MANUFACTURED 

HOUSING 

STANDARDS

Testing and 

Performance 

Evaluation Could 

Better Ensure Safe 

Indoor Air Quality

Report to Congressional Requesters

October 2012

GAO-13-52

United States Government Accountability Office

GAO
United States Government Accountability Office

Highlights of GAO-13-52, a report to 

congressional requesters

October 2012

MANUFACTURED HOUSING STANDARDS 

Testing and Performance Evaluation Could Better 

Ensure Safe Indoor Air Quality

Why GAO Did This Study

Since 1976, HUD has been 

responsible for developing construction 

and safety standards (the HUD Code) 

for manufactured homes. Concerns 

have been raised by Congress and 

others about existing HUD code 

requirements that are intended to 

ensure proper indoor air quality, 

including protecting occupants from 

potential carbon monoxide exposure. 

As requested, GAO examined 

1) existing standards for separating air 

intakes and exhaust vents in both 

manufactured and site-built homes; 

2) reasons for differences in ventilation 

standards for manufactured and sitebuilt homes; and 3) the number of 

manufactured homes built, the 

distances between their air intakes and 

exhaust vents, and the performance of 

their ventilation systems. GAO 

reviewed documentation from HUD 

and building standards organizations to 

determine differences in requirements 

tied to ventilation and air quality, 

reviewed the rulemaking process and 

status of proposed updates to 

manufactured housing standards 

related to ventilation and air quality, 

analyzed data on the occupancy of 

manufactured houses subject to HUD’s 

standards, assessed HUD’s efforts to 

ensure compliance with certain 

standards, and interviewed agency 

officials and indoor air quality experts.

What GAO Recommends

GAO recommends that HUD test the 

performance of installed ventilation 

systems and reassess its ventilation 

standards. HUD generally agreed with 

both recommendations and stated that 

it would bring them before the MHCC 

for consideration.

What GAO Found

Key standards for manufactured homes provide a lower margin of safety against 

a carbon monoxide exposure incident than those for site-built homes, which are 

constructed at their permanent locations. For instance, the Department of 

Housing and Urban Development (HUD) Code requires a minimum 3-foot 

separation between air intakes and exhaust vents, while industry standards for 

site-built homes have required a greater distance for many years. The industry 

standards call for a greater separation between air intakes and exhaust vents to 

help reduce the risk that contaminants such as carbon monoxide will re-enter the 

home. Indoor air quality experts whom GAO interviewed stated that the exhaust 

of an improperly operating furnace combined with unique wind conditions could, 

in rare cases, present a risk of carbon monoxide exposure. GAO analysis shows 

that increasing the separation between an air intake and exhaust vents, using 

industry standards, can significantly dilute concentrations of contaminants.

The primary reason for the differences in ventilation standards for manufactured 

homes and site-built homes is the HUD Code has not been updated since 2005 

and has not kept pace with standards tied to ventilation and air quality for sitebuilt homes. For example, updates to standards for site-built homes made in 

2003 requiring a greater separation between intakes and exhaust vents are only 

now being considered by HUD for manufactured homes. This update was 

recommended to HUD in 2010 by the Manufactured Housing Consensus 

Committee (MHCC), which is responsible for recommending proposed rule 

changes to HUD. Similarly, requirements for carbon monoxide detectors adopted 

in industry standards for site-built homes and recommended by the MHCC in

2009 have yet to be incorporated in the HUD Code. HUD did publish a proposed 

rule in the Federal Register in 2010 to update aspects of the HUD Code but has 

not issued a final rule because the rulemaking process is ongoing. Additional 

proposals, including the two above related to indoor air quality, are under 

consideration by HUD, but have not yet been published as proposed rules.

An estimated 5.5 million occupied manufactured homes were built under the 

HUD Code, according to 2009 American Housing Survey data. Although HUD 

retains copies of approved designs of manufactured homes, the agency does not 

maintain data on the actual distances between the air intakes and exhaust vents 

of each home. Further, once ventilation systems are installed in manufactured 

homes, HUD does not require manufacturers to test their performance. For 

example, manufacturers are not required to determine if the systems meet the 

requirements for the whole-house ventilation airflow rate, which quantifies the 

volume of air exchanged in the home over time. Without performance testing of 

the installed ventilation systems, HUD cannot fully ensure that the systems 

installed in manufactured homes are meeting performance specifications. In 

addition, HUD’s standard for the whole-house airflow rate provided by 

mechanical ventilation was initially established assuming a certain level of natural 

air infiltration. This whole-house airflow rate standard has not changed since 

1993. Air quality experts and research suggest that homes are increasingly being 

built with less air leakage, reducing the expected level of natural air infiltration. 

However, HUD has not reassessed the whole-house ventilation airflow rate 

standard to determine whether it continues to be sufficient to assure adequate air 

quality.

View GAO-13-52. For more information, 

contact Mathew J. Scirè at (202) 512-8678 or 

sciremj@gao.gov.
Page i GAO-13-52 Manufactured Housing Standards

Letter 1 

Scope and Methodology 2

Background 3

Ventilation and Air Quality Standards Differ between Manufactured 

and Site-Built Homes 7

Updates to the HUD Code on Key Standards Related to Ventilation 

and Air Quality Lag behind Industry Standards for Site-Built 

Homes 11

Millions of Manufactured Homes Have Been Built under the HUD 

Code but HUD Has Not Tested the Performance of Installed 

Ventilation Systems 15

Conclusions 18

Recommendations for Executive Action 20

Agency Comments and Our Evaluation 20

Appendix I Comparison of Different Separation Distances Between Air Intakes 

and Exhaust Vents and Impacts on Potential Carbon Monoxide 

Exposure 23

Appendix II Comments from the Department of Housing and Urban Development 27

Appendix III GAO Contact and Staff Acknowledgements 29

Table

Table 1: Parameters Used in Our Analysis 25

Figures

Figure 1: HUD Label 5

Figure 2: HUD Code’s Separation Requirement for Fresh Air 

Intakes and Exhaust Vents, 2012 7

Figure 3: Air Intake Contaminant Concentration at Different Wind 

Speds for Various Exhaust Vent to Air Intake Separations 9

Figure 4: GAO Analysis of HUD Processes for Updating the HUD 

Code 12

Contents
Page ii GAO-13-52 Manufactured Housing Standards

Figure 5: Contaminant Concentration at Air Intake for Different 

Wind Speeds and Various Exhaust Vent to Air Intake 

Separations 26

This is a work of the U.S. government and is not subject to copyright protection in the 

