Conference Paper

NREL/CP-550-48163

August 2010

Ducts in the Attic? What Were 

They Thinking?

Preprint

David Roberts and Jon Winkler

National Renewable Energy Laboratory

Presented at ACEEE Summer Study

Pacific Grove, California

August 15–20, 2010
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Ducts in the Attic? What Were They Thinking?

Dave Roberts and Jon Winkler, National Renewable Energy Laboratory 

ABSTRACT 

As energy-efficiency efforts focus increasingly on existing homes, we scratch our heads 

about construction decisions made 30, 40, 50-years ago and ask: “What were they thinking?” A 

logical follow-on question is: “What will folks think in 2050 about the homes we’re building 

today?” This question can lead to a lively discussion, but the current practice that we find most 

alarming is placing ducts in the attic.

In this paper, we explore through literature and analysis the impact duct location has on

cooling load, peak demand, and energy cost in hot climates. For a typical new home in these 

climates, we estimate that locating ducts in attics rather than inside conditioned space increases 

the cooling load 0.5 to 1 ton, increases cooling costs 15% and increases demand by 0.75 kW. 

The aggregate demand to service duct loss in homes built in Houston, Las Vegas, and Phoenix 

during the period 2000 through 2009 is estimated to be 700 MW. 

We present options for building homes with ducts in conditioned space and demonstrate 

that these options compare favorably with other common approaches to achieving electricity 

peak demand and consumption savings in homes. 

Background

Heat exchangers are designed to transfer as much heat as possible from one fluid to 

another. The heat exchanger we commonly find in a solar storage tank is an immersed coiled 

tube; we move solar heated water through the tube and it heats the water in the tank. In a good 

solar storage system, we place the coil at the bottom of the tank and strive for temperature 

stratification in the tank so we bring the hottest water in the system (from the collectors) in 

contact with the coldest water in the system (from the cold-water mains, settled at the bottom of 

the storage tank). 

This is oddly similar to the configuration we see in many homes being built in coolingdominated climates – we place tubes (with hundreds of square feet of surface area) carrying the 

coldest fluid in the system (55°F air leaving the air-conditioning unit) immersed in the hottest 

fluid in the system (150°F attic air). The only difference is, in this system we don’t want heat 

exchange. Heat exchange is a bad thing because we don’t need to cool the attic air (it’s outside 

our enclosure) and we certainly don’t want to heat the air we just paid the electric utility to cool. 

So, to overcome this serious design flaw we create energy codes that require us to put some goop 

on joints and wrap an inch or so of insulation around the tubes. Feels a bit like bubble gum and a 

bandage − MacGyver would be proud. In fact it reminds us of some of the old homes we see 

where someone stuffed wadded-up newspapers into the walls or poured sawdust on the attic floor 

in an attempt to improve on construction practices of the time. You can almost hear the old-timer 

as he’s working to “fix” things – “What the heck were they thinking Myrtle?” 

1
Figure 1. Ducts in an Unconditioned, Vented Attic

Source: ducts.lbl.gov. Used by permission of LBL.

We know placing ducts in a vented, unconditioned attic is a bad idea because we’ve 

modeled it (Siegel, Walker & Sherman 2000; Walker 2001; Hendron et al. 2002; Hedrick 2003b; 

Kinney 2005). We know it’s a bad idea because we’ve measured it (Jump, Walker & Modera 

1996; Hedrick 2003a). In fact, we know even when it is done pretty well (which is rare), it 

reduces system efficiency by about 20% − not a trivial number. People spend big bucks trying to 

save 20%. Or even bigger bucks to produce that amount of electricity with sexy solar panels. 

And the worst part is that it’s a pretty permanent situation. Fifty years from now, after we’ve 

evolved to the point where we agree this is a bad idea, it will be really hard to change things. It’s 

kind of like figuring out how to go back and insulate walls in millions of homes built in the first 

half of the 20th century. Actually, it’s worse than that.

And here’s what’s really strange. Electric utilities hand out money for things that have far 

less impact. They might give you $500 to upgrade to a SEER 15 air conditioner because it’s 

cheaper than building a new power plant. We’ll take it! Then we’ll take our fancy SEER 15 air 

conditioner (which will probably last 15 years or so and is really easy to upgrade) and hook it up 

to a lousy delivery system (which will likely be around for 100 years and is really hard to 

improve), effectively turning it into a SEER 12 unit (Neal 1998). 

