Expert Meeting: 

Recommended Approaches 

to Humidity Control in High 

Performance Homes

Armin Rudd

Building Science Corporation (BSC)

July 2013
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iii

Expert Meeting: Recommended Approaches to Humidity Control in 

High Performance Homes

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: 

Armin Rudd 

Building Science Corporation 

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Engebrecht 

Prepared under Subcontract No. KNDJ-0-40337-00 

July 2013 
iv

Acknowledgements 

The author appreciates the presentations made by the invited speakers including: Hugh 

Henderson of CDH Energy Corp.; Philip Fairey of Florida Solar Energy Center; and Michael 

Sypolt of IBACOS.

Definitions

BSC Building Science Corporation 

AHRI Air Conditioning Heating and Refrigeration Institute

AHAM Association of Home Appliance Manufacturers

RH Relative humidity

NRC Natural Resources Canada 

USDOE United States Department of Energy

NREL National Renewable Energy Laboratory 

CFI Central-fan-integrated [supply ventilation] 

ERV Energy recovery ventilator 

HRV Heat recovery ventilator
v 

[This page left blank]
vi 

Table of Contents

Acknowledgements ................................................................................................................................... iv

Definitions ................................................................................................................................................... iv

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................ viii

1 Introduction ........................................................................................................................................... 1

2 Logistics ................................................................................................................................................ 2

3 Research Questions............................................................................................................................. 2

4 Objectives.............................................................................................................................................. 3

5 Agenda ................................................................................................................................................... 4

6 Agenda Summaries .............................................................................................................................. 4

ASHRAE RP 1449: Energy Efficiency and Cost Assessment of Humidity Control Options 

for Residential Buildings .....................................................................................................4

Humidity Control and Ventilation ...........................................................................................17

EnergyPlus Humidity Control and Ventilation Modeling Analysis ........................................25

7 Answers to Research Questions ...................................................................................................... 29

8 Action Items ........................................................................................................................................ 32

9 References .......................................................................................................................................... 34
vii 

List of Figures

Figure 1. Computer modeling improvements allowing more realistic simulation results. ................. 5

Figure 2. Simulation framework that allows for multiple temperature control, humidity control, 

ventilation, and fan recirculation systems to operate at the same time......................................... 5

Figure 3. Simulation framework that allows for multiple temperature control, humidity control, 

ventilation, and fan recirculation systems to operate at the same time......................................... 6

Figure 4. Calculation of the combined airflow of infiltration, duct leakage, and balanced and 

unbalanced ventilation......................................................................................................................... 7

Figure 5. Conventional cooling system and indoor temperature and RH response with heating, 

floating, and cooling hours in the same day. .................................................................................... 7

Figure 6. RH limit expressed as hours over 60% RH and 4 hour events over 60% RH. ...................... 8

Figure 7. Impact of exhaust ventilation rate at 50%, 100% and 150% of the ASHRAE Standard 62.2-

2010 whole-building ventilation rate. ................................................................................................. 9

Figure 8. Simulation results for enhanced Systems modeled in HERS 70 level houses. ................. 10

Figure 9. A check of the simulation response with moisture capacitance turned off (set to zero). . 11

Figure 10. Measured and simulated indoor vs. outdoor humidity trends with indoor moisture 

capacitance. ........................................................................................................................................ 12

Figure 11. The impact of overcooling by three degrees “sweeps” the higher RH hours down and to 

the left.................................................................................................................................................. 12

Figure 12. Locating the air distribution system ducts inside conditioned space saves overall 

energy, but also increases the need for supplemental dehumidification. ................................... 13

Figure 13. In Miami, hours of elevated indoor humidity occur mostly in morning hours between 

late November and March.................................................................................................................. 14

Figure 14. Hours of elevated indoor humidity is strongly related to internal moisture gains and 

internal sensible gains....................................................................................................................... 15

Figure 15. Hours of elevated indoor humidity is weakly related to moisture capacitance. .............. 15

Figure 16. Hours of elevated indoor humidity is strongly related to heating setpoint...................... 16

Figure 17. Interactive Internet access is made available to retrieve customized sets of the 

simulation data. .................................................................................................................................. 16

Figure 18. Twelve cities and five climate zones simulated for a ventilation study including 

humidity impacts. ............................................................................................................................... 17

Figure 19. Outline of simulation prototype assumptions. .................................................................... 18

Figure 20. Internal sensible (16.9 kWh/day) and latent gains schedule used in the simulations..... 19

Figure 21. For a week at the end of June and beginning of July on Orlando, the cooling system 

and space RH is showing an expected response. .......................................................................... 20

Figure 22. Hours above 60% RH for exhaust and ERV ventilation in Orlando. .................................. 20

Figure 23. Hours above 60% by climate and space conditioning mode. ............................................ 21

Figure 24. Hours of elevated indoor RH is a strong function of the selected RH limit and climate. 22

Figure 25. ASHRAE 62.2-2010 addendum r ventilation rate raises median RH compared to 

conventional dwelling without mechanical ventilation. ................................................................. 22

Figure 26. Predicted required moisture removal by supplemental dehumidification, by ventilation 

system type (energy recovery ventilator, exhaust, CFI supply).................................................... 23

Figure 27. Predicted cost of supplemental dehumidification............................................................... 24

Figure 28. NREL BEopt E+ program used to generate the building geometry and the base IDF files 

(input description). ............................................................................................................................. 25

Figure 29. Indoor RH vs. hour of year comparison between EnergyPlusPlus (blue symbols) and 

EnergyGauge USA (V3.0.01P) simulation results. .......................................................................... 26

Figure 30. Energy consumption results by ventilation system type, ventilation rate, and building 

enclosure tightness............................................................................................................................ 27

Figure 31. An ERV reduces the number of hours with lowest and highest indoor RH, and a very 

large reduction in hours above 60% RH compared to the FSEC and CDH models. .................... 28

Figure 32. Window condensation potential during Orlando floating hours ....................................... 29

Figure 33. Overview of recommended dehumidifier test conditions. ................................................. 33

Unless otherwise noted, all figures were created by BSC. 
viii

List of Tables 

Table 1. List of meeting participants......................................................................................................... 2

Table 2. Expert Meeting Agenda................................................................................................................ 4

Unless otherwise noted, all tables were created by BSC.
1 

1 Introduction

As homes get more energy efficient, cooling systems have longer off-times due to lowered 

sensible heat gain. Overall, this is good, and produces a 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 will be required to maintain the relative humidity (RH) below 

acceptable levels.

Extensive field testing was done with builder partners in Texas and Florida in 2001 to 2007 

(Rudd et al. 2003, BSC 2004, Rudd et al. 2005). That testing revealed that supplemental 

dehumidification was required in high performance Building America-level homes in order to 

maintain indoor RH below 60% year-round. 

