Measured Cooling Performance 

and Potential for Buried Duct 

Condensation in a 1991 Central 

Florida Retrofit Home 

D. Chasar and C. Withers

BA-PIRC

February 2013
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iii

Measured Cooling Performance and Potential for Buried Duct 

Condensation in a 1991 Central Florida Retrofit Home

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: 

Dave Chasar and Charles R. Withers, Jr.

Building America Partnership for Improved Residential Construction

Florida Solar Energy Center

1679 Clearlake Road 

Cocoa, Florida 32922 

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40339-00 

February 2013
iv

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v 

Contents

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

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

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

Executive Summary................................................................................................................................. viii

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

2 Background........................................................................................................................................... 2

3 Monitoring Equipment and Measurements........................................................................................ 4

4 Analysis ................................................................................................................................................. 6

4.1 Cooling Energy Performance Comparison ..........................................................................6

4.2 Buried-Duct Condensation Analysis....................................................................................9

5 Conclusions ........................................................................................................................................ 15

References................................................................................................................................................. 16
vi 

List of Figures

Figure 1. 1991 wood-frame retrofit home located in Cocoa, Florida ..................................................... 2

Figure 2. Photos of buried duct (left) and exposed moisture sensing strip (highlighted, right)........ 4

Figure 3. Pre- and post-retrofit cooling performance comparison........................................................ 6

Figure 4. Unadjusted average daily cooling performance comparison ................................................ 8

Figure 5. Pre- and post-retrofit average daily electric demand.............................................................. 9

Figure 6. Relative duct moisture levels during one pre-retrofit and two post-retrofit periods......... 11

Figure 7. Outdoor and attic dewpoint temperatures. ............................................................................ 12

Figure 8. Pre- and post-retrofit attic duct drybulb temperatures during similar outdoor 

conditions............................................................................................................................................ 13

Figure 9. Pre- and post-retrofit attic duct surface moisture levels during similar outdoor 

conditions............................................................................................................................................ 13

Figure 10. Post-retrofit duct conditions during worst-case dew point temperatures ....................... 14

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

List of Tables

Table 1. Construction and Equipment Details ......................................................................................... 3

Table 2. Sensors and Datalogger .............................................................................................................. 5

Table 3. Average Daily Data, Summer Results Comparison .................................................................. 7

Table 4. Moisture Strip Readings Derived from Initial Laboratory Observations .............................. 11

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

Definitions

A/C Air conditioning

ACH50 Air changes per hour at 50 Pascals

AHU Air handler unit

BA-PIRC Building America Partnership for Improved Residential Construction

cfm25 Cubic feet per minute at 25 Pascals

FSEC Florida Solar Energy Center

kW Kilowatt

kWh Kilowatt-hour

mV Millivolt

Pa Pascal

Qn Normalized duct leakage to outside (cfm 25), out/ft2

R A measure of resistance to the flow of heat through a given thickness of a 

material (as insulation) with higher numbers indicating better insulating 

properties 

RH Relative humidity

RTD Resistance temperature device

SEER Seasonal energy efficiency ratio

T Temperature

U A measure of the flow of heat through an insulating or building material

V Volt

Wh Watt-hour

ZEH Zero energy home
viii

Executive Summary

This research analyzed measured energy savings from a moderate retrofit by comparing postretrofit performance with measured pre-retrofit monitored data. The test home, occupied by a 

Florida Solar Energy Center researcher and five family members, was fully instrumented to yield 

detailed energy, temperature, and humidity measurements. Two years of pre-retrofit data were

available to characterize improvements in cooling energy and electric demand savings from an 

upgraded heat pump (seasonal energy efficiency ratio [SEER] 10 to SEER 16) and increased 

attic insulation (R-19 to R-38). Additional efforts investigated condensation on supply duct 

surfaces during periods of high dew point temperatures. This potential side effect, specific to the 

hot-humid climates, is a known consequence of flex ducts being buried in cellulose insulation. 

