Measure Guideline: Air 

Sealing Mechanical Closets 

in Slab-On-Grade Homes

Bruce Dickson

IBACOS, Inc.

February 2012
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i

Measure Guideline: Air Sealing Mechanical Closets in 

Slab-On-Grade Homes

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

Bruce Dickson

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, Pennsylvania 15222

Technical Monitor: Michael Gestwick

Task Order KNDJ-0-40341-02

February 2012
ii

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iii

Contents

List of Figures ............................................................................................................................................ iv

List of Tables ............................................................................................................................................... v

Definitions................................................................................................................................................... vi

Executive Summary.................................................................................................................................... 1

1 Strategies Covered in this Measure Guide ........................................................................................ 3

1.1 Audience for this Measure Guide ........................................................................................3

1.2 Industry Relevance for this Measure Guide.........................................................................3

2 Common Practice and Inherent Risk Issues ..................................................................................... 3

2.1 Indoor Air Quality................................................................................................................4

2.2 Building and Equipment Durability.....................................................................................5

2.3 Occupant Comfort and Energy Efficiency...........................................................................5

3 Air Sealing a Total Equipment Replacement..................................................................................... 6

3.1 Procedure .............................................................................................................................7

4 Air Sealing the Existing Equipment and Mechanical Closet.......................................................... 14

5 Verification Testing ............................................................................................................................ 19

6 Selection Criteria for Maximum Energy Efficiency ......................................................................... 20

6.1 Performance .......................................................................................................................20

6.2 System Interactions............................................................................................................20

6.3 Cost Effectiveness..............................................................................................................20

6.4 Codes and Standards..........................................................................................................21

7 Efficiency, Homeowner Awareness and Education........................................................................ 21

8 Conclusion .......................................................................................................................................... 21

References................................................................................................................................................. 22
iv

List of Figures

Figure 1. Typical space conditioning strategies in existing slab on grade homes.............................. 2

Figure 2a. Leakage paths into the system................................................................................................ 5

Figure 2b. Cut away view. .......................................................................................................................... 6

Figure 3a. Closet with air handler and support shelf removed.............................................................. 7

Figure 3b. Install air barrier product (gypsum board) on interior framing............................................ 8

Figure 4. Install the properly-sized and sealed ductboard plenum box................................................ 9

Figure 5a. Air handler installed on the prefabricated metal stand over the return air plenum......... 10

Figure 5b. Seal the return plenum to the air handler. ........................................................................... 11

Figure 6a. Interior view - Air seal around the plenum and any penetrations through the ceiling.... 12

Figure 6b. Exterior view of sealing around supply plenum at ceiling. ................................................ 13

Figure 7a. Air leakage pathways. ............................................................................................................ 14

Figure 7b. Interior view............................................................................................................................. 15

Figure 8a. Install appropriate blocking in any open stud cavities at the support shelf. ................... 16

Figure 8b. Make repairs at the ceiling lid to enable good sealing between components. ................ 17

Figure 9a. Seal all joints and seams in framing and sheathing. .......................................................... 18

Figure 9b. Interior view showing sealing at the supply plenum. ......................................................... 19

* Unless otherwise noted, all figures were created by IBACOS.
v

List of Tables

No table entries found.
vi

Definitions

Air Barrier A material or structural element that inhibits air flow 

into and out of a building's envelope or shell.

Air Handler A device used to circulate heated or cooled air 

throughout the space of a home.

Backdrafting The flow of air down a flue/chimney and into a house 

caused by low indoor air pressure that can occur when 

using several fans or fireplaces and/or if the house is 

very tight.

Blower Door A calibrated fan that is used to measure the airtightness 

level of building envelopes and to diagnose and 

demonstrate air leakage problems. ( Source: The Energy 

Conservatory)

Bulkhead/Dropped Soffit Boxed in area constructed to hang down from a ceiling 

to house beams, ductwork, or plumbing systems.

Caulking A material used to seal areas of potential air leakage into 

or out of a building envelope.

Condensate The liquid resulting when water vapor contacts a cool 

surface; also the liquid resulting when a vaporized 

working fluid (such as a refrigerant) is cooled or 

depressurized.

Condensation The process by which water in air changes from a vapor 

to a liquid due to a change in temperature or pressure; 

occurs when water vapor reaches its dew point 

(condensation point); also used to express the existence 

of liquid water on a surface.