Off-the-shelf, stand-alone supplemental dehumidification systems were employed to address this 

problem while working with manufacturing partners on supplemental dehumidification 

integrated with the central space conditioning system. These companies began to offer integrated 

supplemental dehumidification solutions that allow year-round indoor RH control between 50% 

and 60%. This was intended to enable further reduction in sensible cooling load. through further 

efficiency improvements, without the risk of elevated indoor humidity. 

While these advancements have been important and needed in the residential space conditioning 

industry, supplemental dehumidification technology continues to improve and evolve, and the 

market for these products is still in its infancy. Design capacity prediction is subject to many 

unknowns and requires continued research to fully quantify. 

Models that can accurately simulate the performance of humidity control systems in high 

performance buildings are needed to understand a wide range of scenarios related to the 

economics and operational success of low-energy homes with supplemental dehumidification. 

Simulation of supplemental dehumidification needs and performance in buildings is complex. It 

requires a model that controls both indoor temperature and RH, and depends on many still 

somewhat sketchy inputs, such as: internal moisture generation and other moisture loads

including effects of construction moisture drying and rain wetting under solar loading; building 

moisture capacitance and the effects of building material moisture adsorption/desorption under 

solar loading; and detailed dehumidification 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 programs. 

Building Science Corporation (BSC) hosted an expert meeting, “Recommended Approaches to 

Humidity Control in High Performance Homes,” on October 16, 2012, in Westford, 

Massachusetts, to bring together experts in the field of residential humidity control to address 

modeling issues for dehumidification. 
2 

2 Logistics 

The expert meeting was held at the Westford Regency Inn and Conference Center, in Westford, 

Massachusetts, on October 16, 2012. The meeting was also made available to attendees by 

webinar, and was recorded. See Table 1 for a list of attendees.

Table 1. List of Meeting Participants. 

 

3 Research Questions

A list of research questions that were compiled and distributed with the meeting invitation is as 

follows: 

• What are the important humidity control conclusions from recent residential 

dehumidification systems modeling, with a wide range of building and equipment 

sensitivity, using a customized <2 minute time-step TRNSYS model with temperature 

and humidity control? 

• What are the important humidity control conclusions from recent residential ventilation 

modeling efforts using the temperature and humidity control logic of the Energy Plus 

version of the NREL BEopt program? 

• What are the important indoor humidity conclusions that can be drawn from recent 

residential ventilation modeling efforts using the temperature-only control logic of the 

FSEC EnergyGauge USA program? 

Last name First name Company Email

Attended In Person (P) 

or by Webinar (W)

Bergey Daniel BSC daniel@buildingscience.com P

Bloemer John Research Products jb@aprilaire.com P

Fairey Philip FSEC pfairey@fsec.ucf.edu P

Harriman Lew Mason Grant lewharriman@masongrant.com P

Henderson Hugh CDH Energy hugh@cdhenergy.com P

Kerrigan Philip BSC phil@buildingscience.com P

Metzger Cheryn NREL Cheryn.Metzger@nrel.gov P

Pettit Betsy BSC betsy@buildingscience.com P

Prahl Duncan IBACOS dprahl@ibacos.com P

Rudd Armin BSC arudd@buildingscience.com P

Sypolt Michael IBACOS msypolt@ibacos.com P

Winkler John NREL Jon.Winkler@nrel.gov P

Cottrell Glenn IBACOS gcottrell@ibacos.com W

Dentz Jordan The Levy Partnership jdentz@research-alliance.org W

Griffiths Dianne Steven Winter Associates dgriffiths@swinter.com W

Grisolia Anthony IBACOS agrisolia@ibacos.com W

Hudon Kate NREL kate.hudon@nrel.gov W

Mittereder Nick Ibacos nmittereder@ibacos.com W

Puttagunta Srikanth Steven Winter Associates sri@swinter.com W

Tabares Paulo NREL paulo.tabares@nrel.gov W
3 

• How do the results from these different programs compare?

• Is it generally agreed that controlling to less than 60% RH is the appropriate humidity 

control point for high performance homes, and why? 

• Is it generally agreed that annual hours above 60% RH is the appropriate humidity 

control performance metric to use to compare system performance and to compare 

required supplemental dehumidification energy? Does that metric give generally the same 

result as looking at 4-hour and 8-hour events above 60% RH?

• For a range of climates, ventilation systems, and space conditioning equipment in high 

performance homes, what is the magnitude of hours above 60% RH, the magnitude of 

supplemental dehumidification energy required to control to less than 60% RH, the time 

of year occurrence of elevated indoor humidity and supplemental dehumidification, and 

the space conditioning mode (heating, cooling, floating) during which most periods of 

elevated indoor humidity and supplemental dehumidification occur? 

4 Objectives 

The topic of this meeting was “Recommended Approaches to Humidity Control in High 

Performance Homes,” and focused on dehumidification requirements. Presentations and 

discussions centered on computer simulation and field experience with these systems, with the 

goal of developing foundational information to support the development of a Building America 

Measure Guideline on this topic.

The expert meeting was designed to bring together experts in the field of residential indoor 

humidity control by dehumidification to discuss the following objectives: 

• Compare and contrast the state-of-the art in modeling residential supplemental 

dehumidification requirements in high performance homes. 

• Come to agreement on an acceptable RH control system within a model and the primary 

metric to be used for evaluating the success of a given humidity control system. 

• Quantitatively identify general trends as to the performance of, and cost to operate, 

supplemental dehumidification systems in high performance homes in humid climates.
4 

5 Agenda

Table 2 shows the expert meeting agenda. 

Table 2. Expert Meeting Agenda.

Time Speaker Topic

8:00 to 8:10 Armin Rudd

Building Science Corp

Welcome and Introduction

8:10 to 9:00 Hugh Henderson

CDH Energy Corp

ASHRAE RP 1449

Energy Efficiency and Cost Assessment of 

Humidity Control Options for Residential Buildings

9:00 to 9:50 Philip Fairey

Florida Solar Energy Center

Humidity Control and Ventilation

9:50 to 10:00 Break

10:00 to 10:50 Michael Sypolt and Duncan 

Prahl

IBACOS

EnergyPlus Humidity Control and Ventilation Modeling 

Analysis

10:50 to 12:00 Armin Rudd

Building Science Corp

Facilitated Open Discussion, Action Items, and Final 

Remarks

6 Agenda Summaries

Discussions occurred during and after each of the three invited presentations and in the 

facilitated discussion period that followed. 

ASHRAE RP 1449: Energy Efficiency and Cost Assessment of Humidity Control 

Options for Residential Buildings

(Hugh Henderson, CDH Energy Corp.) 