Cooling energy savings of 30% were measured during the summer of 2011 (June to September)

and compared to the previous two summers (2009 and 2010) using a least squares linear 

regression that accounts for differences in indoor and outdoor temperatures. Directly comparing 

the daily average total building and cooling energy use, unadjusted for weather differences, 

showed a 26% total building energy reduction and 37% less cooling energy use after retrofit 

measures were in place. Summer electricity peak demand was reduced by 37% on a wholebuilding basis and 46% for the submetered air-conditioning system. The whole-building peak 

demand was reduced to essentially the same level once required by the old SEER 10 air 

conditioning alone, 2.82 kW.

The presence of condensation on attic ducts buried in blown insulation was investigated with 

moisture-sensing strips mounted to exterior duct surfaces. Four distinct ranges—from a dry 

surface to a fully wetted, condensing condition—were identified to indicate the presence of 

moisture. Pre-retrofit measurements showed very low condensation potential, although these 

were limited to a relatively short 3-week period and moderate dew point temperatures. Data 

recorded after the ducts were buried showed strong signs of condensation during periods of high 

dew point (mid-summer), but it was reduced during the fall when dew point temperatures 

dropped. Visual inspection of the ducts showed no evidence of damage from accumulated 

moisture. The relatively high set point temperature (79°F) of the study home, however, might 

indicate that greater potential for damage could exist at lower thermostat set points. 
1 

1 Introduction

The Florida Solar Energy Center (FSEC) conducted energy performance monitoring of two 

existing residences in Central Florida that were undergoing various retrofits. These homes were 

occupied by FSEC researchers and were fully instrumented to yield detailed energy, temperature, 

and humidity measurements. The data gave feedback about the performance of two levels of 

retrofit in two types of homes in a hot-humid climate. This report covers a moderate-level retrofit 

and includes 2 years of pre-retrofit data to characterize the impact of improvements. The other 

home, covered in a separate report (Parker and Sherwin 2012), is a deep energy retrofit that has 

performed at near zero energy with a photovoltaic system and extensive envelope improvements.
2 

2 Background 

The 2,200-ft2, four-bedroom wood frame home discussed in this report was built in 1991 and is 

located in Cocoa, Florida (Figure 1). It is occupied by a family of six. It has a 12-year-old flat 

plate solar water heater and a 6-year-old white metal roof but has otherwise had no energyrelated improvements. During roof replacement, attic venting was eliminated. Measured cooling 

savings from the replacement of dark shingles with a white metal roof amounted to 19% and are

documented in an FSEC report (Chasar, Moyer, and Martin 2006). 

Figure 1. Wood frame retrofit home located in Cocoa, Florida

The most recent retrofit measures were completed in May and June 2011. The 20-year-old 

seasonal energy efficiency ratio (SEER) 10, split heat pump was replaced with a SEER 16, 2-

speed heat pump. In addition, blown-in cellulose insulation (R-19) was added over existing R-19 

ceiling batts, which brought the total ceiling insulation level to R-38. Flex duct runouts buried in 

the cellulose were monitored for condensation. 

These two measures have been part of many retrofits documented by FSEC over the past few 

years. They typically represent a significant portion of the 30%–50% total energy reduction goal 

sought by the Building America Program and might be broadly applicable to many hot-humid 

climate homes built from the 1980s to the early 1990s. Burying attic flex ducts in blown 

insulation is a common practice where replacement of existing ductwork is not required. 

Although ducts buried in insulation are expected to benefit from reduced heat gains and losses, 

there remain unresolved durability and air quality implications with this practice when 

implemented in hot-humid climates. Buried ducts have an increased potential for condensation 

on exterior surfaces of ducts, especially in humid climates, that could result in moisture damage 

or mold growth. A method of determining the presence of condensation was employed in several 

key locations to provide feedback about this phenomenon. 

