Efficient Multifamily Homes

in a Hot-Humid Climate by 

Atlantic Housing Partners 

D. Chasar and E. Martin

Building America-Partnership for Improved Residential 

Construction (BA-PIRC) 

April 2013
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iii

Efficient Multifamily Homes in a Hot Humid Climate by 

Atlantic Housing Partners

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 Eric Martin

BA-PIRC / Florida Solar Energy Center

Florida Solar Energy Center

1679 Clearlake Road 

Cocoa, Florida 32922 

NREL Technical Monitor: Stacey Rothgeb 

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

April 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........................................................................................................................................... 1

3 Energy Efficiency Specifications........................................................................................................ 3

3.1 Development of the High Performance Package .................................................................3

3.2 Enhancement of the High Performance Package .................................................................5

4 Heat Pump Water Heater Survey Results .......................................................................................... 6

5 BEopt Analysis ..................................................................................................................................... 7

6 Apartment Rating Results ................................................................................................................... 8

7 Conclusions ........................................................................................................................................ 10

References................................................................................................................................................. 11

Appendix A – Duct Sealing Procedures ................................................................................................. 12

Appendix B – Heat Pump Water Heater Survey Results....................................................................... 13
vi 

List of Figures

Figure 1. Cambridge Cove II Development, completed in 2008. ............................................................ 2

Figure 2. Plan detail for outdoor air ventilation and future dehumidifier (Source: Slocum Platts 

Architects, reprinted with permission)............................................................................................... 3

Figure 3. Estimated savings from improvements to a three bedroom apartment (assumes 

$0.13/kilowatt-hour (kWh) and $2.15/therm). ..................................................................................... 4

Figure 4. Plan view of the two most common types of AHP HPWH installations—breezeway 

installation on left (back to back units) and kitchen installation on right (Source: Slocum Platts 

Architects, reprinted with permission)............................................................................................... 7

Figure 5. BEopt analysis results for second floor, 937 ft2 2-bedroom apartment unit. ....................... 8

Figure 6. Distribution of HERS Index for 934 AHP-rated units............................................................... 9

Figure 7. Duct sealing procedures (Source: Slocum Platts Architects, reprinted with permission 12

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

List of Tables

Table 1. Construction and Equipment Details. ........................................................................................ 5

Table 2. BEopt E+ 1.4 Analysis of Typical Two Bedroom (997 ft2) Apartment Unit. ............................ 8

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

Definitions

A/C Air conditioning/air conditioner

AHP Atlantic Housing Partners

BA-PIRC Building America Partnership for Improved Residential Construction

CFM Cubic feet per minute

EF Energy Factor

FSEC Florida Solar Energy Center

HERS Home Energy Rating System

HPWH Heat Pump Water Heaters

IAQ Indoor Air Quality

HVAC Heating, ventilation, and air conditioning

kW Kilowatt

kWh Kilowatt-hour

MBtu/yr Million British Thermal Units Per Year

PV Photovoltaic

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 

SEER Seasonal energy efficiency ratio

SHGC Solar Heat Gain Coefficient

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

Utility Allowance Cost of utilities in federally assisted public housing established either by 

the local Public Housing Agency or by an engineering-based 

methodology 
viii

Executive Summary

With assistance from the Florida Solar Energy Center (FSEC) and its Building America 

Partnership for Improved Residential Construction (BA-PIRC), Atlantic Housing Partners (AHP) 

has implemented a high performance, systems-engineered package of measures in the new 

construction multifamily housing sector in the hot-humid climate zone. This report demonstrates 

how the initiative achieves Building America (BA) goals of 30%-50% energy savings. In 

addition, the report discusses the role of utility allowance calculations, used as part of the lowincome housing tax credit process, to value those energy savings. 

Results of energy modeling are presented to demonstrate that the specification package achieves 

40% energy savings in typical application. Results from Home Energy Rating System (HERS) 

ratings from more than 1,000 apartment units are included to show consistency of application on 

a large scale. A primary consistency has been communication of design intent via detailed 

schematics and step-by-step instructions in construction documents. 

