Impact of Residential Mechanical 

Ventilation on Energy Cost and 

Humidity Control

Eric Martin 

for 

Building Science Corporation, 

Florida Solar Energy Center/BA-PIRC, IBACOS

January 2014
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, subcontractors, or 

affiliated partners makes any warranty, express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or 

represents that its use would not infringe privately owned rights. Reference herein to any specific commercial 

product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency 

thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the 

United States government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
iii

Impact of Residential Mechanical Ventilation on Energy Cost and 

Humidity Control

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

Eric Martin for

Building Science Corporation, 

Florida Solar Energy Center/BA-PIRC, IBACOS

NREL Technical Monitors: Cheryn Metzger, Stacey Rothgeb, and Michael Gestwick

January 2014 
iv

[This page left blank]
v 

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

1 White Paper Introduction and Residential Ventilation Overview .................................................... 1

1.1 Ventilation Goals .................................................................................................................1

1.2 The Role of Pollutant Source Control ..................................................................................2

1.3 Ventilation Considerations in Humid Climates ...................................................................3

1.4 Review of Ventilation in Residential Codes and Standards ................................................3

1.4.1 Local Mechanical Exhaust Flow Requirements ......................................................3

1.4.2 Whole-Building Mechanical Ventilation Rate Requirements .................................4

1.4.3 Whole-Building Mechanical Ventilation Air Distribution Requirements ...............7

1.4.4 Whole-Building Mechanical Ventilation Air Source Requirements .......................8

1.5 Review of Building America Teams’ Experience With Ventilation Approaches ...............9

1.6 Review of Other Previous Modeling Efforts .....................................................................10

2 Technical Approach for Assessment of Energy and Indoor Humidity Impacts of Residential 

Ventilation ........................................................................................................................................... 12

2.1 Residential Whole-Building Ventilation Systems Evaluated ............................................13

2.1.1 Exhaust Only ..........................................................................................................13

2.1.2 Supply Only ...........................................................................................................14

2.1.3 Balanced Heat Recovery Ventilation and Energy Recovery Ventilation ..............14

2.2 Modeling Assumptions ......................................................................................................14

2.2.1 Constants ................................................................................................................14

2.2.2 Variables ................................................................................................................17

3 Modeling Results................................................................................................................................ 19

3.1 Energy Performance...........................................................................................................19

3.1.1 Energy Performance as a Function of Ventilation Rate .........................................19

3.1.2 Energy Performance as a Function of Ventilation System Type ...........................20

3.1.3 Air Change Rates as a Function of Building Air Leakage .....................................21

3.2 Humidity Control Performance..........................................................................................21

3.2.1 Humidity Control Performance as a Function of Space Conditioning System 

Activity ..................................................................................................................22

3.2.2 Humidity Control Performance as a Function of Infiltration Rate ........................23

3.2.3 Humidity Control Performance as a Function of Ventilation System Type ..........24

3.2.4 Humidity Control Performance as a Function of Building Air Leakage ...............26

3.2.5 Humidity Control Performance as a Function of Internal Moisture Generation ...26

3.2.6 Humidity Control Performance as a Function of Relative Humidity Set Point .....27

4 Conclusions ........................................................................................................................................ 28

4.1 Energy Consumption Related Conclusions .......................................................................28

4.2 Humidity Control Related Conclusions .............................................................................29

5 Key Gaps and Areas of Ongoing Research ..................................................................................... 31

References................................................................................................................................................. 33
vi 

List of Figures

Figure 1. Total annual operating cost for a DOE Challenge Home controlled to < 60% RH over a 

range of ventilation rates..................................................................................................................... x

Figure 2. Total annual operating cost per year for a DOE Challenge Home over a range of 

ventilation system types..................................................................................................................... xi

Figure 3. Hours of elevated indoor RH are a strong function of the selected RH limit and climate 

using EGUSA ....................................................................................................................................... xi

Figure 4. NREL BEopt E+ program used to generate the building geometry and the base IDF files 

(input description) .............................................................................................................................. 12

Figure 5. Schematic overview of typical local and whole-house ventilation equipment (a home 

typically has only one whole-house system) .................................................................................. 13

Figure 6. Hourly internal gains schedule................................................................................................ 16

Figure 7. Average residential electricity and gas utility rates by state for 2011 ................................ 17

Figure 8. Total annual operating cost for a DOE Challenge Home controlled to < 60% RH over a 

range of ventilation rates................................................................................................................... 19

Figure 9. Total annual space conditioning cost difference for a DOE Challenge Home controlled to 

< 60% RH over a range of ventilation rates using EGUSA............................................................. 20

Figure 10. Total annual operating cost per year for a DOE Challenge Home over a range of 

ventilation system types.................................................................................................................... 20

Figure 11. EGUSA differences in air exchange rate between balanced and unbalanced ventilation21

Figure 12. Indoor RH versus hour of year comparison between E+ (blue symbols) and EGUSA 

(V3.0.01P) simulation results............................................................................................................. 22

Figure 13. For a week at the end of June and beginning of July on Orlando, the cooling system 

and space RH are showing an expected response using EGUSA ................................................ 22

Figure 14. Hours of elevated indoor RH is a strong function space conditioning system activity 

and climate.......................................................................................................................................... 23

Figure 15. ASHRAE 62.2-2013 ventilation rate raises median RH compared to a conventional 

dwelling without mechanical ventilation using EGUSA ................................................................. 24

Figure 16. Hours above 60% RH for exhaust and ERV ventilation in Orlando ................................... 24

Figure 17. Annual hours above 60% RH by ventilation system type................................................... 25

Figure 18. For unbalanced ventilation, the supplemental dehumidification requirement increases 

as the building air leakage decreases using EGUSA ..................................................................... 26

Figure 19. Effect of increasing internal moisture generation from 12 to 15 lb/day using EGUSA ... 27

Figure 20. Hours of elevated indoor RH are a strong function of the selected RH limit and climate 

using EGUSA ...................................................................................................................................... 27

Figure 21. Total annual operating cost for a DOE Challenge Home controlled to < 60% RH over a 

range of ventilation rates................................................................................................................... 28

Figure 22. Total annual operating cost per year for a DOE Challenge Home over a range of 

ventilation system types.................................................................................................................... 29

Figure 23. ASHRAE 62.2-2013 ventilation rate significantly increases number of hours between 

60% and 65% RH compared to a conventional dwelling without mechanical ventilation using 

EGUSA................................................................................................................................................. 30
vii 

List of Tables

Table 1. Continuous Whole-House Mechanical Ventilation System Airflow Rate Requirements of 

the 2012 IRC*......................................................................................................................................... 4

Table 2. Ventilation Rates for All Group R Occupancies Four and Fewer Stories (CFM)*.................. 5

Table 3. Normal Operating Exhaust Capacity of Principal Ventilation Fan*......................................... 6

Table 4. Comparison of Fan Flows Required by ASHRAE 62.2-2010 and 2013.................................. 18
viii

Definitions

AHU Air handling unit

BSC Building Science Corporation

CFI Central fan integrated supply ventilation

DOE U.S. Department of Energy

E+ EnergyPlus

EMPD Effective moisture penetration depth 

EGUSA EnergyGauge USA

EPA U.S. Environmental Protection Agency

ERV Energy recovery ventilator

HAMT Heat and moisture transport

HUD U.S. Department of Housing and Urban Development

HRV Heat recovery ventilator

IECC International Energy Conservation Code

IMC International Mechanical Code

IRC International Residential Code

MC Moisture capacitance

NBC National Building Code of Canada

NREL National Renewable Energy Laboratory

RH relative humidity

WAVIAQ Washington State Ventilation and Indoor Air Quality Code
ix

Executive Summary

The U.S. Department of Energy (DOE) Building America program has been conducting research 

leading to cost-effective high performance homes since the early 1990s. Optimizing whole-house 

mechanical ventilation as part of the program’s systems engineered approach to constructing 

housing has been an important subject of the program’s research. Ventilation in residential 

buildings is one component of an effective, comprehensive strategy for creation and maintenance 

of a comfortable and healthy indoor air environment. Indoor air pollution control begins with 

avoiding the placement of items of known high pollutant emissions inside the living 

environment. It follows with local exhaust in areas such as kitchens, and bathrooms, where high 

emissions from pollution sources cannot be avoided, and it ends with whole-building controlled 

mechanical ventilation to dilute remaining indoor air pollutants with fresher outdoor air.

Several codes and standards require residential ventilation. Definitions include airflow rate 

requirements for local exhaust, whole-building mechanical ventilation rate requirements, sources 

of ventilation air, and distribution requirements. While a consistency of intent can be inferred, 

differences among codes and standards, along with the absence of residential ventilation 

requirements in many local codes, indicate the difficulty and uncertainty involved with a generic 

specification. If the primary indoor environment concern is the occupant’s annual average 

exposure to pollutants, certain questions must be answered relating to the ability of ventilation to 

minimize that exposure:

• What are the health impacts of pollutants of concern? Are odor and moisture of concern, 

which have shorter term, rather than annual, exposure issues? 

• What are the limits of whole-house residential ventilation systems to resolve the 

concerns? To what extent can such systems improve indoor air quality, by minimizing 

concentrations of chemical and particulate pollutants, beyond what can be accomplished 

through intermittent spot ventilation and source control? 

In any climate, year-round indoor humidity control reduces condensation potential and relative 

humidity (RH), exceeding comfort conditions. As homes become more energy efficient, space 

conditioning systems with lower capacities can be used to maintain space temperature. Overall, 

this is good, and produces significant net energy and cost savings. However, this often requires a 

change to conventional residential space conditioning system design in humid climates. While 

the sensible cooling load is lower, and can be dealt with in the conventional way, the latent 

(moisture) load in high performance homes remains nearly unchanged due to ventilation 

requirements and internal moisture generation by occupants and their activities. Therefore, at 

times when there is no need to lower the space air temperature, supplemental dehumidification 

may be required to maintain acceptable RH. 

The study described in this report is based on building energy modeling with an important focus 

on the energy and indoor humidity impacts of ventilation. The modeling tools used were 

EnergyPlus version 7.1 (E+) and EnergyGauge USA (EGUSA). Twelve U.S. cities and five 

climate zones were represented. A total of 864 simulations (2*2*2*3*3*12 = 864) were run 

using two building archetypes, two building leakage rates, two building orientations, three 

ventilation systems, three ventilation rates, and 12 climates. Modeling of rates lower than those 
x 

promulgated by ASHRAE standards is not to suggest that such rates provide adequate indoor air 

quality—that is outside the scope of this report. The rates are chosen purely to evaluate the 

energy and moisture impacts of varying rates. 

Energy-related conclusions of the simulation work include determining the difference in 

total space conditioning operating cost for a DOE Challenge Home controlled to < 60% RH

(Figure 1). For homes with 75% of the ASHRAE 62.2-2013 ventilation rate requirement 

compared to 100% of that ventilation rate requirement, the decreased cost was about $45/yr 

averaged over all climates, enclosure tightness, and ventilation systems simulated, or a savings of 

10% of total space conditioning energy. The study found that the 75% ASHRAE 62.2-2013 

ventilation rate requirement is roughly equal to 94% of the ASHRAE 62.2-2010 requirement. At 

50% of the ASHRAE 62.2-2013 ventilation rate requirement, the average additional savings is 

about another $35/yr for an average total savings of 15% of space conditioning energy compared 

with 100% of that ventilation rate requirement. 