Table 1 lists home construction and equipment details. Building and duct airtightness tests were 

performed before and after the retrofit measures were implemented. An evaluation of the original 

R-6 ductwork showed only limited degradation of the duct board trunk and flex duct runouts. 

Repairs were performed at flex duct collar connections, reducing duct leakage from 77 to 55 
3 

cfm25 to out, as shown in Table 1. During each test for duct leakage to outside with the house 

held at –25 Pa (with respect to out), the pressure in the house was recorded at –20.5 Pa with 

respect to the attic. This measurement gives an indication of the reduction in attic venting 

because in a home with a fully vented attic, it would typically be –25 Pa (same as to out). Even 

though the natural attic venting in the study home was eliminated, then, the attic was by no 

means sealed and maintained some connection with the outside. 

Table 1. Construction and Equipment Details

Component Pre-Retrofit Post-Retrofit

Construction Date 1991

Construction Type Wood frame

Floor Type/Area (ft

2

) Slab-on-grade/2,200

Attic/Roof Type Unvented /white metal

Window Type Single-pane, clear glass, aluminum frame

Glass/Floor Area 11%

Insulation 

Attic/Wall/Floor R-19/R-11/R-0 R-38/R-11/R-0 

Exterior Wall Cladding Natural cedar (medium-dark)

Space Conditioning 3-ton, SEER 10, heat pump, 

20 years old

3-ton, SEER 16, 2-speed heat 

pump

Thermostat Standard

Ventilation None

Water Heater Active direct solar, 80-gal electric backup

ENERGY STAR

Appliances H-axis clothes washer and second refrigerator

Other Appliances Refrigerator, dishwasher, clothes dryer

Fluorescent Lighting 70%

Occupancy 6, (2 adults, 2 teens, 2 adolescents)

Infiltration (ACH50) 4.7 4.8

Duct Leakage (cfm25) 173 (total)/77 (out) 158 (total)/55 (out)

Duct Leakage (Qn) 0.035 0.025
4 

3 Monitoring Equipment and Measurements

Energy use, temperature, and relative humidity (RH) conditions were recorded with a Campbell 

Scientific data logger both pre- and post-retrofit. Energy use measurements included wholehouse and indoor and outdoor components of the split heat pump. The difference between 

interior and exterior temperature measurements was useful in characterizing cooling energy 

performance and isolating the influence of thermostat set points. 

Various other measurements were used to examine additional impacts from the retrofit measures, 

including attic and interior temperatures as well as RH and supply air temperature. The ducts 

were fitted on the outer surface with thermocouples and condensate-sensing, resistive strips 

before being buried within the R-19 cellulose that was blown over the existing fiberglass batts. 

The photos in Figure 2 show the placement of instrumentation on the duct and the extent of duct 

coverage by the cellulose insulation. These readings, along with attic air conditions, were 

evaluated for potential and frequency of condensation. The ducts were also visually inspected for 

any noticeable physical effects.

 

Figure 2. Photos of buried duct (left) and exposed moisture sensing strip (highlighted, right) 

A Campbell Scientific data logger had been in place at this home for several years. It has been 

used for various experiments including a baseline cooling performance compared to high 

performance homes (Chasar et al. 2006) and the before and after effects of replacing dark asphalt 

shingles with white metal roofing (Chasar, Moyer, and Martin 2006). 

The data logging system was upgraded in 2010 to a new model CR1000 logger and WattNode 

energy meters. Current data channels include energy use measurements as well as indoor, 

outdoor, and attic conditions (Table 2). The logger operates at a 30-s scan rate and stores data in 

15-min time steps.
5 

Table 2. Sensors and Data Logger 

Measurement Equipment Used

Indoor and Attic T/RH Vaisala RTD/thin-film probe

Outdoor T/RH Vaisala RTD/thin-film probe

Other Temps (duct surface T) Type-T thermocouple

Duct Condensation Resistive surface strips

Whole House and HVAC Energy Use WattNode electric energy meters

Data Collection and Storage Campbell Scientific data logger

Note: T, temperature; RTD, resistance temperature device
6 

4 Analysis 

4.1 Cooling Energy Performance Comparison

To normalize results for weather and occupant thermostat settings, best-fit linear regressions 

were used to characterize cooling energy use versus temperature difference (T_outdoor minus 