United States. The published product may be reproduced and distributed in its entirety 

without further permission from GAO. However, because this work may contain 

copyrighted images or other material, permission from the copyright holder may be 

necessary if you wish to reproduce this material separately.
Page 1 GAO-13-52 Manufactured Housing Standards

United States Government Accountability Office

Washington, DC 20548

October 24, 2012

The Honorable James P. Moran

Ranking Member

Subcommittee on Interior, Environment, and Related Agencies

Committee on Appropriations

House of Representatives

The Honorable John W. Olver

Ranking Member

Subcommittee on Transportation, Housing, and Urban Development, 

 and Related Agencies

Committee on Appropriations

House of Representatives

The Department of Housing and Urban Development (HUD) certified 

manufactured homes offer a lower-cost option to traditional site-built 

homes. In 2010, an estimated 18 million individuals lived in manufactured 

homes, which are defined as transportable structures of at least 320 

square feet built on permanent chassis structures.1

In your request, you raised concerns that the HUD Code’s ventilation 

standards, particularly the separation distance between fresh air intakes 

and exhaust vents that is intended to assure that contaminants do not 

reenter a home, may not be keeping pace with standards for site-built 

homes. For this review, we examined 1) existing standards for separating 

air intakes and exhaust vents in both manufactured and site-built homes; 

2) reasons for differences in ventilation standards for manufactured and 

site-built homes; and 3) the number of HUD manufactured homes built, 

Since 1976, all 

manufactured homes (formerly called “mobile homes”) have been 

required to meet HUD’s Manufactured Home Construction and Safety 

Standards (the HUD Code), the only building code that preempts state 

and local building codes. The HUD Code covers body and frame 

requirements, thermal protection, plumbing, electrical, ventilation, and 

other aspects of the home. Every home built to the HUD Code is 

identified with a red metal tag, known as the HUD certification label.

 

1

This estimate is from the U.S. Census Bureau’s 2010 American Community Survey 

(ACS). The margins of error for these estimates are +/- 87,000.

 
 

Page 2 GAO-13-52 Manufactured Housing Standards

including the distances between their air intakes and exhaust vents, and 

the performance of their ventilation systems.

To examine standards associated with the separation distance between 

air intakes and exhaust vents in homes, we collected and analyzed 

information on existing standards for home ventilation systems. 

Specifically, we collected and analyzed documentary information on 

construction and safety standards for manufactured homes and 

interviewed representatives from HUD to ascertain how such standards 

were developed. Likewise, we collected information on commonlyaccepted industry standards for site-built homes and the evolution of 

those standards. We also interviewed subject area experts performing

research on ventilation and air quality issues for manufactured homes to 

understand factors affecting the indoor air quality of manufactured and 

site-built homes. Further, we contacted state administrative agency 

officials in 5 of the 37 states that administer manufactured housing 

consumer programs for information on indoor air quality complaints. We 

chose administrative agencies in New York, Pennsylvania, Tennessee 

and Texas based on different climate zones and numbers of placed 

manufactured homes. We added Utah because carbon monoxide 

poisoning had allegedly occurred in a manufactured home. We selected 

states with relatively high numbers of manufactured homes, using data 

from the Census Bureau’s 2010 Manufactured Homes Survey. 

Additionally, we compared different separation distances between air 

intakes and exhaust vents and the potential for carbon monoxide 

exposure using modeling methods from the American Society of Heating, 

Refrigerating, and Air-Conditioning Engineers (ASHRAE). Appendix I 

contains a more detailed description of our comparison.

To assess the rationale for having different ventilation standards for 

manufactured and site-built homes, we collected documentary information 

and contacted officials from HUD, industry associations, and 

organizations that set building standards. Specifically, we identified the 

reasons behind the differences in ventilation standards for manufactured 

versus site-built homes, the evolution of these standards over time, and 

the current status of new or revised standards that have been proposed 

for manufactured homes.

To determine the number of manufactured homes that have been built to 

HUD standards, we analyzed U.S. Census Bureau estimates on the 

number of occupied manufactured homes built since 1975 and relevant 

characteristics of the homes. We assessed the reliability of the Census 

Scope and 

Methodology 
 

Page 3 GAO-13-52 Manufactured Housing Standards

Bureau’s 2009 American Housing Survey and 2010 American Community 

Survey by reviewing information on the data and interviewing 

knowledgeable officials on the quality of the data. We determined the 

data were sufficiently reliable for the purposes of this report. To identify 

and obtain available data on the design and performance of ventilation 

systems in manufactured homes, we sought information from HUD on the 

distances between intake and exhaust vents and the performance of the 

ventilation systems. We also contacted officials from industry 

associations, researchers, and companies in the manufactured home 

industry to identify common design and performance specifications that 

they used for manufactured homes. Further, we collected information 

from organizations that conduct testing and certify ventilation systems for 

manufactured homes. 

We conducted this performance audit from December 2011 through 

October 2012 in accordance with generally accepted government auditing 

standards. Those standards require that we plan and perform the audit to 

obtain sufficient, appropriate evidence to provide a reasonable basis for 

our findings and conclusions based on our audit objectives. We believe 

that the evidence obtained provides a reasonable basis for our findings 

and conclusions based on our audit objectives.

HUD is responsible for enforcing the federal manufactured home 

construction and safety standards that it established under The National 

Manufactured Housing Construction and Safety Standards Act of 1974.2

 

2

Pub. L. No. 93-383, Title VI.

The act authorized HUD to develop construction and safety standards for 

manufactured homes and to oversee the enforcement of the standards 

through inspections and review of building plans. HUD developed the 

Manufactured Housing Construction and Safety Standards, commonly 

known as the HUD Code, basing them in substantial part on the National 

Background 

HUD Oversight and 

Standards for 

Manufactured Homes
 

Page 4 GAO-13-52 Manufactured Housing Standards

Fire Protection Association (NFPA) standards for manufactured homes 

(NFPA 501).3 The HUD Code was implemented in 1976.4

Because a manufactured home can be moved from one state to another, 

the HUD Code is applied nationwide and preempts state and local codes. 

As a result, state and local building authorities may not apply their own 

codes to manufactured homes for components covered by the HUD 

Code, such as ventilation systems. Unlike site-built homes, which are 

constructed at their permanent locations, manufactured homes are 

constructed in factories and must have a permanent chassis so that they 

can be moved on wheels to retailers or consumers in different states and 

localities, where they are placed on temporary or permanent foundations. 

Manufactured homes differ from modular homes, which are another type 

of prefabricated home and are often designed and constructed by the 

same manufacturers that construct manufactured homes. Like site-built 

homes, modular homes are built to state and local building codes. But 

unlike manufactured homes that are required to be moved to the site and 

remain on wheeled chassis, modular home sections or modules are 

transported on truck beds and assembled on site.

