HVAC Ducts Shall Not Be Run within Exterior Walls

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Proper Duct Installation =
Professionally-Installed Comfort Delivery Ducts

Technical Description: 

Conditioned air is often lost in transition from comfort equipment to living spaces because of poorly designed duct layouts and poor installation practices, which cause homeowners to pay for conditioned air that never reaches the living spaces of their home. Poor duct layouts with overly long duct runs, numerous branches, and sharp bends, and poor installation with sagging, twisted, or crushed ducts, can severely limit air flow, reducing system performance and increasing energy costs. Professionally installed comfort delivery systems should be installed with compact duct layouts featuring short, straight duct runs with properly supported ducts, in accordance with industry standards. The ducts should be properly insulated, completely air sealed, and free from kinks and sharp bends that restrict air flow.

Alternate Terms

Optimum-Flow Comfort Delivery Duct Installation
Energy Saving Comfort Delivery Duct Installation
Professionally-Installed Comfort Delivery Ducts
Sales Message
Professionally-installed comfort delivery ducts help ensure that heating and cooling can flow optimally to each room. This means less wasted energy along with enhanced room-by-room comfort. Wouldn’t you agree it’s important that your comfort delivery system is installed to deliver optimum performance?

Climate

No climate specific information applies. 

Description

In the past, it was common practice to run ducts inside a wall cavity of an exterior frame wall. It is sometimes done today. However, we have learned that this practice will create an energy penalty and can cause durability issues within the wall cavity.

Ducts in exterior wall cavities experience unnecessary heat gain and loss through conduction, convection, and radiation, which creates an extra load on the HVAC equipment. Heat transfer through conduction takes place when the surface of the duct is in contact with the exterior sheathing. If there is an air space between the exterior sheathing and the duct, heat gain or loss will occur as air moves around the duct (convection). At the same time, the duct loses or gains heat from the exterior siding through radiant heat transfer.

Durability issues occur when humid air moves through cavity spaces which contain ducts that are carrying cool air because the humid air in the cavity space can condense on the exterior surfaces of the cool ducts. This scenario is a common occurrence in older homes in humid climates where cooling has been added to a forced air heating system that has ducts located in the exterior framed walls.

In older homes with poorly insulated walls and single-pane windows, supply registers are often located at or on exterior walls to condition the cold walls and cold air that would leak in at the windows. In new homes with better air sealing and insulation and higher performance double-pane windows, there is less heat transfer through exterior walls and less air leakage in and around windows. Exterior supply air throws are no longer necessary to maintain comfort; shorter duct runs with interior throws are preferred for improved energy efficiency and better HVAC performance. Duct layout should be considered at the design stage and the duct system should be designed for short, compact duct runs with interior throws and appropriately selected registers to achieve comfort across each room.

Another concern with ducts in exterior walls is adequate space. Standard wall cavities are not deep enough to house flex duct and may not be deep enough to house sheet metal duct, which should be wrapped in a minimum of 1 inch (R-4) of insulation. Rectangular fiber board duct might fit but analysis should be done to confirm that the space would accommodate the duct size required to meet the designed air flow per ACCA Manual D.

If ducts must be run in exterior wall cavity spaces, then R-6 rigid foam board insulation should be placed on the exterior side of the duct, between the exterior sheathing and the insulated duct. This rigid foam takes the place of the cavity insulation which is displaced by the duct and is in addition to any rigid foam used over or in place of the external wall sheathing. If ducts must run in exterior walls, make sure the interior and exterior air barrier are continuous and air tight.

Ducts should not be located in exterior wall cavities

Figure 1 - Ducts should not be located in exterior wall cavities. (Image courtesy of CalcsPlus)

 

How to Correctly Locate Ducts 

1.  Plan duct layout at the design stage. Design short, compact duct runs that do not run along exterior walls. Use floor or ceiling register locations. Be sure that the register box is air sealed to the floor or ceiling framing assembly. The register or grille should be removable so the register box can be cleaned periodically.

Install supply registers in floors or ceilings to avoid routing ducts through exterior walls

Figure 2 - Install supply registers in floors or ceilings to avoid routing ducts through exterior walls. (Image courtesy of CalcsPlus)

2.  Or, if wall register locations are desired, run ducts in interior walls. Be sure the wall cavity space being used to house the ductwork is totally sealed from a vented attic or crawlspace.

If wall registers are desired, install HVAC ducts in interior wall cavities

Figure 3 -  If wall registers are desired, install HVAC ducts in interior wall cavities. (Image courtesy of CalcsPlus)

3.  If ductwork must run inside an exterior wall cavity (not recommended), install continuous R-6 foam insulation board along the wall between the ductwork and the siding. Fill the cavity space around the duct with insulation. Ensure that a continuous air barrier exists along the interior and exterior of the exterior wall containing the duct by gluing exterior sheathing and drywall to framing, mudding and taping drywall seams, taping rigid foam sheathing seams, and caulking or foam air sealing around any wall penetrations.