A number of fundamental computer modeling improvements were made to the 

TRNSYS/TRNSED model that CDH had previously developed for desiccant and dehumidifier 

systems. The model improvements, listed in Figure 1, allowed for more accurate, real-world 

simulation results. The short time-step model allowed precise determination of when mechanical 

equipment would be on or off according to actual control strategies. Referring to Figure 2, space 

conditioning equipment, ventilation equipment, and dehumidification equipment could all be 

operating at the same time, with all the heat and moisture consequences accounted for in the 

same simulation time interval.
5 

Modeling Approach

• Using TRNSYS model first developed for 

Desiccant and DH systems 

• Uses small time step (72 sec)

– No more part load curves (either ON or OFF)

– Thermostat dynamics (for cooling)

– Detailed control of all components (e.g., CFIS)

• Type 56 for multi-zone building

– Combined duct leakage & infiltration calcs

• Robust equipment models (performance 

maps)

Figure 1. Computer modeling improvements allowing more realistic simulation results. 

Updated Simulation Framework

Figure 2. Simulation framework that allows for multiple temperature control, humidity control, 

ventilation, and fan recirculation systems to operate at the same time. 
6 

Moisture evaporation from wet cooling coils is modeled to realistically account for cooling and 

dehumidification system latent degradation (moisture added back to the ducts and conditioned 

space) when the cooling compressor is off. 

COIL1_TEST_4B_10B_16B_22B 08/30/02 07:42:04 Cycle #1 (Comp ON time: 45.0 minutes)

0 20 40 60 80 100

time (minutes)

-20

-10

0

10

20

30

Capacity (MBtu/h)

 Compressor

 Sensible

 Latent Addition

 Latent Removal

Moisture Evaporation Models

• Some degradation in AUTO fan mode 

– Off-cycle airflow is assumed to be ~3 cfm

• Significant degradation when fan is ON 

DOE Sponsored project 

in 2003-2006

FSEC-CR-1641-06

Figure 3. Simulation framework that allows for multiple temperature control, humidity control, 

ventilation, and fan recirculation systems to operate at the same time. 

The combined airflow of infiltration, duct leakage, and balanced and unbalanced ventilation is 

accounted for, as shown in Figure 4. 

Figure 5 illustrates the conventional cooling system and indoor temperature and RH response 

with heating, cooling, and floating hours in the same day. Floating hours are times when no 

space conditioning is active because the house remains between the heating and cooling control 

setpoints. For the warm-humid climate, the heating and cooling thermostat setpoints were 73o

F 

and 78o

F). The expected space conditioning equipment operation, and the temperature and RH 

response, is shown. 
7 

Integrated Infiltration, Ventilation 

and Duct Leakage

• Currently integrate infiltration and 

ventilation (balanced, unbalanced) 

cfmin = Vent inlet + return duct leaks 

cfmout = Exhaust + Supply duct leaks 

cfmbalanced = MIN(cfmin, cfmout)

cfmunbalanced = MAX(cfmin, cfmout) - cfmbalanced

cfminf = infiltration flow calculated for 

building for the timestep

cfmcombined = MAX(cfmunbalanced , cfminf + 0.5* cfmunbalanced )

+ cfmbalanced

Figure 4. Calculation of the combined airflow of infiltration, duct leakage, and balanced and 

unbalanced ventilation. 

Conv AC

(System 1)

z1h100s1rh50v1

22: 0: 2: 4: 6: 8: 10: 12: 14: 16: 18: 20: 22: 0:

22 23 24

January

0

5

10

15

 Heat

 Cool

 Dehum

 Vent

 AHU

 DFan

 VDmpr

 EFan

 HRV

z1h100s1rh50v1

22: 0: 2: 4: 6: 8: 10: 12: 14: 16: 18: 20: 22: 0:

22 23 24

70

72

74

76

78

Space Temp (F) 22: 0: 2: 4: 6: 8: 10: 12: 14: 16: 18: 20: 22: 0:

22 23 24

52

54

56

58

60

62

64

Space RH (%)

Figure 5. Conventional cooling system and indoor temperature and RH response with heating, 

floating, and cooling hours in the same day. 
8 

Hrs Over 

60%

No of 

4 hr 

Events 

-

200 

400 

600 

800 

1,000 

1,200 

1,400 

1,600 

1,800 

Z0-Orlando Z1-Miami Z2-Houston Z3-Atlanta Z4-Nashville Z5-Indianapolis

Hours Above 60% RH 

System 1 - Exh Fan (V1)

HERS50 HERS70 HERS85 HERS100 HERS130 HERS130 No Vnt

-

10 

20 

30 

40 

50 

60 

70 

Z0-Orlando Z1-Miami Z2-Houston Z3-Atlanta Z4-Nashville Z5-Indianapolis

No of Events > 4 hrs 

System 1 - Exh Fan (V1)

HERS50 HERS70 HERS85 HERS100 HERS130 HERS130 No Vnt

Figure 6. RH limit expressed as hours over 60% RH and 4 hour events over 60% RH. 

A number of references (ASHRAE Standard 55-2010, Balaras and Balaras 2007, Wolkoff and 

Kjaergaard 2007) refer to indoor RH between 30% and 60% as comfortable, healthy, and 

recommended for human occupancy. In Rudd and Henderson 2007, based on measured data 

from over 40 sites, an analysis approach was presented whereby hours over 60% RH and 4-hour 

events over 60% RH were examined. The logic was that if many of the hours over 60% RH 

occurred sporadically, and did not accumulate to long events, then perhaps both ways of 

characterizing unacceptably elevated indoor humidity would be needed. It was shown that hours 

over 60% RH and 4-hour events over 60% RH had essentially the same result profile and gave 

essentially the same answer. That was because most hours over 60% RH did not occur 

sporadically, but in lumped time periods. A mostly similar comparison was found when 

extending the analysis to 8-hour events over 60% RH. Referring to Figure 6, the same result was 

found in the computer simulations presented here. 

Figure 7 shows a sensitivity of ventilation rate for a house rated at a HERS Index of 70 with 

exhaust ventilation and no cooling system dehumidification enhancements or supplemental 

dehumidification. The ventilation rate was simulated at 50%, 100%, and 150% of the ASHRAE 

Standard 62.2-2010 rate. Total space conditioning energy cost typically went down about 10% 

when the ventilation rate was reduced by 50%, and went up about 10% when the ventilation rate 

was increased by 50%. However, for the hot-humid climates of Miami, Orlando, and Houston, 

the hours where indoor RH was above 60% varied much more, and sometimes in different 

directions. In Orlando, reducing the ventilation rate increased the hours above 60% RH by 72 

hours because of less drying potential in wintertime, but increasing the ventilation rate increased 

the hours above 60% RH by 252 hours. In both Miami and Houston, reducing the ventilation rate 
9 

reduced the hours above 60% RH by 63% and 23%, respectively, and increasing the ventilation 

rate increased the hours above 60% RH by 76% and 35%, respectively. 