Conditioned Space The interior space of a building that is heated or cooled.

Dehumidify The process of removing moisture from air.

Efficiency The ratio of output divided by input. (Source: 

Residential Energy John Krigger and Chris Dorsi)

Energy Retrofit The process of improving the overall energy efficiency 

and comfort level of a structure through upgrades to the 

thermal envelope and mechanical systems.

Expanding Spray Foam Polyurethane or polyisocyanurate foam material that is 

used to seal cracks and holes to prevent air passage and 

provide insulation.
vii

Latent Cooling Load The amount of energy used to remove moisture in the 

air, including from outside air infiltration and from 

indoor sources such as occupants, plants, cooking, 

showering, etc.

Return Air Plenum Ducting or constructed space that is designed to direct 

air from the living space into the air handler to be 

conditioned and re-distributed to the home.

Smoke Pencil A non-toxic smoke generator used to detect drafts and 

air leaks in a home.

Static Pressure The air pressure within a duct system during operation 

that results from duct type, layout, fan capacity and 

other associated restrictions ( balancing dampers, 

turning vanes, filters, etc.).

Unconditioned Space The spaces within a home that do not have any heating 

or cooling.
1

Executive Summary

With rising utility costs, homeowners are considering upgrades to increase energy efficiency 

while maintaining, or even increasing the level of comfort in the home. These upgrades can be as 

extensive as a whole house energy retrofit, where improvements are made to the air tightness and 

insulation levels of the thermal envelope and mechanical systems are removed and replaced; or,

as minimal as a single component upgrade such as new windows or 

new appliances.

In many areas of the country, a slab-on-grade ranch style home is 

often constructed with an unconditioned attic that is insulated at the 

ceiling level and vented with soffit and ridge vents. The space 

conditioning system in this type of home is typically located in the 

unconditioned attic or in a centrally located utility closet within the 

living space of the home. In either scenario, the supply air ductwork is routed throughout the 

attic space, delivering conditioned air to individual rooms through the ceiling. The return air for 

these systems is fed to the air handler from either a single, central source or from ductwork that 

is routed from various rooms in the home. Air handlers that are located 

in attics draw return air from the living space through a return grille or 

grilles that have been cut through and installed in the ceiling plane. If 

the air handler is installed in a closet within the living space, it is 

typically an upflow unit that takes return air from beneath the air 

handler and distributes the conditioned air through supply ductwork that is routed throughout the 

attic. Normally, the cutout for the supply plenum that passes through the ceiling to reach the attic 

space is cut out in very rough fashion and not sealed well or is chinked with fiberglass. Chinking 

the gap with fiberglass or mineral wool is not an effective air barrier between the closet and 

unconditioned attic. Large gaps to the attic and any air leakage paths from the closet to the return 

plenum allows the operating air handler to draw hot, humid attic air 

into the conditioning system. Properly sealing all gaps between the

supply plenum that penetrates the ceiling and all potential air leakage 

pathways from the closet space to the return plenum with expanding 

spray foam or caulking will improve the overall efficiency of the 

equipment and possibly comfort conditions within the home.

Definition: Energy 

Retrofit - The process of 

improving the overall 

energy efficiency and 

comfort level of a structure 

through upgrades to the 

thermal envelope and 

mechanical systems.

Definition: Unconditioned 

Space - The spaces within 

a home that do not have 

any heating or cooling.

Definition: Conditioned 

Space - The interior space 

of a building that is heated 

or cooled.
2

Figure 1. Typical space conditioning strategies in existing slab on grade homes. 

Due to extreme temperatures in the attic, equipment and ductwork located there do not perform 

as efficiently as they would in conditioned space. A home designed to achieve Building America 

performance goals of energy savings would typically bring all equipment and ducts inside the 

conditioned enclosure. 

However, while ductwork located within the conditioned living space 

versus the attic space is more energy efficient and installing it in 

conditioned space is very possible, most retrofit projects do not present 

ideal conditions for this type of strategy. During a retrofit, installing 

the ductwork within the living space is impractical in many instances, 

often requiring the retrofit contractor to insulate the attic roof deck, 

which can be costly, or lowering the ceiling by adding many bulkheads

(dropped soffits) throughout the house to accommodate the ductwork.