T_indoor). These methods are well established to estimate combined HVAC equipment and 

building efficiencies (Kissock, Haberl, and Claridge 2003). Uncertainties in the method have 

been documented by Cummings, Parker, and Sutherland (2010). Figure 3 shows the pre- and 

post-retrofit results. It includes reference to two other Central Florida homes built in 1998 where 

cooling performance was similarly measured from 1998 to 2002. Located in Lakeland, Florida, 

these reference homes were monitored by FSEC to show the impact of extensive envelope and 

equipment improvements between homes with otherwise identical building style, layout, and 

orientation (Parker et al. 1998). The Lakeland Control Home provides an upper boundary for 

cooling energy use in a typical, code-minimum, mid-90s home with a SEER 10 heat pump. 

Cooling energy use in the Lakeland Zero Energy Home (ZEH) provides a lower boundary for 

comparison purposes. Data from this home (the first to use the ZEH label) continue to compare 

favorably with more recent ZEHs. The home even outperforms many Building America 

prototype homes with cooling efficiencies that are higher than its SEER 14 system 

(Chasar et al. 2006). 

 

Figure 3. Pre- and post-retrofit cooling performance comparison 
7 

The pre-retrofit regression results in Figure 3 (red data) are composed of a data set from the two 

summers before improvements (2009 and 2010) and include a total of 219 data points. Each 

point represents the total cooling energy use (air handler unit [AHU] + condenser unit) for 1 day 

(y-axis) with respect to the outdoor minus indoor average temperature difference for the same 

day (x-axis). The resulting regression line has a similar y-intercept (12.4 versus 11.9) to that of 

the Lakeland Control Home; however, it has a noticeably lower slope (1.1 versus 1.9). This may 

reflect similar equipment efficiencies (both were SEER 10), but enhanced levels of building 

envelope performance. A similar effect was found with data taken from this residence in 2004 

and 2005 that documented the impact of replacing dark shingles with a white metal roof (Chasar, 

Moyer, and Martin 2006). 

Data from 2011, after retrofit implementation (blue data), includes 102 data points. It shows the 

relative performance improvements from both envelope and equipment retrofits (SEER 16, 

additional R-19 attic insulation and slightly tighter ducts). Twenty-six days were removed from 

the data sets over the three summers because of data logger downtime and known occupant 

influences such as vacation periods and major cooking or party events. Otherwise, all collected 

data were included in the analysis. An average savings of 30% was derived by applying the 

measured daily delta Ts for the 2011 summer period from June through September to each bestfit equation. The magnitude of this savings in the study home would amount to 1,072 kWh (or 

8.9 kWh/day) for this 4-month summer period. 

Table 3 directly compares average daily energy use and temperatures from the two pre-retrofit 

years (2009 and 2010) with post-retrofit data (2011). These values were unadjusted for weather 

differences between years but were taken from equivalent periods from June 20 to September 30. 

The results show a 27% total building energy reduction and 39% less cooling energy use after 

retrofit measures were in place. Average daily outdoor temperatures were 1°F to 2°F higher 

during the pre-retrofit years than during the post-retrofit period. Indoor temperatures were 

maintained at a stable 79.1°F to 79.5°F over the full study period with a relatively constant 

thermostat setting. Table 3 and Figure 4 also show how the variable-speed AHU effectively used 

about one-half the energy of the single-speed unit it replaced. Condenser unit energy was 

reduced by about one-third. 

