Flexible Residential Test 

Facility: Impact of Infiltration 

and Ventilation on Measured 

Heating Season Energy and 

Moisture Levels 

R. Vieira, D. Parker, P. Fairey III, J. Sherwin, 

C. Withers, and D. Hoak

BA-PIRC

September 2013
NOTICE

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iii

Flexible Residential Test Facility:

Impact of Infiltration and Ventilation on Measured Heating Season 

Energy and Moisture Levels

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: 

R. Vieira, D. Parker, P. Fairey III, J. Sherwin, C. Withers Jr., and D. Hoak

BA-PIRC/Florida Solar Energy Center 

1679 Clearlake Road, Cocoa, FL 32922 

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40339-02 

September 2013
iv

[This page left blank]
v 

Contents

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

List of Tables .............................................................................................................................................. vi

Definitions .................................................................................................................................................. vii

Acknowledgments ................................................................................................................................... viii

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

1 Background ........................................................................................................................................... 1

1.1 Facility .................................................................................................................................1

1.1.1 Internal Gains ...........................................................................................................4

2 Infiltration Experiment ......................................................................................................................... 6

2.1 House Leakiness ..................................................................................................................6

2.2 Data Collection ....................................................................................................................6

3 Winter Infiltration Test Results Without Mechanical Ventilation..................................................... 8

3.1 Tight Home Moisture Problems.........................................................................................10

3.2 Results With Reduced Internal Moisture Generation ........................................................13

4 Winter Infiltration Test Results With Mechanical Ventilation in Tight Building........................... 16

4.1 Infiltration Measurements ..................................................................................................16

4.2 2012–2013 Winter Results .................................................................................................16

4.3 Condensation During Winter 2012–2013 ..........................................................................18

5 Conclusions ........................................................................................................................................ 21

References................................................................................................................................................. 22
vi 

List of Figures

Figure 1. Completed flexible residential test structures on FSEC campus.......................................... 2

Figure 2. Ceiling penetration for planned leakage .................................................................................. 3

Figure 3. Attic view of hole that diffuses airflow ..................................................................................... 3

Figure 4. Windows are shimmed to allow leakage .................................................................................. 3

Figure 5. Daily load schedule for both homes ......................................................................................... 5

Figure 6. January outdoor and indoor temperatures .............................................................................. 8

Figure 7. Twenty-four hour average cycle during January test period................................................. 9

Figure 8. Comparison of leaky and tight home heating energy use for January 2012 by day of 

month ..................................................................................................................................................... 9

Figure 9. Average day heating energy use during January for leaky and tight homes..................... 10

Figure 10. Dew points for tight and leaky homes for January 2012 .................................................... 11

Figure 11. Condensation occurs on all windows of tight building ...................................................... 11

Figure 12. Leaky building had no observed condensation .................................................................. 12

Figure 13. There appeared to be more condensation on the glass portion than on the screened 

portion of each tight home window.................................................................................................. 12

Figure 14. Considerable condensation accumulated............................................................................ 12

Figure 15. Tight home condensation residue by sliding glass doors ................................................. 13

Figure 16. Mold surrounding a tight home window............................................................................... 13

Figure 17. February outdoor dry bulb temperature during cold time period...................................... 14

Figure 18. Heating energy use during February cold time period ....................................................... 14

Figure 19. Dew point temperatures for February experiment .............................................................. 15

Figure 20. Tight house interior dew point temperature and outside dry bulb temperature. The large 

difference can lead to condensation on the interior of the single-pane windows. ..................... 15

Figure 21. Ambient conditions winter 2012–2013.................................................................................. 17

Figure 22. Heating energy use winter 2012–2013 .................................................................................. 17

Figure 23. Air exchange rate winter 2012–2013 ..................................................................................... 17

Figure 24. Unvented tight house north-facing window condensation January 23, 2013 .................. 18

Figure 25. Unvented tight house dew point and window temperatures January 23, 2013................ 19

Figure 26. Unvented tight house dew point and window temperatures February 17, 2013 .............. 19

Unless otherwise noted, all figures were created by BA-PIRC. 