Figure 1. Total annual operating cost for a DOE Challenge Home 

controlled to < 60% RH over a range of ventilation rates

Total annual operating cost for a DOE Challenge Home varied less than $90/yr among all 

ventilation systems in every climate (Figure 2). Compared to exhaust ventilation as the base, a 

balanced energy recovery ventilator (installed such that it does not require coincident air 

handling unit fan operation to avoid short-circuiting of ventilation air) ranged from $0/yr to 

$40/yr more, with an average of about $20/yr more for 100% of the ASHRAE 62.2-2010 

requirement at 3 ACH50. However, at the larger mechanical ventilation rates required in the 1.5 

ACH50 homes, the ERV saved an average of $20/yr compared to exhaust ventilation, in all but 

the dry climate of Phoenix and mild climate of Los Angeles, where it continued to consume 

more energy than exhaust. Central fan integrated supply ventilation ranged from about $11/yr to 

$93/yr more than exhaust ventilation, with an average of about $40/yr. 

Supplemental dehumidification controlled to 60% RH was predicted to be $10–$58/yr for the 

warm-humid climates of Charleston, Houston, and Orlando, and the marine climate of Los 

Angeles. However, a caveat is provided—this value is predicated on an operating dehumidifier 

energy factor of 1.47 L/kWh and recent field data indicate that conventional dehumidifiers 

operate closer to 0.8 L/kWh (Mattison and Korn 2012), which would tend to double this cost. 

Additionally, dehumidifiers tend to operate on a large humidity dead band, which means that 
xi

maintaining humidity below 60% would likely require humidity set points near 55%, which 

could increase dehumidification costs by as much as 40% in humid climates. Also, with 

unbalanced mechanical ventilation, air change goes up as building leakage goes down from 3.0 

ACH50 to 1.5 ACH50, resulting in more needed supplemental dehumidification. The increase in 

air exchange rate from unbalanced to balanced ventilation was about 24% for the 3.0 ACH50 

houses and about 10% for the 1.5 ACH50 houses. 

Figure 2. Total annual operating cost per year for a DOE Challenge Home 

over a range of ventilation system types

Humidity control related conclusions include validating that the hours of elevated indoor RH are

a strong function of the selected RH limit and climate (Figure 3). For example, in Los Angeles, 

nearly all of the hours above 60% RH are also below 65% RH. However, this is not the case for 

Charleston, Houston, and Orlando. Hours above 60% RH during a particular space conditioning 

mode (heating, cooling, floating) were also a strong function of climate. Most of the hours 

significantly above 60% RH occur during floating hours, which occur mostly during fall, winter, 

and spring in Orlando. 

Figure 3. Hours of elevated indoor RH are a strong function of the selected 

RH limit and climate using EGUSA
xii

A key gap and area of ongoing research is cost-effective application of ASHRAE Standard 62.2-

2013 with systems and approaches that reduce energy consumption, improve humidity control, 

and improve indoor air quality. Identifying methods to achieve such objectives would benefit the 

U.S. Environmental Protection Agency Indoor airPLUS program required by the DOE Challenge 

Home. Research should be conducted into methods that include considerations for: 

• Managing indoor air pollutants in ways other than outdoor air exchange.

• Quantifying exposure to pollutants, and mitigation of that exposure, over a time scale 

shorter than the annual average.

• Accounting for the quality of the outdoor air for different outdoor environs and 

ventilation and filtration systems types. 

• Accounting for ventilation air distribution effectiveness. 

• Application of demand-based ventilation systems.

Models that can accurately simulate the performance of advanced ventilation control systems in 

high performance buildings, along with associated humidity control systems in humid climates, 

are needed to understand a wide range of scenarios related to the economics and operational 

success of low-energy homes. Accurate simulation of ventilation and space conditioning control 

systems that operate on subhourly time scales coupled with the interacting heat and mass transfer 

effects is complex. Accurate and meaningful results depend on many still somewhat unknown 

inputs, such as: 

• Internal moisture generation rates 

• Other moisture loads, including effects of construction moisture and rain wetting under 

solar loading 

• Building moisture capacitance and the impacts of building material moisture 

adsorption/desorption 

• Detailed space conditioning equipment performance maps. 
1 

1 White Paper Introduction and Residential Ventilation Overview

The U.S. Department of Energy (DOE) Building America program has been conducting research 

leading to cost-effective high performance homes since the early 1990s. Optimizing whole-house 

mechanical ventilation as part of the program’s systems engineered approach to constructing 

housing has been an important subject of the program’s research. Tens of thousands of homes 

have been constructed in partnership with Building America, and many different approaches to 

whole-house mechanical ventilation have been incorporated and evaluated. 

In the last decade, national programs have begun to utilize Building America research as a basis 

to define and label high performance homes, including DOE Challenge Home, U.S. 

Environmental Protection Agency (EPA) ENERGY STAR®, and U.S. Green Building Council 

Leadership in Energy and Environmental Design for Homes. These programs include wholehouse mechanical ventilation as a required element. However, some programs, as well as some 

codes and standards, require minimum ventilation rates significantly higher than what many

Building America builder partners have historically incorporated as part of the research 

collaboration. While ventilation is critically important to indoor air quality, whole-house 

mechanical ventilation can also have impacts on energy use, comfort, and durability. In northern 

cold climates, the energy use to condition outdoor air for ventilation can be significant; in 

southern climates, there are serious concerns related to indoor humidity control that may warrant 

additional energy use for dehumidification. 

Data directly relating the effect of ventilation on occupant health are scarce. Ventilation systems 

have often been deemed effective in homebuilding practice as long as occupants are satisfied 

with odor and moisture control. However, this has little to do with whether or not there are 

pollutants present that may cause health issues because many pollutants are colorless and 

odorless, and many can be controlled through direct source removal or abatement. While 

evaluating details related to the effect of whole-house mechanical ventilation on occupant health 

is outside the scope of the Building America program, evaluating details relating to the effect of 

whole-house mechanical ventilation on energy use, comfort, and durability remains a priority of 

the program. This report intends to review how ventilation is handled in residential codes and 

standards, review some of the Building America program’s practical experience with the 

application of mechanical ventilation in high performance research homes, and describe the 

results of simulations that were conducted to answer the following questions involving the effect 

of application of ventilation in high performance Building America Homes: 

• What is the relationship between ventilation rate and energy required to maintain 

comfort?

• How do those relationships vary as a function of enclosure airtightness? 

• How do those relationships vary climatically? 

• How do those relationships vary as a function of ventilation system type? 

1.1 Ventilation Goals

Ventilation in residential buildings is one component of an effective, comprehensive strategy for 

creation and maintenance of a comfortable and healthy indoor air environment. Indoor air 
2 

pollutant control begins with avoiding the placement of items of known high pollutant emission 

inside the living environment. It follows with local exhaust in areas where high emission from 

pollutant sources cannot be avoided, such as kitchens, toilet rooms, and bathrooms (wet rooms), 

and it ends with whole-building controlled mechanical ventilation to dilute remaining indoor 

pollutants with fresher outdoor air. 

Whole-building ventilation is accomplished where outdoor air is distributed in a controlled 

manner to dilute more polluted indoor air. Since pollutants are dispersed and usually at low 

concentration, there is no practical way to capture and exhaust them as is typical in a toilet room, 

over a shower, or over a kitchen cooktop. Rather, dilution through delivery and distribution of 

outside air is used to reduce the concentration of dispersed pollutants inside the home. Wholebuilding ventilation can be operated continuously at a lower rate, or intermittently at a higher rate 

and still deliver the same volume of dilution air.

While whole-building ventilation is needed for improved indoor air quality, ventilation rates that 

are higher than needed will waste energy. In dry climates, and during wintertime in cold 

climates, high ventilation rates can cause the house to be too dry, causing occupants to want 

humidification equipment that may not have been necessary if the ventilation rate were lower. 

Likewise, in hot, humid climates, high ventilation rates will increase indoor humidity, requiring 

additional dehumidification. 

Complications begin when establishing the whole-building ventilation rate needed while also 

accounting for control of dangerous sources, system effectiveness factors related to balanced 

versus unbalanced ventilation systems, the distribution of ventilation air, and the source of 

ventilation air. There is a real need to establish a true accounting for these factors because 

excessive ventilation impacts energy consumption and humidity control in cold (too dry) and hot 

humid climates (to wet) alike. 

1.2 The Role of Pollutant Source Control 

The impact of source control should be further investigated, as it could reduce ventilation 

requirements in various programs that promote healthy indoor environments such as the EPA 

Indoor airPLUS and DOE Challenge Home programs. In this context, pollutant source control 

refers to controlling the introduction of materials into the dwelling, and controlling the frequency 

of activities inside the dwelling, that cause emissions of pollutants. 

If the primary indoor environment concern is the occupant’s annual average exposure to 

pollutants, certain questions must be answered relating to the ability of ventilation to minimize 

that exposure: 

• What are the health impacts of pollutants of concern? Are odor and moisture of concern, 

which have shorter term, rather than annual, exposure issues? 

• What are the limits of whole-house residential ventilation systems to resolve that 

concern? To what extent can such systems improve indoor air quality, by minimizing 

concentrations of chemical and particulate pollutants, beyond what can be accomplished 

through intermittent spot ventilation and source control? 
3 

For example, research has shown that formaldehyde can act as a constant concentration pollution 

source; i.e., as outdoor air exchange is increased the concentration of formaldehyde remains 

mostly unchanged because the emission rate increases (Weisel et al. 2005). This presents a 

limitation with respect to the ability of ventilation to minimize an occupant’s acute exposure, 

however may reduce the extent of long term exposure by allowing for faster offgassing, such that 

the total offgassing of materials is completed sooner. 

A recently published study by Lawrence Berkley National Laboratory measured a decrease in 

formaldehyde concentration with increasing air exchange rate, but found the reduction to be less 

than proportional, indicating that emission rates may have been buffered by the elevated indoor 

concentration of formaldehyde (Willem et al. 2013). Such studies point to the importance of 

source control as a primary strategy to minimize exposure to indoor pollutants. If introduction of 

sources of pollutants into the living space is not prevented, or if subsequent emission is not 

removed at the source, even the internal mixing caused by a residential ventilation system, or a 

heating/cooling system, or internal temperature differences could exacerbate the local exposure, 

and the problem. 

1.3 Ventilation Considerations in Humid Climates

In any climate, year-round indoor humidity control reduces condensation potential and relative 

humidity (RH), exceeding comfort conditions. Areas of high moisture generation such as 

kitchens, bathrooms, toilet rooms, and laundries, should be exhausted at the source. When 

outdoor conditions are right, whole-house ventilation can serve to dilute remaining indoor 

moisture with drier outdoor air. 

