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The concept of energy conservation in buildings is not 

new. Throughout history building owners have dealt 

with changing fuel supplies and the need for effi cient 

use of these fuels. Gone are the days of the cheap and 

abundant energy of the 1950’s. Today with energy 

resources being depleted and the concern over the 

effect of greenhouse gases on climate change, owners 

of historic buildings are seeking ways to make their 

buildings more energy effi cient. These concerns are key 

components of sustainability — a term that generally 

refers to the ability to maintain the environmental, 

social, and economic needs for human existence. The 

topic of sustainable or “green” building practices is too 

broad to cover in this brief. Rather, this preservation 

brief is intended to help property owners, preservation 

professionals, and stewards of historic buildings make 

informed decisions when considering energy effi ciency 

improvements to historic buildings. 

Sound energy improvement measures must take into 

consideration not only potential energy savings, but 

also the protection of the historic property’s materials 

and features. This guidance is provided in accordance 

with the Secretary of the Interior’s Standards for 

Rehabilitation to ensure that the architectural integrity 

of the historic property is preserved. Achieving a 

successful retrofi t project must balance the goals of 

energy effi ciency with the least impact to the historic 

building. Planning must entail a holistic approach that 

considers the entire building envelope, its systems and 

components, its site and environment, and a careful 

evaluation of the effects of the measures undertaken. 

Treatments common to new construction need to be 

evaluated carefully before implementing them in historic 

buildings in order to avoid inappropriate alteration of 

important architectural features and irreparable damage 

to historic building materials. This brief targets primarily 

small-to medium-size historic buildings, both residential 

and commercial. However, the general decision-making 

principles outlined here apply to buildings of any size 

and complexity. 

Inherent Energy Effi cient Features 

of Historic Buildings

Before implementing any energy conservation measures, 

the existing energy-effi cient characteristics of a historic 

building should be assessed. Buildings are more 

than the sum of their individual components. The 

design, materials, type of construction, size, shape, site 

orientation, surrounding landscape, and climate all 

play a role in how buildings perform. Historic building 

construction methods and materials often maximized 

natural sources of heat, light and ventilation to respond 

to local climatic conditions. The key to a successful 

rehabilitation project is to understand and identify the 

existing energy-effi cient aspects of the historic building 

and how they function, as well as to understand and 

identify its character-defi ning features to ensure they 

are preserved. Whether rehabilitated for a new or 

continuing use, it is important to utilize the historic 

building’s inherent sustainable qualities as they were 

intended to ensure that they function effectively together 

with any new treatments added to further improve 

energy effi ciency.

Windows, courtyards, and light wells

Operable windows, interior courtyards, clerestories, 

skylights, rooftop ventilators, cupolas, and other features 

that provide natural ventilation and light can reduce 

energy consumption. Whenever these devices can be 

used to provide natural ventilation and light, they save 

energy by reducing the need to use mechanical systems 

and interior artifi cial lighting. 

Historically, builders dealt with the potential heat loss 

and gain from windows in a variety of ways depending 

on the climate. In cold climates where winter heat 

loss from buildings was the primary consideration 

before mechanical systems were introduced, windows 

were limited to those necessary for adequate light and 

ventilation. In historic buildings where the ratio of glass 

Improving Energy Effi ciency 

in Historic Buildings 

Jo Ellen Hensley and Antonio Aguilar

PRESERVATION 3 BRIEFS

National Park Service

U.S. Department of the Interior

Technical Preservation Services
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to wall is less than 20%, the potential heat loss through 

the windows is likely minimal; consequently, they are 

more energy effi cient than most recent construction. In 

hot climates, numerous windows provided valuable 

ventilation, while features such as wide roof overhangs, 

awnings, interior or exterior shutters, venetian blinds, 

shades, curtains and drapes signifi cantly reduced heat 

gain through the windows. Historic windows can play 

an important role in the effi cient operation of a building 

and should be retained.

New architectural styles, beginning with the 

International Style of the 1920’s, brought about an 

increase in the percentage of glazing in the total 

building envelope. By the 1950’s, with the advent of 

the glass curtain wall, glazing constituted nearly 100% 

of a building’s exterior walls in many buildings. While 

many early modern buildings continued to use operable 

windows as a way to provide natural ventilation, greater 

reliance on mechanical heating and air conditioning 

systems eventually reduced the function of exterior 

glazing to providing light only, particularly in 

commercial, offi ce, and institutional buildings. 

Walls

Thick masonry walls typical of the late-nineteenth 

and early-twentieth centuries have inherent thermal 

characteristics that keep the buildings cooler in 

the summer and warmer in the winter. Walls with 

substantial mass have the advantage of high thermal 

inertia, which reduces the rate of heat transfer through 

the wall. For instance, a wall with high thermal inertia, 

subjected to solar radiation for an hour, will absorb the 

heat at its outside surface, but slowly transfer it to the 

interior over a period as long as six hours. Conversely, a 

wall having the equivalent thermal resistance (R-value), 

but a substantially lower thermal inertia, will transfer 

the heat in perhaps as little as two hours. Heavy 

masonry walls also reduce the need for summer cooling. 

High thermal inertia is the reason many older public and 

commercial buildings without air conditioning still feel 

cool during the summer. The heat from the midday sun 

does not penetrate the buildings until late afternoon and 

evening, when it is less likely to be occupied or when 

exterior temperatures have fallen. Heavy masonry walls 

are also effective in moderating internal temperatures 

in the winter by dampening the overall peaks of heat 

gain and loss resulting in a fl atter and more tolerable 

daily cycle. In areas that require cooling during the day 

and heating at night, masonry walls can help spread 

out excess heat gain from the day to cover some of the 

needed heating for the evening and night hours. 