Impact of Ventilation Rate 

HERS70

Exh Vent Hours 

Above 

60% RH 

 AC 

Runtime 

(hrs) 

 AC EER 

(Btu/Wh) 

 AC 

Energy 

(kWh) 

 Htg 

Energy 

(kWh) 

 AHU Fan 

Energy 

(kWh) 

 Exh 

Fan 

Energy 

(kWh) 

 HRV 

Energy 

(kWh) 

 Total 

Electric 

w/o HT 

(kWh) 

 Total 

Costs w 

Furnace 

($) 

50% Vent 678 1,774 16.6 3,093 554 594 - 91 3,778 419 91%

100% Vent 606 1,851 16.7 3,270 686 622 - 203 4,095 460 100%

150% Vent 858 1,915 16.8 3,419 862 646 - 305 4,370 500 109%

50% Vent 159 2,981 16.6 4,181 25 783 - 91 5,055 512 93%

100% Vent 433 3,122 16.7 4,430 42 820 - 203 5,453 554 100%

150% Vent 763 3,247 16.7 4,647 55 853 - 305 5,805 590 107%

50% Vent 164 2,307 16.3 3,241 1,792 635 - 91 3,967 411 92%

100% Vent 214 2,391 16.4 3,403 2,045 661 - 203 4,267 446 100%

150% Vent 288 2,461 16.5 3,537 2,522 687 - 305 4,529 488 109%

50% Vent 27 1,525 16.5 2,098 4,126 468 - 91 2,657 512 91%

100% Vent 15 1,558 16.6 2,167 4,612 485 - 203 2,855 560 100%

150% Vent 40 1,577 16.6 2,214 5,251 500 - 305 3,019 614 110%

Orlando

Miami

Houston

Atlanta

Figure 7. Impact of exhaust ventilation rate at 50%, 100% and 150% of the ASHRAE Standard 62.2-

2010 whole-building ventilation rate. 

A sensitivity comparing a conventional cooling system in a HERS 70 level house with cooling 

systems modified to increase latent capacity and a 50% RH setpoint is shown in Figure 8. 

Enhanced AC represents three degrees of overcooling and lowered airflow (210 cfm/ton vs. 375 

cfm/ton). This showed a reduction in hours over 60% RH of about 95% in Miami, 75% in 

Houston, and 50% in Orlando. However, field experience has shown that three degrees of 

overcooling (cooling to three degrees below the requested setpoint) causes comfort complaints. 

Note that the total space conditioning system operating cost is not directly comparable between 

the conventional system and the enhanced system because the enhanced system was modeled 

with a more efficient cooling system, as typically dictated in the market for systems with variable 

airflow.

Heat pipes (passive system for pre-cooling air entering the evaporator and reheating air leaving 

the evaporator) show a reduction in hours over 60% RH of about 90% in Miami, 60% in 

Houston, and 30% in Orlando. Due to the increased static pressure and associated increase in fan 

energy consumption associated with the heat pipe system, the total space conditioning system 

energy consumption and operating cost increased by 20%-25%.
10 

With an indoor RH setpoint of 50%, the Humiditrol system by Lennox (employing overcooling 

by three degrees and refrigerant subcooling reheat) was able to control indoor RH below 60% in 

Miami and Houston, but still showed 32 hours above 60% RH in Orlando. Those hours occurred 

when overcooling had reached its limit. 

Enhanced Systems

HERS70

 Hours 

Above 

60% RH 

 AC 

Runtime 

(hrs) 

 AC EER 

(Btu/Wh) 

 AC 

Energy 

(kWh) 

 Htg 

Energy 

(kWh) 

 AHU Fan 

Energy 

(kWh) 

 Exh Fan 

Energy 

(kWh) 

 Total 

Electric 

w/o HT 

(kWh) 

 Total 

Costs w 

Furnace 

($) 

Conv AC 606 1,851 16.7 3,270 686 622 203 4,095 460

Enhanced AC 308 3,826 20.8 2,756 699 196 203 3,155 366

AC w/ HPs 418 2,311 16.0 3,799 689 1,126 203 5,128 565

Lennox Humiditrol 32 4,000 17.0 3,374 703 265 203 3,843 436

Conv AC 433 3,122 16.7 4,430 42 820 203 5,453 554

Enhanced AC 20 5,721 19.9 3,772 48 339 203 4,315 439

AC w/ HPs 46 3,846 16.0 5,064 42 1,482 203 6,749 685

Lennox Humiditrol - 5,686 16.9 4,413 42 432 203 5,048 513

Conv AC 214 2,391 16.4 3,403 2,045 661 203 4,267 446

Enhanced AC 53 4,016 19.3 2,914 2,045 342 203 3,459 378

AC w/ HPs 93 2,918 15.9 3,837 2,044 1,157 203 5,197 525

Lennox Humiditrol - 4,061 17.3 3,233 2,039 380 203 3,817 408

Conv AC 15 1,558 16.6 2,167 4,612 485 203 2,855 560

Enhanced AC 15 2,619 19.7 1,819 4,594 270 203 2,292 501

AC w/ HPs

Lennox Humiditrol - 2,662 19.2 1,879 4,595 274 203 2,356 508

Miami

Houston

Atlanta

Orlando

Figure 8. Simulation results for enhanced Systems modeled in HERS 70 level houses. 
11 

Turn off capacitance, Check Gains

0.000 0.005 0.010 0.015 0.020

Outdoor Humidity (lb/lb)

0.000

0.005

0.010

0.015

0.020 Indoor Humidity (lb/lb)

∆w = Qinternal = 0.33 lb/h 

ρ x 60 x cfm 0.075 x 60 x 58 cfm

Figure 9. A check of the simulation response with moisture capacitance turned off (set to zero). 

Figure 9 provides a graphical check of the expected moisture modeling result with moisture 

capacitance turned off (set to zero). All values to the right of the solid line represent hours where 

moisture was being removed from the indoor air by the cooling system. All values between the 

dotted and solid lines represent hours where cooling was not active and internal moisture 

generation pushed the indoor humidity up beyond the outdoor humidity. If there was only 

ventilation and no internal moisture generation, the values would follow the solid line. Values to 

the left of the dotted line would be indicative of moisture capacitance, as shown in Figure 10 

with measured and simulated indoor vs. outdoor humidity trends with indoor moisture 

capacitance. 

A graphic illustration of the impact of overcooling is shown in Figure 11. Comparing the two 

plots, one can see how overcooling by three degrees moves or “sweeps” the higher RH hours 

down and to the left. 
12 

Comparing Humidity Trends

2010

0.000 0.005 0.010 0.015 0.020 0.025

Outdoor Humidity (lb/lb)

0.000

0.005

0.010

0.015

0.020

0.025 Indoor Humidity (lb/lb)

0.000 0.005 0.010 0.015 0.020 0.025

Outdoor Humidity (lb/lb)

0.000

0.005

0.010

0.015

0.020

0.025 Indoor Humidity (lb/lb)

Measured Simulated

∆w ~ 0.002 lb/lb

Figure 10. Measured and simulated indoor vs. outdoor humidity trends with indoor moisture 

capacitance. 