Most of us agree it’s better and cheaper to reduce demand than to increase supply. So 

let’s see what it saves, what it’s worth, and what it costs to move ducts inside the building 

enclosure, and compare that to other popular efficiency measures that electric utilities commonly 

pay for. 

2
What it Saves

Savings Reported from Previous Studies and Reports

Table 1 provides an overview of the energy, demand, and cost savings estimated from 

some earlier studies. Long and short of it – ducts in attics are about 80% efficient at best, add 1⁄2 

to 1 ton to the cooling load, and increase peak demand by about 1 kW. 

Table 1. Sampling of Measured and Predicted Savings From Previous Studies

Reference Type Highlights

Jump, Walker & Modera 1996 Measured Average distribution system efficiency of 76% after repair in 

Sacramento, CA.

Siegel, Walker & Sherman 2000 Modeled Predicts 1.0 kW electric demand savings and 55% daily energy 

savings when moving typical new duct system from vented attic 

to cathedralized attic space and reducing A/C sizing in 

Sacramento, CA.

Walker 2001 Modeled Shows distribution system efficiencies of ~75% for R-4.2, 10% 

leakage duct system in attics during cooling season in 

Sacramento and Phoenix.

Hendron et al. 2002 Modeled At duct leakage levels allowed under 2009 IECC, modeling 

indicates about 20% cooling energy savings and 1.0 kW of 

demand reduction from cathedralizing attic in Las Vegas.

Hedrick 2003a Measured Measured duct leakage-to-outside averaged 29 cfm25 in 16 CA 

homes with ducts inside conditioned space – some using drop 

ceilings, some with cathedralized attics – a fraction of the 

leakage allowed by code and commonly found in new homes 

with ducts in the attic.

Hedrick 2003b Modeled California statewide average energy savings for a 2-story, singlefamily home is predicted to be 3,400 kWh/year from moving 

ducts inside conditioned space. Associated predicted demand 

savings ranged from 0.8 to 3.3 kW, depending on climate.

Kinney 2005 Modeled Predicts about $220 to $240 in annual energy cost savings when 

reducing duct leakage to outside from 30% to 4% and reducing 

conductive losses using R-30 insulation (essentially moving 

ducts inside conditioned space) in Phoenix and Las Vegas.

Our Analysis

We looked at three cooling-dominated climates (Houston, Phoenix, and Las Vegas) 

where standard construction practice is to place ducts in the attic. In the first part of the analysis 

we used ASHRAE Standard 152-2004 (ASHRAE 2004) calculation procedures to estimate the 

energy penalty associated with locating the ducts in the attic. We then used an annual building 

simulation tool (BEopt 0.9 software1

 

1 The BEopt software tool was developed at NREL to identify optimal building energy designs aimed at minimizing 

the total of the amortized cost of improvements and the cost of energy. It produces designs that minimize combined 

construction and energy costs by using the DOE-2.2 and TRNSYS energy simulation programs to automate a 

sequential search technique for locating least-cost solutions on a path toward net zero energy. The software and 

underlying methodology are described in detail by Christensen et al. (2005, 2006) and Horowitz et al. (2008).

) to estimate the annual energy savings and peak demand 

reduction associated with relocating the ducts to conditioned space.

3
The prototypical house used in our analysis is a 2-story, 2500 sq. ft., slab-on-grade home 

with R-13 walls, R-30 vented attic, and 0.3 solar heat gain coefficient (SHGC) windows. The 

house has a SEER 13 air-conditioner, gas furnace, complies with ASHRAE Standard 62.2 for 

ventilation requirements, and is assumed to be fairly tight with a 0.0003 specific leakage area 

(SLA). The ducts are located in the vented2

ASHRAE Standard 152-2004 establishes a methodology for calculating the distribution 

system efficiency (DSE)

attic and assumed to be well insulated and well 

sealed with R-8 insulation and 5.5% leakage on the supply side and 4.5% leakage on the return 

side. This duct system, characterized in the BEopt software as “tight,” can be considered pretty 

well constructed, exceeding the minimum requirements of the 2009 IECC. 

3 of a ducted system. We used appropriate cooling system capacities 

and air flow rates for our example home and Standard 152 procedures to calculate DSEs for the 

three locations. The results are shown in Table 2. 

