Skip to main content

Sealing Duct Boots to Floor or Drywall

Scope

Caulk air seals the boot to the ceiling
Caulk air seals the boot to the ceiling

Air-seal around all duct shafts and flues installed through ceilings, walls, and flooring to keep conditioned air from leaking into unconditioned space. 

  • To air-seal around duct boots, use caulk, canned foam, or rigid air barrier material cut to fit and caulked or foamed in place around the duct shafts. 

See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

Description

Connections between grilles, registers and ducts at ceilings, floors or knee walls typically leak where the boot does not seal tightly to the grille or gypsum board. Air from the attic, basement, or crawlspace can leak in or out where the ducts connect to the boot.

If the gap between boots and gypsum board opening or subfloor openings is kept to less than 3/8-inch, a bead of sealant or mastic may be used to seal the gap. Where gaps are larger than 3/8-inch, fiberglass mesh tape and mastic should both be used. The optimum approach is to keep the gaps to less than 3/8-inch and use a bead of sealant. This requires careful coordination with the drywall contractor to make sure that the rough openings for the boots are cut no more than 3/8-inch bigger than the actual boot size on all sides.

Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the IECC code require that builders meet specified infiltration rates at the whole-house level. See the Compliance tab for links to these programs.

For more about sealing ducts, see the guides Sealed and Insulated Flex Ducts, Metal Ducts, and Fiberboard Ducts.

How to Seal a Duct Boot to the Ceiling

  1. Seal all sides of the duct boot to the gypsum board with spray foam or caulk (Figures 1, 2, and 3). Apply mastic or metal tape to all duct seams and joints.
  2. Cover boot with insulation.
  3. Add insulation to the specified attic insulation depth.
Spray foam air seals the boot to the ceiling.
Figure 1. Spray foam air seals the boot to the ceiling. (Source: Building Science Corporation.)

 

Caulk air seals the boot to the ceiling.
Figure 2. Caulk air seals the boot to the ceiling. (Source: Building Science Corporation.)

 

Seal duct boot to ceiling.
Figure 3. Seal duct boot to ceiling. (Source: Building Science Corporation.)

 

How to Seal the Duct Boot to the Floor

  1. Mechanically attach the boot to framing around the hole in the flooring. Caulk, spray foam, or mastic seal between floor opening and boot with caulk or fiberglass mesh and mastic.
  2. Cover boot with insulation.

Ensuring Success

Blower door testing, which is conducted as part of the whole-house energy performance test-out, may help indicate whether duct openings to unconditioned space (such as an attic) have been successfully sealed. Duct blaster testing may also help to indicate leaks. An infrared camera can be used in conjunction with the blower door testing to detect air leakage and heat loss at the duct and flue shaft openings, if a sufficient temperature difference exists between the unconditioned and the conditioned space of the house. An experienced technician can also check for air leaks around registers with a smoke pencil or by feeling with the back of the hand.

Region

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18.
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)

 

Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

 

Training

Right and Wrong Images

Duct insulation is installed over boot
Duct insulation is installed over boot
Duct insulation is installed over boot
View Image Download
Source
Courtesy Of
EPA
Boot to floor connection not sealed
Boot to floor connection not sealed
Boot to floor connection not sealed
View Image Download
Source
Courtesy Of
EPA
Boot to floor connection sealed
Boot to floor connection sealed
Boot to floor connection sealed
View Image Download
Source
Courtesy Of
EPA
Boot to drywall connection sealed
Boot to drywall connection sealed
Boot to drywall connection sealed
View Image Download
Source
Courtesy Of
EPA
Right – Several potential sources of air leakage into the attic have been air sealed; canned spray foam was used to seal around duct boots, along seams in the drywall, and along top plates.
Right – Several potential sources of air leakage into the attic have been air sealed; canned spray foam was used to seal around duct boots, along seams in the drywall, and along top plates.
Right – Several potential sources of air leakage into the attic have been air sealed; canned spray foam was used to seal around duct boots, along seams in the drywall, and along top plates.
View Image Download
Source

Compliance

ENERGY STAR New Homes and Apartments

DOE Zero Energy Ready Home

International Energy Conservation Code (IECC)

International Residential Code (IRC)

Air Conditioning Contractors of America (ACCA) Standard 9

Retrofit

SCOPE

Air seal around duct boots in the ceiling or floor of an existing home, to reduce air leakage to or from the conditioned space and to minimize the entry or unconditioned unfiltered air.