Definition:

Bulkhead/Dropped Soffit -

Boxed in area constructed 

to hang down from a 

ceiling to house beam, 

ductwork, or plumbing 

systems.
3

1 Strategies Covered in this Measure Guide

This measure guide covers two fundamental retrofit strategies for air sealing around air handling 

systems that are located within the living space in an enclosed closet. It includes the design and 

installation details necessary to effectively seal the air handler closet and central return system to 

maximize the efficiency and safety of the space conditioning system. This guide will discuss and 

demonstrate best practice methods associated with the air sealing of a mechanical closet where 

all of the equipment is removed and being replaced, and a closet where the equipment is to 

remain and existing conditions are sealed.

1.1 Audience for this Measure Guide

The audiences who will benefit most from this measure guide include the homeowner, the 

retrofit contractor, the mechanical contractor, and the equipment manufacturer.

1.2 Industry Relevance for this Measure Guide

The market demand for improved comfort, indoor air quality, and energy efficiency has 

increased. As energy costs rise, homeowners are becoming less tolerant of the loss of comfort 

and energy efficiency due to poor design and installation methods and are looking for ways to 

improve the performance and comfort of their homes.

2 Common Practice and Inherent Risk Issues

In a home where the air handler is located in a mechanical closet within the living space, it is not 

uncommon for the return air box or plenum area beneath the unit to be constructed of open 

framing, where no air barrier product (typically gypsum wallboard) has 

been applied against the studs. In some cases, this open framing may 

extend all the way to the attic. Without any air barrier on the interior of 

the closet, air is free to move within the wall cavities and potentially 

travel to and from any adjacent unconditioned spaces. If these 

conditions exist, in almost all circumstances the air will be drawn into 

the air handler if the air has already been drawn into the closet space.

In addition, the ceiling plane above the air handler may or may not 

have an air barrier “lid’ installed. If there is a lid, often the cutout for the supply plenum, which 

passes through the ceiling to reach the attic space, is cut out in very rough fashion and left 

unsealed. This allows the free movement of attic air into the mechanical closet and vice versa.

If these inadequate, yet common practices exist in the home, there are potential risk issues that 

could affect the occupants of the house in terms of poor indoor air quality, building and 

equipment durability problems, occupant comfort, equipment inefficiency, and reduced system 

performance. Through these open building cavities and unsealed areas, hot, humid air is readily 

drawn down through the mechanical closet and delivered to the air handler from the open return 

plenum. This undesirable air within the system can make the air handler work longer or harder to 

remove the negative effects; or worse yet, this air passes through the system and is mixed with 

the conditioned air and distributed throughout the house. 

Definition: Return Air 

Plenum - Ducting or preconstructed space that is 

designed to direct air from 

the living space into the air 

handler to be conditioned 

and re-distributed to the 

home.
4

2.1 Indoor Air Quality

A leaky mechanical closet with a bottom-feed return air system can draw unwanted air from the 

attic and building cavities into the duct system. This air may contain pollutants, such as dirt, dust, 

mold, and other allergens as well as unwanted moisture. If the air handler (furnace) is a gas-fired, 

natural-draft vented (B-vent) furnace, there is a high risk of backdrafting, or drawing exhaust 

gases back down the flue by the surrounding negative pressure from the leaky return. 

Backdrafting can lead to unhealthy exhaust gases being drawn into the living space of the house. 

To ensure combustion safety and reduce health risks due to 

backdrafting, the following life-safety strategies are critically 

important if the home has a gas-fired, natural-draft vented (B-vent) 

furnace. The first and best strategy is to replace any natural draft 

appliances with sealed combustion, direct vent furnaces, or all electric 

heat pumps. This equipment does not backdraft. If a naturally vented 

(B-vent) furnace is installed and not being replaced, a thorough 

inspection by a qualified mechanical contractor should be completed to ensure proper venting 

and combustion air sourcing before proceeding with the retrofit. If existing natural draft-vented 

appliances (B-vent) will not be replaced, then it is critical to confirm that the home has a 

combustion air source that meets or exceeds the International Mechanical Code requirements for 

combustion air from the indoors, and is fitted with carbon monoxide 

alarms complying with UL 2034 in close proximity to the combustion 

appliances and outside each separate sleeping area in the immediate 

vicinity of the bedrooms. The 2006 International Mechanical Code

702.1 All air from indoors. Combustion and dilution air shall be 

permitted to be obtained entirely from the indoors in buildings that are 

not of unusually tight construction. In buildings of unusually tight 

construction, combustion air shall be obtained from the outdoors in 

accordance with Section 703, 705, 706 or 707.