As homes become more energy efficient, space conditioning systems with lower capacities can 

be used to maintain space temperature. Overall, this is good, and produces significant net energy 

and cost savings. However, this situation requires a change to conventional residential space 

conditioning system design in humid climates. While the sensible cooling load is lower, and can 

be dealt with in the conventional way, the latent (moisture) load in high performance homes 

remains nearly unchanged due to ventilation requirements and internal moisture generation by 

occupants and their activities. Therefore, at times when there is no need to lower the space air 

temperature, supplemental dehumidification may be required to maintain RH at acceptable 

levels.

1.4 Review of Ventilation in Residential Codes and Standards

Several codes and standards require residential ventilation. Definitions include airflow rate 

requirements for local exhaust, whole-building mechanical ventilation rate requirements, sources 

of ventilation air, and distribution requirements. While a consistency of intent can be inferred, 

differences among codes and standards, along with the absence of residential ventilation 

requirements in many local codes, indicate the difficulty and uncertainty involved with a generic 

specification.

1.4.1 Local Mechanical Exhaust Flow Requirements

1.4.1.1 Kitchen

The 2012 International Residential Code (IRC), the 2012 International Mechanical Code (IMC), 

the Minnesota Building Code (which points to the IRC), the Washington State Ventilation and 

Indoor Air Quality Code (WAVIAQ), and ASHRAE 62.2-2010 require 100 CFM to outside 
4 

intermittent, or 25 CFM continuous in a kitchen. The U.S. Department of Housing and Urban 

Development (HUD) Code and the National Building Code of Canada (NBC) require rated 

exhaust capacity of 100 CFM in a kitchen. 

1.4.1.2 Bathroom

The 2012 IRC, the 2012 IMC, the Minnesota Building Code, the WAVIAQ, and ASHRAE 62.2-

2010 require 50 CFM to outside intermittent, or 20 CFM continuous in a bathroom (having a 

shower, tub, spa, or other bathing fixture). The HUD Code and the NBC require rated exhaust 

capacity of 50 CFM in a bathroom. 

The 2012 IRC, the 2012 IMC, the Minnesota Building Code, and the WAVIAQ require 50 CFM

to outside intermittent, or 20 CFM continuous in a toilet room. The HUD Code and the NBC

require rated exhaust capacity of 50 CFM in a toilet room (water closet).

While ASHRAE 62.2-2013 does require intermittent ventilation of bathrooms, it does not require 

mechanical ventilation for toilet rooms, and does not consider odor an indoor pollutant of 

concern. 

1.4.2 Whole-Building Mechanical Ventilation Rate Requirements

1.4.2.1 International Residential Code 2012

For all new construction with measured building air leakage of < 5.0 ACH50, the IRC requires a 

whole-house mechanical ventilation system consisting of one or more supply or exhaust fans, or 

a combination of such, providing a whole-house mechanical ventilation rate according to Table 1

below, either continuous or intermittent within a time period not less than 1 hour in every 4 

hours. The rates shown in the table are based on the ASHRAE 62.2-2010 rate equation, but are 

generally higher than that because the groupings cover a wide range of floor area. For example, 

the 1,501–3,000 ft2

 house category captures most U.S. housing in one category with a ventilation 

rate requirement that is 60% higher than required by ASHRAE 62.2-2010 for a 1,501-ft2

, twobedroom house. For an average 2,000-ft2

, three-bedroom house, the rate is 20% higher. 

Table 1. Continuous Whole-House Mechanical Ventilation System 

Airflow Rate Requirements of the 2012 IRC*

Dwelling Unit 

Floor Area 

(ft2)

Number of Bedrooms

0–1 2–3 4–5 6–7 > 7

Airflow in CFM

< 1,500 30 45 60 75 90

1,501–3,000 45 60 75 90 105

3,001–4,500 60 75 90 105 120

4,501–6,000 75 90 105 120 135

6,001–7,500 90 105 120 135 150

> 7,500 105 120 135 150 165

*Adapted from Table M1507.3.3(1)

1.4.2.2 International Mechanical Code 2012

For all new construction with measured building air leakage of < 5.0 ACH50, the IMC 2012 

requires a balanced mechanical ventilation system while not prohibiting negative or positive 

building pressure with respect to outdoors. The mechanical ventilation must be provided by a 
5 

method of supply air and return or exhaust air, and the amount of supply air shall be 

approximately equal to the amount of return and exhaust air. For single and multiple private 

dwellings, the required airflow rate must be calculated based on 0.35 ACH, but not less than 

15 CFM/person, with two people counted for the first bedroom and one for each additional 

bedroom. 

1.4.2.3 Minnesota Building Code 2009

The Minnesota Building Code requires a total ventilation rate (capacity) equal to total ventilation 

rate (CFM) = (0.02)(square feet of conditioned space) + (15)(number of bedrooms + 1) for each 

1-hour period. For a 2000-ft2

, three-bedroom house, that would be 100 CFM. For heat recovery 

ventilators (HRVs) and energy recovery ventilators (ERVs), the average hourly ventilation 

capacity must account for any reduction of exhaust or outdoor air intake, or both, for defrost or 

other equipment cycling per Home Ventilation Institute Standard 920. However, the continuous 

(or averaged over each hour) ventilation rate (operating), must be not less than 50% of the total 

ventilation rate and not less than 40 CFM. In other words, the Minnesota code requires a total 

ventilation rate capacity of two times the ASHRAE 62.2-2010 rate and an operating ventilation 

rate equal to the ASHRAE 62.2-2010 rate. A maximum airflow limit occurs when local 

ventilation requirements are being met by the continuous ventilation system, in which case shall 

not be capable of operating at more than the total ventilation rate.

1.4.2.4 Washington State Ventilation and Indoor Air Quality Code

The WAVIAQ requires minimum and maximum whole-house ventilation rates according to the 

table below. The rates shown in the Table 2 are also generally higher than the ASHRAE 62.2-

2010 rates. For example, the 1,501–2,000 ft2 house category captures most U.S. housing in one 

category with a minimum ventilation rate requirement that is 73% higher than required by 

ASHRAE 62.2-2010 for a 1,501-ft2, two-bedroom house. For an average 2,000-ft2, threebedroom house, the minimum rate is 60% higher.

Table 2. Ventilation Rates for All Group R Occupancies Four and Fewer Stories (CFM)*

Floor Area 

(ft2)

Number of Bedrooms

≤ 2 3 4 5 6 7 8

Min Max Min Max Min Max Min Max Min Max Min Max Min Max

< 500 50 75 65 98 80 120 95 143 110 165 125 188 140 210

501–1000 55 83 70 105 85 128 100 150 115 173 130 195 145 218

1001–1500 60 90 75 113 90 135 105 158 120 180 135 203 150 225

1501-2000 65 98 80 120 95 143 110 165 125 188 140 210 155 233

2001–2500 70 105 85 128 100 150 115 173 130 195 145 218 160 240

2501-3000 75 113 90 135 105 158 120 180 135 203 150 225 165 248

3001–3500 80 120 95 143 110 165 125 188 140 210 155 233 170 255

3501–4000 85 128 100 150 115 173 130 195 145 218 160 240 175 263

4001–5000 95 143 110 165 125 188 140 210 155 233 170 255 185 278

5001–6000 105 158 120 180 135 203 150 225 165 248 180 270 195 293

6001–7000 115 173 130 195 145 218 160 240 175 263 190 285 205 308

7001–8000 125 188 140 210 155 233 170 255 185 278 200 300 215 323

8001–9000 135 203 150 225 165 248 180 270 195 293 210 315 225 338

> 9000 145 210 160 240 175 263 190 285 205 308 220 330 235 353

*Adapted from Table 3-2 of the WAVIAQ
6 

1.4.2.5 National Building Code of Canada 2005

The NBC requires a principal ventilation fan to have minimum and maximum exhaust capacities 

shown in Table 3 below, depending solely on the number of bedrooms. 

Table 3. Normal Operating Exhaust Capacity of Principal Ventilation Fan*

Number of Bedrooms 

in Dwelling Unit

Normal Operating Exhaust Capacity of 

Principal Ventilation Fan (CFM)

Minimum Maximum

1 34 51

2 38 59

3 47 68

4 55 81

5 64 95

*Adapted from Table 9.32.3.3 of the 2005 NBC

1.4.2.6 Housing and Urban Development Code 2008

The HUD Code requires that each manufactured home be provided with whole-house ventilation 

having a minimum capacity of 0.035 CFM/ft2 of interior floor space or its hourly average 

equivalent, but not less than 50 CFM or more than 90 CFM. 

1.4.2.7 ASHRAE Standard 62.2-2013

The ASHRAE Standard 62.2-2013 determines the whole-house ventilation rate based on two 

factors—the number of bedrooms and the conditioned floor area. The bedroom part amounts to 

7.5 CFM for each occupant, with the number of occupants being counted as the number of 

bedrooms plus one. The “plus one” comes from presuming that the first bedroom will likely have 

two occupants. The floor area part amounts to 3 CFM per 100 ft2 of floor area. These two parts 

(the people and floor area parts) are added together. For example, a three-bedroom, 2,000-ft2

house would require 90 CFM, calculated as: (7.5)(3+1) + (2000)(0.03) = 30 + 60 = 90 CFM. 

Without taking any credit for measured infiltration, that is an 80% increase over the 62.2-2010 

requirement. Where a blower door test is conducted, and the annual average infiltration rate is 

calculated, up to two-thirds of the infiltration rate can be deducted from the whole-building 

ventilation rate to arrive at a mechanical ventilation fan rate. 

The whole-building ventilation can be continuous or intermittent, and must have on/off override 

control readily accessible to the occupant. For a continuous ventilation fan flow rate requirement 

of 90 CFM, the intermittent ventilation fan flow rate for a system operating one-half of the time 

would have to be 180 CFM. Also, the full effective amount of ventilation must be provided 

within a 4-hour cycle time or a higher ventilation rate must be used based on an effectiveness 

factor. Whole-building ventilation fans that are not remotely mounted, or are not part of the 

central space conditioning system, must comply with a sound level requirement of 1 sone or less. 

ASHRAE Standard 62.2 accounts for ventilation effectiveness only in regards to system runtime. 

While the maximum ventilation air delivery cycle time is truncated to 1 day, the runtime 

effectiveness values are based on calculations that would allow the ventilation system to be off 

for months without any decrease in effectiveness, because the evaluation metric is locked to 

annual average exposure. That approach ignores shorter term indoor air quality effects of odor 

and sensory irritation, which are nevertheless stated parts of an acceptable indoor air quality 
7 

approach in the ASHRAE Standard 62.2 Scope, and definitions of “acceptable indoor air 

quality” and “air cleaning.” 

1.4.3 Whole-Building Mechanical Ventilation Air Distribution Requirements

1.4.3.1 International Mechanical Code 2012

The 2012 IMC requires outdoor air such that the minimum outdoor airflow rate shall be 

determined in accordance with a specification table in the IMC Section 403.3. Ventilation supply 

systems shall be designed to deliver the required rate of outdoor airflow to the breathing zone 

within each occupiable space. 

1.4.3.2 Minnesota Building Code 2009

The Minnesota Building Code requires ventilation air distribution and circulation such that 

outdoor air is delivered to each habitable space by a forced air circulation system, separate duct 

system, individual inlets, or a passive opening. When outdoor air is directly ducted to a forcedair circulation system, circulation of 0.075 CFM/ft2

 must be maintained on average each hour. 