List of Tables

Table 1. Building Tightness Comparison of Tight House Configuration (Tight Duct System)........... 4

Table 2. Building Tightness Comparison of Leaky House Configuration (Tight Duct System) ......... 4

Table 3. Leakage Measurements in Flexible Residential Test Facility for Experiment ....................... 6

Table 4. Heating, Ventilation, and Air Conditioning and Power Measurement Sensor Plan............... 7

Table 5. Average Measured Air Exchange Winter 2012–2013 .............................................................. 18

Unless otherwise noted, all tables were created by BA-PIRC. 
vii 

Definitions

ACH50 Air changes per hour at 50 Pascals

BA-PIRC Building America Partnership for Improved Residential Construction

C Air leakage coefficient

CFM Cubic feet per minute

CO2 Carbon dioxide

ELA Effective leakage area

EqLa Equivalent leakage area

FSEC Florida Solar Energy Center

kWh Kilowatt hour

n Exponent in the building leakage curve

Pa Pascal

Qn Normalized air distribution system leakage

RH Relative humidity

T Temperature
viii

Acknowledgments

The authors acknowledge and sincerely appreciate the editing done by Danielle R. Daniel. 
ix

Executive Summary

Perhaps no residential topic has been discussed as much among building scientists as air 

infiltration and required mechanical ventilation. Usually, envelope tightening is recommended as 

an efficiency strategy for retrofitting an existing home. In humid climates, reducing infiltration 

can reduce summer moisture loads. However, during drier weather when the air conditioner is 

not running, lower infiltration can lead to potential condensation. Even though severe cold 

weather is rare in the Deep South, the prevalence of single-pane windows can lead to 

condensation events. 

As part of a long-term experiment exploring retrofit measures, the savings from reducing air 

infiltration, with and without the addition of mechanical ventilation, are being studied. In 2011–

2012, two identical laboratory homes designed to model existing Florida building stock were 

sealed and tested to 2.2 ACH50. Then, one was made leaky with 70% leakage through the attic 

and 30% through the windows, to a tested value of 8 ACH50. Reduced energy use was measured 

in the tighter home (2.2 ACH50) in the range of 15.8%–18.6% relative to the leaky (8 ACH50) 

home. 

Internal moisture loads resulted in higher dew points inside the tight home than in the leaky 

home. Window condensation and mold growth occurred inside the tight home. Even cutting 

internal moisture gains in half to 6.05 lb/day, the dew point of the tight home was more than 

15°F higher than the outside dry bulb temperature. The homes have single-pane glass 

representative of older central Florida homes. There are a few factors that may limit the 

representation of the moisture results: 

• The laboratory homes have very little moisture capacitance, as they contain no interior 

walls, no furnishings, and no carpeting (slab is exposed). 

• There was neither mechanical ventilation nor operation of windows. The weather was 

largely comfortable for much of the test period and many residents would likely open up 

the homes for most periods during this time. 

• The homes were only one year old when the testing took place. There is likely still some 

drying out of the slab and concrete block walls typical of new homes, not existing homes. 

A second winter of testing was conducted in 2012–2013 with the tight home alternating between 

two-week periods with mechanical ventilation (63 CFM supply air continuously) and not having 

ventilation. The leaky east building remained the same with no ventilation. Both buildings used a 

schedule of 11 lb/day of internal moisture generation. Winter condensation was observed again 

when the supply ventilation fan was off. Inside window temperatures (measured for the second 

winter collection period) were lower than the inside dew point on cold winter nights. However, 

condensation was not observed when the ventilation fan was on, or in the leaky home. Heating 

energy use in the tight but ventilated home was 15% higher than in the leaky home with natural 

air infiltration only. 