If HVAC duct must be installed in an exterior wall, separate it from the exterior with at least R-6 of continuous rigid insulation

Figure 4 - If HVAC duct must be installed in an exterior wall, separate it from the exterior with at least R-6 of continuous rigid insulation. (Image courtesy of CalcsPlus)

Ensuring Success

Inspect to ensure that ducts are not installed in exterior wall cavity spaces, if at all possible. The best way to ensure this is to design the duct layout early and to accommodate space for ducts in the framing layout, with short, straight duct runs and throws along interior walls and preferably with ducts and air handler located within the conditioned space. If ducts must be run in exterior wall cavity spaces, inspect to ensure that R-6 rigid foam board insulation is located between the exterior sheathing and the insulated duct. Also, if ducts must run in exterior walls, make sure the interior and exterior air barriers are continuous and air tight.

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Scope

HVAC ducts, cavities used as ducts, and combustion inlets and outlets may pass perpendicularly through exterior walls but shall not be run within exterior walls unless at least R-6 continuous insulation is provided on exterior side of the cavity

Do not install HVAC ducts in exterior walls.

Zero Energy Ready Home Notes

The U.S. Department of Energy’s Zero Energy Ready Home Program requires that builders comply with the ENERGY STAR Certified Homes requirements; see below and see the Compliance tab. Builders must also meet the requirements of the U.S. Environmental Protection Agency’s Indoor airPLUS Program. The Indoor airPLUS checklist (Item 4.3 Location of Air-Handling Equipment and Ductwork) dictates that air-handling equipment or ductwork should not be located in garages, but notes that ducts and equipment may be located in framing spaces or building cavities adjacent to garage walls or ceilings if they are separated from the garage space with a continuous air barrier (see ENERGY STAR Thermal Enclosure System Rater Checklist).

ENERGY STAR Certified Homes Notes

The ENERGY STAR Certified Homes HVAC System Quality Installation Rater Checklist dictates the following (Item 2.6): HVAC ducts, cavities used as ducts, and combustion inlets and outlets may pass perpendicularly through exterior walls but shall not be run within exterior walls unless at least R-6 continuous insulation is provided on the exterior side of the cavity, along with an interior and exterior air barrier where required by the Thermal Enclosure System Rater Checklist.

If installing supply ducts within the walls, verify that there is enough space in the wall cavity for a duct capable of outputting the necessary air flow. Typically, only double-wall assemblies will have enough depth to allow for proper duct size and adequate insulation.

Training

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Presentations

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Compliance

DOE Zero Energy Ready Home

The U.S. Department of Energy’s Zero Energy Ready Home Program requires that builders comply with the ENERGY STAR Certified Homes requirements. Builders must also meet the requirements of the U.S. Environmental Protection Agency’s Indoor airPLUS Program. The Indoor airPLUS checklist (Item 4.3 Location of Air-Handling Equipment and Ductwork) dictates that air-handling equipment or ductwork should not be located in garages, but notes that ducts and equipment may be located in framing spaces or building cavities adjacent to garage walls or ceilings if they are separated from the garage space with a continuous air barrier (see ENERGY STAR Thermal Enclosure System Rater Checklist).

ENERGY STAR Certified Homes

The ENERGY STAR Certified Homes HVAC System Quality Installation Rater Checklist dictates the following (Item 2.6): HVAC ducts, cavities used as ducts, and combustion inlets and outlets may pass perpendicularly through exterior walls but shall not be run within exterior walls unless at least R-6 continuous insulation is provided on the exterior side of the cavity, along with an interior and exterior air barrier where required by the Thermal Enclosure System Rater Checklist.

If installing supply ducts within the walls, verify that there is enough space in the wall cavity for a duct capable of outputting the necessary air flow. Typically, only double-wall assemblies will have enough depth to allow for proper duct size and adequate insulation.

2012 IRC

Section M1601.1.1 Above-ground duct systems. Stud wall cavities in building envelope exterior walls cannot be used as air plenums.*

*Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code.

More Info.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: August, 2012

    Case study describing how a properly designed duct system is essential for keeping a home comfortable, managing indoor air quality, and maximizing the performance of heating and cooling equipment.

References and Resources*

  1. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December, 2013

    Standard outlining industry procedure for sizing residential duct systems.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: April, 2014

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: June, 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: November, 2013

    Document outlining specifications that were developed by the U.S. Environmental Protection Agency (EPA) to recognize new homes equipped with a comprehensive set of indoor air quality (IAQ) features.

Last Updated: 06/04/2014

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