Roofs 

Roof construction and design in historic buildings, 

particularly vernacular buildings, are strongly 

Fig. 1. A decorative, stained glass skylight allows natural daylight 

into the interior.

Fig. 2. Upper and lower shutters control daylight and provide privacy.

Fig. 3. Stone walls with substantial mass have high thermal inertia.
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infl uenced by the conditions of the local climate. Wide 

overhangs that sometimes extend to create porches 

minimize the heat gain from the sun in warmer climates, 

while steep, sloping roofs with minimal or no overhang 

prevail in colder climates to allow for shedding snow 

and increasing benefi cial solar heat gain through the 

windows. Materials and color also infl uence the thermal 

performance of roofs. Metal and light colored roofs, for 

example, refl ect sunlight and thereby reduce the heat 

gain from solar radiation. 

Floor Plans 

The fl oor plan of many historic buildings, particularly 

traditional vernacular ones, was also designed to 

respond to the local climate. In cold climates, rooms 

with low ceilings were clustered around central 

chimneys to share the heat, while small windows 

with interior shutters reduced drafts and heat loss. In 

warmer climates, wide central halls with tall ceilings, 

breezeways, and large porches all maximized 

air circulation.

Landscape

Site orientation was another factor considered especially 

in the location of a historic building on its property. 

In cold climates, buildings were oriented against 

northern winds while buildings in warm climates were 

sited to take advantage of prevailing breezes. Evergreen 

trees planted on the north side of buildings shielded 

from winter winds; deciduous trees planted to the 

south provided summer shade and maximum sun in 

the winter. 

Energy Audit

Before implementing any measures to improve the 

thermal performance of a historic building, an energy 

audit should be undertaken to evaluate the current 

energy use of the building and identify defi ciencies 

in the building envelope or mechanical systems. In 

some areas, the local utility company may offer a free 

simple audit, however a more in-depth audit should 

be obtained from a professional energy auditor. The 

goal of the audit is to establish a baseline of building 

performance data to serve as a reference point 

when evaluating the effectiveness of future energy 

improvements. It is important to hire an independent 

auditor who does not have fi nancial interests in the 

results, such as a product vendor. 

An energy auditor fi rst documents the current energy 

use patterns in the building to establish an energy use 

history. This initial step includes obtaining the billing 

history from the local utility company over a one- or 

two-year period, as well as documenting the number 

of building occupants, how the building is used, and 

the type of fuel consumed. The location of any existing 

insulation is recorded and the approximate R-value of 

various components of the building envelope including 

walls, ceilings, fl oors, doors, windows and skylights 

is calculated. The building envelope is inspected to 

identify areas of air infi ltration and air loss. The type 

and age of mechanical systems and major appliances 

are also recorded. 

Tools such as a blower door test or infrared 

thermography are useful to identify specifi c areas of 

infi ltration, lack of insulation and thermal bridging. 

Mechanical depressurization along with infrared 

thermography is extremely useful in identifying 

locations of air leakage and heat loss followed by the use 

of tracer smoke to isolate specifi c air leaks. These tests 

are often challenging to perform on buildings and must 

be undertaken by experienced professionals to avoid 

Fig. 4. A typical 

New England saltbox 

features a steeply 

sloping roof to shed 

snow and a fl oor plan 

organized around a 

central chimney to 

conserve heat.

Lean-to 

Addition

Parlor Hall

Fig. 5. The side porches of this house in Charleston, SC, shade the 

large windows and provide outdoor living spaces that take advantage 

of sea breezes.
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misleading or inaccurate results. There are professional 

standards for audits, those of the Building Performance 

Institute (BPI) being the most widely used. 

The energy auditor then produces a detailed report 

that documents the fi ndings of the audit and includes 

specifi c recommendations for upgrades such as air 

sealing, adding insulation, general repairs, lighting, and 

improvements to or replacement of mechanical systems 

or major appliances. Cost estimates are provided 

for each of the improvements including the cost of 

implementation, potential operating cost savings, and, 

importantly, the anticipated payback period. Armed 

with this information, historic building owners can 

start to make informed decisions on how to improve 

the performance of their buildings. Usually the auditor 

fi nds a few locations where there is major air leakage; 

large “holes” that are unique to a particular building 

and require equipment to fi nd them. These anomalies 

are often invisible to the people who use the building on 

a regular basis. It is important to retest the performance 

of the building following the implementation of any 

upgrades undertaken as a result of an energy audit to 

ensure that the upgrades are performing as expected.

Prioritizing Energy Upgrades 

When implementing energy upgrades, efforts should 

be concentrated on improvements that will provide the 

most payback for the money expended and the least 

compromise to the historic character of the building. 

Some upgrades recommended in energy audits may not 

be introduced into a historic building feasibly without 

damaging historic fabric or altering the appearance 

of signifi cant features. Removing historic siding and 

replacing it with new siding to introduce insulation 

into the wall cavity of a frame building or replacing 

repairable historic windows are examples of treatments 

that should not be undertaken on historic buildings. 