In addition, this report describes some successes and challenges with the use of heat pump water 

heaters (HPWH) on a widespread basis, a key measure toward achieving the documented 

efficiency. Overall, the information provided in this report is most useful to multifamily 

developers looking to duplicate success through incorporating energy efficient strategies, and 

methods to value that efficiency in a rental, rather than homeownership, setting. 
1 

1 Introduction

Atlantic Housing Partners (AHP) is an affordable multifamily developer that has participated in

research with the Florida Solar Energy Center (FSEC) at various levels since 2003. Currently, 

AHP has a substantial portfolio of affordable multifamily rentals that includes more than 24,000 

units at 112 communities, primarily in Florida. As a U.S. Department of Energy (DOE) Building 

America (BA) research team, the Building America Partnership for Improved Residential 

Construction (BA-PIRC) has worked with AHP to steadily improve the efficiency and durability 

of these units’ construction in recent years. The goal of the BA program is to conduct research to 

develop market-ready energy solutions that improve the efficiency of new and existing homes by 

30%-50% (compared to 2009 energy codes for new homes and pre-retrofit energy use for 

existing homes). Through targeted research, industry partnerships, and collaboration with related 

DOE residential initiatives, BA works to make cost-effective energy efficient homes a reality for 

all Americans. Along with energy savings, the program also focuses on solutions that lead to: 

• Improved occupant health through effective indoor air quality 

• Higher comfort levels in all rooms throughout the home 

• Durable and moisture-resistant building designs 

• Increased builder profitability. 

This report demonstrates achievement of these goals by documenting efficient multifamily 

housing that is built cost effectively and on a large scale. While BA has conducted similar efforts 

for attached housing in hot-dry (Dakin, 2012), cold (Aldrich, 2012), and marine (Gordon, 2012) 

climates, little work has been done to demonstrate achievement of BA goals in this housing 

sector in hot-humid climates. The report presents results of energy modeling that document 

typical performance of the specification package, along with Home Energy Rating System 

(HERS) ratings conducted on a large scale to show consistency of application. A key aspect of 

this research includes how efficiency packages can be optimized in the context of multifamily 

rental housing built with low-income housing tax credits. This funding mechanism requires that 

the total cost of rent plus utilities be capped according to affordability criteria for the region. 

2 Background 

In 2003, FSEC assisted AHP with their first ENERGY STAR community, which consisted of 

208 units in Gainesville, Florida. In addition, since 2008, FSEC has assisted AHP with the 

installation of over 500 kilowatts (kW) of photovoltaic (PV) energy systems in ten communities. 

The systems are used to offset common area energy use (clubhouse, pool, etc.) at select AHP

communities in Florida (see Figure 1 example).
2 

Figure 1. Cambridge Cove II Development, completed in 2008. 

Much of the early assistance provided to AHP targeted non-energy-related concerns, such as 

high wintertime indoor humidity and building envelope moisture problems. Measurements at 

several Florida developments showed a pattern of high indoor humidity levels in some 

apartments during winter months. The worst problems were found in first floor, north-facing 

units where low solar loads resulted in the tendency for air conditioning (A/C) capacity to be 

oversized. Assistance with A/C sizing procedures led to an improved match with cooling loads, 

which varied widely by apartment size, orientation, and location in the two- to three-story 

designs. Initial results yielded a size reduction of roughly 1-ton per apartment at one 

development. Additional modifications to address the problem included addition of an outside air 

duct providing 30 to 75 CFM (depending on apartment size) of supply ventilation to the return 

plenum during space conditioning operation. The team also added utilities and space clearance to 

allow for addition of a supplemental dehumidifier (if needed). A schematic of the air handler 

closet incorporating these features is shown in Figure 2. FSEC and its partners have successfully 

used this mechanical ventilation strategy, involving the intermittent introduction of limited 

quantities of conditioned outside air, on thousands of homes to accomplish a number of 

objectives (Chandra, et al 2008). This feature was subsequently added as a standard feature in 

AHP construction, and no moisture-related complaints have been reported since these 

modifications were implemented.
3 

Figure 2. Plan detail for outdoor air ventilation and future dehumidifier (Source: Slocum Platts 

Architects, reprinted with permission).