Space Conditions

Daily Indoor Space Conditions: z1h100s1rh50v1

60 65 70 75 80 85 90

Dry Bulb Temperature (F)

0.000

0.005

0.010

0.015

0.020

0.025 Humidity Ratio (lb/lb)

40% 

50% 

60% 

70% 

80% 

Hours Above 50% = 5361

Hours Above 55% = 2399

Hours Above 60% = 1303

Hours Above 65% = 543

Hours Above 70% = 107

All Hrs

Cooling Hrs

Impact of 

Overcooling (S2) in Miami

Figure 11. The impact of overcooling by three degrees “sweeps” the higher RH hours down and to 

the left. 

Locating the air distribution system ducts inside conditioned space saves energy overall, but, 

with the reduced sensible cooling load, also comes an increased need for supplemental 

dehumidification. This impact is illustrated in Figure 12, where ducts in the attic were modeled 
13 

with 5% leakage (60% supply side, 40% return side). For the HERS 70 and HERS 50 efficiency 

levels, moving the ducts inside conditioned space increases the hours above 60% RH indoors by 

25%-50%. A point of discussion in the meeting was that this difference may not be so large if the 

model were to account for the moisture desorption from wood framing materials that typically 

increase the attic humidity ratio over that of the outdoors during the late morning to early 

afternoon hours in warm-humid climates. In that case, return duct leakage would increase the 

moisture source for the ducts-in-attic configuration. This will be addressed in a simplified way 

by running an additional sensitivity with the attic dewpoint temperature being forced to 10oF 

over outdoors from 10 am to 1 pm between May 15 and October 15. However, this may not 

change the result that much since examining Figure 13 shows that, in Miami, for example, most 

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

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

RH. 

Location of Ducts

-

200 

400 

600 

800 

1,000 

1,200 

Z0-Orlando Z1-Miami Z2-Houston Z3-Atlanta Z4-Nashville Z5-Indianapolis

Hours Above 60% RH 

Location of Ducts

In Attic In Space In Attic In Space

HERS 50 HERS 70

Figure 12. Locating the air distribution system ducts inside conditioned space saves overall 

energy, but also increases the need for supplemental dehumidification. 
14 

Shade Plot of Humidity Bins

z1h100s1rh50v1

Day (MAX/MIN = 5.00/ 0.00 )

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

2006

0

2

4

6

8

10

12

14

16

18

20

22

24

Hour of Day

Shades of Gray

0 – below 55% 

1 – 55-60% 

2 – 60-65% 

3 – 65-70% 

4 – 75-80% 

5 – above 80% 

Figure 13. In Miami, hours of elevated indoor humidity occur mostly in morning hours between 

late November and March. 

As shown in Figure 14, internal moisture generation (latent gain) and internal heat generation 

(sensible gain) strongly impact 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. Internal sensible heat generation of 21 

kWh/day is typical for older homes with less efficient lighting and appliances, while 15 kWh/day 

is more typical of high performance homes with compact fluorescent or LED lighting and 

ENERGY STAR® rated appliances. Similar to ducts in hot attics, very high internal sensible heat 

gain drives the cooling system to operate more often and for longer runtimes, reducing indoor 

humidity. 

Moisture capacitance does smooth hourly fluctuations in indoor humidity but it turns out that it 

has a relatively weak impact on hours of elevated indoor humidity (Figure 15). In Figure 15, the 

15x, 30x, and 45x designations refer to internal moisture mass of 15, 30, and 45 times the 

moisture mass held in the house air volume alone. Moisture capacitance has its greatest impact 

when the cooling system is operating enough to intermittently drive a significant difference in 

indoor humidity, but that is not when most hours of elevated indoor humidity occur. Most of 

those hours occur when the space is floating between the cooling and heating setpoints. In that 

case, referring to Figure 16, raising the heating setpoint has a significant impact on reducing 
15 

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 & Sensible Gains

-

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

10.7 kWh/day 21.3 kWh/day 32 kWh/day

Hours Above 60% RH 

Orlando: Moisture & Sensible Gains

6 lb/day 12 lb/day 18 lb/day 24 lb/day

Figure 14. Hours of elevated indoor humidity is strongly related to internal moisture gains and 

internal sensible gains. 

Moisture Gains & Capacitance

-

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

15x 30x 45x

Hours Above 60% RH 

Orlando: Moisture Gains & Capacitance

6 lb/day 12 lb/day 18 lb/day 24 lb/day

Figure 15. Hours of elevated indoor humidity is weakly related to moisture capacitance. 
16 

Sensible & Heating Set Pts

-

500 

1,000 

1,500 

2,000 

2,500 

3,000 

10.7 kWh/day 21.3 kWh/day 32 kWh/day

Hours Above 60% RH 

Orlando: Sensible Gains & Htg Set Pts

70F 72F 74F

Figure 16. Hours of elevated indoor humidity is strongly related to heating setpoint. 

Figure 17 shows how these data are being made available on the Internet via an interactive web 

application where the user can choose simulation inputs and outputs to suit a particular interest. 

Web Access to Results

cloud.cdhenergy.com/rp1449

Figure 17. Interactive Internet access is made available to retrieve customized sets of the 

simulation data. 
17 

Humidity Control and Ventilation

(Philip Fairey, Florida Solar Energy Center) 

For a Building America ventilation white paper study with an important focus on indoor 

humidity impacts, twelve U.S. cities and five climate zones (Figure 18) were simulated using a 

customized version of the FSEC EnergyGauge USA (V3.0.01P) computer program, based on the 

DOE2 hourly simulation engine. Some of the custom routines were designed to improve the 

modeling of indoor humidity while using the temperature-only control capability of DOE2. 

Lumped moisture capacitance was modeled as 10 times the air mass capacitance. Latent 

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

cycles was not modeled.

7 | Building America Partnership for Improved Residential Construction ba-pirc.org

Warm-Humid Zone Orlando, FL

Warm-Humid Zone Houston, TX

Warm-Humid Zone Charleston, SC

Mixed-Humid Zone Baltimore, MD

Mixed-Humid Zone Kansas City, MO

Mixed-Humid Zone Charlotte, NC

Cold-Humid Zone Minneapolis, MN

Cold-Humid Zone Chicago, IL

Dry Zone Phoenix, AZ

Dry Zone Denver, CO

Marine Zone Los Angeles, CA

Marine Zone Seattle, WA

Selected Climates

2 archetypes * 3 vent rates * 2 leakages * 2 orientations 

* 3 vent systems * 12 climates = 864 simulation runs

Figure 18. Twelve cities and five climate zones simulated for a ventilation study including 

humidity impacts. 