When outdoor air is not directly ducted to a forced-air circulation system, circulation of 0.15 

CFM/ft2

 must be maintained on average each hour. 

1.4.3.3 Washington State Ventilation and Indoor Air Quality Code 2009

The WAVIAQ requires the introduction and distribution of outdoor air and the removal of indoor 

air by mechanical means. It further requires that outdoor air be distributed to each habitable room 

by means such as individual inlets, separate duct systems, or a forced-air system. Conflictingly, 

in homes with exhaust-only ventilation systems without outdoor air inlets, the home must have a 

ducted forced-air heating system that communicates with all habitable rooms and the interior 

doors must be undercut to a minimum of 1⁄2 in. above the surface of the finish floor covering; 

however, nothing is mentioned about a minimum interval of ducted forced-air heating system 

communication with all habitable spaces. This will typically leave days and weeks on end with 

minimal ventilation air distribution.

1.4.3.4 National Building Code of Canada 2005

In the NBC, for ventilation systems not used in conjunction with a forced-air heating system, an 

outdoor air supply ventilation fan is required with the same rated capacity as the principal 

(exhaust) ventilation fan to distribute outdoor air directly to all bedrooms through a system of 

supply ducts. Where an exhaust-only system is installed via the principal ventilation fan, the 

exhaust fan control must be wired so that activation of the exhaust fan automatically activates the 

circulation fan of the forced-air distribution system required at its rated capacity but not less than 

5 times the rated capacity of the exhaust fan. Alternatively, interlocking the forced-air 

distribution system’s circulation fan with the principal (exhaust) ventilation fan can be 

accomplished where the forced-air distribution system is equipped with a control that 

automatically activates the circulation fan at user-selected intervals.

1.4.3.5 Housing and Urban Development Code 2008

The HUD Code requires that ventilation system be designed to ensure that outdoor air is 

distributed to all bedrooms and main living areas. 
8 

1.4.3.6 ASHRAE Standard 62.2-2013

ASHRAE 62.2 does not attempt to address the issue of delivery of outdoor airflow to each space, 

or the breathing zone within each occupiable space, or forced-air circulation of ventilation air at 

all. It simply makes an assumption that for all ventilation system cases, the entire house is a 

single, well-mixed zone, focusing only on annual average occupant exposure. 

1.4.4 Whole-Building Mechanical Ventilation Air Source Requirements

1.4.4.1 International Mechanical Code 2012

The IMC 2012 requires an approximately balanced ventilation system with the ventilation supply 

system designed to deliver the required rate of outdoor airflow to the breathing zone within each 

occupiable space.

1.4.4.2 Washington State Ventilation and Indoor Air Quality Code 2009

The WAVIAQ requires that the ventilation system have direct outdoor air inlets, and that they be 

screened and located so as not to take air from the following contaminated areas, unless an 

exhaust only ventilation system has a ducted forced-air heating system that communicates with 

all habitable rooms and the interior doors are undercut to a minimum of 1⁄2- in. above the surface 

of the finish floor covering: 

• Closer than 10 ft from an appliance vent outlet, unless such vent outlet is 3 ft above the 

outdoor air inlet 

• Where it will pick up objectionable odors, fumes, or flammable vapors 

• A hazardous or unsanitary location 

• A room or space having any fuel-burning appliances therein

• Closer than 10 ft from a vent opening of a plumbing drainage system, unless the vent 

opening is at least 3 ft above the air inlet

• Attics, crawlspaces, or garages.

This provision creates a significant inconsistency, since it is presumably acceptable to take 

outdoor air from all the above contaminated locations for exhaust-only systems but not for any 

other system. To be consistent, the WAVIAQ would need to also require that for exhaust-only 

ventilation systems, the path of least resistance must always be the outdoor air inlet(s) and not 

elements of the building enclosure, while understandably excluding high wind infiltration 

conditions. To make that possible, very large inlets would be required, and would create 

significant comfort and energy problems. 

1.4.4.3 National Building Code of Canada 2005

The NBC stipulates that outdoor supply air supply be connected directly to the outside. 

1.4.4.4 Housing and Urban Development Code 2008 

The HUD Code requires that the ventilation system be balanced, and designed to exchange air 

directly with the exterior of the home. It specifically prohibits air drawn from the space 

underneath the home, through the floor, walls, or ceiling/roof systems. Except for high wind 

effects, positive pressure is not allowed in cold climates, and negative pressure is not allowed in 

warm-humid climates.
9 

1.4.4.5 ASHRAE Standard 62.2-2013

ASHRAE Standard 62.2-2013 requires that supply and balanced ventilation systems draw 

outdoor air from a known fresh air location, but it does not include any requirement for exhaust 

ventilation systems. Therefore, makeup air for exhaust ventilation air comes from the paths of 

least resistance, which could be through a garage, attic, crawlspace, basement, or other soil 

contact location. To be consistent, the Standard would need to require intentional makeup air 

inlets, or require a supply system that provides makeup air from a known fresh air location 

whenever the whole-building exhaust ventilation system was operating.

1.5 Review of Building America Teams’ Experience With Ventilation Approaches 

As stated in the preceding review of codes and standards, ASHRAE Standard 62.2 wholebuilding ventilation does not account for shorter than annual average indoor air quality effects 

(such as odor, moisture, and sensory irritation), nor does it address system effectiveness factors 

related to outdoor air distribution, mixing with filtration, or source of outdoor air. Research 

conducted by the DOE Building America program has resulted in widespread use of systems that 

combine reduced rates with various elements of effectiveness.

Building Science Corporation (BSC) experience with whole-building controlled mechanical 

ventilation in tens of thousands of high performance homes in locations all across the United 

States has shown that drawing outdoor air from a known fresh air location, conditioning that air 

by filtration and sometimes heating or cooling, tempering that ventilation air by mixing it with 

central system return air, and fully distributing that air on at least an hourly average basis is a 

practical and effective way to mitigate odor complaints in all climates and an effective way to 

mitigate moisture buildup in mixed and cold climates. For more than 15 years, BSC builder 

partners have been installing systems capable of meeting more than ASHRAE Standard 62.2 

ventilation rates, but typically running those systems at one third to one half that rate, resulting in 

satisfied builders and homeowners in both production and custom housing (Rudd and Lstiburek 

1999, 2001, 2008). BSC attributes that satisfaction at the lower ventilation rates to the full 

distribution and whole-house mixing of outdoor air drawn from a known fresh air source with 

filtration (Hendron et al. 2006, 2007; Rudd and Lstiburek 2000; Townsend et al. 2009a, 2009b). 

BA-PIRC (formerly BAIHP) worked with site and factory builders constructing custom, 

production, affordable, and multifamily homes to implement supply-based mechanical 

ventilation through the introduction of outdoor air into the return side of centrally ducted, forcedair, space conditioning systems. This approach, combined with rightsized heating/cooling 

systems and properly operating bathroom and kitchen exhaust fans (ducted to the outdoors) has 

been implemented in thousands of homes, primarily in the southeastern United States, since 1997 

and has effectively controlled odors, maintained comfort, and proven effective at minimizing 

wintertime moisture buildup (Chandra et al. 2008). Similar to BSC’s approach, these systems 

draw outdoor air from a known fresh air location, filter the air, temper the air by mixing it with 

central system return air, and fully distribute the air. Systems have been commissioned to deliver 

approximately 30%–70% of ASHRAE Standard 62.2-2010 rates, enough to create a slight 

positive pressure in the home with respect to outdoors; however, only while the central HVAC 

system is running to satisfy a heating or cooling requirement. Therefore, operation of the 

ventilation system is intermittent, especially during periods of limited to no HVAC runtime. In 

the Southeast, these periods typically coincide with increased natural ventilation through more 

frequent window operation, and the system has gained the acceptance of homeowners and 
10 

builders alike in terms of comfort, durability, energy consumption, and perceived odor and 

moisture control. However, most of these systems do not meet the whole-house mechanical 

ventilation requirements of ASHRAE 62.2-2010. 

1.6 Review of Other Previous Modeling Efforts 

In the publication by BSC (Rudd and Walker 2007), a comprehensive set of simulations were

performed to examine the relative operating costs and air change rates of various residential 

ventilation methods. The simulations included six major U.S. climate zones, three house sizes, 

two house types (standard-performance International Energy Conservation Code [IECC] and 

higher-performance), and 13 ventilation systems. The key results for the high performance 

houses (most comparable to the houses in this study) were:

• For both the Standard-Performance and Higher-Performance house, ASHRAE Standard 

62.2 can be met using a simple exhaust-only system for an annual operating cost ranging 

from a savings of $225 to a cost of $150, all compared to a Standard-Performance house 

having approximately the same annual average air change through natural infiltration 

alone. The highest cost increase and the highest annual average air change increase 

occurred in the milder climates. 

• For the same rated fan flow, the greatest increase in air change rate is for balanced 

systems (HRV/ERV), the least change is for exhaust-only systems, and supply-only 

systems are between those two.

• HRVs/ERVs sized to meet ASHRAE Standard 62.2, and as commonly installed to 

require coincident operation of the central air handling unit (AHU) fan, tend to require 

the greatest operating cost due to higher air change and fan energy consumption.

• Depending on climate and electricity cost, the cost to provide ventilation air distribution 

and thermal comfort mixing using a central fan cycling system at a 20 minute per hour 

minimum was between –$20 and +$90 compared to the same house in the same climate 

without the minimum central fan cycling. The higher costs were in hotter climates with 

more expensive electricity.

• In humid climates, the ASHRAE Standard 62.2 compliant ventilation systems increased 

the median RH by about 15% compared to the Standard-Performance house without 

mechanical ventilation. It was clear that, with or without mechanical ventilation, 

supplemental dehumidification would be required to control elevated indoor RH yearround. 

ASHRAE Research Project (RP) 1449 (Rudd et al. (2013) completed a comprehensive simulation 

analysis of energy and humidity control performance of various options in humid climates for 

residential buildings with and without ventilation. Results show the effects of duct location, 

thermostat set points, and modeled moisture capacitance on humidity control: 

• Duct location has a major influence on humidity control, and locating the air distribution 

system ducts inside conditioned space saves energy overall, but, with the reduced 

sensible cooling load, and hence reduced latent capacity, also comes an increased need 

for supplemental dehumidification in warm-humid climates. Moving the ducts inside 

conditioned space compared to ducts in the attic with 5% leakage (3% supply side, 2% 
11 

return side) increases the hours above 60% RH indoors by 30%–50% for a DOE 

Challenge Home. That was simulated without accounting for the moisture desorption 

from wood framing materials that typically increases the attic humidity ratio over that of 

the outdoors during the late morning to early afternoon hours in warm-humid climates 

(about 10°F dew-point temperature over outdoors from 10:00 a.m. to 1:00 p.m. between 

May 15 and October 15). That effect would tend to lessen the difference, since return 

duct leakage with higher attic humidity would increase the moisture source for the ductsin-attic configuration. However, additional simulations done on this as a sensitivity study

showed that the results were nearly the same. The reason is that most elevated indoor 

RH hours occur in night and early morning hours between late November and March. 