See the U.S. Department of Energy Standard Work Specifications for additional guidance attaching and sealing duct boots.

For guidance on working in attics and crawlspaces, see the Pre-Retrofit Assessment of Attics, Ceilings and Roofs, and Pre-Retrofit Assessment of Basements and Crawlspaces in the Building America Solution Center.

DESCRIPTION

If sealing from the attic or crawlspace side, Remove insulation as needed to access areas to be sealed. Seal with caulk or fiberglass mesh tape and mastic.

If sealing from within the house, remove the register grille. Seal the seam between the duct boot and the flooring or sealing framing and finish material. Use caulk for gaps that are less than 3/8-inch. Use fiberglass mesh tape and mastic where gaps are larger than 3/8-inch.

How to Seal Duct Boot to Ceiling with Fiberglass Mesh Tape and Mastic (for gaps larger than 3/8 inch).

  1. Remove register grille and wipe seam clean to remove dust and grease from edges of duct boot.
  2. Apply fiberglass mesh tape over seam between duct boot and ceiling drywall (Figures 1, 2, and 3).
  3. Cover tape with mastic.
Air seal duct boot to ceiling by installing fiberglass mesh tape and mastic over seam.
Figure 1. Air seal duct boot to ceiling by installing fiberglass mesh tape and mastic over seam. (Source: NM Energy $mart Academy 2014).
Duct boot is air sealed to ceiling by covering the seam with fiberglass mesh tape and mastic.
Figure 2. Duct boot is air sealed to ceiling by covering the seam with fiberglass mesh tape and mastic. (Source: NM Energy $mart Academy 2014).
Fiberglass mesh tape is installed around a duct boot in preparation for air sealing with mastic.
Figure 3. Fiberglass mesh tape is installed around a duct boot in preparation for air sealing with mastic. (Source: NM Energy $mart Academy 2014).

 

 

COMPLIANCE 

See Compliance tab. 

More Info

References and Resources

Flexible Duct Performance and Installation Standards

Publication Date
·
Author(s)
Air Diffusion Council
·
Organization(s)
ADC
Description
Standard providing a comprehensive approach to evaluating, selecting, specifying and installing flexible duct in HVAC systems.

Measure Guideline: Sealing and Insulating Ducts in Existing Homes

Publication Date
·
Author(s)
Aldrich Robb,
Puttagunta Srikanth
·
Organization(s)
Consortium for Advanced Residential Buildings,
CARB,
National Renewable Energy Laboratory,
NREL,
Steven Winter Associates,
SWA
Description
Report describing the sealing and insulating of HVAC duct systems in existing homes.

Selecting Caulk

Publication Date
·
Author(s)
Robinson Bill
·
Organization(s)
Journal of Light Construction,
JLC
Description
Article describing various types of caulks and sealants, their formulas and best uses.

*For non-dated media, such as websites, the date listed is the date accessed.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Pacific Northwest National Laboratory

Building Science Corporation

Sales

Building Science-to-Sales Translator

Tight Duct Sealing

Image(s)

Technical Description

As conditioned air flows from heating and cooling equipment to the spaces where people live, excessive leaks compromise comfort, health, and durability while increasing energy costs. This is because conditioned air escapes ducts supplying heating and cooling, and potentially contaminated air is pulled into the ducts, returning air to the heating and cooling equipment. Professionally sealed comfort delivery systems are significantly air-tight at all seams and connections. Sealing helps conditioned air get where it is needed, at the correct temperature, without pulling in contaminants from crawlspaces and attics.

Alternate Terms

Optimum-Flow Comfort Delivery System
Contaminant-Sealed Comfort Delivery System
Energy-Saving Comfort Delivery System
Professionally Sealed Comfort Delivery System
Last Updated

Questions? Comments? Contact our webmaster.

Disclaimer

This content is a work created with funding provided by the United States Department of Energy under Contract no DE-AC05-076RL01830 for the operation of Pacific Northwest National Laboratory. The information and guidance provided by Pacific Northwest National Laboratory (PNNL) in the content are intended solely for educational purposes only and do not constitute formal training or certification. It is provided with the explicit understanding that neither the United States Government nor the United States Department of Energy, nor the Contractor, nor any or their employees, nor any jurisdiction or organization that has cooperated in the development of these materials, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness or any information, apparatus, product, software, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Viewers assumes full responsibility for all actions that they may take from information provided in this content including ensuring the safety, code compliance, and proper functionality of any products they choose to install. Installation and use of such products should be performed in accordance with local regulations and manufacturer instructions.