3 Energy Efficiency Specifications

3.1 Development of the High Performance Package

One of the first steps to enhancing the performance of AHP multifamily units was revision of a 

duct sealing procedure and incorporation into the mechanical plan section of construction 

documents. This included step-by-step procedures (see Appendix A) on the use of foil tape, 

mastic, glass fab mesh, duct board components, and flex duct take-offs, as well as guidance on 

sealing interfaces between ducts and drywall. As shown in Figure 2, proper air sealing practices

during construction are particularly important where ducts are located between living spaces with 

limited access on the first and second floors. Ducts in third floor units are located in the vented 

attic. Duct testing results from over 900 units showed average leakage to out (Qn) of 0.04 or 4 

CFM per 100 ft2

 of floor area. Limited random duct testing performed on AHP construction prior 

to these advancements revealed that many units were already meeting high performance leakage 

targets of 3 to 6 CFM per 100 ft2, but occasional outliers had much higher leakage rates. New 
4 

procedures implemented on current construction have all but eliminated the high leakage 

outliers.

The team performed analysis to show the relative energy savings of various potential 

enhancements in a typical AHP dwelling unit. At the time of construction, AHP developments 

were built with a roughly even mix of all-electric and gas-electric utilities. Figure 3 shows the 

resulting savings from various measures in a typical top-floor, three-bedroom apartment at a 

development with both electric and gas utilities. Measures added to AHP’s standard construction 

included radiant barrier, programmable thermostats, and ENERGY STAR appliances. The more 

costly improvements, such as improved water heating and enhanced A/C efficiency, were not 

implemented initially. 

Estimated Annual Savings

Individual

$28 $12

$44

$3

$28

$57

$99

$66

$99

$125

$216

$-

$50

$100

$150

$200

$250

Rad. Barrier

Program T-stat

EStar Refrig

Windows (1)Windows (2)

50% Fluor.80% Fluor.

SEER 15 A/CSEER 17 A/C

EF.83 Gas HW

ICS Solar

Figure 3. Estimated savings from improvements to a three bedroom apartment (assumes 

$0.13/kilowatt-hour (kWh) and $2.15/therm). 

For some of AHP’s developments, incentives from the natural gas utility encouraged the use of 

multiple gas appliances including oven/range, dryer, hot water and space heating. FSEC and 

AHP began investigating a change from standard tank gas water heaters with an energy factor 

(EF) of 0.62 to tankless gas with an EF of 0.82. AHP had already made hydronic space heating 

sourced from the water heater a common practice in developments where gas was present. A 

side-by-side field comparison of tank and tankless gas water heaters conducted by FSEC in 2009 

showed a 20% measured savings for the tankless unit. This upgrade was never implemented on a 

large scale, for even with the utility incentives, the cost was considered prohibitive by the 

developer. 
5 

3.2 Enhancement of the High Performance Package

AHP primarily builds projects with funding through low-income housing tax credits, which 

require limiting the total cost of rent plus utilities to specified amounts according to affordability 

criteria for the project’s region. The allowance for each apartment’s utility cost is typically 

estimated by local housing authorities, or a utility company, based on comparable properties. In 

2010, a simulation-based alternative was introduced by the funding agency enabling customized 

energy cost estimates that account for energy efficiency measures that may not be present in 

otherwise comparable properties. Reduced annual energy use projected by the simulation 

alternative provides the basis for lower monthly utility allowances. It also provides a 

corresponding increase in income for the developer, given that total cost paid by the tenant 

remains fixed. 