Some nighttime and rainy periods during mild summer conditions also produce elevated 

indoor RH. 

o Similar to ducts in hot attics, very high internal sensible heat gain experienced by 

older homes (simulated at 21 kWh/day) drives the cooling system to operate more 

often and for longer runtimes, reducing indoor humidity. High performance 

homes, with compact fluorescent lamps or light-emitting diodes and ENERGY 

STAR-rated appliances, have lower internal sensible heat generation than older 

homes (simulated at 15 kWh/day), further complicating humidity control. 

• Thermostat set points have a major influence on humidity control. For the warm-humid 

climate, shifting the heating and cooling thermostat set points up by 2°F (to 73°F and 

78°F, respectively) would tend to increase the hours above 60% RH at the edge of 

cooling demand due to less space conditioning runtime, and decrease the hours above 

60% RH at the edge of heating demand. Raising the heating set point has a significant 

impact on reducing indoor RH (even though it does nothing to reduce the absolute 

humidity) because it keeps the air from getting as cold, stopping the rise in RH. 

• Moisture capacitance is a secondary factor in increasing hours above 60% RH, and 

moisture capacitance factors of 10–30 times the air mass capacity do not show significant 

difference in indoor RH. 
12 

2 Technical Approach for Assessment of Energy and Indoor 

Humidity Impacts of Residential Ventilation

This study was based on building energy modeling with an important focus on indoor humidity 

impacts. The modeling tools used were EnergyPlus version 7.1 (E+) and EnergyGauge USA

(EGUSA). Twelve U.S. cities and five climate zones were represented. A total of 864 

simulations (2*2*2*3*3*12 = 864) were run using two building archetypes, two building 

leakage rates, two building orientations, three ventilation systems, three ventilation rates, and 12 

climates. For both the E+ and EGUSA simulations, custom scripts were written to automate 

parameter modification for the hundreds of simulations, which could be run overnight. The 

following sections explain the details of these parametric inputs. 

The BEopt program (E+ version) was used to generate the building geometry and the base IDFs 

(input data files) for the E+ simulations. Figure 4 shows the building geometry for the two-story 

house. 

Figure 4. NREL BEopt E+ program used to generate the building geometry and 

the base IDF files (input description)

The EGUSA program is based on the DOE-2 hourly simulation engine and was a custom version 

for this project (V3.0.01P). Some of the custom routines were designed to improve the modeling 

of indoor humidity while using the temperature-only control capability of DOE-2. Latent 

degradation due to evaporation of moisture from wet cooling coils during cooling system off 

cycles (Shirey et al. 2006) was not modeled (moisture added back to the ducts and conditioned 

space when the cooling compressor is off and fan remains on). 
13 

2.1 Residential Whole-Building Ventilation Systems Evaluated

2.1.1 Exhaust Only

Exhaust whole-house ventilation systems expel inside air directly to outdoors, tending to 

depressurize the interior space relative to outdoors (Figure 5). Exhaust systems draw outdoor air 

from whatever building enclosure leaks create the path of least resistance. The exhaust 

ventilation system modeled here was a single-point exhaust system such as a high quality 

exhaust fan installed in a master bathroom, family bathroom, toilet room, or laundry room. 

Figure 5. Schematic overview of typical local and whole-house ventilation equipment 

(a home typically has only one whole-house system) 

(Rudd 2011)
14 

2.1.2 Supply Only 

Supply whole-house ventilation systems draw outdoor air from a known location and deliver it to 

the interior living space. This known location should be selected to maximize the ventilation air 

quality. The air can be treated before being distributed to the living space (heated, cooled, 

dehumidified, filtered, or cleaned).This study used a central fan integrated (CFI) supply 

ventilation system that provides ventilation air through a duct that extends from outdoors to the 

return air side of the central heating and cooling system AHU, with a motorized outdoor air 

damper, and with an automatic timer control to ensure ventilation air is periodically supplied 

when heating and cooling have been inactive and to limit outdoor air introduction to a maximum 

regardless of how long the fan operates. 

2.1.3 Balanced Heat Recovery Ventilation and Energy Recovery Ventilation

Balanced whole-house ventilation systems both exhaust and supply in roughly equal amounts. 

Inside air is exhausted to the outdoors and outdoor air is supplied indoors. In current practice, 

most balanced ventilation systems are HRVs or ERVs. Balanced ventilation systems with HRVs 

use a heat exchanger to transfer heat between the exhaust air stream and the outdoor air supply 

stream. With HRVs, no moisture is exchanged between the air streams. This means that in cold 

months, the heating load due to ventilation will be less, and in hot months, only the sensible 

cooling load due to ventilation will be less.

The balanced ventilation system modeled here is an ERV. ERVs operate the same as HRVs with 

the exception that both heat and moisture are exchanged between the exhaust air stream and the 

outdoor air supply stream. This means that in cold, dry months, the heating load due to 

ventilation will be less, and the house indoor moisture level will be higher than it otherwise

would have been without energy recovery. In hot, humid months, the total cooling load (both 

sensible and latent) due to ventilation will be less. While less heat and moisture will come in 

from outdoors, an ERV can neither cool nor dehumidify the interior space. A good way to think 

of this is that the heat and moisture tend to remain on the side from which they came. Another 

important point about using ERVs in humid climates is that, at times of the year when indoor 

humidity is the highest, there is usually a small difference between indoor and outdoor humidity, 

minimizing the latent exchange effect of the ERV.

The ERV modeled here was configured with ducts completely separate from the central space 

conditioning system. Therefore, no central AHU operation was needed coincident with the ERV 

operation as is the case in the majority of installations.

2.2 Modeling Assumptions

2.2.1 Constants

2.2.1.1 No Window Ventilation, Only Mechanical Ventilation and Natural Infiltration

The models assumed that windows were closed the entire year. Window openings were not 

included in the simulations because they could easily confound the results. The worst case 

scenario would be that occupants may choose not to open windows for a variety of reasons. In 

addition, the simulations assumed that there was no added infiltration induced by duct leakage to 

the outside. This assumption was made for similar reasons of confounding the results as well as 

the goal that duct leakage to the outdoors should be effectively eliminated in a DOE Challenge 

Home.
15 

2.2.1.2 Infiltration Model and Combined Airflow 

The infiltration rate in EGUSA is simulated using the Sherman-Grimsrud model and Shelter 

class 4, and is simulated in E+ using the Walker Wilson model. Unbalanced mechanical 

ventilation airflow and the infiltration airflow are added in quadrature (by summing the squares, 

and then taking the square root). Balanced mechanical ventilation airflow and infiltration airflow 

are summed directly. The combined airflow of infiltration and balanced and unbalanced 

ventilation was accounted for. 

2.2.1.3 Thermal Enclosure Specifications

The objective was to make each simulated home qualify for the DOE Challenge Home. The 

DOE Challenge Home is generally a combination of EPA’s ENERGY STAR version 3, Indoor 

airPLUS, and Water Sense requirements, and the IECC 2012 requirements, resulting in a Home 

Energy Rating System Index in the low- to mid-50s range. The Home Energy Rating System 

Index for the homes being simulated here fell in the mid- to upper-50s range depending on 

climate zone.

2.2.1.4 Space Conditioning Equipment

Following the requirements of the DOE Challenge Home, the space conditioning air duct system 

and AHU were located in conditioned space with zero leakage to the outdoors. The space 

conditioning equipment efficiency matched the DOE Challenge home requirements. 

2.2.1.5 Ventilation System Airflow and Power

The mechanical ventilation airflow met the requirement of ASHRAE Standard 62.2-2013 every 

hour. Exhaust fan power was set to 0.3 Watts/CFM. The ERV airflow was left slightly 

unbalanced, with a 10% difference between supply and exhaust, with the larger value set equal to 

the Qfan requirement (regarding Qfan, refer to the Section 2.2.2.6). The ERV fan power was set 

equal to 0.5 W/CFM*(supply CFM + return CFM). ERV effectiveness was set to 60% per the

DOE Challenge Home criteria. The CFI supply ventilation duty cycle was set to 33% for both 

the minimum and maximum operation times, and only AHU energy in excess of that needed for 

heating and cooling was counted as ventilation energy. The CFI outdoor airflow was set equal to 

Qfan/duty cycle fraction.

1 AHU fan power was set at 0.5 Watts/CFM for SEER < 14 (assuming 

permanent split capacitor fan at > 0.5 in. water column external static pressure), and 0.375 

watts/CFM for SEER >14 (assuming electronically commutated motor fan). Ventilation fan heat 

was added to the indoor space for supply ventilation air stream (CFI and supply side of ERV) but 

not for exhaust ventilation air stream (exhaust fan and exhaust side of ERV). 

2.2.1.6 Building Geometry and Glazing Area

A 15% window-floor-area ratio was used for all homes. 

2.2.1.7 Internal Heat and Moisture Generation and Schedule 

Internal gains for all homes were set at: sensible heat gain = 57,717 Btu/day, and latent heat gain 

= 12.09 lb/day (12,698 Btu/day). The hourly schedule for both heat and moisture generation is 

shown in Figure 6. 

 

1 The CFI systems modeled in this study met the airflow requirements of ASHRAE 62.2-2013. CFI systems are 

typically not operated with airflows required by ASHRAE 62.2, due to various practical reasons, although there is 

nothing inherent with this system to preclude that. The system is included in this modeling study, as it is a popular 

system with builders for some reasons not accounted for in 62.2.
16 

Figure 6. Hourly internal gains schedule

2.2.1.8 Internal Moisture Capacitance 

Indoor moisture capacitance was simulated using an air mass multiplier of 10 (i.e., the indoor air 

mass for moisture storage purposes is increased by 10 times). 

2.2.1.9 Temperature Control Set Points

Thermostat temperature control set points were: heating = 71°F and cooling = 76°F.

2.2.1.10 Indoor Humidity Control Set Point

The indoor humidity control set point was 60% RH. 

2.2.1.11 Electricity and Gas Utility Costs

Electricity and gas utility costs were applied equally for all cities as $0.12/kWh and $1.20/therm 

(1 therm is approximately equal to 1 ccf or 100 ft3 of gas at standard pressure and temperature). 

Figure 7 shows those rates to be generally reasonable assumptions according to the U.S. Energy 

Information Agency data for 2011. 
17 

Figure 7. Average residential electricity and gas utility rates by state for 2011 

(Source: EIA.gov)

2.2.2 Variables

2.2.2.1 U.S. Climates/Cities

• Warm-humid zones (2A, 3A): Orlando, Florida; Houston, Texas; Charleston, 

South Carolina 

• Mixed-humid zones (4A): Baltimore, Maryland; Kansas City, Missouri; Charlotte, 

North Carolina 

• Cold-humid zones (5A and 6A): Chicago, Illinois; Minneapolis, Minnesota 

• Dry zones (2B and 5B): Phoenix, Arizona; Denver, Colorado 

• Marine zones (3C and 4C): Los Angeles, California; Seattle, Washington. 

2.2.2.2 House Type and Size

Two house types and sizes were simulated as follows: 

• 2000-ft2, one-story, three-bedroom, slab-on-grade, wood frame

• 2400-ft2, two-story, three-bedroom, slab-on-grade, wood frame. 