‘(IMC, 2006)’ requirements are below:

702.2 Air from the same room or space. The room or space containing fuel burning appliances 

shall be in unconfined space as defined in Section 202.

702.3 Air from adjacent spaces. Where the volume of the room in which the fuel-burning 

appliances are located does not comply with Section 702.2, additional inside combustion and 

dilution air shall be obtained by opening the room to adjacent spaces so that the combined 

volume of all communicating spaces meets the volumetric requirement of Section 702.2. 

Openings connecting the spaces shall comply with Sections 702.3.1 and 702.3.2.

702.3.1 Number and location of openings. Two openings shall be provided, one within 1 foot 

(305 mm) of the ceiling of the room and one within 1 foot (305 mm) of the floor.

702.3.2 Size of openings. The net free area of each opening, calculated in accordance with 

Section 708, shall be a minimum of 1 square inch per 1,000 Btu/h (2201 mm2

/kW) of input 

rating of the fuel burning appliances drawing combustion and dilution air from the 

communicating spaces and shall not be less than 100 square inches (64516 mm2).

Definition: Backdrafting -

The flow of air down a 

flue/chimney and into a 

house caused by low 

indoor air pressure that can 

occur when using several 

fans or fireplaces and/or if 

the house is very tight.

Warning: If there are 

naturally vented (B-vent) 

appliances located in the 

closet that are not going to 

be replaced a professional 

mechanical contractor 

should be hired to conduct 

a thorough inspection of all 

venting and make up air 

strategies. Any issues must 

be corrected before 

proceeding with the 

retrofit. 
5

2.2 Building and Equipment Durability

In a poorly sealed or unsealed mechanical closet, the air handler will have the potential to draw 

warm, moist air from the attic space and through the building cavities from outside. Drawing 

warm, moist air from the attic will generate more condensate on the air conditioning coil, which 

will have to be removed by a condensate pump and then drained. If the pump or drain fails, the 

result will be leaks or clogs, increasing the risk of water damage. In addition, this moist attic air 

will tend to increase the relative humidity within the home and may lead to condensation related 

issues on building surfaces.

2.3 Occupant Comfort and Energy Efficiency

In slab-on-grade homes during the cooling season, the conditioned living space of the home will 

typically be maintained between 68°–72°F degrees, while attic temperatures may exceed 110°F

degrees. When the mechanical closet is not sealed well and there is a lot of air transfer between 

the attic space and closet, there is great potential for the system to draw 

warm, moist air from the attic and building cavities into the air handler 

(see Figures 2a and 2b). Drawing warm, moist air into the system will 

increase the latent cooling load of the system and require the air 

handler to work much harder to cool and dehumidify the air being delivered to the living space. 

Figure 2a. Leakage paths into the system.

Definition: Dehumidify -

The process of removing 

moisture from air.
6

Figure 2b. Cut away view.

Homeowners will have higher utility bills due to the increased latent 

cooling load placed on the system. Homeowners may also experience 

reduced comfort due to the higher temperature of air from the attic 

entering the living space. In addition, the air handler may not 

sufficiently remove moisture from the living space, increasing indoor 

humidity and further decreasing homeowner comfort. 

3 Air Sealing a Total Equipment Replacement

In this first retrofit strategy, the homeowner will have the existing air handling system, which is 

located in a mechanical closet within the living space, removed and replaced with a more 

efficient unit into the same closet. This scenario also includes removal of the return air plenum 

that was located beneath the unit to ensure that a completely sealed system is reinstalled. If 

possible, it is best to plan for and install a full-size closet door to facilitate the installation and 

ease of maintenance of the new equipment.

Definition: Latent Cooling 

Load - The amount of 

energy used to remove 

moisture in the air, 

including from outside air 

infiltration and from indoor 

sources such as occupants, 

plants, cooking, showering, 

etc.
7

3.1 Procedure

After removing the air handler and the shelf support, inspect the interior construction of the 

mechanical closet for sufficient air sealing (see Figure 3a). It will be necessary to sheathe any 

exposed wall or ceiling framing. The most convenient product for this purpose is gypsum 

wallboard, installed conventionally, and at a minimum with all seams and corners having the 

initial base coat of joint compound and taping.