The new incentive offered by simulation-based utility allowances caused AHP to re-examine the 

cost effectiveness of further efficiency upgrades. The wider availability of heat pump water 

heaters (HPWH) offered by AHP’s primary appliance supplier and analysis by FSEC showing 

greater savings with an all-electric design resulted in the developer suspending construction of 

properties with gas utilities. The enhanced, all-electric package also included a 15 SEER heat 

pump and progressively greater amounts of fluorescent lighting. Table 1 shows the details of this 

package including average performance testing results and HERS Indices from 1,086 of their 

most recently constructed units. 

Table 1. Construction and Equipment Details. 

Component Component Type and/or Value

Construction styles 1-story and 2-story townhomes and single level apartments in 

3-story buildings

Construction type Wood-frame

Floor type /area (ft2) Slab-on-grade / 720 (1BR) to 1660 (4BR)

Attic /roof type Vented / medium colored shingle

Window type U-value 0.35, SHGC* 0.30

Glass/floor area 6 to 10%

Insulation attic/wall/floor R-30 / R-15 / R-0

Exterior wall cladding Medium colored cement board

Space conditioning SEER 15 heat pump

Thermostat Programmable

Ventilation 30 to 75 CFM** (runtime at return plenum)

Water heater 50-gal HPWH (in all but one project)***

ENERGY STAR appliances Refrigerator, dishwasher

Fluorescent lighting 84% average (range: 10% to 100%)

Infiltration (ACH50) 6.0 average (range: 2.3 to 13.2)

Duct leakage Qn 0.039 average (range: 0.008 to 0.154)

HERS Index 59.2 average (range 51 to 77)

* Solar heat gain coefficient

* *Cubic feet per minute

*** Standard electric tank water heaters were used in one project totaling 90 units
6 

4 Heat Pump Water Heater Survey Results

AHP began installing HPWHs in most new-construction units in 2010. To date, there have been 

996 HPWHs installed at nine Atlantic communities, with several more developments planned for 

2013. An informal survey was conducted with managers at seven communities to determine the 

success of HPWHs over standard electric units. Survey results are shown in Appendix B. 

The number one complaint from tenants was the noise level generated by the evaporator fan. 

This problem was sufficient in some cases such that the heat pump feature was overridden by 

community management by placing the unit in standard heating mode. Standard mode is the least 

efficient setting where heating is provided via the electric resistance element as in a standard 

electric water heater. These cases were found primarily to exist in apartments where the HPWH 

was located in an interior closet where sound transmission is largely unimpeded. Many Atlantic 

apartment units use an exterior closet accessible from a common breezeway for easy 

maintenance, and this layout generated far fewer complaints. A plan view of these two 

installation scenarios is illustrated in Figure 4. The overall experience with HPWH has been 

positive, and AHP continues to install them on a large scale except where floor plans place the 

unit inside the apartment, especially in proximity to bedroom areas. Standard electric water 

heaters are used in these cases, which exist on a limited basis.
7 

Figure 4. Plan view of the two most common types of AHP HPWH installations—breezeway 

installation on left (back to back units) and kitchen installation on right (Source: Slocum Platts 

Architects, reprinted with permission).

5 BEopt Analysis

There are a limited number of apartment styles produced by AHP, and the majority of them are 

single-level units grouped in three-story buildings. Variations in home energy rating results were 

noted between floor levels, with the best scores occurring at the bottom level and the worst 

scores at the top level. This variation was also noted during analysis with BEopt E+ 1.4 with the 

lower and middle floor showing greater savings than the top floor. The primary drivers of this 

phenomenon are thought to be the additional ceiling cooling load and higher duct leakage to the 

Heat Pump Water

Heater Locations
8 

outside present in third floor units, which are not present on lower building levels. Ground 

coupling of the slab seems to provide benefit to first floor units compared to second floor units. 