2.2.2.3 Glazing Orientation

The worst-case and best-case solar gain orientations were simulated to evaluate the impact that 

could have on indoor humidity levels. The indoor humidity levels would be impacted due to 

more or less cooling demand driven by solar gain.

2.2.2.4 Building Airtightness

Buildings with airtightness of 1.5 ACH50 and 3.0 ACH50 were simulated. 

0.0

1.0

2.0

3.0

4.0

5.0

0

2

4

6

8

10

12

14

16

FL TX AZ SC CA MD KS NC WA IL CO MN

Natural Gas Rate ($/ccf)

Electric Rate ($/kWh)

EIA Electric and Gas Utility Rates

by State average for 2011

Electric Natural Gas
18 

2.2.2.5 Ventilation System Type 

Exhaust-only, supply-only, and balanced HRV/ERV ventilation systems were simulated.

2.2.2.6 Mechanical Ventilation Rate

Three mechanical ventilation rates were evaluated: 100%, 75%, and 50% of ASHRAE 62.2-

2013. Modeling of the reduced rates is not to suggest that such rates provide adequate indoor air 

quality—that is outside the scope of this report. The rates are chosen purely to evaluate the 

energy and moisture impacts of varying rates. The 75% rate was chosen as it closely represents 

ASHRAE 62.2-2010 prior to the recent 62.2-2013, and the 50% rate was chosen as 

representative of what many Building America production builder partners have historically 

incorporated in various research and demonstration homes. The ventilation fan flow rate is 

climate dependent because a credit for measured infiltration is allowed as follows:

Qfan = Qvent – Qinfil

Qvent = 0.03*CFA + 7.5*(Nbr+1) 

Qinfil = climate dependent calculation based on procedures specified by ASHRAE 62.2-2013, 

where Qinfil cannot be more than two thirds of Qvent 

On average, for all sets of ACH50 conditions, the fan flow rates required by 62.2-2013 compared 

with 62.2-2010 requirements as follows: 

100% of 2013 ≈ 140% of 2010

75% of 2013 ≈ 94% of 2010 

50% of 2013 ≈ 48% of 2010 

Table 4. Comparison of Fan Flows Required by ASHRAE 62.2-2010 and 2013

CLIMATE ZONE LOCATION

ASHRAE

WSF*

62.2-

2013

fan cfm

% diff

from 

62.2-

2010

fan cfm

62.2-

2013

fan cfm

% diff

from 

62.2-

2010

fan cfm

62.2-

2013

fan cfm

% diff

from 

62.2-

2010

fan cfm

62.2-

2013

fan cfm

% diff

from 

62.2-

2010

fan cfm

Warm-Humid Zone Orlando, FL 0.39 73 35% 88 62% 71 42% 81 61%

Warm-Humid Zone Houston, TX 0.40 72 34% 87 61% 71 41% 80 61%

Warm-Humid Zone Charleston, SC 0.43 70 30% 86 59% 69 38% 80 59%

Mixed-Humid Zone Baltimore, MD 0.50 65 20% 83 55% 66 31% 78 56%

Mixed-Humid Zone Kansas City, MO 0.60 58 7% 80 48% 61 22% 75 51%

Mixed-Humid Zone Charlotte, NC 0.43 70 30% 86 59% 69 38% 80 59%

Cold-Humid Zone Minneapolis, MN 0.63 55 2% 79 46% 59 19% 75 49%

Cold-Humid Zone Chicago, IL 0.60 58 7% 80 48% 61 22% 75 51%

Dry Zone Phoenix, AZ 0.43 70 30% 86 59% 69 38% 80 59%

Dry Zone Denver, CO 0.61 57 5% 79 47% 60 21% 75 50%

Marine Zone Los Angeles, CA 0.42 71 31% 86 60% 70 39% 80 60%

Marine Zone Seattle, WA 0.56 61 12% 81 50% 63 26% 76 53%

average of climates: 65 20% 83 55% 66 31% 78 56%

2-story, 62.2-2010 fan cfm=54 1-story, 62.2-2010 fan cfm=50

3.0 ach50 1.5 ach50 3.0 ach50 1.5 ach50
19 

3 Modeling Results

3.1 Energy Performance

3.1.1 Energy Performance as a Function of Ventilation Rate 

Removal of indoor moisture by supplemental dehumidification was approximated from the 

EGUSA simulations by a post-processing routine that assumed that during hours over an RH 

limit, the net moisture gain during that hour due to air exchange and internal moisture generation 

would have to be removed. That strategy is only an approximation because the heat and mass 

transfer interactions that a real-time dehumidifier would have on the space conditions and the 

cooling system are not accounted for. For example, while a dehumidifier is operating, it is 

removing moisture and adding heat to the space, both of which drive the RH down. Additionally, 

heat added from the dehumidifier may raise the space temperature enough to cause the cooling 

system to come on, which then will also remove moisture. Those interactions are not accounted 

for in the EGUSA model. They are accounted for in the E+ model; however, they were found to 

have a minor effect on annual space conditioning energy use. 

The predicted lb/yr values from the EGUSA modeling were converted to $/yr by an assumed 

dehumidifier energy factor of 1.47 L/kWh and $0.12/kWh. Based on that, supplemental 

dehumidification was predicted to be $10–$30/yr for the warm-humid climates of Charleston, 

Houston, and Orlando, and the marine climate of Los Angeles. The E+ modeling yielded similar 

results. 

Figure 8 shows the total annual operating cost for a DOE Challenge Home controlled to < 60% 

RH over a range of ventilation rates from 100% of the ASHRAE 62.2-2013 requirement to 75% 

and 50% of that. The 75% ventilation rate values generally correspond to the 62.2-2010 rates. As 

shown in Figure 9, that difference is exclusively space conditioning energy use, and is about 

$45/yr averaged over the climates, enclosure tightness and ventilation systems simulated, or a 

savings of 10% of total space conditioning energy. Dropping to 50% of the ASHRAE 62.2-2013 

rate can save about another $35/yr on average for a total average savings of 15% of space 

conditioning energy. 

Figure 8. Total annual operating cost for a DOE Challenge Home 

controlled to < 60% RH over a range of ventilation rates
20 

Figure 9. Total annual space conditioning cost difference for a DOE Challenge Home 

controlled to < 60% RH over a range of ventilation rates using EGUSA

3.1.2 Energy Performance as a Function of Ventilation System Type 

Total annual operating cost per year for a DOE Challenge Home varied less than $90/yr among 

all ventilation systems in every climate (Figure 10). Compared to exhaust ventilation as a 

reference, EGUSA results show balanced ERV ventilation (installed such that it does not require 

coincident AHU fan operation to avoid short-circuiting of ventilation air) ranged from $0/yr to 

$40/yr more, with an average of about $20/yr more for 100% of the ASHRAE 62.2-2010 

requirement at 3 ACH50. However, at greater mechanical ventilation rates required in the 1.5 

ACH50 house,2 the ERV saved an average of $20/yr over exhaust ventilation, in all but the dry 

climate of Phoenix and the mild climate of Los Angeles, where it continued to consume more 

energy than exhaust (not shown). E+ results show savings from balanced ERV ventilation 

compared to exhaust ventilation at 3 ACH50, primarily in cold/mixed climates. The cause of the 

discrepancy between E+ and EGUSA is unknown. CFI supply ventilation ranged from about 

$11/yr to $93/yr more than exhaust ventilation, with an average of about $40/yr. 

Figure 10. Total annual operating cost per year for a DOE Challenge Home 

over a range of ventilation system types

 

2 The weather-dependent, natural component of the total ventilation requirement is smaller in the 1.5 ACH50 house. 

Therefore, a larger continuous, mechanical component is required. 
21 

3.1.3 Air Change Rates as a Function of Building Air Leakage

For the three ACH50 houses, air change rates were slightly higher (about 0.01 ACH) for onestory, unbalanced ventilation system houses than for two-story houses. Air change rates were 

mostly the same between one- and two-story, balanced ventilation system houses. The difference 

between balanced and unbalanced ventilation was about 0.05 ACH, with balanced being higher 

as expected.

For the 1.5 ACH50 houses, air change rates were about 0.02 ACH higher for one-story, 

unbalanced ventilation system houses than for two-story houses, and were about 0.01 ACH

higher for one-story houses with balanced ventilation. 

The difference in air exchange rate between balanced and unbalanced ventilation was about 24% 

for the 3.0 ACH50 houses and about 10% for the 1.5 ACH50 houses (Figure 11).

Figure 11. EGUSA differences in air exchange rate between balanced and unbalanced ventilation

Figure 11 shows that for unbalanced ventilation, total air exchange increases as building leakage 

decreases. The reason is that the ventilation and infiltration flows were combined in quadrature 

in the simulations while the required mechanical ventilation was calculated using the ASHRAE 

procedure, which is a simple subtraction of the estimated infiltration rate from the total 

ventilation rate requirement. As a result, as the envelope leakage is reduced, the portion of the 

total ventilation rate coming from mechanical ventilation increases. Simultaneously, the use of 

quadrature to combine the ventilation and infiltration rates in the simulation model causes the 

infiltration contribution to be discounted more in the 3 ACH50 home than in the 1.5 ACH50 

home, resulting in a slightly greater total air exchange rate for unbalanced ventilation in the 

tighter home.

3.2 Humidity Control Performance

The E+ model results were compared to the EGUSA model results, and initially some input 

differences were corrected. However, some result differences still exist and remain mostly 

unexplained. Figure 12 shows an example of the E+ results generally showing a greater number 

of hours over 60% RH indoors, and hours that extend to higher RH, compared to the EGUSA 

results and much of BSC’s field data (Rudd et al. 2003, 2005; Rudd and Henderson 2007). These 

might be due to differences in the infiltration model. There may also be differences in the 

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

Air Exchange Rate (1/h)

1.5 ach50; 100%; North

1-Sty ERV 2-Sty ERV 1-Sty EXH 2-Sty EXH
22 

thermal energy balance model, which could lead to different loads, as another possibility. The E+

moisture modeling inputs were double checked without finding anything significantly wrong. 

The moisture capacitance factor and moisture generation rate were consistent with the EGUSA 

inputs. While there is a noticeable difference between the two models, the trends are for the most 

part very similar. 

Figure 12. Indoor RH versus hour of year comparison between E+ (blue symbols) and 

EGUSA (V3.0.01P) simulation results

3.2.1 Humidity Control Performance as a Function of Space Conditioning 

System Activity

Figure 13 illustrates the indoor RH response with cooling load using EGUSA. The expected 

space conditioning equipment operation, and the temperature and RH response, are shown. 

Figure 13. For a week at the end of June and beginning of July on Orlando, 

the cooling system and space RH are showing an expected response using EGUSA
23 

The hours above 60% RH during a particular space conditioning mode (heating, cooling, 

floating) was a strong function of climate. Floating hours are times when no space conditioning 

is active because the house remains between the heating and cooling control set points. The 

EGUSA results in Figure 14 show that the hours above 60% RH during heating are below 200 in 

every climate, but the hours above 60% RH while floating can be equal to or significantly greater 

than the hours above 60% RH during cooling and that is very dependent on the climate. For 

example, in Orlando, the hours above 60% RH are nearly equal between cooling hours and 

floating hours, but in Charleston and Houston, the hours above 60% RH are 35%–50% greater 

for floating hours. However, the cooling hours above 60% RH are generally < 65% RH while, in 

the warm-humid climates, the floating hours are 5%–15% RH above 60% RH. 