The 1974 act was amended by the Manufactured Housing Improvement 

Act of 2000 to create a balanced consensus process for establishing and 

revising manufactured home building standards.5

The MHCC consists of voting members representing seven producers or 

retailers of manufactured housing; seven representatives of consumer 

interests, such as owners of manufactured homes; and seven general 

interest and public official members. In addition to construction and safety 

standards, the MHCC also develops proposed model installation 

The amendment 

established the Manufactured Housing Consensus Committee (MHCC), a 

federal advisory committee established to provide recommendations to 

the HUD Secretary on new standards and revisions of current standards.

 

3

NFPA is a nonprofit organization that focuses on the prevention of fire and other hazards 

through codes and standards, research, training, and education. First published in 1940, 

NFPA 501 Standard for Manufactured Housing establishes minimum criteria for 

manufactured homes and is updated periodically. The NFPA has a HUD contract to 

provide administrative, managerial, and technical support to the MHCC.

4

24 C.F.R. Part 3280.

5

Pub. L. No. 106–569, Title VI.
 

Page 5 GAO-13-52 Manufactured Housing Standards

standards for the manufactured housing industry. Two-thirds of MHCC 

members must approve a proposal before the committee recommends it 

to HUD.

As part of the oversight of manufactured homes, HUD approves certain 

state agencies and private third-party entities to inspect manufactured 

housing plants and determine whether manufacturers are complying with 

the HUD Code. Each manufacturer contracts with two types of third-party 

entities, a Design Approval Primary Inspection Agency (DAPIA) and Inplant Production Primary Inspection Agency (IPIA). DAPIAs review and 

approve all home designs, design changes, and each plant’s quality 

assurance manuals. To ensure that homes comply with the HUD Code, 

manufacturers contract with IPIAs to approve and monitor their quality 

control programs by inspecting each home at some stage of production in 

the plant. IPIAs issue the HUD label that is attached to each section of 

the home upon completion (see fig. 1).

Figure 1: HUD Label

Builders of manufactured homes, as well as site-built and modular 

homes, are designing more tightly constructed homes that have less 

natural air infiltration in order to reduce energy costs for consumers. 

Without proper ventilation, a home’s occupants may be exposed to a 

buildup of harmful contaminants, such as carbon monoxide, 

Ventilation Standards Used 

for Manufactured Homes
 

Page 6 GAO-13-52 Manufactured Housing Standards

formaldehyde, and mold.6

To prevent indoor contamination and compensate for less natural air in 

manufactured homes, the HUD Code requires a whole-house ventilation 

system consisting of either mechanical ventilation or systems combining 

mechanical and passive ventilation.

In addition to the HUD Code, multiple federal 

agencies have an interest in and have made efforts to improve indoor air 

quality in buildings.

7

To help ensure that contaminants that accumulate indoors are adequately 

vented out of the home and replaced with fresh outside air, the HUD 

Code requires a certain whole-house ventilation airflow rate. This rate 

specifies the volume of air that should be “replaced” over time—that is, 

fresh air drawn into the ventilation system as contaminated air is expelled. 

HUD requires an airflow rate of least 0.035 cubic feet per minute for each 

square foot of interior floor space or its hourly average equivalent. The 

ventilation system is required to produce between 50 and 90 cubic feet 

per minute to achieve this whole-house airflow rate.

Mechanical ventilation uses fans 

and ducts to bring fresh air into the home or draw contaminated air to the 

outdoors. Passive ventilation takes place naturally through windows, 

doors, and other air leakage sites. One common mechanical ventilation 

approach uses an outdoor air intake that is connected to the air 

distribution system return duct and that carries fresh air into the home 

whenever the furnace fan operates. Another approach uses a wholehouse exhaust fan, which draws contaminants from the home to the 

outdoors. In both cases, the system releases air to the outdoors through 

an exhaust vent, preventing the build-up of contaminants indoors. 

Although mechanical ventilation is becoming more common, most older 

site-built homes have relied on passive ventilation.

8

 

6

Carbon monoxide is particularly hazardous because it is colorless, odorless, and 

tasteless. According to Centers for Disease Control (CDC) estimates, from 1999 to 2004 

an average of 439 persons died annually from unintentional, non-fire-related carbon 

monoxide poisoning. CDC estimates from 2000 to 2009 show that about 78 percent 

(53,039) of carbon monoxide exposure-related incidents occurred in the home.

7

24 C.F.R. § 3280.103(b)

8

24 C.F.R. § 3280.103(b).
 

Page 7 GAO-13-52 Manufactured Housing Standards

Standards exist for separating air intakes and exhaust vents in both 

manufactured and site-built homes, but standards for the two types of 

homes differ. Since 1976, HUD’s ventilation system specifications for 

manufactured homes have required that any fresh air intake be at least 3 

feet from any exhaust vent—for example, from a gas furnace exhaust or 

plumbing system vent. (See fig. 2.)

Figure 2: HUD Code’s Separation Requirement for Fresh Air Intakes and Exhaust 

Vents, 2012

Compared with the HUD Code, industry standards commonly used for 

site-built homes recommend a greater distance between air intakes and 

exhaust vents. State and local building codes commonly cite generally 

accepted ventilation standards from ASHRAE and the International 

Residential Code (IRC). Specifically, ASHRAE requires a 10-foot 

horizontal separation between any air intake and exhaust vent. 

Additionally, IRC requires either a 10-foot horizontal separation or a 

minimum vertical separation of 3 feet if the horizontal separation is less 

than 10 feet.

Industry standards that address the separation distance between air 

intakes and exhaust vents were created to reduce the possibility that 

contaminants in the exhaust, such as carbon monoxide, would reenter the 

building. Experts associated with testing and establishing ventilation 

Ventilation and Air 

Quality Standards 

Differ between 

Manufactured and 

Site-Built Homes
 

Page 8 GAO-13-52 Manufactured Housing Standards

standards said that increasing the distance between the air intakes and 

exhaust vents would improve the margin of safety, making it less likely 

that contaminants, such as carbon monoxide, could reenter the home. 

Experts explained, however, that such an event would occur rarely and 

would require a combination of several factors. They noted that the 

exhaust of a properly functioning furnace would contain relatively low

levels of carbon monoxide (e.g. less than 4 parts per million (ppm)). They 

also noted an improperly functioning furnace, however, could produce 

hazardous carbon monoxide levels that, coupled with unique wind 

conditions, could result in carbon monoxide gas reentering a home 

through a fresh air intake. Because carbon monoxide is produced by 

incomplete combustion in fuel-burning devices such as gas furnaces, 

contamination is most likely in homes utilizing combustible fuels for 

heating and with an outdoor air intake connected to the air distribution 

duct system.