Table 3. Average Daily Data, Summer Results Comparison 

Average Site Energy (kWh/day) Average Daily 

Temperature (°F) 

Year Total Building AHU Condenser Unit Total A/C Indoor Outdoor 

2009 56.5 4.2 27.9 32.1 79.1 81.5 

2010 57.7 5.0 30.4 35.4 79.5 82.3 

2011 41.7 2.3 18.5 20.7 79.3 80.3 

Savings Over Previous Years (%) 

2009 26 46 34 35 

2010 28 55 39 41 

Combined 27 51 37 39 

Note: A/C, air conditioning 
8 

Figure 4. Unadjusted average daily cooling performance comparison

Electric demand was also analyzed over the same summer periods. An average daily demand 

profile was created (Figure 5) by averaging 15-min power draws and plotting the pre- and postretrofit values. The figure shows what an average 24-h day would look like before and after 

energy improvements took place. Average 15-min peak demand was reduced by 37% on a 

whole-building basis; submetered A/C peak demand (combined AHU and condenser unit 

energy) was reduced by 46%. The peak summer demand window for utilities is typically from 

hours 14 to 16 (2 p.m. to 4 p.m.), and reductions during this period were slightly reduced at 34% 

for whole-building demand and 45% for A/C peak demand. 

24.4 22.3 20.9

27.9 30.4

18.5

4.2 5.0

2.3

0

10

20

30

40

50

60

70

2009 2010 2011

Average Energy (kWh/day)

Average Daily Summer Energy Comparison

Measured data from Jun 20 - Sep 30

AHU

Condenser

non-HVAC
9 

Figure 5. Pre- and post-retrofit average daily electric demand

4.2 Buried-Duct Condensation Analysis

Increasing the level of attic insulation is a common retrofit measure and can be highly cost 

effective in poorly insulated attics. Where flex duct systems are used for air distribution, these 

can often be found lying directly on top of existing insulation. This was the case for all the 

flex duct runouts in the study home, which were attached to a flex and ductboard trunk line 

elevated above the attic insulation and connected to the AHU, which was mounted in the 

conditioned space.

Labor expense is reduced by reusing existing flex ducts and simply applying blown attic 

insulation on top of the old ductwork. This effectively adds insulation to the duct system, which 

can deliver efficiency gains of its own but also reduces the exterior surface temperature, raising 

the probability of surface condensation during high dew point periods.

Covering supply ducts in attics with blown insulation results in colder external supply duct 

surfaces, which increases the potential for condensation during summer cooling periods. 

Condensation on duct exterior surfaces is primarily a building degradation concern. In a worstcase scenario, long cooling cycles would keep the duct surface cold, causing moisture to 

accumulate long enough that it builds up and runs off of the duct and onto other building 

materials. Wood, drywall, and some insulation materials could stay damp enough to support 

mold growth and eventually rot if substantial drying cycles do not occur. 
10 

Exterior duct condensation is not expected to cause an increase in cooling energy use, unless 

significant perforations in the outer duct jacket permit water to run into the insulation. Water 

would not dry out readily and would tend to accumulate over time, resulting in wet insulation 

that would drastically diminish the effective R-value. Although this is possible, it is not likely in 

most cases. Water most likely will accumulate on the exterior duct surface and then eventually 

run off the jacket if the surface is cold for long enough. 

A simple and relatively inexpensive monitoring method was desired that would be able to 

differentiate a wet duct surface from dry and determine the amount of time the duct was wet. The 

estimated duct surface moisture was a relative evaluation simply classified as dry, surface 

humidity >90%, surface condensation, and surface saturated. This phenomenon was investigated 

by evaluating attic air conditions as well as duct surface and supply air temperatures. In addition, 

duct surface moisture levels were estimated by measuring the voltage drop across two metallic 

strips imbedded onto a thin porous fabric substrate that was adhered to the duct surface. Voltage 

varied with the change in resistance on the strip as a function of the moisture level with the 

maximum occurring when the strip was fully wetted from condensation. This same principle is 

commonly used in moisture meters (James 1988). Resistive strips were attached to the exterior 

surface of a 5-in.-diameter R-6 flex duct in two locations (side and bottom). The side location 

would be at the 3 or 9 o’clock position if looking down the duct in direction of air flow. The 

bottom would be at the 6 o’clock position and represents the most likely location for 

condensation because it would have a colder surface temperature than the sides or top of duct 

covered by less insulation. The side location allows the bottom to be compared to a lower 

potential condensation area. The flex duct chosen for this investigation was the one nearest the 

cooling coil, meaning that the moisture-sensing strips were within 5 ft of the cooling coil. This 

represented the worst-case condensation potential with the coldest supply air passing through 

the duct. 