A detailed set of simulation prototype assumptions were laid down to facilitate identical inputs 

for the FSEC and IBACOS simulations. Figure 19 lists many of those assumptions. The HERS 

Index of the configured home (which varied some by climate) was in the mid to upper 50’s, 

falling between the HERS 50 and HERS 70 homes of the CDH simulations. The heating and 

cooling thermostat setpoints were shifted 2 degrees lower for the FSEC and IBACOS 

simulations (71°F and 76oF) which would tend to decrease the hours above 60% RH at the edge 

of cooling demand and increase the hours above 60% RH at the edge of heating demand. 
18 

3 | Building America Partnership for Improved Residential Construction ba-pirc.org

• Two home archetypes:

– 2000 ft2, 1-story, 3-bedroom, slab-on-grade, frame 

– 2400 ft2, 2-story, 3-bedroom, slab-on-grade, frame 

• Both homes:

– 15% window-floor-area ratio 

– DOE Challenge Home qualification

• HERS Index: mid to upper 50s

• Duct system and AHU located in conditioned space with 

zero leakage

• Thermostat set points: heating = 71F; cooling = 76F

• Mechanical ventilation is continuous (every hour)

• Window opening for natural ventilation is not allowed

Archetype Homes

4 | Building America Partnership for Improved Residential Construction ba-pirc.org

• Two enclosure leakage rates:

– 3 ach50 (tight)

– 1.5 ach50 (very tight)

• Best case and worst case window orientations

• Best case window area distribution:

– N & S window area = 35% of total

– E & W window area = 15% of total

• Worst case is rotated 90o

• 16” roof overhang on all sides of hip roof with vented 

attic

• Infiltration heights (requires modification of DOE-2)

– 1-story = 9 feet

– 2-story = 17.5 feet

Other Archetype Assumptions

Figure 19. Outline of simulation prototype assumptions. 

Internal moisture and sensible heat gains were modeled according to the magnitude and schedule 

provided in Figure 20. The daily total sensible gain (16.9 kWh) was 20% less than that used in 

the CDH Energy simulations (21.3 kWh/day), which, according to Figure 14, would tend to 

increase hours above 60% RH by about 8%. The latent gains of 12 lb/day were the same for 

both. 
19 

9 | Building America Partnership for Improved Residential Construction ba-pirc.org

Internal Gains Schedule

57,717 Btu sensible gains + 12,698 Btu (12.09 lb) latent gains

Figure 20. Internal sensible (16.9 kWh/day) and latent gains schedule used in the simulations. 

Figure 22 shows analysis of simulation results for exhaust and ERV ventilation in Orlando. The 

hours above 60% RH are hours where the RH was above 60%. They 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. In an hour where both heating 

and cooling occurred, the one with the greater runtime was designated. The simulation assumed 

that the windows were closed and the space conditioning system was used all year.

As shown in Figure 22, most of the hours significantly above 60% RH occur during floating 

hours, which occur during fall, winter, and spring in Orlando. The ERV nearly eliminated the 

cooling hours above 60% RH, but it had little effect on the floating hours above 60% RH. That is 

because the cooling system forces a greater absolute 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 absolute humidity side would be transferred to the lower absolute 

humidity side, 60% of a small absolute humidity difference is still a small amount of moisture. In 

other words, the ERV is ineffective in keeping indoor RH down during floating hours when the 

difference between indoor and outdoor absolute humidity is small.
20 

12 | Building America Partnership for Improved Residential Construction ba-pirc.org

Cooling Load and RH

Figure 21. For a week at the end of June and beginning of July on Orlando, the cooling system 

and space RH is showing an expected response. 

14 | Building America Partnership for Improved Residential Construction ba-pirc.org

Orlando: Exhaust vs. ERV

Figure 22. Hours above 60% RH for exhaust and ERV ventilation in Orlando. 
21 

19 | Building America Partnership for Improved Residential Construction ba-pirc.org

Seasonal Impacts

HVAC condition for hours above 60% is strong function of climate

Figure 23. Hours above 60% by climate and space conditioning mode. 

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

floating) was a strong function of climate. Figure 23 shows 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% to 50% greater for floating hours. However, the cooling hours above 60% RH are 

generally less than 65% RH while, in the warm-humid climates, the floating hours above 60% 

RH are generally in the range of 65% to 75% RH. 

Figure 24 shows that, the hours of elevated indoor RH is 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. 

Referring to Figure 25, mechanical ventilation, operated at the ASHRAE 62.2-2010 addendum r 

rate, in a 3 ach50 house, raises the annual median indoor RH by almost 10% RH compared to a 7 

ach50 house without mechanical ventilation in Orlando. That is because infiltration drivers are 

generally weak in that climate during floating hours (when it is still humid outside and the 

cooling system is not removing moisture), but mechanical ventilation forces a minimum air 

exchange. Note that most of the RH increase due to ventilation is between 60%-65% RH; the 

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

dehumidification would be needed in either case anyway. 
22 

20 | Building America Partnership for Improved Residential Construction ba-pirc.org

Climate Severity

Climate Severity is a function of selected RH limit 

Figure 24. Hours of elevated indoor RH is a strong function of the selected RH limit and climate. 

31 | Building America Partnership for Improved Residential Construction ba-pirc.org

2009 IECC Envelope Leakage

ASHRAE 62.2 exhaust ventilation raises median RH almost 10%

Internal gains: 57,717 Btu/day + 12.09 lb H2O/day

Figure 25. ASHRAE 62.2-2010 addendum r ventilation rate raises median RH compared to 

conventional dwelling without mechanical ventilation. 
23 

25 | Building America Partnership for Improved Residential Construction ba-pirc.org

Moisture Excess by Vent Type

ERV systems significantly reduce moisture excess at 60% RH limit 

Figure 26. Predicted required moisture removal by supplemental dehumidification, by ventilation 

system type (energy recovery ventilator, exhaust, CFI supply). 

Shown in Figure 26, removal of indoor moisture by supplemental dehumidification was 

approximated by the FSEC simulations by a post-processing routine that assumed that during 

hours over a 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 FSEC model, but are accounted for in the CDH model. 

The predicted lb/y values shown in Figure 26 are converted to $/y in Figure 27 by an assumed 

dehumidifier Energy Factor of 1.47 L/kWh and $0.12/kWh. Based on that, supplemental 

dehumidification was predicted to be between $10 and $30/yr for the warm-humid climates of 

Charleston, Houston, and Orlando, and the marine climate of Los Angeles.
24 

26 | Building America Partnership for Improved Residential Construction ba-pirc.org

Cost of 60% RH Limit

deH2O EF = 1.47 L/kWh; electricity = $0.12/kWh

Theoretical costs could be a factor of 2 low with poor field performance

Figure 27. Predicted cost of supplemental dehumidification. 
25 

EnergyPlus Humidity Control and Ventilation Modeling Analysis

(Michael Sypolt and Duncan Prahl, IBACOS)

IBACOS ran simulations in EnergyPlus that are parallel to those that FSEC ran in the 

customized version of EnergyGaugeUSA. The NREL BEopt E+ program was used to generate 

the geometry and base IDF files. A custom script modified parameters for about 860 runs which 

could be run overnight. 