Our analysis of scenarios involving the dispersion of exhaust containing 

carbon monoxide demonstrated that the contaminant was less likely to 

reenter a building when the separation distance between an air intake 

and exhaust vent was increased. Using calculations recommended by 

ASHRAE, we quantified the difference in contaminant concentrations at 

the air intake as a function of wind speeds for various exhaust vent to air 

intake separations. As shown in figure 3, across a range of wind speeds, 

contaminant concentrations at air intakes placed either 10 feet 

horizontally or 3 feet horizontally and 3 feet vertically from an exhaust 

vent are less than contaminant concentrations that would be expected at 

an air intake separated just 3 feet horizontally (HUD Code requirement) 

from an exhaust vent. The figure also shows that the effect of separation 

distance on concentration levels is particularly evident at low wind 

speeds. For example, in a light 1 mph wind, exhaust with a carbon 

monoxide concentration of 200 parts per million (ppm) would be diluted to 

approximately 50 ppm at an air intake separated 3 feet horizontally from 

the exhaust vent. Increasing the separation between the exhaust vent 

and air intake to either 10 feet horizontally or 3 feet horizontally and 3 feet 

vertically in a 1 mph wind results in carbon monoxide concentrations of 

less than 10 ppm at the air intake.
 

Page 9 GAO-13-52 Manufactured Housing Standards

Figure 3: Air Intake Contaminant Concentration at Different Wind Speds for Various Exhaust Vent to Air Intake Separations

Note: The lines shown in the figure were generated using an exhaust-to-intake dilution calculation, 

based on recommendations from ASHRAE members, with parameters typically found in 

manufactured housing. Among the parameters used in our scenario, we assumed a capped exhaust 

vent with a diameter of 4 inches, which emitted exhaust at a speed of 1000 feet per minute, and 

which contained a CO concentration of 200 parts per million (ppm). The dilution levels determined by 

use of the calculation will vary based on changing these and other parameters. See Appendix I for a 

more detailed discussion of our use of the ASHRAE dilution equations.

These results were consistent with statements made by indoor air quality 

experts that increasing the distance between air intakes and exhaust 

vents reduces the concentration of contaminants such as carbon 

monoxide from reentering a home.

Concerns about the risk of carbon monoxide reentering the fresh air 

intake of a manufactured home were presented to the MHCC in July 

2009, including an explanation of the differences between current industry 

standards and the HUD Code. That is, the MHCC viewed a presentation 

prepared by an individual who claimed an incident of carbon monoxide 

reentering a home built to the existing HUD Code requiring a separation 

distance of 3 feet.

HUD officials told us that the agency, in its oversight capacity for the 

Manufactured Housing Program, had not received any reports, other than 

the incident described above, either directly or from state administrative 
 

Page 10 GAO-13-52 Manufactured Housing Standards

agencies, on cases of carbon monoxide reentering manufactured homes

through fresh air intakes. We also contacted several state administrative 

agencies in the five states we reviewed, all of which stated that they had 

not received reports from consumers of any incidents of carbon monoxide 

reentering a home through a fresh air intake. HUD has not conducted or 

sponsored research to determine an appropriate separation distance 

between the fresh air intakes and exhaust vents since the HUD Code was 

established in 1976, according to HUD officials. However, HUD has 

sponsored research related to whole-house ventilation that discusses 

airflow rate issues.

Industry standards also call for carbon monoxide detectors in all site-built 

and modular homes. These detectors are not intended to be used as a 

measure of, or to test for, adequate indoor air quality but are a safety 

device to warn occupants in the event of a dangerous build-up of carbon 

monoxide gasses in the air. IRC and ASHRAE have required carbon 

monoxide detectors for residential site-built and modular homes since 

2009 and 2010, respectively. While the MHCC has recommended this 

standard be required for all new manufactured homes, it is not yet a 

requirement in the HUD Code.
 

Page 11 GAO-13-52 Manufactured Housing Standards

As of April 2012, HUD officials stated they were reviewing 84 MHCC 

recommendations on changes to the HUD Code. However, time delays 

associated with considering updates to the HUD Code continue to 

exacerbate differences between standards for manufactured and site-built 

homes. These include MHCC recommendations and proposed changes 

related to ventilation and indoor air quality that HUD has begun 

considering to bring the HUD Code in line with industry standards. 

However, HUD’s process for considering and approving proposed 

changes, which includes development and consideration of proposed 

changes by the MHCC, can take many years (see fig. 4). Despite the 

outstanding proposed changes that are being considered, HUD has not 

adopted any changes to the HUD Code since 2005. Another set of 

proposed updates to the HUD Code were published in the Federal 

Register in 2010, and a final rule is currently under review at HUD, 

according to HUD officials. Further, other proposed updates being 

considered by HUD have not yet been published in the Federal Register

as proposed updates to the HUD Code. These include MHCC safety 

designated items related to air quality and ventilation.

Updates to the HUD 

Code on Key 

Standards Related to 

Ventilation and Air 

Quality Lag behind 

Industry Standards 

for Site-Built Homes
 

Page 12 GAO-13-52 Manufactured Housing Standards

Figure 4: GAO Analysis of HUD Processes for Updating the HUD Code

MHCC recommended in 2010 that HUD increase the separation distance 

between air intakes and exhaust vents. HUD officials said that HUD is 

now considering this recommendation to increase the separation distance 

toward industry standards, which were modified in 2003. The 

requirements for this separation distance in the HUD Code have 

remained unchanged since 1976, when HUD first established the HUD 

Code. We found that IRC and ASHRAE standards on ventilation were 

periodically updated about every 3 years.
 

Page 13 GAO-13-52 Manufactured Housing Standards

Specifically, in the 2003 version of its residential ventilation standards, 

ASHRAE stated several reasons for the need to update the standards, 

including:

• recognition by government agencies on ties between indoor air quality 

and health effects;

• increasingly more energy-efficient, air-tight homes being constructed;

• increased concerns over residential indoor air quality and ventilation; 

and

• the importance of adequate whole-house ventilation to improve air 

quality.