Figure 14. Hours of elevated indoor RH is a strong function 

space conditioning system activity and climate

The E+ results, while having a greater number of hours over 60% RH, show similar trends 

compared with the EGUSA results. The exception is in Los Angeles, where the floating hours 

are more than double those predicted by the EGUSA model. 

3.2.2 Humidity Control Performance as a Function of Infiltration Rate 

Referring to Figure 15, mechanical ventilation, operated at the ASHRAE 62.2-2013 rate, in a 3 

ACH50 DOE Challenge level house, raises the annual median indoor RH by almost 10% 

compared to a 7 ACH503

 house without mechanical ventilation in Orlando. That is because 

infiltration drivers are quite seasonally dependent with greater infiltration rates in winter and 

smaller infiltration rates in summer, but mechanical ventilation forces a minimum air exchange 

year round. Note that most of the RH increase due to ventilation is 60%–65% RH; the hours 

between 65% and 75% RH mostly remain the same. This indicates that some supplemental 

dehumidification would be needed in either case to maintain RH below 60%. 

 

3 High performance houses built to 2012 code or the DOE Challenge Home program would likely not be built with 

natural infiltration > 5 ACH50.
24 

Figure 15. ASHRAE 62.2-2013 ventilation rate raises median RH compared to a 

conventional dwelling without mechanical ventilation using EGUSA

3.2.3 Humidity Control Performance as a Function of Ventilation System Type

Figure 16 shows analysis of simulation results for exhaust and ERV ventilation in Orlando. The 

hours above 60% RH are distinguished by heating, cooling, and floating hours. A heating hour 

was where any heating occurred during that hour, a cooling hour was where any cooling 

occurred during that hour, and a floating hour was an hour where no heating or cooling occurred 

during that hour. 

Figure 16. Hours above 60% RH for exhaust and ERV ventilation in Orlando
25 

These data demonstrate the difference between the results from E+ and EGUSA. The exhaustonly cases are similar, however E+ results indicate more hours above 60% RH and hours with 

higher RH compared to EGUSA. The ERV case shows that E+ predicts much lower RH due to 

the use of an ERV than EGUSA. EGUSA also predicts most of the hours significantly above 

60% RH occur during floating hours, which occur mostly during fall, winter, and spring in 

warm-humid climates, and sometimes during summer nights and summer extended rainy periods 

where sensible heat gain is lowered and internal moisture generation stays the same. EGUSA 

shows that in Orlando, the ERV nearly eliminates the cooling hours above 60% RH, but it has 

little effect on the floating hours above 60% RH. E+ shows that the floating hours are well below 

60% RH, and that most hours above 60% RH are cooling hours. The E+ model is suspect, as the 

cooling system forces a greater humidity difference between the indoors and outdoors, which 

makes the ERV transfer more moisture to the exhaust stream. However, even though the ERV 

was modeled with a constant 60% effectiveness, meaning that 60% of the moisture from the 

higher humidity side would be transferred to the lower humidity side, 60% of a small humidity 

difference is still a small amount of moisture. The EGUSA model shows that the ERV is 

ineffective in keeping indoor RH down during floating hours when the difference between 

indoor and outdoor absolute humidity is small, as would be expected. The ERV can be slightly 

counterproductive in winter in Orlando and Houston due to recovery of indoor moisture when 

humidity is lower outdoors than indoors. 

Figure 17 illustrates the annual hours above 60% RH by ventilation system type. The EGUSA 

results from the three warm-humid climates of Charleston, Houston, and Orlando stand out with 

more hours of elevated indoor RH. ERV ventilation generally showed one third to one half fewer 

hours above 60% RH compared to CFI supply and exhaust ventilation. The CFI system shows 

slightly fewer hours above 60% than the exhaust system because it is a supply ventilation system 

that adds fan heat to the space, increasing the call for cooling, which removes additional 

moisture.

Figure 17. Annual hours above 60% RH by ventilation system type

The E+ results show that an ERV appears to virtually eliminate the number of hours with RH 

above 60% compared to the EGUSA results. The amount of reduction in elevated RH by the E+

model is suspect, but the cause is unclear. For both models, the ERV moisture performance was 

modeled pretty simply as a constant latent recovery effectiveness value applied to the actual 
26 

indoor to outdoor humidity ratio difference. The EGUSA and E+ models configured the ERV as 

separately ducted from the central air distribution system.

If the ERV was configured as being connected between the return and supply of the central AHU 

(as most ERVs are to reduce installation cost), requiring that the AHU operate coincident with 

the ERV, it is expected that the hours above 60% RH would be higher due to moisture 

evaporation from the cooling coil. 

3.2.4 Humidity Control Performance as a Function of Building Air Leakage

With unbalanced mechanical ventilation, total air change goes up as the building gets tighter and 

leakage decreases from 3.0 ACH50 to 1.5 ACH50. This is because the variable, natural 

component is smaller, and the consistent, mechanical component is larger. The result of that is 

shown in Figure 18 where the amount of supplemental dehumidification needed increases as the 

building air leakage decreases.

Figure 18. For unbalanced ventilation, the supplemental dehumidification requirement 

increases as the building air leakage decreases using EGUSA

3.2.5 Humidity Control Performance as a Function of Internal Moisture 

Generation

Internal moisture generation (latent gain) strongly impacts the predicted hours of elevated indoor 

humidity. Moisture generation of 12 lb/day produces indoor humidity results that seem to fit 

reasonably well with monitored data for three- to four-bedroom dwellings. More than twice that 

is a worst-case peak design value suggested by ASHRAE Standard 160. While that may be 

useful for sizing considerations in extreme cases, it is not useful for annual simulation of indoor 

humidity and supplemental dehumidification energy consumption. Figure 19 illustrates the 

sensitivity of the hours above 60% RH to an internal moisture generation change from 12 lb/day 

to 15 lb/day. 
27 

Figure 19. Effect of increasing internal moisture generation from 12 to 15 lb/day using EGUSA

3.2.6 Humidity Control Performance as a Function of Relative Humidity Set Point

Figure 20 shows that the hours of elevated indoor RH are a strong function of the selected RH 

limit and climate. For example, in Los Angeles nearly all of the hours above 60% RH are also 

below 65% RH, but that is not true for Charleston, Houston, and Orlando. 

Figure 20. Hours of elevated indoor RH are a strong function of the 

selected RH limit and climate using EGUSA
28 

4 Conclusions

4.1 Energy Consumption Related Conclusions

The difference in total space conditioning operating cost for a DOE Challenge Home controlled 

to < 60% RH with 75% of the ASHRAE 62.2-2013 requirement compared to 100% of that

ventilation rate was about $45/yr averaged over all climates, enclosure tightness and ventilation 

systems simulated, or a savings of 10% of total space conditioning energy (Figure 21). (The 75% 

ventilation rate generally corresponds to the 62.2-2010 rate.) At 50% of the ASHRAE 62.2-2013 

ventilation rate requirement, the average additional savings is about another $35/yr for an 

average total savings of 15% of space conditioning energy compared with 100% of that

ventilation rate requirement.

Figure 21. Total annual operating cost for a DOE Challenge Home 

controlled to < 60% RH over a range of ventilation rates

Total annual operating cost for a DOE Challenge Home varied less than $90/yr among all 

ventilation systems in every climate (Figure 22). Compared to exhaust ventilation as a reference, 

balanced ERV ventilation (installed such that it does not require coincident AHU fan operation 

to avoid short-circuiting of ventilation air) ranged from $0/yr to $40/yr more, with an average of 

about $20/yr more for 100% of the ASHRAE 62.2-2010 requirement at 3 ACH 50. However, at 

greater mechanical ventilation rates required in the 1.5 ACH50 house, the ERV saved an average 

of $20/yr over exhaust ventilation, in all but the dry climate of Phoenix and the mild climate of 

Los Angeles, where it continued to consume more energy than exhaust. CFI supply ventilation 

ranged from about $11/yr to $93/yr more than exhaust ventilation, with an average of about 

$40/yr. 
29 

Figure 22. Total annual operating cost per year for a 

DOE Challenge Home over a range of ventilation system types

Supplemental dehumidification controlled to 60% RH was predicted to be $10–$58/yr for the 

warm-humid climates of Charleston, Houston, and Orlando, and the marine climate of Los 

Angeles. However, a caveat was provided that indicates that this value is predicated on an 

operating dehumidifier energy factor of 1.47 L/kWh, and recent field data indicate that 

conventional dehumidifiers operate closer to 0.8 L/kWh (Mattison and Korn 2012), which would 

tend to double this cost. Additionally, dehumidifiers tend to operate on a large humidity dead 

band, which means that maintaining humidity below 60% would likely require humidity set 

points near 55%, which could dramatically increase dehumidification costs. Also, with 

unbalanced mechanical ventilation, air change goes up as building leakage goes down from 3.0 

ACH50 to 1.5 ACH50, resulting in more needed supplemental dehumidification. The increase in 

air exchange rate from unbalanced to balanced ventilation was about 24% for the 3.0 ACH50

houses and about 10% for the 1.5 ACH50 houses. 

4.2 Humidity Control Related Conclusions

Hours of elevated indoor RH are a strong function of the selected RH limit and climate. For 

example, in Los Angeles, nearly all of the hours above 60% RH are also below 65% RH, but that 

is not true for Charleston, Houston, and Orlando. Hours above 60% RH during a particular space 

conditioning mode (heating, cooling, floating) was also a strong function of climate. Most of the 

hours significantly above 60% RH occur during floating hours, which occur mostly during fall, 

winter, and spring in Orlando, and sometimes during summer nights and summer extended rainy 

periods when sensible heat gain is lowered and internal moisture generation stays the same.

Mechanical ventilation, operated at the ASHRAE 62.2-2013 rate (about 40% more than 

ASHRAE 62.2-2010), in a 3 ACH50 house, raises the annual median indoor RH by almost 10% 

RH compared to a 7 ACH50 house with approximately the same annual average air exchange but 

without mechanical ventilation in Orlando. However, as seen in Figure 23, most of the RH 

increase due to ventilation is between 60% and 65% RH; the hours between 65% and 75% RH 

mostly remain the same. This indicates that the frequency of extreme RH events, which are of 

greater health/durability concern than median events, is not increased. This also indicates that 

some supplemental dehumidification would be needed in either case to maintain RH below 60%. 

In the 7 ACH50 house, the ventilation rate throughout the year is variable, and dependent on 
30 

weather related driving forces. As seen in Figure 23, in Orlando weak summertime driving forces 

result in little need for supplemental dehumidification during the cooling season. 

Figure 23. ASHRAE 62.2-2013 ventilation rate significantly increases number of 

hours between 60% and 65% RH compared to a conventional dwelling 

without mechanical ventilation using EGUSA

Durability analysis for Orlando, for a standard double-pane window with vinyl frame, showed 

that center-of-glass window condensation problems are highly unlikely to occur because the 

simulations show that it would require wintertime indoor RH of ≥ 75%, which is highly unlikely 

to occur in homes. With a less thermally efficient metal window frame, the condensation risk 

would be greater. 