Figure 28. NREL BEopt E+ program used to generate the building geometry and the base IDF files 

(input description). 

The model results were compared to the FSEC model results, and some input differences were 

corrected. Some result differences still exist. The EnergyPlus results generally show a greater 

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

FSEC (see Figure 29) and CDH simulations, and compared to much of BSC field data (Rudd et 

al. 2003, BSC 2004, Rudd et al. 2005). These might be due to differences in the infiltration 

model, or the infiltration-ventilation superposition. There may also be differences in the thermal 

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

EnergyPlus moisture modeling inputs were double checked by IBACOS and re-checked by 

FSEC without finding any significant discrepancies. The lumped moisture capacitance factor and 

moisture generation rate were consistent with the FSEC and CDH inputs. 
26 

Figure 29. Indoor RH vs. hour of year comparison between EnergyPlusPlus (blue symbols) and 

EnergyGauge USA (V3.0.01P) simulation results. 

Figure 30, shows the energy consumption results by ventilation system type, ventilation rate, and 

building enclosure tightness. EnergyPlus shows the differences to be relatively small in each 

case.

Referring to Figure 31, using the IBACOS EnergyPlus model, an ERV (not integrated with the 

central AHU) reduces the number of hours with lowest and highest indoor RH in Orlando. 

Overall, there is a reduction in indoor RH, and a very large reduction in hours above 60% RH 

compared to the FSEC (not integrated with the central AHU) and CDH (integrated with the 

central AHU) models. This result is suspect, but the cause is unclear. In each case, the ERV

moisture performance was modeled pretty simply as a constant effectiveness value applied to the 

actual indoor to outdoor humidity ratio difference. Of the three, the CDH model shows the 

highest hours above 60% RH because the ERV was configured as being connected between the 

return and supply of the central AHU, as most ERVs are installed. That installation configuration 

requires the AHU fan to operate coincident with the ERV fan (in this case a 50% duty cycle), 

which increased energy consumption and moisture evaporation when the cooling coil was wet. 

The CDH model calculates the amount of water retained on the cooling coil as a function of 

cooling runtime. Then, it calculates coil moisture evaporation starting each time the compressor 

0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000 4000 5000 6000 7000 8000

Houston Exhaust 100% ASHRAE

EG (V3.0.01P)

EnergyPlus
27 

stops, and when the fan runs without the compressor. The FSEC and IBACOS models 

configured the ERV as separately ducted from the central air distribution system, and did not 

model coil moisture evaporation when the compressor was off. 

Figure 30. Energy consumption results by ventilation system type, ventilation rate, and building 

enclosure tightness. 
28 

Figure 31. An ERV reduces the number of hours with lowest and highest indoor RH, and a very 

large reduction in hours above 60% RH compared to the FSEC and CDH models. 

Analysis was conducted to look at window condensation potential and how that could impact a 

selected indoor RH limit from a durability perspective. Previous research (Arena et al. 2010) 

reported the highest occurrence of visible mold or moisture damage was on or around windows 

and in bathrooms. The window that was modeled was standard double pane with vinyl frame. 

For that window in Orlando, indoor RH above 75% shows risk of center-of-glass window 

condensation, especially during hours when the space conditions were floating between the 

heating and cooling set points (Figure 32). With a less thermally efficient window frame (nonthermally broken aluminum is common in Florida due to high wind resistance requirements), a 

lower indoor RH threshold would be needed to avoid condensation on or near the window frame. 
29 

Figure 32. Window condensation potential during Orlando floating hours

 

7 Answers to Research Questions

• What are the important humidity control conclusions from recent residential 

dehumidification systems modeling, with a wide range of building and equipment 

sensitivity, using a customized <2 minute time-step TRNSYS model with temperature 

and humidity control? 

o Internal moisture generation, internal sensible heat generation, heating setpoint 

temperature, and air distribution system duct location (inside or outside of 

conditioned space) are the major influencers of elevated indoor RH in high 

performance homes in warm-humid climates.

o Mechanical ventilation is a secondary factor in increasing hours above 60% RH.

o Moisture capacitance is a secondary factor, and lumped moisture mass factors of 

10 to 30 times the air mass capacity do not show significant difference in indoor 

RH.

o Supplemental dehumidification by either a stand-alone dehumidifier, ducted 

dehumidifier, or gas-fired desiccant dehumidifier (e.g. NovelAire ComfortDry 

400 in supply air stream) was effective in eliminating hours over 60% RH. In 

Orlando, Miami, and Houston, the difference in total HVAC operating cost 

between a HERS 70 level house with those systems was 15%, 11%, and 1%, 

respectively. The gas-fired desiccant system showed the lowest operating cost, 

followed by the ducted dehumidifier and the stand-alone dehumidifier.

o Cooling system enhancement by subcooling condenser reheat was effective in

eliminating hours over 60% RH in Miami and Houston, and nearly so in Orlando.
30 

o Heat pipe cooling system enhancement was less effective than three degrees 

overcooling plus low evaporator coil airflow (210 cfm/ton). Both were more 

effective in Miami and Houston than in Orlando, but even in Orlando the hours 

over 60% RH were reduced by about half (600 down to about 350 on average). 

However, questions remain as to whether the overcooling can be tolerated from a 

comfort perspective.

o Two-speed and variable speed cooling systems do not appreciably reduce hours 

above 60% RH unless coupled with lower than standard cooling airflow 

(cfm/ton).

• What are the important indoor humidity conclusions that can be drawn from recent 

residential ventilation modeling efforts using the temperature-only control logic of the 

FSEC EnergyGauge USA program? 

o Most of the hours significantly above 60% RH occur during floating hours, which 

occur during fall, winter, and spring in Orlando.

o In Orlando, an ERV nearly eliminated the cooling hours above 60% RH, but it 

had little effect on the floating hours above 60% RH.

o Hours above 60% RH during a particular space conditioning mode (heating, 

cooling, floating) was a strong function of climate.

o Hours of elevated indoor RH is 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.

o Mechanical ventilation, operated at the ASHRAE 62.2-2010 addendum r rate 

(about 25% more than before addendum r), in a 3 ach50 house, raises the annual 

median indoor RH by almost 10% RH compared to a 7 ach50 house without 

mechanical ventilation in Orlando. However, most of the RH increase due to 

ventilation is between 60%-65% RH; the hours between 65%-75% RH mostly 

remain the same. This indicates that some supplemental dehumidification would 

be needed in either case anyway.

o Supplemental dehumidification was predicted to be between $10 and $30/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 EF of 1.47 L/kWh and recent 

field data indicates 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 deadband, which means that 

maintaining humidity below 60% would likely require humidity setpoints near 

55%, which would dramatically increase dehumidification costs.
31 

• What are the important humidity control conclusions from recent residential ventilation 

modeling efforts using the temperature and humidity control logic of the Energy Plus 

version of the NREL BEopt program? 

o Energy consumption by ventilation system type, ventilation rate, and building 

enclosure tightness showed relatively small differences in each case compared to 

total HVAC energy consumption.

o Durability analysis for Orlando, for a standard double pane window with vinyl 

frame, showed that indoor RH above 75% risks center-of-glass window 

condensation, especially during hours when the space conditions are floating 

between the heating and cooling set points. With a less thermally efficient metal 

window frame, the risk would be greater.