A common misconception is that replacing windows 

alone will result in major energy savings. This argument, 

often used to sell replacement windows, is simply not 

true. Although it varies from building to building, the 

U.S. Department of Energy (DOE) has documented that 

air loss attributable to windows in most buildings is 

only about 10% of the total air loss. Studies have shown 

that window replacement does not pay for itself in 

energy savings in a reasonable length of time. Moreover, 

there are ways to improve the performance of historic 

windows that do not require their replacement. 

In addition, historic windows can usually be repaired 

and are, thus, sustainable, while most new windows 

cannot be repaired, or even recycled, and may wind up 

in landfi lls. 

When considering energy upgrades, it is imperative to 

get a clear picture of what an improvement will cost 

initially and how long it will take to pay back the cost 

in energy savings. Therefore, the life cycle cost of the 

improvement must be considered as well as its impact 

on historic fabric. Reducing infi ltration around existing 

windows and doors, sealing penetrations in the building 

envelope, and adding insulation — particularly in the 

attic where it has little impact on historic fabric — can 

result in signifi cant improvements at relatively little 

cost. Updating mechanical systems or changing the way 

in which they are operated can also be cost-effective 

interventions. For example, installing a more effi cient 

mechanical system alone may pay for itself in ten years.

Fig. 6. (left) A blower door is used to depressurize a building by exhausting air at a rate that allows pressure gages and tracer smoke to measure the 

amount and location of air leakage. Photo: Robert Cagnetta, Heritage Restoration, Inc. 

Fig. 7. (center and right) The left thermal image shows the walls of this building before insulating. After insulation was added, the cooler and, thus 

darker exterior walls evidence how much the heat loss has been reduced. Photos: EYP Architecture & Engineering.
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Actions to Improve Energy Effi ciency

Reducing energy demands for heating and cooling 

may be accomplished in two steps. First, implement 

operational changes and upgrades to mechanical 

systems and major appliances — measures that do not 

require making alterations or adding new materials — 

to ensure that a building functions as effi ciently 

as possible. After all these measures have been 

implemented, corrective work or treatments, such as 

weatherization, that require other alterations to the 

building may be considered. 

Establishing Realistic Goals

Energy consumption data gathered by the U.S. Energy 

Information Administration (see chart) shows that 

residential buildings built before 1950 (the largest 

percentage of historic building stock) are about 30 to 

40 percent less energy effi cient than buildings built 

after 2000. Using this as a baseline, a 30 to 40 percent 

upgrade of a historic building’s energy performance 

can be a realistic goal. A 40 percent increase in energy 

effi ciency would of course be a more achievable goal for 

buildings that have had minimal upgrades since their 

original construction, i.e., added insulation, tightening 

of the exterior envelope, or more effi cient mechanical 

equipment. On the other hand, achieving “net zero” 

energy goals as it is currently done with some new 

construction can be a much more diffi cult challenge to 

achieve in a historic retrofi t. Attempting to reach such a 

goal with a historic building would most likely result in 

signifi cant alterations and loss of historic materials. [The 

data for commercial buildings documents that buildings 

in 2003 used approximately the same energy as they did 

before 1920, after reaching their peak in the 1980’s.]

Operational Changes

One of the greatest effects on energy use is user 

behavior. Once an energy audit has established a 

baseline for the current energy use in a building, 

operational changes should be identifi ed to control 

how and when the building is used to minimize the use 

of energy-consuming equipment. These changes can 

range from simple measures such as regular cleaning 

and maintenance of mechanical equipment to installing 

sophisticated controls that cycle equipment on and off 

in specifi ed intervals for maximum performance. The 

following changes are recommended to reduce heating 

and cooling costs.

• Install programmable thermostats. 

• Close off rooms that are not in use and adjust the 

temperature in those rooms.

• Do not condition rooms that do not need to be 

conditioned, thereby reducing the thermal envelope.

• Use insulated shades and curtains to control heat gain 

and loss through windows.

• Use operable windows, shutters, awnings and 

vents as originally intended to control temperature 

and ventilation.

• Take advantage of natural light. 

• Install compact fl uorescent lights (CFL) and lightemitting diode (LED) lights. 

• Install motion sensors and timers for lighting and local 

ventilation, such as bathroom exhaust fans.

• Reduce “phantom” electricity loads by turning 

equipment off when not in use. 

• Clean and service mechanical equipment regularly. 

These measures should be undertaken fi rst to save 

energy in any existing building and are particularly 

appropriate for historic buildings because they do not 

require alterations to historic materials. 

Upgrading Equipment and Appliances

In addition to maximizing the energy effi ciency of 

existing building systems, substantial savings can be 

achieved through upgrading equipment and appliances. 

One should still weigh the operational savings against 

the initial cost of the new equipment, particularly if the 

existing equipment is not near the end of its life. 

Calculator aids that take into account the effi ciency 

of both the existing and new equipment are available 

Residential Energy Use Intensity by Age

Year Built KBtu/sq ft/yr

Prior to 1950 74.5

1950 to 1969 66.0

1970 to 1979 59.4

1980 to 1989 51.9

1990 to 1999 48.2

2000 to 2005 44.7

Source: Residential Energy Consumption Survey, 2005

Fig. 8. Where Air Escapes From a House (by percentage) – Image 

based on data from Energy Savers, U.S. Department of Energy. 

Illustration: Blank Space LLC.
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online to assist in determining the payback. Advance 

planning will allow time to fi nd the most effi cient unit, 

as well as to investigate the availability of any state and 

federal energy credits. As energy prices continue to rise 

and technology advances, options such as the installation 

of a solar hot water heater or geothermal ground 

source or water source heat pumps are becoming more 

economically feasible. Recommendations for upgrading 

equipment and appliances include:

• Upgrade the heating system. It is important to install 

new furnaces that utilize outside combustion air to 

reduce air drawn into the building through uncontrolled 

infi ltration. [All furnaces and boilers are now measured 

by their annual fuel utilization effi ciency or AFUE.] 