Table 2. ASHRAE Standard 152 Calculated Distribution System Efficiency

Houston Phoenix Las Vegas

Equipment Cooling Capacity (kBtu/hr) 36.0 48.0 42.0

Cooling Fan Flow (cfm) 1200 1600 1400

Cooling Supply Duct Leakage (cfm) 66 88 77

Cooling Return Duct Leakage (cfm) 54 72 63

Cooling Design DSE 71% 72% 73% 

Cooing Seasonal DSE 79% 82% 81% 

The average DSE for the three locations on the design day, which would be considered 

the day of the season when cooling demand is highest, is 72%. This means that on the hottest day 

of the summer, 28% of the air-conditioner output is ultimately lost. Over the entire cooling 

season the average loss in cooling capacity is 20%. So in a climate where the air-conditioner is 

likely used for eight to nine months of the year, 20% of the cooling produced by the airconditioner is simply thrown away by the distribution system.

The obvious alternative is to place the ducts in conditioned space. Here, most of the air 

leakage and thermal losses of the distribution system would go into cooling the living space. The 

end result is that the cooling system design load, energy consumption, and peak demand imposed 

on the utility are reduced. 

To estimate reductions in annual cooling energy usage, peak load, and peak demand, 

BEopt 0.9 software was used to simulate the prototype house over the course of an entire year, in 

the three climates previously mentioned, with the ducts first located in the attic and then moved 

to the living space. In BEopt, ducts in living space are assumed to have a DSE of 100% (an 

admittedly optimistic assumption and difficult to achieve in practice). Table 3 shows the percent 

reduction from the baseline case (ducts in the attic) to the case with the ducts located in 

conditioned space.

 

2 Attic vent area set to 1 sq. ft. per 300 sq. ft. of attic floor area.

3 Modera (1993) provides a pretty clear definition of DSE: the ratio of the energy that would be consumed by a 

house using a given piece of heating or cooling equipment, to the energy consumed by that house with the thermal 

distribution system connected to that same piece of equipment. 

4
Table 3. Savings Due to Moving Ducts Inside Living Space

Houston Phoenix Las Vegas

Reduction in Required A/C Capacity 24% 24% 23%

Reduction in Annual Cooling Electricity Usage 17% 16% 14% 

Reduction in Peak Cooling Demand 22% 23% 22% 

Let’s take a closer look at what these savings mean. On average (for this particular 

home), the cooling system can be reduced by nearly 0.8 tons of capacity. The capital cost savings 

associated with this capacity reduction will be realized every time the air-conditioning unit is 

replaced, (about every 15 years), because the ductwork will be used for the life of the home. 

According to the Energy Information Agency, the average price of electricity in these three cities

during 2009 was 12.13 ¢/kWh (EIA 2009), meaning on average the homeowner would save 

more than $80 per year to cool their home. (This does not include additional savings attributed to 

heating the home during the winter.) The peak demand placed on the utility, which occurs on the 

hottest day of the year, is also reduced on average by more than 0.75 kW. These savings 

ultimately reduce the need to construct additional generation and distribution capacity as new 

homes are added to the utility system. Time-of-use rates, likely to become more common with 

the advent of smart grid technologies, will increase the cost of running air-conditioning units 

during periods of peak demand, further bolstering our case for moving ducts out of the attic. 

What it Costs

There are several approaches to building homes in cooling-dominated climates with ducts

located inside the enclosure: (1) cathedralizing the attic – moving the thermal and air barrier 

from the attic floor to the roof deck; (2) using drop ceilings and soffits below the attic floor; (3) 

in 2-story homes, using the space between floors; and (4) using a scissor-truss to create a plenum 

below the attic and above the living space. The fourth approach is not widely used. The first 

three have been used by quite a few builders so there is a fair bit of cost data in the literature. 

Ultimately, the best approach, and associated cost, will depend on each builder’s current 

construction practices.

Figure 2. Schematics of Four Approaches To Moving Ducts Inside Conditioned Space

5
Figure 3. Examples of Dropped Ceiling and Between Floor Locations During Construction

Source: Janet Mcilvane, FSEC (left) (used by permission of FSEC) and IBACOS, Inc. (right) (NREL PIX 14234). 

Kerr (2008) provides an overview of an approach taken by two production builders in the 

Northwest. Both used space between the first and second floors (one used open-web trusses) and 

added furnace closets to get the air handler into conditioned space. The estimated cost to make 

the change was $500. 

Lubliner et al. (2008) report stipulated costs of $650 for utility programs in the Northwest 

developing demand-side programs that encourage relocation of the duct system to the 

conditioned space. They state that one production builder in the Northwest added $675 to the 

price of its homes to cover this cost.