• How do the results from these different programs compare? 

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 CDH, FSEC, and 

IBACOS results show hours above 60% RH in the range of 1000, 2000, and 3000, respectively. 

In terms of maximum indoor RH, the CDH and FSEC results both show about 75% while the 

IBACOS results show about 85%. However, in all cases, supplemental dehumidification would 

be required for high performance, low-energy houses, and the difference in supplemental 

dehumidification energy would probably be less than 150 kWh/yr or less than $20/yr. 

The IBACOS ERV modeling using EnergyPlus is suspect of showing too much reduction in 

elevated indoor humidity hours compared to both the CDH and FSEC models, however, the 

CDH model did not model the ERV as a completely separate system from the central system 

ducts. After a sensitivity run by CDH is done with an ERV as a completely separate system, the 

results will be more directly comparable.

• Is it generally agreed that controlling to less than 60% RH is the appropriate humidity 

control point for high performance homes, and why? 

It was generally agreed that, a dehumidification control setpoint of 55%, in order to keep indoor 

RH from exceeding a 60% RH limit, was the correct strategy for high performance, low-energy 

homes. While it is clear that everything will not fail at once if the indoor RH goes over 60%, a 

60% RH limit provides the best practice coverage for providing comfort and durability over a 

reasonable range of varying factors, such as internal moisture generation rate, and occupant 

comfort perception and susceptibility to illness stemming from elevated indoor humidity. 

Included in the variability of internal moisture generation rate is construction moisture drying. It 

has been BSC’s experience that limiting indoor RH to 60% via supplemental dehumidification is 

a generic enough limit to remove moisture concerns related to the seasonal timing of building 

closure and occupancy in warm-humid climates.

• Is it generally agreed that annual hours above 60% RH is the appropriate humidity 

control performance metric to use to compare system performance and to compare 
32 

required supplemental dehumidification energy? Does that metric give generally the same 

result as looking at 4-hour and 8-hour events above 60% RH? 

It was generally agreed that annual hours above 60% RH is the single most appropriate humidity 

control performance metric to use to compare system performance and to compare required 

supplemental dehumidification energy. That metric does give generally the same result as 

looking at 4-hour and 8-hour events above 60% RH. 

• For a range of climates, ventilation systems, and space conditioning equipment in high 

performance homes, what is the magnitude of hours above 60% RH, the magnitude of 

supplemental dehumidification energy required to control to less than 60% RH, the time 

of year occurrence of elevated indoor humidity and supplemental dehumidification, and 

the space conditioning mode (heating, cooling, floating) during which most periods of 

elevated indoor humidity and supplemental dehumidification occur? 

The answers to this question are given in more detail in section 6, Agenda Summaries, but are

summarized here. The warm-humid climates of Miami, Orlando, Houston, and Charleston show 

a clear need for supplemental dehumidification for high performance homes. Without 

supplemental dehumidification, hours above 60% RH were in the range of 800 to 1800, with 

hours above 65% being about half of that. Most of the hours of elevated indoor humidity occur in 

the mild temperature but humid outdoor conditions of fall and spring, but also occur in winter in 

Orlando and Miami. A smaller number of hours occur during some summer nights and days-long 

rainy periods. Few hours above 60% RH occur during heating hours. Most hours between 60%-

65% RH occur during either cooling or floating hours, and most hours above 65% RH occur 

during floating hours. The supplemental dehumidification energy consumed to keep indoor RH 

below 60%, taken as the difference in total HVAC cost for the same building with and without 

supplemental dehumidification, is relatively small. It is in the range of 250 kWh/yr or less 

($30/yr or less), but necessary to enable deep cuts in sensible heat gain without incurring long 

periods of elevated indoor RH. 

8 Action Items

The following action items were discussed at the end of the meeting: 

• Continue to do more checking to understand the EnergyPlus moisture modeling 

differences compared to the FSEC and CDH modeling. 

• Add a sensitivity run using the CDH model for an ERV not integrated with the central air 

distribution system.

• Add a sensitivity run using the CDH model to further detail the difference between ducts 

in the attic and ducts in the conditioned space, where the attic dew point temperature is 

increased 10o

F between 10 am and 1 pm between May 15 and October 15. 
33 

• Contact Association of Home Appliance Manufacturers (AHAM) to explore the prospect 

of developing a new residential dehumidifier standard encompassing two test points in 

addition to the one test point in their existing standard, and reporting of additional test 

results.

o From discussion of Figure 33, the group proposed a new dehumidifier test 

condition of 75oF and 55% RH to represent spring/fall part load cooling 

conditions, and a new test condition of 65oF and 50% RH to represent basement 

conditions where avoiding high RH or condensation on cool below-grade concrete 

walls and floors is needed. These new test conditions could replace the AHAM 

test conditions or could be in addition to the legacy test condition of 80o

F and 

60% RH in the current AHAM standard if considered necessary on a consensus 

basis, primarily as a transitional measure for comparison to legacy test results. 

The AHAM test point of 80°F/60% RH is not a common indoor environmental 

condition for occupied spaces, limiting its usefulness to consumers and designers. 

Thermodynamics dictate that for the same RH set point, dehumidifier energy 

consumption will be higher at lower, more common indoor dry-bulb temperatures. 

NREL reported performance maps of several dehumidifiers at various operating 

conditions (Winkler et al. 2011). 

Figure 33. Overview of recommended dehumidifier test conditions. 

Indoor Sensible Latent Moisture Moisture

Inlet Outlet Wet-coil Cooling Cooling Removal Total Removal

T/RH/Tdp T/RH/Tdp Airflow Capacity1 Capacity Capacity Power Efficiency2

Test condition represents: (F/%/F) (F/%/F) (cfm) (Btu/h) (Btu/h) (L/h) (kW) (L/kW-h)

Current AHAM test Test 1 80/60/65

Sping/Fall part load cooling3 Test 2 75/55/58

Basement Test 3 65/50/46

1

 Negative cooling capacity denotes net heat added from inlet to outlet

2

 Same units as the USDOE and USEPA Energy Factor for dehumidifiers

3

 All tests with steady wet coil
34 

9 References 

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