Figure 3a. Closet with air handler and support shelf removed.
8

In many cases, a small section of the existing supply plenum will still penetrate the ceiling plane 

in order to connect the new system to the existing supply ductwork in the attic, it is critical that 

the cutout surrounding the plenum is neat and sealable. If it is not, it will be necessary to add a 

new ceiling panel that is cut to fit neatly over the existing lid. When complete, the closet should 

take on the rough appearance of any other closet in the home (see Figure 3b). One coat of 

finishing on the joints and corners, as well as painting, is entirely cosmetic and can be left up to 

aesthetic preference.

Figure 3b. Install air barrier product (gypsum board) on interior framing.
9

After the mechanical closet is completely air sealed, the mechanical contractor who is installing 

the new equipment will need to plan the air filtration strategy and construct a sealed return 

plenum box of the proper size. The best material for the plenum box is insulated fibrous duct 

board; because it is sound absorbent and easy to air seal (see Figure 4). Sealing all joints, seams 

and penetrations in the plenum with UL 181 rated foil tapes or approved mastics will ensure an 

airtight assembly. Fibrous duct construction should conform to the Sheet Metal and Air 

Conditioning Contractors National Association (SMACNA) Fibrous Glass Duct Construction 

Standards or the North America Insulation Manufacturers Association (NAIMA) Fibrous Glass 

Duct Construction Standards.

Figure 4. Install the properly-sized and sealed ductboard plenum box.

The main return duct should be constructed with straight duct modules fabricated from flat 

sections of fibrous glass duct board. Joints, corners, and longitudinal seams need to be sealed 

using a UL 181 approved closure system as outlined in NAIMA’S Fibrous Glass Duct 

Construction Standards, 3rd edition (2002). One of the most effective and easiest to install 

methods for air filtration is to install a high performance filter that is designed to coordinate with 

the size of the return air grille. These types of filters do not require any additional ductwork at 

the unit and their location on the wall allows for easy homeowner maintenance. 
10

Next, install the air handler on a prefabricated metal stand that straddles the return plenum (see 

Figure 5a).

Figure 5a. Air handler installed on the prefabricated metal stand over the return air plenum.
11

This method of installation allows the return plenum to be easily connected and sealed to the air 

handler. It is critical that the connection between the return air opening on the base of the air 

handler is well sealed to the return plenum opening with approved foil tape or mastic (see Figure 

5b). Additionally the same sealing practices must occur between the supply air ductwork and top 

of the unit connection. A thorough sealing job will complete the air-tight connection. An 

alternative to the metal stand method would be to construct a support 

shelf that fits neatly over the return plenum on which the air handler is 

installed. Make sure the front opening of the return plenum that faces 

the living space is sized to accept the grille and filter to deliver the 

designed return airflow with the air filter installed. Note: The grille 

should be sized to yield the designed airflow. If the grille is too small, 

it will lead to higher static pressures, decreased latent-load moisture 

removal, reduced occupant comfort and efficiency, and the potential 

for a whistling noise.

Figure 5b. Seal the return plenum to the air handler.

Definition: Static Pressure 

– The air pressure within a 

duct system during 

operation that results from 

duct type, layout, fan 

capacity and other 

associated restrictions (eg 

balancing dampers, turning 

vanes, filters).
12

Following the installation of the return plenum and air handler, completely seal any gaps around 

the ceiling lid and duct box with spray foam (see Figures 6a and 6b). Proper sealing of any 

exhaust piping and wiring that may penetrate the ceiling plane should be done at this time. Make 

certain that all required safety shields and collars are installed on any naturally vented gas type 

furnaces (B-vent). 

Figure 6a. Interior view - Air seal around the plenum and any penetrations through the ceiling.
13

Figure 6b. Exterior view of sealing around supply plenum at ceiling.

This measure guide is intended to address the air sealing of the mechanical closet and associated 

equipment. Additional information related to further inspections and detailing of the remaining 

ductwork that is located in the attic space can be found in the following reference materials.