Hedrick (2003b) estimated additional construction costs for approaches 1, 2, and 4 using 

three methods of estimation for various house sizes. Cost estimates for approach 4, the plenum 

truss, were quite high, approaching $4000. The costs for approaches 1 and 2, however, ranged 

from $800 downward to a saving of $800 for single-family homes. This did not include the 

savings from downsizing the air conditioner, furnace and air-handling unit, which were estimated 

to be $1100 to drop from a 4-ton to a 3.5-ton unit or from a 3.5-ton unit to a 3-ton unit. 

DOE (2010) reports that Tommy Williams Homes in Gainesville, Florida used ducts in 

conditioned space to meet the Builders Challenge program requirements and produce a net-zero 

energy home. Ken Fonorrow (2010), the builder’s home energy rater, reports the estimated 

additional cost to move the ducts into dropped ceilings is $800, not including savings associated 

with downsizing the air-conditioner. 

How it Compares

The easiest way to put the savings attributed to relocating ducts to conditioned space into 

perspective is to compare it to other energy efficiency upgrades (most of which are often given 

more attention in energy codes and utility programs than the relocation of ductwork). We used

BEopt to compare moving the ducts into the conditioned space to upgrading the wall R-value, 

upgrading the attic floor R-value, using low SHGC windows, and installing a higher SEER airconditioner. 

Of the energy efficiency upgrades included in the analysis, moving the ducts into 

conditioned space led to the most significant reduction in the required air-conditioner capacity.

Incrementally enhancing the building envelope over the prototype home slightly reduces the 

need for cooling, but not significantly compared to relocating the ducts. Upgrading the SEER 

rating of the air-conditioner does nothing to dramatically influence the air-conditioner capacity

6
and only decreases the amount of energy required to provide a certain amount of cooling, thus 

the effect of SEER was not included in Figure 4. 

Figure 4. Reduction in Required Cooling Capacity for Various Efficiency Upgrades

In terms of the amount of electricity required to cool the home over the course of the 

year, relocating the ducts slightly out performs upgrading to even a SEER 17 air-conditioner (see 

Figure 5). However, relocating the ductwork is a one-time expense that is amortized into the cost 

of the mortgage. The air-conditioning unit, by contrast, is an upgrade that will need to be 

repurchased. Again, incrementally enhancing the building envelope goes only so far to reduce 

the energy consumption required to cool the home. 

Relocating the ducts to conditioned space reduces peak demand far more than any other 

energy efficiency upgrade included in the analysis (see Figure 6). The SEER 15 air-conditioner 

reduces peak demand more than the SEER 17 air-conditioner, which is contrary to expectations, 

because the SEER 17 unit is a two-speed unit and can operate at low stage to save energy when 

the cooling load is not as high, the energy efficiency ratio (EER) for the SEER 17 unit at stage 2 

cooling is slightly lower than that for the SEER 15 unit. Thus, on the hottest of days the SEER 

17 unit will draw more power. Upgrading the attic insulation levels and windows does little to 

reduce demand. R-19 walls show modest improvement over the R-13 walls. 

0%

5%

10%

15%

20%

25%

% Reduction in A/C Capacity

Houston Phoenix Las Vegas

7
Figure 5. Reduction in Cooling Electricity Usage for Various Efficiency Upgrades

Figure 6. Reduction in Peak Cooling Demand for Various Efficiency Upgrades

Selecting energy efficiency upgrades should be based not only on the potential savings 

but on the expected total cost, including the initial capital cost, energy bill savings, and 

replacement cost after a given number of years. BEopt was selected to perform this analysis, in 

part, because it is well suited to optimize the order in which these upgrades should be selected. 

Only the results for Houston are shown in Figure 7, but the results for the other two cities display 

similar trends. The total normalized cost relative to the baseline is plotted on the y-axis and the 

annual electricity savings on the x-axis. Total normalized cost includes the amortized 

incremental capital cost of the energy efficiency measure, including replacements during the 30-

year mortgage, and the associated energy cost savings. Values less than 1.0 indicate energy cost 

savings are greater than the incremental mortgage and replacement costs. 