Heating equipment is now more effi cient and gas 

furnaces that used to have a 60% (AFUE) rating can 

now operate at as much as 90 to 97% effi ciency.

• Upgrade the air conditioning system. 

• Replace the water heater. High-effi ciency water heaters 

use far less energy than earlier models, and high-effi ciency 

tankless water heaters heat water on demand and offer 

even greater savings. Point of use water heat can also 

reduce costs and water consumption by reducing the time 

it takes to draw hot water.

• Upgrade appliances. Energy Star appliances, 

particularly refrigerators, washing machines and 

dishwashers can all reduce electricity use and additional 

indoor heating loads. 

Upgrading Building Components 

In addition to operational and mechanical upgrades, it 

can be possible to upgrade many building components 

in a manner that will not jeopardize the historic character 

of the building and can be accomplished at a reasonable 

cost. The goal of these upgrades is to improve the 

thermal performance of the building, resulting in even 

greater energy savings. Retrofi t measures to historic 

buildings should be limited to those that achieve at least 

reasonable energy savings, at reasonable costs, with the 

least impact on the character of the building. 

The following list includes the most common measures 

proposed to improve the thermal performance of 

an existing building; some measures are highly 

recommended for historic buildings, but others are less 

benefi cial, and can even be harmful to a historic building.

Requires Minimal Alteration 

• Reduce air leakage. 

• Add attic insulation.

• Install storm windows.

• Insulate basements and crawlspaces. 

• Seal and insulate ducts and pipes.

• Weather strip doors and add storm doors.

• Add awnings and shading devices where appropriate.

Requires More Alteration 

• Add interior vestibules.

• Replace windows.

• Add insulation to wood-frame walls.

• Add insulation to masonry walls.

• Install cool roofs and green roofs.

The treatments listed fi rst have less potential to 

negatively impact the historic fabric of a building. They 

tend to be less intrusive, are often reversible, and offer 

the highest potential for energy savings. Undertaking 

any of the treatments in the second group, however, may 

pose technical problems and damage to historic building 

materials and architectural features. Their installation 

costs may also outweigh the anticipated energy savings 

and must be evaluated on a case-by-case basis with 

advice from professionals experienced in historic 

preservation and building performance.

Requires Minimal Alteration 

Reduce air leakage. Reducing air leakage (infi ltration 

and exfi ltration) should be the fi rst priority of a 

preservation retrofi t plan. Leakage of air into a building 

can account for 5 to 40 percent of space-conditioning 

costs, which can be one of the largest operational costs 

for buildings.1

 In addition, unwanted air leakage into 

and out of the building can lead to occupant comfort 

issues resulting from drafts. Air infi ltration can be 

especially problematic in historic buildings because it 

is closely linked to increased moisture movement into 

building systems. 

Air fl ow into and out of buildings is driven by three 

primary forces: wind pressure, mechanical pressure 

and the stack effect. Cold outside air that infi ltrates the 

building through big holes, as well as through loose 

windows, doors, and cracks in the outer shell of the 

building, causes the heating system to work harder and 

consume more energy. In a multi-story building, cold 

air that enters the building at lower levels, including 

the basement or crawlspace, will travel up through 

the building and exit out leaky windows, gaps around 

windows and the attic as a result of temperature and 

pressure differential. This pattern of air movement 

Fig. 9. An energy auditor tests the effi ciency of a boiler.
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is called the “stack effect.” Not only is valuable 

conditioned air lost, but damaging moisture may also 

enter the wall cavities and attic spaces. To stop the stack 

effect, the top and bottom of the exterior walls, interfl oor bypasses, and any existing chases or shafts must 

be sealed, or “draft proofed.” The use of spray foam 

sealants in basement and attic cracks is a particularly 

useful technique for reducing air infi ltration. 

Adding weatherstripping to doors and windows, sealing 

open cracks and joints at the base of walls and around 

windows and doors, sealing off recessed lighting fi xtures 

from above, and sealing the intersection of walls and 

attic, will substantially reduce air leakage. When using 

exterior caulk to seal the intersection of siding and doors 

or windows, do not caulk the underside of clapboards 

or below windows to allow any liquid water to escape. 

When infi ltration and, consequently, exfi ltration are 

reduced, mechanical ventilation may be necessary to 

meet occupants’ requirements for fresh air.

Add attic or roof insulation. Heat loss and gain caused 

by increased interior/exterior temperature differentials 

primarily due to the stack effect and solar radiation are 

greatest at the top of a building. Therefore, reducing 

heat transfer through the roof or attic should be one of 

the highest priorities in reducing energy consumption. 

Adding insulation in unoccupied, unfi nished attics 

is not only very effective from an energy-savings 

perspective, but it is also generally simple to install and 

causes minimal disruption to historic materials. The U.S. 

Department of Energy (DOE) provides a recommended 

R-value chart based on climate zones to help determine 

the optimal amount of insulation that should be installed 

in a particular project. Local codes may also have specifi c 

insulation requirements. Insulating trap or access doors 

should not be overlooked. Even though they may be 

small, attic doors can be responsible for substantial 

heat loss and should be addressed as part of any attic 

insulation project.