The moisture sensor tape was calibrated in a laboratory before field installation. The moisture 

sensor was applied to a duct foil jacket surface taken from a flexible duct. The duct jacket was 

then placed in direct contact around a container of water. The sensor had a 2.5-V excitation 

applied every 30 s, and the differential voltage was read across the sensor after each excitation. 

The wetter the surface, the greater the measured voltage across the sensor. Visual observations of 

the duct jacket surface were made for moisture indications of dry, light condensation (fog), small 

droplets of water, and running drops of water. The sensor voltage was noted during these 

observations. Ice was gradually added to water inside the container to create moisture at the duct 

jacket surface. Type T thermocouples were placed on the jacket surface and inside the ice water 

mixture. The temperature and RH of the surrounding air was also measured to determine the dew 

point of the air environment around the sensor at the test surface. 

The sensor was also tested with placement in unconditioned space during periods of high outdoor 

RH without an ice water mixture inside the container. This allowed the characterization of lower 

boundary sensor voltage for RH > 90%, but without visible signs of condensation. 

In the field installation, a 2.5-V, direct-current potential was applied to the sensor, and the 

measured voltage across the sensor was recorded at 30-s intervals. Voltage readings varied from 

a low of a few hundred millivolts under dry conditions to 2,300 mV or more for fully wetted, 

condensing conditions. Initial laboratory experiments found two other significant threshold 
11 

levels: (1) when the surface is still dry but ambient RH is elevated to near 90% RH (800 mV) 

and (2) when the surface initially begins to show condensation (1,600 mV), as shown in Table 4. 

Although rigorous experiments have yet to be conducted with these strips, these thresholds form

a basic framework to evaluate readings in this particular research project. 

Table 4. Moisture Strip Readings Derived from Initial Laboratory Observations

Measured Output (mV) Observed Conditions

400 or Less Dry conditions (ambient below 60% RH)

800 Dry conditions (ambient approximately 90% RH)

1,600 Beginning of surface condensation

2,300 or More Fully wetted surface (condensation)

Figure 6 illustrates relative duct moisture levels using the Table 4 thresholds during pre- and 

post-retrofit periods. Three periods chosen from the data set (one pre-retrofit and two postretrofit) show the impact on moisture readings under various circumstances. Data points in 

Figure 6 were plotted by taking the daily average duct surface moisture (mV) against daily 

average outdoor dew point (°F). Duct moisture data logging began in late May 2011 just 3 weeks 

before the blown insulation was added, as reflected in the relatively small number of blue data 

points clustered near the 500-mV level. Although this data set is small and was taken at moderate 

dew point temperatures, it illustrates the relatively low potential for condensation before burying 

the ducts in insulation. 

Figure 6. Relative duct moisture levels during one pre-retrofit and two post-retrofit periods
12 

A larger data set was collected after the blown attic insulation was applied. These data were

separated into two groups by level of daily average outdoor dew point, which tended to be higher 

from June through September (red data points) than from October through December (green data 

points). Condensation potential is elevated during the summer months when dew points are 

generally 65°F or higher and reduced in the dryer fall and winter months. Interior conditions 

were maintained at a nearly constant 79°F during this study, suggesting that lower thermostat 

settings would create greater potential for duct surface condensation because the ducts would be 

cooled for longer periods and possibly reach lower surface temperatures. 