According to HUD officials, HUD is currently considering the MHCC 

recommendation to increase the distance between the air intakes and 

exhaust vents to a 3-foot vertical separation if the horizontal distance is 

less than 10 feet. MHCC developed the proposal after hearing the public 

testimony in July 2009 alleging that exhaust from a vent reentered a 

manufactured home through the air intake and caused carbon monoxide 

poisoning. MHCC made the recommendation to HUD in April 2010, but as 

of August 2012, HUD had not published a proposed rule in the Federal 

Register. A HUD official explained that HUD had received and is 

considering the MHCC recommendation to revise the current separation 

distance required between air intakes and exhaust vents. That is, HUD 

has not completed its internal clearance process – step 4 in figure 4.

HUD is also considering an MHCC recommendation to require installation 

of carbon monoxide detectors for all new manufactured homes. MHCC 

made this recommendation in December 2009. Similarly, HUD officials 

stated that HUD has not completed the economic analysis or drafted the 

proposed rule for this recommendation. As we have seen, other 

organizations that set industry standards already have this requirement—

the IRC since 2009 and ASHRAE since 2010.

The Manufactured Housing Improvement Act of 2000 established a 1-

year time limit for HUD to publish decisions on MHCC recommendations 

in the Federal Register.

9

 

9

Pub. L. No. 106-569 requires the Secretary of HUD to either accept, reject, or modify 

proposed changes to the HUD Code within 1 year and publishing the decisions for 

notification or further comment in the Federal Register. 

HUD’s website states that statutory language 
 

Page 14 GAO-13-52 Manufactured Housing Standards

requiring the MHCC to submit recommendations to HUD in the form of a 

proposed rule, including an economic analysis, is impractical.

HUD explains that the MHCC does not have the technical expertise to 

present HUD with a rule package that meets the requirements of the 

Federal Register and the Administrative Procedure Act. HUD collects the 

necessary supporting information and then prepares the proposed rule. 

According to HUD, proposals that the MHCC recommends are then 

subject to a review and clearance process—which can be lengthy—first 

within HUD and then are subject to review and approval by the Office of 

Management and Budget. We plan to assess these rulemaking issues, 

including options for accelerating the process, in future work.

We also observed several administrative challenges facing the 

Manufactured Housing Program. For example, NFPA temporarily 

suspended its administration of the MHCC in May 2012, citing that HUD 

had not yet paid invoices supporting the committee. Later, NFPA officials 

stated they had resumed the function for administering the MHCC after 

HUD paid the outstanding funds due. Further, MHCC members we spoke 

to told us that HUD’s Office of Manufactured Housing was understaffed 

and lacking in resources, possibly contributing to delays in updating the 

standards. HUD officials stated that the Manufactured Housing Program 

and its processes can be labor intensive. The program has 10 authorized 

staff positions. The office has been run by an Acting Deputy Administrator 

since July 2011. The Administrator position has been vacant since 2010.

Industry officials also told us that the federal rulemaking process for 

manufactured housing was slow because manufactured housing was not 

a priority for HUD. We found the priority given the program is unclear. 

HUD’s strategic plan does not include the manufactured housing program 

among those contributing to HUD’s priority goal of promoting healthy, 

energy-efficient, and affordable buildings.
 

Page 15 GAO-13-52 Manufactured Housing Standards

HUD does not maintain data on the number of manufactured houses built 

or the distances between vents and air intakes of homes designed with 

an outdoor air intake connected to the air distribution system. American 

Housing Survey estimates showed that in 2009 many of the 

approximately 6.8 million occupied manufactured homes had been built 

under the HUD Code. Specifically, about 5.5 million of these units had 

been built after 1975.10 The 3-foot separation requirement between air 

intakes and exhaust vents has been in effect since 1976 and thus applied 

to these homes. According to the American Housing Survey, about half 

(52 percent) of occupied manufactured homes used electricity as their 

main heating source, and most of the remaining 48 percent used a 

combustible fuel source.11

According to HUD officials, although it does not have data on the actual 

distances between vents and air intakes of constructed homes, HUD 

retains copies of all DAPIA approved design packages, which include 

requirements for this separation. As validated by DAPIAs, designs of 

manufactured homes are developed with the intent of meeting the HUD 

Code. Two large manufacturers we spoke to, which accounted for about 

60 percent of the market, stated that they did not track the distances 

between vents and air intakes of each home, but that their homes met 

HUD standards—that is, the distances were at least 3 feet for homes with 

air intakes.

As we have mentioned, experts said carbon 

monoxide exposure—although rare—is most likely in homes that use a 

combustible fuel source, or nearly half of the manufactured homes 

identified in the survey.

Furthermore, we found that the HUD Code does not require testing of the 

performance of the ventilation system, including the resulting wholehouse airflow rate of homes, either at the plants or once the homes are 

put in place. HUD officials told us that the DAPIAs review designs to help 

ensure the performance of ventilation systems meets the HUD Code. In 

particular, DAPIAs ensure that designs include the required exhaust fans 

to move a certain volume of air that should achieve the specified whole-

 

10The relative margins of error for these estimates are between 4.41 and 13.70 percent.

11Estimates are from the Census Bureau’s 2009 American Housing Survey. The margins 

of error are between 1.40 and 4.70 percent.

Millions of 

Manufactured Homes 

Have Been Built 

under the HUD Code 

but HUD Has Not 

Tested the 

Performance of 

Installed Ventilation 

Systems 

Data on the As-Installed 

Performance of Ventilation 

Systems in Manufactured 

Homes are Limited
 

Page 16 GAO-13-52 Manufactured Housing Standards

house airflow rate.12 HUD officials stated that IPIAs are not required to 

test the whole-house airflow rate either in the plant or in the field because 

the HUD Code does not require it. In contrast, the HUD Code does 

require the inspection and testing of other specified features, such as gas 

lines, smoke alarms installed at the factory, and plumbing systems.13

Air quality experts we spoke with emphasized that a variety of factors 

could impact the whole-house airflow rate. For example, one expert said 

duct leakage and friction could result in airflow losses. Therefore, an 

approved design may assume a certain airflow rate for an exhaust fan, 

but the quality of construction could reduce the whole-house airflow rate 

upon installation. Additionally, several studies by the Department of 

Commerce’s National Institute of Standards and Technology (NIST) 

indicated that the whole-house airflow rate of manufactured homes 

depended on how often the fans were operated, as well as weather and 

climate factors. In 2008 and 2010 NIST reported that some airflow rates 

in a test home were below levels specified in the HUD Code. Because 

HUD does not require testing to validate the performance of ventilation 

systems, specifically the resulting whole-house airflow rates, it does not 

know how the systems are performing or whether they meet HUD 

specifications.

Cooperative research in 2003 involving HUD and the Manufactured 

Housing Research Alliance concluded that it was unclear how well wholehouse ventilation systems in manufactured homes performed with respect 

to the HUD Code’s whole-house airflow rate performance specification.14

 

1224 C.F.R. § 3280.103 (b). The HUD Code also requires mechanical ventilation systems 

to have a manual control and for the instructions to encourage consumers to operate the 

system whenever the home is occupied. 24 C.F.R. § 103(b)(4), (6).