In unfi nished and unheated attics, the insulation 

material is typically placed between the fl oor joists 

using blown-in, batt, or rigid foam insulation. When 

using fi berglass batts faced with a vapor retarder, 

the vapor retarder should be face down towards the 

heated interior. However, the use of a vapor retarder 

is not necessary in attic applications. If additional batt 

insulation is being added over existing insulation that 

is near or above the top of the joists, new un-faced batts 

should be placed perpendicular to the old ones to cover 

the top of the joists and reduce thermal bridging through 

the frame members. In low-pitched roofs, or where 

installing batt insulation is diffi cult, a more complete 

coverage of the attic fl oor may be achieved by using 

blown-in insulation. Unfi nished attics must be properly 

ventilated to allow excess heat to escape.

 

Fig. 10. The pattern of air movement referred to as the ”stack effect”. 

Illustration: Blank Space LLC.

Fig. 11. Air infi ltration and exfi ltration. 

Illustration: Blank Space LLC.

Fig. 12. Recommended energy improvements vary widely based on 

climate. The information contained in this document is based primarily 

on the available data for the Northeast and Mid-Atlantic regions.

DOE Climate Zone Map
8

Radiant barriers may be used in attics to reduce thermal 

radiation across the air space between the roof deck 

and the attic fl oor in order to reduce summer heat gain. 

They are most benefi cial in reducing cooling loads in 

hot climates and consist of a highly refl ective sheet or 

coating, usually aluminum, applied to one or both sides 

of a fl exible material. They are effective only when the 

foil surface faces an air space, and as long as the surface 

remains shiny – that is, free from dirt, dust, condensation 

and oxidation. Radiant barriers should not be installed 

directly over insulation on the attic fl oor, as they can act 

as vapor retarders and trap moisture in the insulation 

unless they are perforated. Their placement should be 

ventilated on both sides.

Insulating the underside of the roof rather than the attic 

fl oor increases the volume of the thermal envelope of 

the building, thus making this treatment inherently less 

energy effi cient. However, when mechanical equipment 

and/or ductwork are housed in an attic space, placing 

the insulation under the roof and treating the attic 

as a conditioned space is strongly recommended. 

This treatment allows the equipment to operate more 

effi ciently and can prevent moisture-related problems 

caused by condensation on the mechanical equipment.

When insulation is placed under the roof, all vents in 

the attic and the intersection between the walls and roof 

rafters must be sealed. Rigid foam or batt insulation 

placed between the roof rafters is a common method of 

insulating the underside of a roof. Open cell spray foam 

(.5lb/cuft) may sometimes be applied under the roof 

deck only when there are no gaps in the sheathing which 

could allow the foam to expand under slates or shingles, 

preventing the re-use of the roofi ng material. Also, if 

roof leaks do occur, they may go undetected until after 

major damage occurs. Consideration must also be given 

to the irreversibility of this procedure because the foam 

enters the pores of the wood. It may be more advisable 

to install a breathable layer of material that will allow for 

future removal without leaving a residue. 

When total roof replacement is required because of 

deterioration, installing rigid foam insulation on top 

of the roof deck before laying the new roofi ng material 

can be simple and effective, particularly on low-pitched 

or fl at roofs. However, the added thickness of the roof 

caused by installing rigid foam can alter the appearance 

of projecting eaves, dormers, and other features. If this 

application would signifi cantly alter the appearance of 

these features, consider other methods. 

Install storm windows. The addition of metal or wood 

exterior or interior storm windows may be advisable 

to increase the thermal performance of the windows 

in ways that weatherstripping and caulking cannot 

address. A single-glazed storm window may only 

increase a single-pane window’s thermal resistance to 

R2, however, that is twice as good as a single-glazed 

window alone. It will make a noticeable contribution 

to the comfort level of the building occupant, with 

the added benefi t of protecting the historic window 

from weathering. Using clear, non-tinted, low-e glass 

in the storm window can further increase the thermal 

performance of the window assembly without the loss of 

historic fabric. Studies have shown that the performance 

of a traditional wood window with the addition of a 

storm window can approach that of a double-glazed 

replacement window.2

 Some storm windows are 

available with insulated low-e glass, offering even 

higher thermal performance without the loss of the 

historic window. Furthermore, a storm window avoids 

the problem of irreparable seal failure on insulated glass 

units (IGUs) used in modern replacement windows. 

While the lifespan of the IGU depends both on the 

quality of the seal and other factors, it is unreasonable to 

expect more than 25 years. Once the seal fails, the sash 

itself will usually need to be entirely replaced.

By providing an additional insulating air space and 

adding a barrier to infi ltration, storm windows improve 

comfort and reduce the potential for condensation on the 

glass. To be effective and compatible, storm windows 

must be tight fi tting; include a sealing gasket around the 

glass; align with the meeting rail of the primary sash; 

match the color of the sash; and be caulked around the 

frame to reduce infi ltration without interfering with any 

weep holes. 

Whether a storm window or the historic window itself, 

the interior window must be the tighter of the two units 

to avoid condensation between the windows that can 

Fig. 14. Sample installation of rigid foam insulation, tapered at the 

edge to avoid altering the appearance of the roof.

Fig. 13. Sample installation of a radiant barrier.

Roof Decking

Air Space

Radiant Barrier

Insulation

Joist
9

occur in a cold climate that requires indoor heating. 