In Orlando, an ERV nearly eliminated the cooling hours above 60% RH, but according to 

EGUSA it had little effect on the floating hours above 60% RH. ERV modeling using E+ is 

suspect, as it shows too much reduction in elevated indoor humidity hours compared to the 

EGUSA models. The most important way in which the results differ between the simulation 

programs is in the frequency and magnitude of elevated indoor RH. For example, in Orlando, the 

EGUSA results show roughly 2000 hours above 60% RH, while the E+ results are closer to 3000 

hours. In terms of maximum indoor RH, the EGUSA results show about 75% while the E+ 

results show about 85%. However, in all cases, supplemental dehumidification would be 

required to maintain RH below 60% in high performance, low-energy houses, and the difference 

in supplemental dehumidification energy between the simulation models would probably be less 

than 150 kWh/yr or less than $20/yr. 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

<10 <15 <20 <25 <30 <35 <40 <45 <50 <55 <60 <65 <70 <75 <80 <85 <90

Hours per Year

RH Bins

Orlando, 1-Sty; 3 ach50; 100%; North; EXH

Cooling Heating Floating

Cooling = 56.5%

Heating = 51.3%

Floating = 55.8%

0

500

1000

1500

2000

2500

3000

3500

4000

4500

<10 <15 <20 <25 <30 <35 <40 <45 <50 <55 <60 <65 <70 <75 <80 <85 <90

Hours per Year

RH Bins

Orlando; 1-Sty; 7 ach50; 0%; North

Cooling Heating Floating

Cooling = 52.7%

Heating = 49.8%

Floating = 54.9%

0

0.5

1

1.5

2

2.5

3

0 30 60 90 120 150 180 210 240 270 300 330 360

Dehumidification (lb/h)

Day of Year

Orlando, 1-Sty; 3 ach50; 100%; North; EXH

Cooling lb H2O = 320

Heating lb H2O = 18

Float lb H2O = 518

0

0.5

1

1.5

2

2.5

3

0 30 60 90 120 150 180 210 240 270 300 330 360

Dehumidification (lb/h)

Day of Year

Orlando; 1-Sty; 7 ach50; 0%; North

Cooling lb H2O = 33

Heating lb H2O = 17

Float lb H2O = 407
31 

5 Key Gaps and Areas of Ongoing Research

A key gap and area of ongoing research is cost-effective application of ASHRAE Standard 62.2-

2013 with systems and approaches that reduce energy consumption, improve humidity control 

performance, and improve indoor air quality. Identifying methods to achieve such objectives 

would benefit the EPA Indoor airPLUS program required by the DOE Challenge Home. 

Research should be conducted into methods that include considerations for: 

• Managing indoor air pollutants in ways other than outdoor air exchange.

• Quantifying exposure to pollutants, and mitigation of that exposure, over a time scale 

shorter than annual average.

• Accounting for the quality of the source of outdoor air for outdoor environs and different 

ventilation systems types. 

• Accounting for ventilation air distribution effectiveness. 

• Application of demand-based ventilation systems. 

Models that can accurately simulate the performance of advanced ventilation control systems in 

high performance buildings, along with associated humidity control systems in humid climates, 

are needed to understand a wide range of scenarios related to the economics and operational 

success of low-energy homes. Accurate simulation of ventilation and space conditioning control 

systems that operate on sub-hourly time scales coupled with the interacting heat and mass 

transfer effects is complex. It depends on many still somewhat unknown inputs, such as: 

• Internal moisture generation rates 

• Other moisture loads including effects of construction moisture and rain wetting under 

solar loading 

• Building moisture capacitance and the impacts of building material moisture 

adsorption/desorption 

• Detailed space conditioning equipment performance maps. 

This capability is improving, but still has a long way to go to be adequately integrated into 

commonly used building design and performance rating software. Conducting research leading 

to quantification of unknown inputs is an important need. 

Refinement to methods to simulate moisture capacitance is also needed. The simulations 

conducted for this study were conducted using the moisture capacitance (MC) modeling method 

where the indoor MC was set equal to 10 times the indoor air capacitance. A recent study by 

Woods et al. (2013) has examined the three principal methods of representing MC in building 

energy simulation models. They are generically referred to as the MC method, the effective 

moisture penetration depth (EMPD) method and the detailed, coupled heat and moisture 

transport (HAMT) method. Using HAMT as the basis of comparison, the other two methods 

were evaluated by Woods et al. In addition, the authors postulated a second EMPD2 model that 

uses two, instead of one, effective penetration depths. The EMPD2 method was found to have 
32 

significant advantages over the EMPD1 model and the MC model. The second layer in the 

EMPD2 model provides for a longer-term moisture storage capacitance in addition to the short 

term capacitance of the EMPD1 model. This substantially improves estimation accuracy 

compared with both the MC model and the EMPD1 model. The EMPD2 model, like the MC 

model and the EMPD1 model, can be used with conduction transfer functions, rather than 

requiring finite element or finite difference HAMT numerical solutions. This significantly 

reduces simulation times compared to HAMT modeling. 
33 

References

Chandra, S.; Parker, D.; Sherwin, J.; Colon, C.; Fonorow, K.; Stroer, D.; Martin, E.; McIlvaine, 

J.; Chasar, D.; Moyer, N.; Thomas-Rees, S.; Hoak, D.; Beal, D.; Gil, C. (2008). “An Overview of 

Building America Industrialized Housing Partnership (BAIHP) Activities in Hot—Humid 

Climates.” Sixteenth Symposium on Improving Building Systems in Hot and Humid Climates, 

15–17 December 2008, Dallas, TX.

Hendron, B.; Anderson, R.; Barley, D.; Hancock, E. (2006). Building America Field Test and 

Analysis Report. Draft NREL report.

Hendron, R.; Rudd, A.; Anderson, R.; Barley, D.; Townsend, A. (2007). “Field Test of Room-toRoom Distribution of Outside Air with Two Residential Ventilation Systems.” IAQ 2007: 

Healthy & Sustainable Buildings Conference Proceedings. American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc. 

Mattison, L; Korn, D. (2012). “Dehumidifiers: A Major Consumer of Residential Electricity.” 

2012 ACEEE Summer Study on Energy Efficiency in Buildings. The Cadmus Group, Inc. 

www.aceee.org/files/proceedings/2012/data/papers/0193-000291.pdf. 

Rudd, A. (2011). Ventilation Guide—Fully Updated. Building Science Press Somerville, MA. 

September. ISBN-10: 0-9755127-6-5. 

Rudd, A., Lstiburek, J. (2000). “Measurement of Ventilation and Interzonal Distribution in 

Single-Family Homes.” ASHRAE Transactions 106(2):709–718, MN-00-10-3, V. 106, Pt. 2., 

Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Rudd, A.; Henderson, H.; Bergey, D.; Shirey, D. (2013). RP-1449: Energy Efficient and Cost 

Assessment of Humidity Control Options for Residential Buildings. Atlanta, GA: ASHRAE.

Rudd, A.; Henderson, H., Jr. (2007). “Monitored Indoor Moisture and Temperature Conditions 

in Humid Climate U.S. Residences.” ASHRAE Transactions (17, Dallas 2007). Atlanta, GA: 

American Society of Heating Refrigeration and Air-Conditioning Engineers. 

Rudd, A.; Lstiburek, J. (1999). “Design Methodology and Economic Evaluation of Central-FanIntegrated Supply Ventilation Systems.” Indoor Air 5:25–30. Air Infiltration and Ventilation 

Center, Coventry, United Kingdom. 

Rudd, A.; Lstiburek, J. (2001). “Clean Breathing in Production Homes.” Home Energy 

Magazine, May/June, Energy Auditor & Retrofiter, Inc., Berkeley, CA. 

Rudd, A.; Lstiburek, J. (2008). “Systems Research on Residential Ventilation.” Proceedings of 

the 2008 ACEEE Summer Study on Energy Efficiency in Buildings, Pacific Grove, CA, August. 

American Council for an Energy Efficient Economy, Washington, D.C. 

Rudd, A.; Lstiburek, J.; Ueno, K. (2003). “Residential Dehumidification and Ventilation Systems 

Research for Hot-Humid Climates.” Proceedings of 24th AIVC and BETEC Conference, 

Ventilation, Humidity Control, and Energy, Washington, D.C., pp. 355–360. 12–14 October, 

2013. Air Infiltration and Ventilation Centre, Brussels, Belgium. 
34 

Rudd, A.; Lstiburek, J.; Ueno, K. (2005). Residential Dehumidification Systems Research for 

Hot-Humid Climates. Golden, CO: National Renewable Energy Laboratory. NREL/SR-550-

36643. www.nrel.gov/docs/fy05osti/36643.pdf. 

Rudd, A. Walker, I. (2007). Whole House Ventilation System Options—Phase 1 Simulation 

Study. Building Science Corporation, ARTI Report No. 30090-01, Final Report, March. 

Arlington, VA: Air-Conditioning and Refrigeration Technology Institute. 

Shirey, D.B.; Henderson, H.I.; Raustad, R. (2006). Understanding the Dehumidification 

Performance of Air-Conditioning Equipment at Part-Load Conditions. Final Report, FSEC-CR1537-05, Cocoa: Florida Solar Energy Center. 

Townsend, A.; Rudd, A.; Lstiburek J. (2009a). “Extension of Ventilation System Tracer Gas 

Testing Using a Calibrated Multi-Zone Airflow Model.” ASHRAE Transactions 115(2). 

Townsend, A.; Rudd, A.; Lstiburek J. (2009b). “A Method for Modifying Ventilation Airflow 

Rates to Achieve Equivalent Occupant Exposure.” ASHRAE Transactions 115(2). 

Weisel, C.P.; Zhang, J.; Turpin, B.J.; Morandi, M.T.; Colome, S.; Stock, T.H.; Spektor, D.M.; 

Korn, L.; Winer, A.; Alimokhtari, S.; Kwon, J.; Mohan, K.; Harrington, R.; Giovanetti, R.; Cui, 

W.; Afshar, M.; Maberti, S.; Shendell, D. (2005). “Relationship of Indoor, Outdoor and Personal 

Air (RIOPA) Study.” Journal of Exposure Analysis and Environmental Epidemiology Mar; 

15(2):123-37. Environmental and Occupational Health Sciences Institute, Piscataway, NJ. 

Willem, H.; Hult, E.; Hotchi, T.; Russel, M.; Maddalena, R.; Singer, B. (2013). Ventilation 

Control of Volatile Organic Compounds in New U.S. Homes: Results of a Controlled Field Study 

in Nine Residential Units. LBNL-6022E. 

Woods, J.; Winkler J.; Christensen, D. (2013). Evaluation of the Effective Moisture Penetration 

Depth (EMPD) Model for Estimating Moisture Buffering in Buildings.” National Renewable 

Energy Laboratory, Golden, CO. NREL/TP-5500-57441. 
buildingamerica.gov

DOE/GO-102014-4290 ▪ January 2014

Printed with a renewable-source ink on paper containing at 

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