• Flexible Duct Performance & Installation Standards. (5th edition 2010) of the Air 

Diffusion Council 

• An Introduction to Residential Duct Systems, http://ducts.lbl.gov/

• Air Seal and Insulate with ENERGY STAR,

http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_sealing

• Guide to Attic Air Sealing – Identifying and Blocking Air Leakage Pathways Providing 

Airtight Closure, http://www.buildingscience.com/documents/guides-and-manuals/gmattic-air-sealing-guide.
14

4 Air Sealing the Existing Equipment and Mechanical Closet

In this second retrofit strategy, the existing equipment will remain in place within the mechanical 

closet, and the mechanical closet, supply plenum penetration through the ceiling, and return air 

plenum will be air sealed. The objective of this retrofit is to eliminate, as best as possible, all

leakage paths from the attic space into the closet and from the closet space to the return air 

plenum beneath the air handling unit. (see Figures 7a and 7b).

Figure 7a. Air leakage pathways.
15

Figure 7b. Interior view.

First, inspect the interior of the closet space for air leakage locations. Start with the closet 

ceiling. Any holes or gaps should be thoroughly sealed with either caulk, or expanding foam. 

Silicone or polyurethane caulks are best (most durable). If existing gaps are too wide (larger than 

3⁄4”), it may be necessary to fit and seal additional sheathing products around the supply plenum 

(see Figures 8a and 8b). If there is exposed wall framing, inspect the top plates for penetrations 

into the attic, and seal them, as necessary. Inspect the air handler platform around the perimeter, 

where the platform is attached to the sidewalls. If necessary, install blocking to close any large 

framing voids—such as between studs—and then seal the blocking with caulk or spray foam. 

Inspect the base of the air handler unit where it rests upon the closet shelf, and seal the perimeter 

with caulking or expanding foam (see Figures 9a and 9b). If possible, and the opening at the 

return air grille is large enough for access, inspect the interior space of the under unit return 

plenum. In particular, look for any gaps or holes that may permit air leakage to other building 

components of the house. Sealing of these areas with expanding foam, mastic, or rated foil tapes 

will greatly improve the overall performance of the system.
16

Figure 8a. Install appropriate blocking in any open stud cavities at the support shelf.
17

Figure 8b. Make repairs at the ceiling lid to enable good sealing between components.
18

Figure 9a. Seal all joints and seams in framing and sheathing.
19

Figure 9b. Interior view showing sealing at the supply plenum.

5 Verification Testing

Diagnostic pressure testing of the house with a blower door, and 

diagnostic pressure testing of the ductwork will help to determine if the 

air sealing efforts have been effective or if further sealing is needed. 

These tests can help to identify the amount and location of air leakage, 

making it easier to complete the sealing process. If the air sealing of the

mechanical closet and central return system is effective, a standard 

depressurization test with a blower door will not yield any air movement 

coming down through the chase way. The use of a smoke pencil or puffer 

is an effective tool to visualize and identify any air leaks for sealing. A 

duct leakage pressurization or depressurization test will also indicate the 

overall tightness of the space conditioning system, as well as whether the 

new return plenum box is airtight.

Definition: Blower Door 

- A calibrated fan that is 

used to measure the 

airtightness level of 

building envelopes and 

to diagnose and 

demonstrate air leakage 

problems. ( Source: The 

Energy Conservatory)

Smoke pencil - A nontoxic smoke generator 

used to detect drafts and 

air leaks in a home.
20

6 Selection Criteria for Maximum Energy Efficiency

6.1 Performance

• Increasing the air tightness of the mechanical closet and return air plenum will result in 

improved occupant comfort of the living space.

• HVAC system load reduction will be achieved by eliminating the addition of warm moist 

attic air into the system. 

• HVAC system efficiency will be increased by ensuring and maintaining the designed 

amount of return air to the system.

• Noise reduction through the use of fibrous insulated ductwork for the return air plenum 

box.

6.2 System Interactions

• Improved air tightness of the thermal envelope.

• HVAC system load reduction, which in turn will reduce energy costs.

• Improved dehumidification of the interior air that will lead to better comfort for the 

homeowner.

6.3 Cost Effectiveness

Sealing the mechanical closet so that it does not have the ability to draw unintentional air from 

the attic or building cavities is one of the easiest and cost-effective solutions to improve the 

performance of the space conditioning system and provide better comfort to the living spaces of 

the home. In either scenario, the cost to make the sealing improvements would be minimal on 

both the labor involved and materials needed.