Condensation is a particular concern if it collects on 

the historic window, as can easily happen with a loosefi tting, storm window. While interior storm windows 

can be as thermally effective as exterior storm windows, 

appropriate gaskets must be used to ensure that 

damage-causing condensation does not form on the 

inside face of the historic window. Opening or removing 

the interior storm windows during non-heating months 

also helps to avoid the negative effects of moisture 

build-up. 

For large, steel industrial windows, the addition of 

interior, insulated sliding glass windows that align 

with the primary vertical mullions has proven to be a 

successful treatment that allows the primary window to 

remain operable. 

Insulate basements and crawlspaces. The fi rst step in 

addressing the insulation of basements and crawl spaces 

is to decide if they are to be part of the conditioned 

space and, therefore, within the thermal envelope of 

the building. If these areas are kept outside the thermal 

envelope of the building and treated as unconditioned 

areas, insulating between the fl oor joists on the 

underside of the subfl oor is generally recommended. 

Alternatively, rigid foam insulation installed over 

the bottom of the fl oor joists on the basement or 

crawlspace side may also be used. All gaps between the 

unconditioned and conditioned areas of the building, 

including the band joists, should be air sealed to prevent 

air infi ltration into the upper levels of the building. 

If the crawlspace contains mechanical equipment, or if 

high levels of moist air enter the crawlspace through 

vents during the summer months, it is advisable to 

include the crawlspace within the thermal boundary of 

the building. As in attics, water vapor can condense on 

ducts and other equipment located in unconditioned 

basements and crawlspaces. In the past, building codes 

routinely required that crawlspaces be treated as nonconditioned spaces and be ventilated. However, this has 

not proven to be a best practice in all cases. Ventilation 

through crawlspace vents does not keep the space dry 

during humid summers. All vents should be sealed and 

access doors weather-stripped. Rigid foam insulation 

installed on the interior face of the wall is recommended 

for basement and crawlspace foundation walls, only 

after all drainage issues have been addressed. Special 

attention should be given to ensure that all the joints 

between the insulation boards are sealed. 

A moisture barrier on exposed dirt in a crawlspace is 

strongly recommended to prevent ground moisture 

from entering the building envelope. Whenever feasible, 

pouring a concrete slab over a moisture barrier in 

crawlspaces or basements with exposed dirt fl oors 

should be considered. 

Seal and insulate ducts and pipes. A surprisingly 

enormous amount of energy is wasted when heated 

or cooled air escapes from supply ducts or when 

hot attic air leaks into air conditioning return ducts. 

Based on data collected in energy audits, as much as 

35 percent of the conditioned air in an average central 

air conditioning system may escape from the ducts.3

Care must be taken to completely seal all connections 

in the duct system and adequately insulate the ducts, 

especially in unconditioned spaces. This loss of 

energy is another reason to treat attics, basements and 

crawlspaces as conditioned spaces. Ducts located in 

unconditioned spaces should be insulated based on the 

recommendations for the appropriate climate zone. Hot 

water pipes and water heaters should be insulated in 

unconditioned spaces to retain heat, and all water pipes 

insulated to prevent freezing in cold climates.

Weather strip doors and add storm doors. Historic 

wood doors are often signifi cant features and should 

always be retained, rather than replaced. While an 

insulated replacement door may have a higher R-value, 

doors represent a small area of the total building 

envelope, and the difference in energy savings after 

replacement would be insignifi cant. The doors and 

frames should, however, have proper maintenance 

including regular painting, and the addition or renewal 

of weatherstripping. Storm doors can improve the 

thermal performance of the historic door in cold 

climates and may be especially recommended for a 

door with glazing. The design of the storm door should 

be compatible with the character of the historic door. 

A fully glazed storm door with a frame that matches 

the color of a historic door is often an appropriate 

choice because it allows for the historic door to remain 

visible. Storm doors are recommended primarily for 

residential buildings. They are not appropriate for 

commercial or industrial buildings. These buildings 

never had storm doors, because the doors were opened 

frequently or remained open for long periods. It may 

also not be appropriate to install a storm door on a 

highly signifi cant entrance door. In some instances, 

Fig. 15. Original steel windows were 

retained and made operable during 

the rehabilitation of this historic mill 

complex. Insulated sliding windows 

were added on the interior to improve 

energy effi ciency.
10

the addition of a storm door could add signifi cant heat 

gain on certain exposures or in hot climates, which could 

degrade the material or fi nish of the historic door.

Add awnings and shading devices. Awnings and other 

shading devices can provide a considerable reduction 

of heat gain through windows and storefronts. Keeping 

existing awnings, or replacing them if previously 

removed, is a relatively easy way to enhance the energy 

performance of a building. Awnings should only be 

installed when they are compatible with the building 

type and character. In building types that did not have 

awnings historically, interior shades, blinds or shutters 

should be considered instead.

A wide range of shades, blinds and shutters is available 

for use in all types of buildings to control heat gain or 

loss through windows, as well as lighting levels. When 

properly installed, shades are a simple and cost-effective 

means of saving energy. Some shade fabrics block only 

a portion of the light coming in — allowing the use of 

natural light — while others block all or most of the 

light. The light-colored or refl ective side of the shades 

should face the window to reduce heat gain. Quilted 

roller shades feature several layers of fi ber batting and 

sealed edges, and these shades act as both insulation 

and an air barrier. They control air infi ltration more 

effectively than other soft window treatments. Pleated or 

cellular shades provide dead air spaces within the cells 

to add insulation value. These shades, however, do not 

measurably control air infi ltration.