Additional data need to be collected during hot and humid weather to determine how well the 

moisture tape performs as a moisture sensor over long-term periods. It still needs to be 

determined if the conduction will change over time as the sensor material ages. Controlled 

laboratory experiments did find that the absolute measure of voltage varied by the length of 

sensor used and the type of material to which it was adhered. In this monitoring experiment, the 

sensor was evaluated for the type of flex duct surface to which it was attached. It is important to 

remember that the sensor is only intended to indicate a relative amount of moisture so that 

adequate warning can be provided to prompt an inspection before building material can become 

damaged. 

Data were selected for 2 days that had similar outdoor dew points representative of common 

summer conditions. Data for the selected days are shown in Figures 7 through 9, one before and 

the other after adding blown insulation to illustrate the impact of burying the ducts. Figure 7 

shows the similarity of outdoor and attic dew points between periods. Pre- and post-retrofit 

outdoor dew points were quite similar in each case, remaining at or above 70°F during the entire 

period. Attic dew point temperatures were also quite similar but varied over the period, with each 

data set experiencing a peak between 3 p.m. and 4 p.m. before declining steadily until 

approximately 7 a.m. the next morning. This is a common characteristic in summertime attics 

where moisture desorbs from wood building materials during solar heating, adding moisture into 

the attic air and resulting in elevated dew points that can exceed the ambient dew point. As the 

attic begins cooling down overnight, the dry attic wood begins to adsorb moisture from the air, 

and the attic air dew point drops (Cleary 1985; Fairey, Swami, and Beal 1988). 

Figure 7. Outdoor and attic dew point temperatures 
13 

Figure 8 compares duct surface temperatures over a daily period. This graph clearly shows how 

duct surface temperature is reduced after being buried in insulation. Dotted lines represent the 

pre-retrofit condition, which shows higher surface temperatures than the post condition, as would 

be expected. The bottom surface of the duct showed an 8°F drop in temperature during peak 

temperatures, and the side surface location showed a 12°F decrease. 

Figure 8. Pre- and post-retrofit attic duct dry bulb temperatures during similar outdoor conditions

Figure 9 shows duct surface moisture levels. Two solid horizontal lines are shown as a reference 

to the degree of wetness. The solid red line at the top indicates a saturated surface dripping with 

water. The solid blue line indicates a lesser degree of moisture with condensation present on the 

surface, but not dripping wet. A clear impact can be seen where adding insulation over the ducts 

results in greater surface moisture levels. The bottom area is coolest and shows the highest 

moisture levels. Gaps in the post-retrofit measured data (dashed lines) can be seen in the graph 

and are unexplained at the time this writing. These gaps occurred only during the post-retrofit 

period when moisture levels were elevated. Further research in the use of moisture-sensing strips 

will help establish the long-term reliability of moisture sensing tapes. 

Figure 9. Pre- and post-retrofit attic duct surface moisture levels during similar outdoor conditions
14 

Figure 10 is a 24-h plot of post-retrofit duct, attic, and outdoor conditions when dew point 

temperatures and A/C runtime were at their highest. This would represent a higher potential for 

attic duct condensation. As seen in Figure 9, the moisture sensor on the bottom duct location 

showed the highest potential for condensation. The daily average reading of the bottom duct 

location in Figure 10 is 1,929 mV. This is high enough to indicate some condensation, but not 

high enough to be considered saturated. A visual inspection of the ducts conducted in late fall 

showed no obvious signs of physical damage from accumulated moisture. Therefore, even 

though the moisture sensors indicate significant periods of elevated moisture during hot and 

humid weather, it was not enough to create any visually observable impact such as material 

damage or mold and mildew. As a caution, however, even though no moisture damage was 

observed, it might still be possible under different circumstances. As mentioned previously, a 

colder thermostat cooling set point will cause long runtime and colder duct surfaces. It is 

possible that such operation in this home could still result in enough moisture accumulation to 

cause moisture-related damage. Conditions will continue to be observed during the next hot and 

humid period. 