According to the study and HUD officials, identifying a typical airflow rate 

for manufactured homes is challenging because factors such as 

temperature, wind, location of the home, and construction type have 

varying impacts on the natural infiltration in similar homes. The study 

recommended that HUD: 

1324 C.F.R. § 3280.706(l)(8), 3280.208(f), 3280.612.

14The Manufactured Housing Research Alliance is now named the Systems Building 

Research Alliance. The mission of this nonprofit organization is to develop new 

technologies to enhance the value, quality, and performance of both manufactured and 

modular homes.
 

Page 17 GAO-13-52 Manufactured Housing Standards

• conduct further research to develop a testing method to determine 

whether specifications for ventilation design and performance were 

being met, and

• publish guidelines for achieving the required airflow rate in 

manufactured homes in a best practices manual to be created for the 

industry.

As of August 2012, MHCC was considering a proposal to adopt an 

industry standard for residential ventilation that includes testing to ensure 

that ventilation systems meet performance specifications. Requirements 

for testing the ventilation system after installation were incorporated in 

ASHRAE’s ventilation standards for residential homes in 2003. The 

current proposal with the MHCC includes testing the delivered airflow of 

the system, using an airflow measuring device to confirm the airflow of 

the system.

HUD officials acknowledged that the recommendations from the 2003 

study had not been implemented and cited funding issues as reasons for 

not implementing them. According to HUD officials, HUD has not received 

negative reports on indoor air quality in manufactured homes that 

occurred as a direct result of the whole-house ventilation systems. 

However, without testing of the actual whole-house airflow rates of 

recently manufactured homes, HUD cannot know whether these homes, 

as built, meet HUD’s requirement of 0.035 cubic feet per minute for each 

square foot of interior floor space. Still, the ventilation standards establish 

standards for airflow, not air quality, although the required airflows are 

intended to enhance air quality in the home.

Measuring the actual airflow achieved by installed ventilation systems 

would not only permit HUD to know whether its standards are being met, 

but also permit HUD to better understand the potential impact ventilation 

systems may have on indoor air quality. In the next section we discuss 

limitations in HUD’s standard.

We found that the performance specifications for the whole-house airflow 

rate in the HUD Code were based on industry standards and assumptions 

from nearly 20 years ago. As we stated earlier, HUD has not tested these 

performance specifications as manufactured homes are built and 

installed. In 1993, HUD first set a standard for the whole-house airflow 

The HUD Code Airflow 

Rate Is Based on 

Standards and Research 

from 1993
 

Page 18 GAO-13-52 Manufactured Housing Standards

rate, basing it on the ASHRAE standards from 1989 and ventilation 

research by NIST and the U.S. Department of Agriculture’s Forest 

Products Laboratory.15

Since 1993, HUD has continued to specify a mechanical whole-house 

ventilation airflow rate of least 0.035 cubic feet per minute for each 

square foot of interior floor space or its hourly average equivalent.

The original whole-house airflow rate requirement 

aimed at replacing about one-third of the air in a manufactured home with 

fresh air each hour (that is, 0.35 total air changes per hour). As part of 

this requirement, HUD also assumed natural infiltration would account for 

0.25 air changes per hour and required mechanical ventilation to provide 

the remaining 0.1 air changes per hour. Therefore, the HUD Code in 1993 

specified that each manufactured home shall be provided with 

mechanical whole-house ventilation having a minimum capacity of 0.035 

cubic feet per minute for each square foot of interior space in order to 

achieve a minimum rate of 0.1 air changes per hour. In 2005, HUD 

removed the natural infiltration assumption but kept the same rate for 

mechanical ventilation.

16 Air 

quality experts emphasized that homes have continued to be built with 

less leakage and greater energy efficiency. Thus, a home built in 1993

and a home built in 2012 may both meet the HUD whole-house airflow 

rate standard (that relates only to ventilation achieved through 

mechanical means) but may not achieve the same level of air quality. 

Nonetheless, HUD has not reconsidered the appropriateness of its 

standard in achieving an acceptable level of air quality, nor specified an 

acceptable level of air quality. Without further research and testing of its 

whole-house airflow rate standard, HUD may not know the effect of new 

design and construction practices on the overall ventilation performance 

and air quality of manufactured homes.

Current standards used for site-built and modular homes offer a greater 

margin of safety against carbon monoxide exposure than the HUD Code 

standards used for manufactured homes. While carbon monoxide 

 

15As of 2010, ASHRAE requires specific fan flow rates depending on the floor area, the 

number of bedrooms, and the number of occupants rather than the fixed airflow rate of 

0.35 air changes per hour.

1624 C.F.R. § 3280.103(b). In 2005 the HUD Code also first required exhaust fans to 

produce between 50 and 90 cubic feet per minute to achieve this airflow rate.

Conclusions 
 

Page 19 GAO-13-52 Manufactured Housing Standards

exposure resulting from exhaust reentering the home through an air 

intake is unlikely, air quality experts we spoke to maintain that industry

standards already required for site-built homes offered a greater margin 

of safety to prevent carbon monoxide from reentering a home than the 

HUD Code does for manufactured homes. Our analysis also confirmed 

that increasing the separation distance between air intakes and exhaust 

vents to industry standards decreased the likelihood of carbon monoxide 

reentering the home. Further, industry standards for site-built homes call 

for the use of a carbon monoxide detector whereas the HUD Code does 

not. In response, the MHCC has forwarded safety-designated 

recommendations to HUD to update the HUD Code to address concerns 

over the separation distance between air intakes and exhaust vents and 

the lack of a requirement for carbon monoxide detectors in manufactured 

homes. 

The differences between the HUD Code and industry standards related to 

home ventilation and indoor air quality are due to the regulatory 

procedures and time it takes HUD to consider and implement proposed 

updates. Although we observed proposals aimed at maintaining similar 

standards for manufactured and site-built homes, proposed updates for 

manufactured homes lagged behind those made by industry for site-built 

and modular homes. HUD’s process for adopting changes to the HUD 

Code involving considerable time for proposals to be considered by the 

MHCC and HUD has resulted in a lack of action even on safety-related 

proposals that the MHCC has put forth, including increasing the 

separation requirement for air intakes and exhaust vents and requiring 

carbon monoxide detectors. Although MHCC has submitted several 

recommendations to HUD, HUD has not implemented any changes to the 

HUD Code since 2005, so actual implementation of recent updates may 

be years away. We will explore rulemaking and other issues with the 

Manufactured Housing Program in further work.