The results indicate that tight construction in humid climates will risk window condensation and 

high interior humidity levels without mechanical ventilation. To reduce condensation potential 

there are steps practitioners may take coincident with tightening an older home. If the efficiency 
x 

measures include window replacement, a low U-value for the window can be selected to avoid 

condensation. Also, mechanical ventilation can be introduced, which will likely reduce humidity 

in the home during winter. Judicious use of operable windows during mild periods with no space 

conditioning will also likely be helpful in reducing moisture problems.
1 

1 Background 

Perhaps no residential topic has been discussed as much among building scientists as air 

infiltration and required mechanical ventilation. Sherman and Walker (2012) write “Although 

there is no national regulation of airtightness, many jurisdictions, regulatory bodies, codes and 

standards associations are beginning to include requirements for limiting envelope... There is 

currently a range of allowable leakage levels that are not the same depending on which code or 

standard is being referenced.” Rudd and Lstiburek (2008) review some of the issues with 

different mechanical ventilation through the years ranging from first cost to poor air distribution 

and high energy use. Ventilation requirements are becoming part of high performance programs 

and new energy codes, usually referencing ASHRAE 62.2 (2010), “Ventilation and Acceptable 

Indoor Air Quality in Low-Rise Residential Buildings.” However, implementation methods are 

varied, and few studies have been done to show the persistence of ventilation equipment run time 

and delivery of recommended air quantity over time. In existing homes, air sealing techniques 

may take place without adding ventilation. 

Usually envelope tightening is recommended as an efficiency strategy for retrofitting an existing 

home. For example, the Energy.Gov website (2012) has a link from its main home page to air 

sealing that indicates “Reducing the amount of air that leaks in and out of your home is a costeffective way to cut heating and cooling costs, improve durability, increase comfort, and create a 

healthier indoor environment.” The site recommends sealing and ventilation. 

In humid climates, reducing infiltration can reduce summer moisture loads. However, during 

drier weather when the air conditioner is not running, lower infiltration can lead to condensation. 

Even though severe cold weather is rare in the Deep South, the prevalence of single-pane 

windows can lead to condensation events. 

As part of a long-term experiment exploring retrofit measures, the savings from reducing air 

infiltration is being studied. This report looks at the wintertime result with and without any 

mechanical ventilation. A separate report examines summer results with and without mechanical 

ventilation. 

1.1 Facility

The State of Florida provided funding for the design and construction of two reconfigurable, 

geometrically identical, full-scale, side-by-side residential building energy research facilities at 

the Florida Solar Energy Center (FSEC), as shown in Figure 1. The Building America 

Partnership for Improved Residential Construction (BA-PIRC) has instrumented these flexible 

research homes and will monitor them to conduct research on advanced building energy 

efficiency technologies under controlled conditions. 

The purpose of the Flexible Residential Test Facility is to provide a controlled research 

environment that serves two main purposes. First, it is used to research and evaluate advanced 

energy efficiency technologies and operational strategies. Second, it serves as a venue to help 

validate building simulation programs and algorithms. Details of the buildings and their 

instrumentation are provided by Vieira and Sherwin (2012). 
2 

Figure 1. Completed flexible residential test structures on FSEC campus

Of particular significance to this report is the substantial effort that went into creating equal air

leakiness in the 1,536 ft2, 14,331 ft3 of volume buildings. Initial construction created reasonably 

tight buildings (3.62 and 3.82 ACH50), but FSEC staff further sealed leakage points until each 

was able to achieve 2.2 ACH50. The air distribution systems were tightened to achieve 13 

CFM25/100 ft2. (Qn = 0.013) in each home. Each home was then configured with controllable 

duct leakage and air leakage. The air leakage was designed to create the type of distribution and 

diffusion of air leakage represented in a number of southern slab-on-grade homes: 

• Both homes were configured with four controllable ceiling leakage sites providing ~70% 

of leakage area needed to achieve ~8 ACH50 (see Figure 2 and Figure 3). Seventy 

percent leakage through the ceiling was able to be verified using a calibrated flow hood 

to measure air through ceiling leak sites when the house was at –50 Pa with reference to 

the outside. 