A typical two-bedroom unit was chosen for analysis, as this apartment style occurs frequently in 

AHP developments. The results in Table 2 and Figure 5 reflect savings over the adjusted BA 

Benchmark (Hendron, 2010), which results from the specifications in Table 1 (Section 3.1). 

Table 2. BEopt E+ 1.4 Analysis of Typical Two Bedroom (997 ft2) Apartment Unit. 

Source Energy (MBtu/yr*)

Unit type 1st Floor 2nd Floor 3rd Floor

BA Benchmark** 161.1 156.5 156.5

Design 89.6 92.9 95.5

Savings 44% 41% 39%

Notes: *Million British thermal units per year 

**adjusted Building America Benchmark (area weighted)

 

Figure 5. BEopt analysis results for second floor, 937 ft2 2-bedroom apartment unit. 

6 Apartment Rating Results

AHP began conducting a HERS rating on each apartment in all new developments to comply 

with requirements defined in the simulation-based allowance calculation method. Results of 

these ratings were compiled for 1,086 Atlantic units from nine central Florida developments 

constructed between 2010 and 2012. Building construction, equipment, and testing details are 

shown in Table 1 (Section 3.1). The values listed represent a typical new Atlantic dwelling unit 

where construction and equipment specifications are largely standardized within the company.
9 

Two equipment items (fluorescent lighting and water heating) were found to vary between 

construction projects. HPWHs are being specified on all new Atlantic projects, but one 90-unit 

project used standard electric tank water heaters instead. Fluorescent lighting was used only 

sparingly in the past (lighting roughly 10% of floor area), but Atlantic has experimented with 

higher levels on new projects and is now specifying 80%-100% fluorescent lighting as standard. 

There was a natural variation in duct and envelope tightness; however, on average, these 

measures were found to be adequate for high performance construction in central Florida. 

Results compiled from 1,086 rated units yielded an average HERS Index of 59.2 and ranged 

from a low of 51 to a high of 77. Figure 6 illustrates the HERS Index variation between 934 fully

rated units. One development of 152 units was removed from this chart, as these units underwent 

“Class 2” Florida ratings. A Class 2 Florida rating consists of an on-site audit to verify 

construction features and equipment efficiencies. This “audit-only” type of rating requires no 

duct or envelope performance testing. Instead, it assumes relatively large default leakage values, 

which resulted in a HERS range of 63 to 77. Performing full energy ratings on these units 

(including duct and envelope testing) would likely have resulted in HERS values similar to the 

remaining apartment sample. Removing the Class 2 ratings from the sample yields an average 

HERS of 57.6 (range 51 to 68), which should represent the average rating for the high 

performance set of specifications implemented by AHP. 

0

20

40

60

80

100

120

140

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

Number of Dwellings

HERS Index

Atlantic Housing Partners

HERS Ratings

Total Ratings = 934

Figure 6. Distribution of HERS Index for 934 AHP-rated units.
10 

7 Conclusions

AHP has made progressive strides to improve the efficiency and durability of their multifamily 

developments. Recent improvements, such as HPWHs and 15 SEER heat pumps, enable 

Atlantic’s new construction units to achieve 40% energy savings above the BA Benchmark, 

correlating to HERS Indices in the 50s and 60s. The commercial viability of the high 

performance specification package is demonstrated through its incorporation across AHP’s entire 

portfolio, in over 1000 units. This viability is made possible, in part, due to a simulation-based 

method for calculating utility allowances used by developments funded by low-income housing 

tax credits. The simulation-based method assigns value to the energy savings, which are returned 

to the developer through increased revenue. Tenants benefit from enhanced humidity control, 

comfort and indoor air quality resulting from a systems-engineered package which includes 

right-sized HVAC systems, mechanical ventilation, and performance testing of duct and 

envelope leakage to manage uncontrolled air flow. 