Retractable awnings and interior shades should be 

kept lowered during the summer to prevent unwanted 

heat gain, but raised in the winter to take advantage 

of the heat gain. Interior shades, especially those that 

have some insulation value, should be lowered at night 

during the winter months.

Light shelves are architectural devices designed to 

maximize daylight coming through windows by 

refl ecting it deeper into the building. These horizontal 

elements are usually mounted on the interior above 

head height in buildings with high ceilings. Although 

they can provide energy savings, they are not compatible 

with most historic buildings. In general, light shelves 

are most likely to be appropriate in some industrial or 

modernist-style buildings, or where the historic integrity 

of interior spaces has been lost and they can be installed 

without being visible from the exterior.

Requires More Alteration 

Add interior vestibules. Vestibules that create a 

secondary air space or “air lock” are effective in 

reducing air infi ltration when the exterior door is 

open. Exterior and interior vestibules are common 

architectural features of many historic buildings and 

should be retained wherever they exist. Adding an 

interior vestibule may also be appropriate in some 

historic buildings. For example, new glazed interior 

vestibules may be compatible changes to historic 

commercial and industrial buildings. New exterior 

vestibules will usually result in too great a change to the 

character of primary entrances, but may be acceptable 

in very limited instances, such as at rear entrances. Even 

in such instances, new vestibules should be compatible 

with the architectural character of the historic building.

Replace windows. Windows are character-defi ning 

features of most historic buildings. As discussed 

previously, the replacement of a historic window with 

a modern insulated unit is not usually a cost-effective 

choice. Historic wood windows have a much longer 

service life than replacement insulated windows, which 

cannot be easily repaired. Therefore, the sustainable 

choice is to repair historic windows and upgrade their 

thermal performance. However, if the historic windows 

are deteriorated beyond repair, if repair is impractical 

because of poor design or material performance, or 

if repair is economically infeasible, then replacement 

windows may be installed that match the historic 

windows in size, design, number of panes, muntin 

profi le, color, refl ective qualities of the glass, and the 

same relationship to the window opening. 

Other options should also be considered before 

undertaking complete window replacement. If only the 

sash is severely deteriorated and the frame is repairable, 

then only the sash may need to be replaced. If the 

limited lifespan of insulated glass is not a concern, the 

Fig. 16. Historic vestibules retain conditioned air in the living spaces. new sash can be made to accommodate double glazing.
11

Where the sashes are sound, but improved thermal 

performance without the use of a storm window 

is desired, some windows may be retrofi tted with 

insulated glass. If the existing sash is of suffi cient 

thickness, it may be routed to accept insulated, clear 

low-e glass without extensive loss of historic material 

or historic character. When insulated glass is added in 

a new or retrofi tted sash, any weights will have to be 

modifi ed to accommodate the signifi cant extra weight.

Wall Insulation

Adding wall insulation must be evaluated as part of 

the overall goal to improve the thermal effi ciency of 

a building and should only be considered after the 

installation of attic and basement insulation. Can this 

goal be achieved without the use of wall insulation? Can 

insulation be added without causing signifi cant loss 

of historic materials or accelerated deterioration of the 

wall assembly? Will it be cost effective? These are basic 

questions that must be answered before a decision is 

made to insulate the walls and may require professional 

evaluation.

Add insulation to wood-frame walls. Wood is 

particularly susceptible to damage from high moisture 

levels; therefore, addressing existing moisture problems 

before the addition of insulation is essential. 

Un-insulated historic wood buildings have a higher 

rate of air infi ltration than modern buildings; while 

this makes older buildings less effi cient thermally, it 

helps dissipate the unwanted moisture and thus keeps 

building assemblies dry. Climate, building geometry, 

the condition of the building materials, construction 

details, and many other factors make it diffi cult to assess 

the impact that adding insulation will have on reducing 

the air fl ow and, hence, the drying rate in a particular 

building. For this reason, predicting the impact of 

adding insulation to wood-frame walls is diffi cult. 

Insulation Installed in the Wall Cavity: When sheathing is 

part of the wall assembly, and after any moisture-related 

problems have been addressed, adding insulation to the 

interior cavity of a wood-frame wall may be considered. 

Adding insulation in a wall where there is no sheathing 

between the siding and studs is more problematic, 

however, because moisture entering the wall cavity 

through cracks and joints by wind-driven rain or 

capillary action will wet the insulation in contact with 

the back of the siding. 

Installing blown-in insulation, either dense-packed 

cellulose or fi berglass, into the wall cavity causes 

the least amount of damage to historic materials and 

fi nishes when there is access to the cavity walls, and 

it is therefore a common method of insulating woodframe walls in existing buildings. In most cases, blowing 

insulation material into the wall cavity requires access 

through the exterior or interior wall surfaces. When 

historic plaster, wood paneling, or other interior 

historic decorative elements are present, accessing the 

cavity from the exterior is recommended by removing 

individual siding boards at the top of each wall cavity. 

In this manner the boards can be reinstalled without 

unsightly drill holes on the exterior. If the plaster is 

deteriorated and will require repair, then the wall cavity 

may be accessed from the interior through holes drilled 

through non-decorative plaster.

Of the materials available, dense-packed cellulose fi ber 

is most commonly used. Its R-value, ability to absorb 

and diffuse moisture, impediment to air fl ow, relatively 

simple installation, and low cost make it a popular 

choice. Cellulose insulation from most manufacturers is 

available in at least two grades that are characterized by 

the type of fi re retardant added to the insulation. The fi re 

retardants are usually: (1) a mix of ammonium sulfate 

and boric acid or (2) boric acid only (termed “borate 

only”). The recommended type of cellulose insulation 

for historic buildings is the “borate only” grade, as 

cellulose treated with sulfates reacts with moisture in the 

air and forms sulfuric acid which corrodes many metals.