HUD does not maintain data on the actual separation distances between 

the fresh air intakes and exhaust vents, but does retain copies of designs 

with DAPIA approval, indicating they comply with the HUD Code. We

noted that 5.5 million occupied manufactured homes were built since the 

HUD Code took effect with a 3-foot separation requirement, and almost 

half of manufactured homes built use combustible furnace fuels. 

However, we found limited data available related to the installed 

performance of the ventilation systems of manufactured homes 

constructed and placed. HUD does not require manufacturers to test the 

performance of the ventilation systems installed in manufactured homes 

to determine whether the systems actually meet their performance 
 

Page 20 GAO-13-52 Manufactured Housing Standards

specifications. Further, while the HUD Code specifies certain inspections 

and tests to validate many other features of manufactured homes, HUD 

does not require manufacturers to conduct performance testing of the 

ventilation system, specifically the required whole-house ventilation 

airflow rate. Other research efforts have previously voiced similar 

concerns over uncertainties as to whether the performance specifications 

of the ventilation system were being met in manufactured homes. 

Ultimately, without testing the whole-house airflow rate for constructed 

manufactured homes, either in the factory or the field, HUD and others 

cannot be assured as to whether the airflow is ventilating the home as 

specified.

We also found that the current HUD code performance specification for 

the whole-house ventilation airflow rate is based on standards and 

research from nearly 20 years ago. Since then, air quality experts and 

research suggest that the industry has improved the construction of 

manufactured homes. To the extent that manufactured homes continue to 

be built tighter and more energy efficient, mechanical ventilation of homes 

becomes more important for ensuring indoor air quality. HUD’s wholehouse airflow rate standard for mechanical ventilation of 0.035 cubic feet 

per minute per square foot of living space has remained unchanged. 

Without further assessment of the impact that potential changes in natural 

air infiltration have on whole-house ventilation, HUD cannot be certain of 

the air quality in manufactured homes.

To better ensure that air ventilation systems in manufactured homes 

perform as specified and meet the HUD Code, we recommend that HUD 

develop an appropriate method to test and validate the performance of 

the ventilation system as part of the HUD certification process.

To ensure that its specification for airflow continues to be appropriate, we 

recommend that HUD reassess the assumptions for the whole-house 

ventilation specification, working with the MHCC, to determine the 

appropriate rates, taking into consideration current natural air infiltration, 

to achieve the whole-house ventilation performance, considering the 

expected impact such ventilation would have on indoor air quality.

We provided a draft of this report to HUD for review and comment. HUD’s 

Acting Assistant Secretary for Housing-Federal Housing Commissioner 

provided written comments that are discussed below and presented in 

Recommendations for 

Executive Action

Agency Comments 

and Our Evaluation
 

Page 21 GAO-13-52 Manufactured Housing Standards

Appendix II. HUD also provided technical comments that were 

incorporated as appropriate.

HUD agreed with both recommendations, noting that it would bring them 

before the MHCC for consideration. HUD also said, however, that it would 

require additional funding and resources. Specific to our recommendation 

that HUD develop an appropriate method to test and validate the 

performance of the ventilation system as part of the HUD certification 

process, HUD agreed that such testing and validation could improve the 

accuracy of system performance. But HUD also questioned the need for 

such testing, given that the current systems were designed and approved 

by private design professionals. HUD further noted that no documentation 

existed showing that ventilation systems in manufactured homes did not 

meet current standards. But as our report points out, HUD lacks such 

documentation because it does not systematically test the systems. HUD 

also stated that it would be impractical to conduct testing at the factory for 

multi-section units. However, testing of the whole-house airflow rate could 

occur in the field as well as at factories, when practicable.

We also recommended reassessing the assumptions for the existing 

whole-house ventilation airflow rate specification. HUD agreed that a 

reassessment of the assumptions used to determine the appropriate rate 

could have a positive impact on indoor air quality. HUD also said that it 

would need to balance changes in these requirements against costs 

incurred by manufacturers and consumers. As we noted in the report, the 

existing whole-house ventilation airflow rate requirement (0.035 cubic feet 

per minute per square foot of living space) is based on assumptions for 

natural air infiltration dating back to 1993. HUD noted in its technical 

comments that no evidence existed to support the argument that the 

ventilation standards for manufactured homes were less effective than 

industry standards. Further, HUD said that its standards provide rates of 

mechanical ventilation that are comparable to those provided by industry 

standards. However, without further assessment of the impact that 

potential changes in natural air infiltration have on whole-house 

ventilation, HUD cannot be certain of the air quality in manufactured 

homes. Reassessing assumptions made nearly 20 years ago would help 

determine whether HUD’s required whole-house ventilation airflow rate 

continues to ensure that manufactured homes are properly ventilated.
 

Page 22 GAO-13-52 Manufactured Housing Standards

We are sending copies of this report to appropriate congressional 

committees and the Secretary of Housing and Urban Development. The 

report also is available at no charge on the GAO website at 

http://www.gao.gov. 

If you or your staff have any questions about this report, please contact 

me at (202) 512-8678 or sciremj@gao.gov. Contact points for our Offices 

of Congressional Relations and Public Affairs may be found on the last 

page of this report. GAO staff who made major contributions to this report 

are listed in appendix III.

Mathew J. Scirè

Director,

Financial Markets and Community Investment
Appendix I: Comparison of Different 

Separation Distances Between Air Intakes and 

Exhaust Vents and Impacts on Potential 

Carbon Monoxide Exposure

Page 23 GAO-13-52 Manufactured Housing Standards

Reentry of exhaust occurs when exhaust leaving its vent finds its way to 

an air intake and gets pulled back into the home. The common mitigation 

practice is to provide adequate horizontal or vertical separation between 

the exhaust vent and the air intake such that any reentered air is diluted 

to a greater extent by the time it reaches habitable areas of the home.

In their Handbook of Fundamentals, the American Society of Heating, 

Refrigerating, and Air-Conditioning Engineers (ASHRAE) provides 

methods for modeling the diluting effect of an exhaust stream mixing with 

open air. ASHRAE officials we spoke with told us that these methods can 

be used to assess solutions to various ventilation problems including 

exhaust air reentry scenarios. Therefore, we used these techniques on a 

simple model of a manufactured home to analyze how much carbon 

monoxide in the exhaust stream of the furnace in the home could be 

diluted by the time it reached an air intake of the home. We analyzed 

different scenarios involving various separation distances between the 

exhaust vent and the air intake to see what effect separation distance 

produced on the amount of dilution provided by open air mixing.