• The remaining 30% of leakage area was achieved using metal shims at all windows 

(Figure 4). The leakage rate of the leaky configuration of the east unit increased from 

6.01 ACH50 to 7.99 ACH50 by adding the shims. 
3 

Figure 2. Ceiling penetration for planned 

leakage Figure 3. Attic view of hole that diffuses airflow

Figure 4. Windows are shimmed to allow leakage

FSEC staff experimented with different configurations of holes and air pathway restrictions until 

they were able to achieve an “n” value in a range of 0.6–0.7 while bringing in 30% of the air 

through the windows and obtaining an ACH50 value near 8. An n value of 0.6–0.7 was 

established since this is the typical range found in measurements in homes across the United 

States. Once the air leakage design was established in the first home, the same design was copied 

in the second home to obtain matched leakage results. 

Airtightness testing was repeated on both homes in the tightest house envelope configuration and 

in the leakiest house envelope configuration at 8 ACH50. The detailed test results are 

summarized in Table 1 and Table 2. These tests were done with a very tight duct system having 

only Qn = 0.013 (0.013 CFM25 leakage to outside per 100 ft2

 of conditioned space).
4 

Table 1. Building Tightness Comparison of Tight House Configuration (Tight Duct System) 

ACH50 CFM50 C n r

2 EqLa ELA

East Laboratory

Tight 2.26 540

(± 0.7%)

36.0

(± 8.6%)

0.692

(± 0.023) 0.9990 55.8(± 3.5%)

29.6

(± 5.5%)

West Laboratory

Tight 2.18 520

(± 0.6%)

36.0

(± 8.9%)

0.683

(± 0.023) 0.9996 53.7(± 3.7%)

28.5

(± 5.8%)

ACH50 Air changes per hour at 50 Pascals

CFM50 Cubic feet per minute at 50 Pascals

C Air leakage coefficient

n Exponent in the building leakage curve

r

2 The coefficient of variation as an indicator of the fit of the least squares equation to the data

EqLA Equivalent leakage area

ELA Effective leakage area

Table 2. Building Tightness Comparison of Leaky House Configuration (Tight Duct System) 

ACH50 CFM50 C n r

2 EqLa ELA

East Laboratory 

Leaky 7.99 1,909 

(± 1.8%) 

177.3 

(± 15.4%) 

0.607 

(± 0.047) 0.9940 197.1 (± 5.9%) 

104.8 

(± 9.6%) 

West Laboratory 

Leaky 8.06 1,926

(± 1.1%)

182.3

(± 11.3%)

0.603

(± 0.031) 0.9962 198.8(± 4.3%)

105.7

(± 7.1%)

1.1.1 Internal Gains

Due to the limited heating season in central Florida, internal gains can provide sufficient heat on 

some winter days, particularly in newer, tighter homes. Furthermore, internal moisture 

generation in a tight home without ventilation can create high moisture loads. The laboratories 

have automated (computer controlled) heat and moisture gains scheduled by time of day based 

on the Residential Energy Services Network lighting, appliance and miscellaneous energy usage 

amendment schedule (RESNET 2011, also see Parker et al. 2010). The hourly schedules utilized 

are shown in Figure 5, and more details of how they were derived and delivered are given in the 

report on the facility (Vieira and Sherwin 2012). 
5 

Figure 5. Daily load schedule for both homes

A second experiment was run whereby the latent loads were cut in half (6 lb/day), maintaining 

the same hourly profile but half the magnitude, in order to observe the effect on condensation 

and humidity levels. 
6 

2 Infiltration Experiment

2.1 House Leakiness

For the infiltration test, the west laboratory home served as the tight home and had the leakage 

sealed off at the ceiling plane. The leaky east laboratory home used the ceiling penetrations and 

window shims to obtain the results shown in Table 3. 

Table 3. Leakage Measurements in Flexible Residential Test Facility for Experiment 

Leakage Parameter Leaky Home—East Tight Home—West

CFM50 1,909 520

ACH50 7.99 2.18

C 177.3 36.0

n 0.607 0.683

R2 0.9970 0.99983

ELA (in.