0%

5%

10%

15%

20%

% Reduction in Cooling Electricity

Houston Phoenix Las Vegas

0%

5%

10%

15%

20%

25%

% Reduction in Peak Cooing Demand

Houston Phoenix Las Vegas

8
The first selected energy efficiency measure is upgrading to low SHGC windows, 

because this option has the most negative slope relative the baseline point. However, the annual 

energy savings are minimal (1%). Moving the ducts inside is the next preferred option. The 

percent energy savings per unit cost are slightly higher than upgrading windows, but moving the 

ducts has the most dramatic effect on annual energy savings. The next preferred option is to 

upgrade to a more efficient SEER 15 air-conditioner. As can be expected, the electric 

consumption is most cost-effectively lowered by moving the ducts inside and upgrading the 

windows before upgrading the air-conditioner. 

The final three points deal with wall and attic insulation levels. R-15 walls do not show 

as a preferred option because of their small incremental benefit over the code R-13 walls and the 

fact that R-19 walls reduce the framing factor by moving to 2x6 studs spaced 24 inches on 

center. There is little energy savings associated with higher attic insulation values in these 

cooling-dominated climates. 

Figure 7. Optimized Energy Efficiency Upgrades for Phoenix

A Utility Perspective

Most electric utilities have programs aimed at reducing system peak demand or overall 

electrical consumption. The programs generally involve utility investment in energy efficiency 

measures. In buildings this usually comes in the form of payments for specific technologies (e.g., 

compact fluorescent lamps) or performance targets (e.g., ENERGY STAR Qualified Home). The 

size of the payments or rebates is usually based, in part, on the avoided cost to the utility of 

generating a kilowatt-hour of electrical energy or building the next kilowatt of capacity. 

Peak demand for utilities in cooling-dominated climates is generally driven by airconditioning. Demand-side programs from these utilities tend to target air-conditioning, paying 

incentives for measures that reduce cooling during periods of system peak. Common measures 

include low solar heat gain windows, high-efficiency air conditioners, and direct load control. In 

existing homes, measures might include solar screens, duct insulation and sealing, and air

conditioner tune-ups. We could not find any utility that offers direct incentives to design and 

build homes with ducts in conditioned space. 

0.88

0.9

0.92

0.94

0.96

0.98

1

0 5 10 15

Normalized Total Annual Costs

Annual Electricity Savings (%)

R50 Attic

Baseline

Ducts Inside

Low SHGC Windows

SEER 15 A/C

R40 Attic

SEER 17 A/C & 

R19 Walls

9
Our analysis indicates payments to incentivize builders to move ducts out of attics would 

be a wise investment for utilities in cooling-dominated climates. For example APS, an electric 

utility serving the Phoenix area, will pay $425 for a SEER 14 air-conditioner (APS 2010). Figure 

6 shows moving the ducts inside will save more than twice the peak demand as the airconditioner, so one might conclude that APS could pay an $800 incentive for a builder to move 

the ducts. Similarly, CenterPoint and Entergy, two utilities serving the Houston area, will pay 

$477/kW and $0.16/kWh for duct sealing in existing homes (DSIRE, 2009). Based on our 

estimates of demand and energy savings for Houston, similar rebates might total $380 for 

moving a duct system from the attic into conditioned space in new homes. And finally, according 

to the DSIRE website (2009), NV Energy, which serves the Las Vegas market, was paying $280 

to upgrade to a SEER 14, 4-ton air conditioner in 2009. Again, referring to the results in Figure 

6, one might conclude that a $500 rebate for homebuilders would be appropriate for moving the 

ducts. 

The BEopt simulations results in Figure 8 emphasize the impact of shaving peak demand. 

The high demand associated with air-conditioning generally occurs over the span of only a few 

hours in the evening. The additional capacity required by the electric utility to meet the peak 

cooling demand is being fully utilized for several hours of the day on the hottest days of the year. 

Thus, the demand offset by relocating the ducts to conditioned space in new construction will 

ultimately reduce the need for additional installed capacity and improve the load factor for the

electric utility.

Figure 8. Cooling Demand Profile During Day of Peak Demand for Houston 

The accumulation of potential savings is startling. According to the U.S. Census Bureau

(2010), approximately 930,000 single-family homes were built between 2000 and 2009 in the 

combined markets of Houston, Phoenix, and Las Vegas. Assuming all these homes were 

constructed with ductwork in the attic, and using our savings estimate of 0.75 kW per home, 

nearly 700 MW of new installed capacity could have been avoided if all these homes had been 

built with ducts inside conditioned space. 