Dilution of contaminants occurs through mixing of the exhaust stream with 

fresh air as the stream leaves its vent, enters open air, is affected by the 

wind, and spreads away from the vent, dispersing contaminants in the 

stream as it progresses. In general, dilution increases with increasing 

wind speed, as might be expected, because a stronger airflow will aid in 

dispersing contaminant particles, and is inversely proportional to both the 

speed with which the exhaust stream is coming out of the vent, as well as 

the size of the vent opening. This makes sense because increasing 

exhaust speed and increasing vent size, naturally, competes against 

dilution by putting more contaminant into the air.

However, the mixing is not straightforward, as many factors can influence 

how much dilution takes place. For example, the airflow closer to the 

ground can be disrupted by obstacles such as trees and buildings, 

inducing turbulence in the flow. At roof level, vents and even the pitch of 

the roof can further complicate the flow, creating a mix of eddies and 

zones where the air might get recirculated and trapped near the roof, thus 

restricting the dilution effect. This is sometimes the case at lower exhaust 

speeds and lower wind speeds, where the exhaust plume will not have 

the momentum to rise very high or get dispersed very quickly by the wind 

and may get pushed down and remain near roof level and the air intake.

In implementing their model, ASHRAE assumes the air intake is 

positioned directly downwind of the exhaust vent and then allows for a 

Appendix I: Comparison of Different Separation 

Distances Between Air Intakes and Exhaust Vents 

and Impacts on Potential Carbon Monoxide 

Exposure
Appendix I: Comparison of Different 

Separation Distances Between Air Intakes and 

Exhaust Vents and Impacts on Potential 

Carbon Monoxide Exposure

Page 24 GAO-13-52 Manufactured Housing Standards

variety of parameters to specify the geometries involved; for example the 

height of the roof, the horizontal and vertical separation distances 

between the exhaust vent and the air intake, and the size of the exhaust 

vent opening. In addition, ASHRAE allows for specifying characteristics of 

the exhaust stream, such as the speed with which the exhaust is exiting 

the vent, and the initial concentration of a contaminant within the stream 

at the vent opening. Finally, ASHRAE models the flow of the air through a 

number of factors that include the wind speed, the downwind, cross-wind, 

and vertical spread of the plume as it progresses downstream, and the 

turbulence intensity of the air, which is controlled by a factor that allows 

for specifying the roughness of the terrain over which the air will be 

flowing.

In our analysis, we used three different scenarios for separation distances 

between the air intake and exhaust vent—3 foot horizontal, 10 foot 

horizontal, and 3 foot horizontal with a 3 foot vertical separation. Table 1 

also shows the specific parameters we used in our analysis. These 

included a manufactured home with a roof height of 10 feet and an 

exhaust vent diameter of 4 inches. We used an initial carbon monoxide 

concentration in the exhaust stream of 200 ppm, which experts have told 

us could occur in the exhaust stream of a malfunctioning furnace.
Appendix I: Comparison of Different 

Separation Distances Between Air Intakes and 

Exhaust Vents and Impacts on Potential 

Carbon Monoxide Exposure

Page 25 GAO-13-52 Manufactured Housing Standards

Table 1: Parameters Used in Our Analysis

Parameter Value Notes

Height of the roof 10 ft Height of the roof off the ground at the gutter.

Horizontal distance between the 

exhaust vent and the air intake

Variable We assessed cases with horizontal separation distances of 3 feet 

and 10 ft.

Vertical separation distance between 

the exhaust vent and the air intake

Variable We assessed cases of no separation (0) and a separation of 3 feet.

Diameter of the exhaust vent opening 4 inches Diameter of the exhaust vent opening. We considered the exhaust 

vent to be capped.

Exhaust stream speed 1000 feet per minute 

(fpm) 

ASHRAE representatives mentioned 100-2000 fpm are typical 

values for residential furnaces.

Initial carbon monoxide concentration 200 ppm Concentration at the exhaust vent opening of an assumed carbon 

monoxide component in the exhaust stream.

Wind speed Variable We assessed scenarios varying the wind speed from 1 to 26 mph 

(about 88 to 2288 fpm).

Roughness factor 2.132 Characterizes airflow over the terrain. This is the value ASHRAE 

specifies for a suburban setting. For comparison purposes, a flat 

desert would have a value of 0.03 while an urban setting would 

have a value of 6.0.

Source: GAO analysis based on the ASHRAE Handbook of Fundamentals. 

Figure 5 represents the results of our analysis and illustrates contaminant 

concentration at the air intake as a function of wind speeds for various 

exhaust vent to air intake separations. Here it can be seen that increasing 

the horizontal separation to 10 feet or adding a 3 foot vertical separation 

to the 3 foot horizontal separation reduces the concentration at the air 

intake more than the 3 foot horizontal separation alone does. For 

example, in a light 1 mph wind, exhaust with a carbon monoxide 

concentration of 200 ppm would be diluted to approximately 50 ppm at an 

air intake separated 3 feet horizontally from the exhaust vent. Increasing 

the separation between the exhaust vent and air intake to either 10 feet 

horizontally or 3 feet horizontally and 3 feet vertically in a 1 mph wind 

results in carbon monoxide concentrations of less than 10 ppm at the air 

intake.
Appendix I: Comparison of Different 

Separation Distances Between Air Intakes and 

Exhaust Vents and Impacts on Potential 

Carbon Monoxide Exposure

Page 26 GAO-13-52 Manufactured Housing Standards

Figure 5: Contaminant Concentration at Air Intake for Different Wind Speeds and Various Exhaust Vent to Air Intake 

Separations
Appendix II: Comments from the Department 

of Housing and Urban Development

Page 27 GAO-13-52 Manufactured Housing Standards

Appendix II: Comments from the Department 

of Housing and Urban Development
Appendix II: Comments from the Department 

of Housing and Urban Development

Page 28 GAO-13-52 Manufactured Housing Standards
Appendix III: GAO Contact and Staff 

Acknowledgements

Page 29 GAO-13-52 Manufactured Housing Standards

Mathew J. Scirè, (202) 512-8678 or sciremj@gao.gov

In addition to the contact named above, Andy Finkel (Assistant Director), 

Mike Armes, James Ashley, Tim Bober, Bill Carrigg, Emily Chalmers, 

Pamela Davidson, Juliann Gorse, Barry Kirby, John McGrail, Marc 

Molino, and Nadine Garrick Raidbard, made key contributions to this 

report.

Appendix III: GAO Contact and Staff 

Acknowledgements 

GAO Contact

Staff 

Acknowledgments

(250641)
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