2

) 104.8 28.5

Specific Leakage Area 0.00053 0.000119

2.2 Data Collection

Details of the measurement capability are provided in the instrumentation plan (Vieira and 

Sherwin 2012). However, of particular interest for this experiment are the space and power 

measurements, which are presented in Table 4. Each of these values is taken every 10 seconds 

and averaged over 15 minutes, or for energy or consumption, summed over 15 minutes. 
7 

Table 4. Heating, Ventilation, and Air Conditioning and Power Measurement Sensor Plan

Performance Location Type Thermocouple

Channels

Millivolt

Channels Pulse

Supply T/RH Duct T/RH Volts 2 

Return T/RH Duct T/RH Volts 2

End of Supply T/RH Duct T/RH Volts 2

Airflow Duct Volts 2

Total 8

Interior Room 

Conditions Location Type ThermocoupleChannels

Millivolt

Channels Pulse

Center T/RH Center T/RH Volts 2

Center mrt Center TC 1

Ceiling Surface Center TC 1

Tstat T/RH Tstat T/RH Volts 2

Total 2 4

Power/Use 

Measurements Location Type ThermocoupleChannels

Millivolt

Channels Pulse

Air Handler Pulse W-h 1

Condenser Pulse W-h 1

Interior Fans Pulse W-h 1

Interior Lights Pulse W-h 1

Outdoor Lights Pulse W-h 1

Water Heater pulse W-h 1

Total Use pulse W-h 1
8 

3 Winter Infiltration Test Results Without Mechanical Ventilation

As shown in Figure 6, temperatures were mild in January 2012 with two cold spells. In order to 

ensure some heating, thermostats in the laboratories were set to 72°F. Temperatures inside the 

two laboratories remained close, as shown in Figure 7, which indicates the profile for an average 

of each hour over the test period shown. Note that during some warm days, the temperature 

increased beyond the winter set point. No cooling was used for this test. 

The heat was supplied by electric resistance furnaces. The energy use for each home for each day 

is shown in Figure 8, and the 24-hour cycle for the average day is shown in Figure 9. Due to the 

mild weather, there are some days without heating. Overall, there is about an 18.6% savings 

from the tighter home. Note that neither home has mechanical ventilation, and the result is based 

on a small amount of heating use. Also, there were some moisture concerns in the tight home. 

The average daily cycle (each hour averaged for the time period) is shown in Figure 6. 

Figure 6. January outdoor and indoor temperatures
9 

Figure 7. Twenty-four hour average cycle during January test period

Figure 8. Comparison of leaky and tight home heating energy use 

for January 2012 by day of month
10 

Figure 9. Average day heating energy use during January for leaky and tight homes

3.1 Tight Home Moisture Problems

Figure 10 plots the dew point inside each home and outdoors during January. Because of internal 

moisture generation, the moisture level in the homes is higher than outside. The leaky home dew 

point is 1°–2°F higher than outside typically, while the tight home dew point is ≥ 10°F higher 

than outside. This resulted in significant hours when the dew point temperature inside the tight 

home was likely higher than the window temperature. Unfortunately, the laboratory did not have 

a thermocouple measuring window temperature (one is being added). However, the resulting 

condensation was significant during cold spells (see Figure 11 to Figure 16). The windows are 

single-pane, typical of most existing residential central Florida homes.
11 

Figure 10. Dew points for tight and leaky homes for January 2012

Figure 11. Condensation occurs on all windows of tight building
12 

Figure 12. Leaky building had no observed condensation

Figure 13. There appeared to be more condensation on the glass portion 

than on the screened portion of each tight home window

Figure 14. Considerable condensation accumulated
13 

Figure 15. Tight home condensation residue by sliding glass doors

Figure 16. Mold surrounding a tight home window

3.2 Results With Reduced Internal Moisture Generation

Immediately following the January test period, the laboratory homes were dried out by 

eliminating internal moisture generation and air conditioning as much as possible. Internal 

moisture generation was cut in half from 12.10 lb/day to 6.05 lb/day. On February 8, the heating 

systems in both laboratory homes were reactivated. A cold snap on February 11–14 (see Figure 

17) allowed additional measurements under the revised internal moisture generation schedule. 
14 

Figure 17. February outdoor dry bulb temperature during cold time period

The heating energy savings in the tight home were similar to the January tests but slightly less, 

with just 15.8% savings, as illustrated in Figure 18. The dew point temperatures inside the tight 

home were still considerably higher than in the leaky home (Figure 19). A peak difference 

between inside dew point and outside ambient of 19°F occurred during this cold spell (Figure 

20). Condensation was observed during this cold spell even with the reduced moisture generation 

inside the home. However, the condensation was short-lived and did not create the mold as in 

earlier winter cold periods. Thermocouples were added to the glass and will be reported in future 

experiments. 