One component of the package, the HPWH, did prove problematic in certain installations. A 

survey conducted with managers at seven communities yielded positive feedback overall 

regarding the success of installing HPWHs over standard electric units; however, some noise 

complaints were received. Steps to resolve those noise complaints have been taken, along with 

steps to prevent them in the future. Through the installation of nearly 1,000 HPWHs at nine 

communities, AHP learned that occupants accept the units, and maintenance is manageable. 

Hence, AHP plans to continue installing HPWHs on a large scale. 
11 

References 

Aldrich, R. (2012). Wisdom Way Solar Village: Design, Construction, and Analysis of a LowEnergy Community. Golden, CO: National Renewable Energy Laboratory. 

Dakin, B.; Backman, C.; Hoeschele, M.; and German, A. (2012). West Village Community: 

Quality Management Processes and Preliminary Heat Pump Water Heater Performance. 

Golden, CO: National Renewable Energy Laboratory. 

Chandra, S.; Parker, D.; Sherwin, J.; Colon, C.; Fonorow, K.; Stroer, D.; Martin, E.; McIlvaine, 

J.; Chasar, D.; Moyer, N.; Thomas-Rees, S.; Hoak, D.; Beal, D.; and Gil, C. (2008). An Overview 

of Building America Industrialized Housing Partnership (BAIHP) Activities in Hot-Humid 

Climates, Sixteenth Symposium on Improving Building Systems in Hot and Humid Climates, 

December 15-17, 2008, in Dallas, TX.

Gordon, A.; Lubliner, M.; Howard, L.; and Kunkle, R. (2012). Evaluation of Savings in Energy Efficient 

Public Housing in the Pacific Northwest. Golden, CO: National Renewable Energy Laboratory. 

Hendron, R; and Engebrecht, C. (2010). Building America House Simulation Protocols. Golden, CO: 

National Renewable Energy Laboratory.
12 

Appendix A – Duct Sealing Procedures 

Figure 7. Duct sealing procedures (Source: Slocum Platts Architects, reprinted with permission
13 

Appendix B – Heat Pump Water Heater Survey Results

No. Question HH HB CC PP SR BV FB

1 Have there been any tenant-related complaints? yes yes yes yes yes yes yes

a insufficient supply or lack of hot water?

some units 

with small 

families

no no yes, 1 unit

complaints 

from 8-9 

residents

yes, 2 units no

b noticeable noise from unit? fan blower fan blower fan blower fan blower about 25 

complaints fan blower several asked to turn off

c noticeable cold air near unit?

some have 

used space 

heating

yes yes yes yes no no

2 Have you noticed any blocking of transfer grills 

connected to the water heater location? no no no no no no no

3

Have there been any issues with the air filter or 

condensate drainage and is maintenance 

scheduled to check these items? 

No issues to 

date. 

Maintenance 

scheduled 

every quarter.

Some 

condensate 

not connected 

to drain line. 

Maintenance 

scheduled 

every quarter.

Some 

condensate 

not connected 

to drain line. 

Maintenance 

scheduled 

every quarter.

Some 

condensate 

blockage. 

Checked 

during 

Quarterly 

Maintenance.

No issues to 

date. 

Maintenance 

scheduled 

every quarter.

Some scum 

buildup in 

drain pans. 

Maintenance 

scheduled 

every quarter.

Some 

condensate 

blockage. 

Checked 

during 

Quarterly 

Maintenance.

4 Has the temperature been altered from the 

factory preset of 120degF? no no no no no some raised to 125F no

5 Have units been altered from default mode? no no no yes no yes yes

· eHeat (most efficient)

· standard (least efficient, same as 

traditional)

yes, due to 

noise

yes, due to 

noise

yes, due to 

noise

· hybrid (default mode)

· high demand (less efficient than hybrid)

6 Have any of the following features been used? no no no no no yes yes

high demand (to increase speed of hot water 

output) no no

vacation (to save additional energy when vacant) no yes

"stop cold air" (puts unit in traditional heating 

mode)

yes, due to 

noise no
DOE/GO-102013-3887 ▪ April 2013

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