Figure 10. Post-retrofit duct conditions during worst-case dew point temperatures 
15 

5 Conclusions

This research provides a measured example of energy savings from a moderate residential 

retrofit in a hot-humid climate with relatively high occupancy (six persons). The retrofit 

measures employed are common and often cost-effective choices for older homes in need of A/C

replacement and with relatively low attic insulation levels. Although homeowners can expect to 

benefit from reduced cooling energy costs (30% in this case), electric utilities will value the 

substantially reduced peak summer demand, which in this case averaged 46% for cooling only 

and 37% on a whole-home basis. Demand reductions during the typical utility summer window 

(2 p.m. to 4 p.m.) were slightly reduced at 34% for whole-building demand and 45% for A/C 

peak demand. 

Moisture damage from condensation on attic ducts buried in blown insulation was found to be a 

real possibility, but the consequences from one cooling season appear to be minimal. The duct 

surface moisture levels increased from dry during the uncovered duct period to periods of 

condensation after covering the ducts with insulation from late June through September. About 

45% of these days had sensor indications of duct surface condensation. After September, the 

outdoor dew points dropped enough to result in dry duct surfaces during air conditioning. 

Although this study home did not exhibit physical evidence of moisture damage, concerns about 

covering attic ducts with blown insulation over the long term remain. Further study is needed to 

investigate how the potential moisture damage increases with vented attic and lower cooling set 

points. This should include additional data on duct surface condensation in the standard 

(unburied) condition, especially at higher dew point temperatures that were limited in this 

investigation. Investigation of this phenomenon at lower thermostat set point temperatures than 

the temperature of the study home (79°F) might reveal greater potential for damage. The study 

home’s unvented attic may also have reduced the chance of condensation by limiting infiltration 

of summertime, higher dew point outside air. Additional laboratory work under controlled 

conditions would also yield a clearer interpretation of readings from the moisture-sensing strips, 

which were limited in this study to four distinct ranges.
16 

References 

Chasar, D., et al. (2006). “Cooling Performance Assessment of Building America Homes.” 

Proceedings of the Fifteenth Symposium on Improving Building Systems in Hot Humid Climates.

FSEC-CR-1673-06. Cocoa, FL: FSEC. 

Chasar, D.; Moyer, N.; Martin, E. (2006). Energy Efficient Renovations of Storm Damaged 

Residences—Florida Case Studies. FSEC-CR-1648-06. Cocoa, FL: FSEC. 

Cleary, P.G. (1985). “Moisture Control by Attic Ventilation- An In-Situ Study.” ASHRAE 

Transactions (91). 

Cummings, J.; Parker, D.; Sutherland, K. (2010). Evaluation of Bias Issues within RegressionBased Inverse Modeling Methods Against Climate and Building Characteristics Using Synthetic 

Data. FSEC-CR-1863-10. Cocoa, FL: FSEC. 

Fairey, P.; Swami, M.; Beal, D. (1988). RBS Technology Task 3 Report. FSEC-CR-211-88. 

Cocoa, FL: FSEC.

James, W.L. (1988). Electric Moisture Meters for Wood. United States Department of 

Agriculture Forest Service General Technical Report. FPL-GTR-6. Washington, DC: Forest 

Products Laboratory. 

Kissock, J.K.; Haberl, J.; Claridge, D. (2003). “Inverse Modeling.” ASHRAE Transactions – 

Research (109:2); pp. 425–434. 

Parker, D.S.; Sherwin, J. (2012). Achieving Very High Efficiency and Net Zero Energy in an 

Existing Home in a Hot-Humid Climate: Long-Term Utility and Monitoring Data. Washington, 

DC: U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. 

Parker, D.S. et al. (1998). Field Evaluation of Efficient Building Technology with Photovoltaic 

Power Production in New Florida Residential Housing. FSEC-CR-1044-98. Cocoa, FL: FSEC. 
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