Figure 18. Heating energy use during February cold time period
15 

Figure 19. Dew point temperatures for February experiment

Figure 20. Tight house interior dew point temperature and outside dry bulb temperature. The large 

difference can lead to condensation on the interior of the single-pane windows.
16 

4 Winter Infiltration Test Results With Mechanical Ventilation in 

Tight Building

For the winter of 2012–2013, a supply fan set for 63 CFM continuously was placed in the tight 

west building. The leaky east building remained the same with no ventilation. Both buildings 

used a schedule of 11 lb/day of internal moisture generation. This second winter of data was run 

with the fan on for about a 2-week cycle and then off for about a 2-week cycle. This allowed for 

repeating results from the first winter. Window temperature measurements were also added for 

the second winter.

4.1 Infiltration Measurements

Carbon dioxide (CO2) dosing and measurement equipment were installed to measure infiltration 

in the two homes. The dosing equipment consisted of two parallel CO2 tanks located in the 

garage and precisely metered on their release by digital flow meters that are recorded by 

instrumentation. Measurements came from Vaisala CO2 transmitters with a full-range accuracy 

of + 2%. 

Ambient outdoor CO2 sensors were also installed. Because the buildings are unoccupied, it is 

possible to measure the in situ building infiltration rate using the CO2 gas emission rate as a 

tracer gas. The rise in the concentration of indoor CO2 can be used to then estimate the building 

infiltration and ventilation rate with outdoor air. An initial problem with this setup was 

experienced when it became apparent that the outdoor CO2 sensor had to be quite remote from 

the homes to prevent the effluent CO2 from the leaking buildings from corrupting the reference 

outdoor CO2 measurement. This tended to be more a problem with the ventilated west home, so 

the sensor outside the east home has been used as the ambient level for all analysis.

4.2 2012–2013 Winter Results

There were 60 days of good winter data collected continuously from December 24, 2012 through 

February 21, 2013. On February 22, the east oven controller that is used to deliver internal loads 

started to experience inconsistencies, and data from an early March cold period cannot be 

included. Figure 21 shows the ambient conditions during this period. Keeping in mind this is 

central Florida, there were four periods of heating. Figure 22 shows the daily watt-hours of 

electric resistance heating used in each unit as well as the status of the mechanical ventilation 

system in the tight building. A value of 1 represents the fan on all day, 0 off all day. For 

transition days, it depends on when the fan was turned on or off. For example, if the fan was 

turned off at 8:00 a.m., a value of 0.33 is shown. Only two transition days occurred on days with 

heating. Figure 23 shows the matching measured air exchange rate using the CO2 sensors. The 

average values are shown in Table 5.
17 

Figure 21. Ambient conditions winter 2012–2013 

Figure 22. Heating energy use winter 2012–2013

Figure 23. Air exchange rate winter 2012–2013
18 

Table 5. Average Measured Air Exchange Winter 2012–2013

Ventilation 

Status

Data Points 

(15-min Measurements)

Leaky Home—East

ACH Average From 

CO2 Measurements

Tight Home—West

ACH Average From 

CO2 Measurements

Off 2,624 0.320 0.053

On 3,096 0.292 0.334

The measured heating energy use consistently shows greater heating use in the ventilated unit 

when the ventilation is on. The results for when the ventilation system is off are not as clear 

during the two cold spells in 2012–2013 as they were in 2011–2012. To estimate savings from 

one house to the next, only days with 1,000 Wh of heating were included to eliminate any 

significant measurement error during low standby time. Days of ventilation transition were not 

included. This left 11 days of heating when the vent fan was off. The total heating used in the 

leaky unit those days was 70.205 kWh, and the tight unit used 70.398 kWh with no significant 

difference.

