become familiar with the materials and tools necessary to properly seal duct systems.

Why do ducts need to be sealed?

• The air in ducts is moved by a powerful fan. Duct leakage can create pressure imbalances in the building envelope, causing combustion safety issues and comfort issues.

• Duct leakage outside the building envelope decreases the efficiency of the entire HVAC system.

• Duct leakage can cause moisture and mold problems. Leaks in return ducts can draw in air from the attic, crawl space or attached garage.

Master Bedroom

Bedroom Bath

Utility Kitchen

Living Room

Whole House Return in Hallway

Supp

ly

Supply

SupplySupply

Supply Supply

Supply

Supply

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Closed doors that prevent supply air from getting back to a return cause positive pressures in those rooms...

meanwhile starving the return for air, causing negativepressures in the zone where the return is located

DuDuct lct leeaakkagageecan cause the same problems

Combustion Appliance Zone (CAZ)

• Air sealing or duct sealing may affect combustion-appliance venting by increasing house pressures or reducing the available supply of combustion air.

• After all weatherization materials have been installed, all crews or contractors must conduct worst-case draft testing and check the safety of all combustion appliances.

Combustion Appliance Zone (CAZ)

Return at furnace contributed to CAZ problem: -8 pa

Gas water heater and clothes dryer in same room

Replaced solid door with louver door – CAZ went down to -4 pa

Air Leakage Requires All of the Following:

•Air •Hole •Pressure Difference

1 cfm in = 1 cfm out

Air Moves from High Pressure Areas to Low Pressure Areas

Air Moves through Ducts at a High Pressure Relative to the Outside of Ducts

Ergo…

A hole in a duct leaks more air than the same size hole in the building envelope because the pressure difference across the hole in the duct is much greater.

Conditions Necessary for Mold Growth:

•Mold Spores Present•Available Nutrient Base•Temperature Range: 35° to 120° F•Relative Humidity of 70% or above at surfaceDuct leakage can increase the relative humidity inside the building envelope, crawl space, or attic enough to encourage mold growth.

Condensation at leaky duct/wall connection during cooling season

Quantifying Duct Leakage

• The amount of duct leakage can be determined by using a calibrated fan such as the Duct Blaster.

By itself, measures total duct leakage.

Used with the blower door, can isolate duct leakage to outside, supply leakage, and return leakage.

Finding Duct LeakagePressure pan testing with the blower door will help to find the biggest leaks.

Reversing flow on the blower door (pressurizing the house) is a good way to find leaks outside the building envelope.

A digital camera or a “see-snake” is useful for looking inside the ducts.

Finding Duct Leakage

Blowing smoke through a closed system helps to visually find duct leakage

Prioritizing Duct Leakage

• Ducts located outside the thermal boundary or in a buffer zone like a ventilated attic or crawl space should be sealed.

• Leaks closer to the air handler are under higher pressure and are more important than leaks further away

Prioritizing Duct Leakage - in relative order of importance

• Seal all return leaks in the combustion appliance zone to prevent this leakage from depressurizing the combustion zone and causing backdrafting.

• Plenum/air handler connection.• Connections at branch takeoffs, especially the tabs. • Joints in elbows.• Connections to boots.• Floor/wall/ceiling-to-boot connection

Prioritizing Duct Leakage - Beware!

• Floor and ceiling cavities used as return-air plenums. If possible, replace these with a duct and a filter grill or a grill straight off the furnace.

• Any building cavity used as a supply duct. If possible, replace these with a duct.

• These are nearly impossible to seal because you can hardly ever access all the framing details connecting these cavities to the outside.

Sealing a building cavity used as a duct

Entire cavity painted with mastic


It’s always good to have a tall skinny guy on the crew for those hard-to-get-to places.


Entire cavity painted with mastic

Panned return

Panned Return

Prioritizing Duct Leakage – Mobile Homes• Base of furnace closet where furnace is seated over duct.• End caps.• Boot/duct and boot/floor connections.• Crossover ducts in double-wide mobile homes.• Branch lines.• Ducts connecting outdoor unit for central cooling.• Plumbing/electrical penetrations.• Belly returns should be closed off.

Typical Existing MH Furnace/Duct Connection

Sealed (from inside) MH Furnace/Duct Connection

Sealed (from below) MH Furnace/Duct Connection

MH End Cap - Leaky

Mobile Home – unsealed register

Mobile Home – sealed register with boot

Prioritizing Duct Leakage – Small MultiFamily

• Party walls and between-floor joist spaces are usually very connected to the outside, so duct leaks in those areas are difficult to seal.

• Ducts are often run in drop ceilings and soffits, but those are often open or connected to the outside.


This whole closet is the return plenum

Would you want to circulate what is in this closet throughout the apartment?

Note condensation from line that runs to outside HVAC unit

The zonal pressure in this cavity is 42 pa



Panned supply ducts in crawl space

Pressure Pans

39.5 pa


Register in upstairs apartment-gap between duct and wall

Pressure pan-

38.7 pa


This is the “duct”

Sealing Ducts - Materials

• Joints should rely on mechanical fasteners to prevent joint movement or separation.

• Tape should never be expected to hold a joint together nor expected to resist the force of compacted insulation or joint movement.

• Aluminum foil or cloth duct tape are not good materials for duct sealing - their adhesive often fails after a short time.


Duct mastic is a preferred duct sealing material because of its superior durability and adhesion.

Apply at least 1/16-inch thick and use reinforcing mesh for all joints wider than 1/8 inch or joints that may experience some movement.

Using mastic to seal ductwork




Return in ceiling


Return in ceiling-sealed


Two-part foam may be used to seal ducts in unconditioned areas.

Two-part foam insulates as well as seals, saving time.

Crew should wear respirator when applying two-part foam in poorly ventilated areas.


Using two-part foam to seal ductwork

Using two-part foam to seal ductwork


Using two-part foam to seal ductwork – one product to seal and insulate ducts and walls



Take care to get total coverage when using two-part foam to seal ducts –sometimes it is better to use mastic to seal when the hand fits and the foam gun won’t go.

Sealing Ducts – Duct Chases

If chase opening is large, seal with a rigid barrier such as fire-rated foam board, plywood or drywall, and seal the barrier to ducts with caulk or foam.

Smaller cracks betweenthe barrier and surrounding materials may be foamed or caulked.

Sealing Ducts

• Flex-duct-to-metal joints: • Apply mastic to the metal sleeve • Clamp the flex-duct’s inner liner over this strip of mastic

with a plastic strap, using a strap tensioner.• Clamp the insulation and outer liner with another strap.• Support ducts with duct hangers to take the weight off the

joints.

Using mastic to seal flexduct

Take-offs/connectors should be mechanically fastened to trunk


Mastic should be applied to connector, then the duct slipped over the connector


Duct should be secured with a strap.

Fiberglass mesh tape should be used on connections with a wide gap or junctions which may experience movement.


Mastic may applied using a brush or by hand


Insulation may then be pulled down, then secured with another strap

Sealing Ducts - Panned returns (or supplies)

• Mechanically fasten pan to joist if needed.• Seal all edges and seams of panning material, including

connections to ducts, or main trunk.• Seal top of joist where it connects to the sub-floor. Gaps in

the sub-floor must be sealed if present.• Seal all accessible seams and connections from the return

inside the house.

That’s all folks!

Thank you for coming!

Duct Demonstration in Exhibit Hall

Block 5 – Wednesday, 2:00-3:30pm

Block 8 – Thursday, 10:15 – 11:45 am

weston basic duct sealing doe - weatherization. weston...zone like a ventilated attic or crawl space...

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