0

0.5

1

1.5

2

2.5

3

0 3 6 9 12 15 18 21 24

A/C Electric Demand (kW)

Hour of Day

Baseline Ducts Inside SEER 15 A/C

10
Summary

 Building a home in a hot climate with ducts located in the attic is a bad idea. Moving the 

ducts inside the thermal enclosure during construction is relative inexpensive and will reduce 

electric consumption for cooling about 15%, will reduce demand by about 0.75 kW, and will 

reduce needed cooling capacity by 0.5 to 1 tons. Spending money to move the ducts inside is a 

better investment than other commonly incented energy efficiency measures.

References

[APS] Arizona Public Service. 2010. APS Green Choice Residential AC Rebate Program 

http://www.aps.com/main/green/choice/choice_3.html?source=prgj. Phoenix, Ariz.: 

Arizona Public Service.

[ASHRAE] American Society of Heating, Refrigerating, and Air-Conditioning Engineers. 2004. 

Standard 152-2004 -- Method of Test for Determining the Design and Seasonal 

Efficiencies of Residential Thermal Distribution Systems (ANSI Approved). Atlanta, Ga.: 

American Society of Heating, Refrigerating, and Air-Conditioning Engineers. 

CenterPoint Energy. 2010. Residential Standard Offer Program. 

http://www.centerpointenergy.com/services/electricity/residential/energyefficiencyprogra

ms/residentialstandardoffer/. Houston, Tex.: CenterPoint Energy, Inc. 

Christensen, C., S. Horowitz, T. Givler, A. Courtney, and G. Barker. 2005. BEopt: Software for 

Identifying Optimal Building Designs on the Path to Zero Net Energy. Golden, Colo.: 

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Christensen, C. R. Anderson, S. Horowitz, A. Courtney, and J. Spencer. 2006. BEopt Software 

for Building Energy Optimization: Features and Capabilities. Golden, Colo.: National 

Renewable Energy Laboratory, NREL/TP-550-39929. 

[DOE] Department of Energy. 2010. Tommy Williams Homes Zero Energy Home Longleaf. 

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[DSIRE] Database of State Incentives for Renewables & Efficiency. 2009. www.dsireusa.org. 

Raleigh, N.C.: N.C. Solar Center / N.C. State University / College of Engineering. 

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Calif.: California Energy Commission 500-03-082-A-29. 

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Technical Information Report. Sacramento, Calif.: California Energy Commission 500-

03-082-A-31. 

11
Hendron, R., R. Anderson, P. Reeves, & E. Hancock. 2002. Thermal Performance of Unvented 

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Lubliner, M., R. Kerr, A. Gordon, and C. Murray. 2008. “Moving Ducts Inside: Big Builders, 

Scientists Find Common Ground.” In Proceedings of the ACEEE 2008 Summer Study on 

Energy Efficiency in Buildings, 1.152-1.163. Washington, D.C.: American Council for an 

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Modera, M. 1993. “One Size Fits All: A Thermal Distribution Efficiency Standard.” In Home 

Energy September/October 1993. Berkeley, Calif.: Home Energy Magazine. 

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1. REPORT DATE (DD-MM-YYYY)

August 2010

2. REPORT TYPE

Conference paper

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Ducts in the Attic? What Were They Thinking? Preprint

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

D. Roberts and J. Winkler

5d. PROJECT NUMBER

NREL/CP-550-48163

5e. TASK NUMBER

BE10.1001

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/CP-550-48163

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

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National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

As energy-efficiency efforts focus increasingly on existing homes, we scratch our heads about construction decisions 

made 30, 40, 50-years ago and ask: “What were they thinking?” A logical follow-on question is: “What will folks think 

in 2050 about the homes we’re building today?” This question can lead to a lively discussion, but the current practice 

that we find most alarming is placing ducts in the attic. In this paper, we explore through literature and analysis the 

impact duct location has on cooling load, peak demand, and energy cost in hot climates. For a typical new home in 

these climates, we estimate that locating ducts in attics rather than inside conditioned space increases the cooling 

load 0.5 to 1 ton, increases cooling costs 15% and increases demand by 0.75 kW. The aggregate demand to service 

duct loss in homes built in Houston, Las Vegas, and Phoenix during the period 2000 through 2009 is estimated to be 

700 MW. We present options for building homes with ducts in conditioned space and demonstrate that these options 

compare favorably with other common approaches to achieving electricity peak demand and consumption savings in 

homes.

15. SUBJECT TERMS

duct; ventilation; attic; conditioned space; duct location

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OF ABSTRACT

UL

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Unclassified

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