There were 16 days of significant heating when the homes were ventilated. The total heating 

used in the leaky unit those days was 95.6 kWh, and the tight ventilated unit used 110.2 kWh or 

a 15.3% increase in heating energy use in the tight ventilated home relative to the leaky home.

A key preliminary finding was that while the impact of air infiltration on heating was unclear in 

the 2013 winter season, there were clear indications that mechanical ventilation led to an 

approximately 15% increase in space heating. 

4.3 Condensation During Winter 2012–2013

As anticipated, condensation was observed in the tight unit during times of non-venting and not 

in the leaky home (see Figure 24 of condensation forming on January 23). Condensation was not 

observed in the tight unit during times of ventilation. 

Figure 24. Unvented tight house north-facing window condensation January 23, 2013
19 

Dew point and window temperature data are shown for two such days, January 23 and February

17 in Figures 25 and 26. The dew point temperature in the tight west home is ≥ 10°F higher than 

the leaky east home. The tight home window temperature is at times lower than the dew point 

leading to the observed condensation. Two interior window temperature measurements are 

shown from the same North-facing window, one from the lower portion of the single-hung 

window that was semi-protected by a window screen, and one from the upper half that did not 

have a screen.

Figure 25. Unvented tight house dew point and window temperatures January 23, 2013

Figure 26. Unvented tight house dew point and window temperatures February 17, 2013
20 

All of the temperatures are consistently 3°–4°F below the inside dew point during the night, such 

that condensation can form. Note that at night the temperature of the upper glass is cooler than 

the lower glass, indicating some thermal insulation influence from the insect screen. During the 

day the temperatures become more even, perhaps due to more indirect solar heating on the top 

portion of the window. 
21 

5 Conclusions

Two identical laboratory homes designed to model existing Florida building stock were sealed 

and tested to 2.2 ACH50. Then, one was made leaky with 70% leakage through the attic and 

30% through the windows, to a tested value of 8 ACH50. Two winters of data were recorded. In 

the first winter, there was no mechanical ventilation in either home. In the second winter, a 

mechanical ventilation system was operated in the tighter home during most of the cold periods. 

For the first winter, reduced energy use was measured in the tighter home (2.2 ACH50) in the 

range of 15.8%– 18.6% relative to the leaky (8 ACH50) home. Internal moisture loads resulted 

in higher dew points inside the tight home than the leaky home. The homes have single-pane 

glass representative of older central Florida homes. Window condensation and mold growth 

occurred inside the tight home. Even cutting internal moisture gains in half to 6.05 lb/day, the 

dew point of the tight home was more than 15°F higher than the outside dry bulb temperature. 

There are a few factors that may limit the representation of these results regarding condensation: 

• The laboratory homes have very little moisture capacitance, as they contain no interior 

walls, no furnishings, and no carpeting (slab is exposed). 

• There was neither mechanical ventilation nor operation of windows. The weather was 

largely comfortable for much of the test period, and some residents would likely open up 

the homes for most periods during this time. 

• The homes were only one year old when the testing took place. There is likely still some 

drying out of the slab and concrete block walls not typical of older homes. 

The results presented alter just one variable—the air infiltration rate. To reduce condensation 

problems, there are steps practitioners may take coincident with tightening an older home. If the 

efficiency measures include window replacement, a low U-value for the window can be selected 

to avoid condensation. Also, mechanical ventilation can be introduced, which will likely reduce 

humidity in the home during winter. Judicious use of operable windows during mild periods with 

no space conditioning will also likely be helpful in reducing moisture problems. 

For the second winter, condensation was observed again when the supply ventilation fan was off. 

Inside window temperatures were lower than the inside dew point on cold winter nights. 

However, condensation was not observed when the ventilation fan was on, or in the leaky home. 

This indicates that tight construction in humid climates will risk window condensation and high 

interior humidity levels without mechanical ventilation. 

Heating energy use in the tight, but ventilated home was 15% higher than in the leaky home with 

natural air infiltration only. This may argue for heat recovery ventilation for such systems to 

offset heating related impacts.
22 

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DOE/GO-102013-4041 ▪ September 2013

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