Estimated costs for the work effort and associated materials to conduct the first total replacement 

scenario for a standard 4’ by 4’ closet with an 8’ ceiling height would be in the neighborhood of 

$1,200.00 and would include costs for the demolition of the existing support shelf, hanging and 

taping of the gypsum wallboard, new fibrous ductboard return plenum and incidental caulking 

and foam. The second scenario, where existing equipment is to remain and the comprehensive air 

sealing of the closet will take place, will have an estimated cost of $500.00 to install any 

necessary blocking and gypsum wallboard and to perform a thorough air seal with expanding 

foam and caulking.

In addition to the costs of the construction activities, there could be 

additional costs to conduct air leakage testing and mechanical system 

safety inspections. These costs vary on the complexity of the home but 

should generally fall into the $300.00 range for testing and $100.00 for 

the safety inspection.

Note: Costing is intended 

to provide a reference for 

the air sealing retrofit 

activities and are rough 

estimates. Costs will vary 

depending upon house 

type, complexity, location, 

and trade base.
21

6.4 Codes and Standards

The 2009 International Energy Conservation Code (IECC) and the 2009 International Residential 

Code (IRC) have several new mandatory requirements for air sealing in new construction and 

additions. These codes apply to new construction where adopted by local jurisdictions. In 

general, these requirements do not apply to retrofit projects unless the project adds living space 

to the building or changes the building’s energy load. The existing, unaltered portions of the 

structure are not required to comply with all of the requirements of the 2009 IECC or IRC. 

However, Building America recommends implementing these requirements in existing portions 

of your home wherever they are applicable and your budget allows or health and safety concerns 

make them necessary.

The requirements regarding new buildings can be summarized in this section excerpted from 

IECC, Chapter 4, Section 402.4, Air Leakage (mandatory). Builders can see IECC 2009, Chapter

4 “Residential,” and IRC 2009, Chapter 11 “Energy Efficiency,” for more details.

7 Efficiency, Homeowner Awareness and Education

Informing and educating the homeowners to the importance of the purpose of the mechanical 

closet and all components is critical for long-term functionality of the space. It is best to instruct

the homeowners that the mechanical closet is specifically to house the heating and cooling 

equipment for the home and that it is not an additional storage closet. In particular, chemicals 

and volatile organic compounds (such as found in paints, finishes, and cleaning supplies) should 

be stored away from the space conditioning equipment and ductwork.

The location of the air handler within the closet makes service and routine maintenance such as

filter changes easily accomplished.

8 Conclusion

This air sealing measure guide represents one of the easier and cost-effective retrofit actions that 

a homeowner can either contract out to a retrofit contractor or implement themselves. Either 

way, air sealing and isolating the mechanical closet from the attic space and return air plenum 

will save energy, increase homeowner comfort, and minimize any building durability issues.
22

References

2006 International Mechanical Code – 702.1, 702.2, 702.3, 702.3.1, 702.3.2.

2009 International Energy Conservation Code (IECC).

2009 International Residential Code (IRC).

2010 Building America Best Practice Series Volume 10 Retrofit Techniques and Technologies –

Air Sealing. A Guide For Contractors To Share with Homeowners.

Air Seal and Insulate with ENERGY STAR. U.S. Environmental Protection Agency, ENERGY 

STAR. http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_sealing.

Fibrous Glass Duct Liner Standard - Design, Fabrication and Installation Guidelines Third 

Edition 2002 NAIMA – North American Insulation Manufacturers Association.

Fibrous Glass Duct Construction Standards 7th Edition - SMACNA – Sheet Metal and Air 

Conditioning Contractors National Association.

Glossary of Energy-Related Terms. U.S. Department of Energy, Energy Efficiency & Renewable 

Energy. http://www1.eere.energy.gov/site_administration/glossary.html.

Guide to Attic Air Sealing – Identifying and Blocking Air Leakage Pathways Providing Airtight 

Closure. Somerville, MA: Building Science Corporation. 

http://www.buildingscience.com/documents/guides-and-manuals/gm-attic-air-sealing-guide.

An Introduction to Residential Duct Systems. Berkeley, CA: Lawrence Berkeley National 

Laboratory. http://ducts.lbl.gov/.

Retrofit Techniques and Technologies: Air Sealing – A Guide for Contractors to Share with 

Homeowners. U.S. Department of Energy, Building America Best Practice Series. April 12, 

2010. 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_airsealing_report.

pdf.
DOE/GO-102012-3385 ▪ February 2012

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.