Optimum conditions for installing insulation inside 

the wall cavity occur in buildings where either the 

exterior materials or interior fi nishes have been lost, 

or where the materials are deteriorated beyond repair 

and total replacement is necessary. However, wholesale 

removal of historic materials either on the exterior or 

interior face of a historic wall to facilitate insulation is 

Fig. 17. Illustration of insulation from the 1889 trade catalog 

“The Uses of Mineral Wool in Architecture, Car Building and Steam 

Engineering”. Collection Centre Canadien d’Architecture/Canadian 

Centre for Architecture, Montreal, Canada.
12

not recommended. Even when the exterior materials, 

such as wood siding, could potentially be reinstalled, 

this method, no matter how carefully executed, usually 

results in damage to, and loss of historic materials.

If the wall cavity is open, the opportunity to properly 

install batt insulation is available. A tight fi t between 

the insulation and the adjacent building components 

is critical to the performance of the insulation. Batt 

insulation must be cut to the exact length of the cavity. 

A batt that is too short creates air spaces above and 

beneath the batt, allowing convection. A batt that is too 

long will bunch up, creating air pockets. Air pockets 

and convection currents signifi cantly reduce the thermal 

performance of insulation. Each wall cavity should be 

completely fi lled. Unfaced, friction-fi t batt insulation 

fl uffed to fi ll the entire wall cavity is recommended. Any 

air gaps between the insulation and the framing or other 

assembly components must be avoided. Batts should be 

split around wiring, pipes, ducts and other elements in 

the wall rather than be pushed or compressed around 

obstacles.

When adding insulation to the sidewalls, the band joist 

area between fl oors in multi-story, platform-framed 

buildings should be included in the sidewall insulation 

retrofi t. The R-value of the insulation installed in the 

band joist area should be at least equal to the R-value of 

the insulation in the adjacent wall cavities. In balloonframed buildings, the wall cavity is continuous between 

fl oors except where fi re stops have been inserted. 

The use of spray foam or foamed-in-place insulation

would appear to have great potential for application 

in historic wood-frame buildings due to their ability to 

fl ow into wall cavities and around irregular obstacles. 

Their high R-value and function as an air barrier make 

them a tempting choice. However, their use presents 

several problems. The injected material bonds tightly to 

historic materials making its removal diffi cult, especially 

if it is encased in an existing wall. The pressure caused 

by the expansion rate of these foams within a wall can 

also damage historic material, including breaking the 

plaster keys or cracking existing plaster fi nishes.

Insulation Installed on Either Side of the Wall: Batt, rigid 

foam board, and spray foam insulation are commonly 

added to the interior face of walls in existing buildings 

by furring-out the walls to accommodate the additional 

thickness. However, this often requires the destruction 

or alteration of important architectural features, such 

as cornices, base boards, and window trim, and the 

removal or covering of plaster or other historic wall 

fi nishes. Insulation installed in this manner is only 

recommended in buildings where interior spaces and 

features lack architectural distinction or have lost 

signifi cance due to previous alterations. 

Adding rigid foam insulation to the exterior face of 

wood-frame buildings, while common practice in 

new construction, is never an appropriate treatment 

for historic buildings. Exterior installation of the foam 

boards requires removal of the existing siding and trim 

to install one or more layers of polyisocyanurate or 

polystyrene foam panels. Depending on the amount 

of insulation added for the particular climate, the wall 

thickness may be dramatically increased by moving 

Band Joist

Fig. 18. Dense-packed cellulose insulation is being blown in through 

holes drilled in the sheathing. Once the operation has been completed, 

the shingles will be reinstalled. Photo: Edward Minch.

Fig. 20. The walls 

have been furred 

out inappropriately 

around the historic 

window trim creating 

an appearance the 

interior never had 

historically. 

Fig. 19. Platform framing (left) and Balloon framing (right).
13

the siding as much as 4 inches out from the sheathing. 

Even if the historic siding and trim could be removed 

and reapplied without signifi cant damage, the historic 

relationship of windows to walls, walls to eaves, 

and eaves to roof would be altered, which would 

compromise the architectural integrity and appearance 

of the historic building.

Solid Masonry Walls: As with frame buildings, installing 

insulation on the interior walls of a historic masonry 

structure should be avoided when it would involve 

covering or removing important architectural features 

and fi nishes, or when the added thickness would 

signifi cantly alter the historic character of the interior. 

The addition of insulation on solid masonry walls 

in cold climates results in a decreased drying rate, 

an increased frequency of freeze-thaw cycles, and 

prolonged periods of warmer and colder temperatures 

of the masonry. These changes can have a direct effect on 

the durability of materials.

Depending on the type of masonry, exterior masonry 

walls can absorb a signifi cant amount of water when it 

rains. Masonry walls dry both toward the exterior and 

the interior. When insulation is added to the interior side 

of a masonry wall, the insulation material reduces the 

drying rate of the wall toward the interior, causing the 

wall to stay wet for longer periods of time. Depending 

on the local climate, this could result in damage to 

the historic masonry, damage to interior fi nishes, and 

deterioration of wood or steel structural components 

What about moisture?

The issue of moisture in insulated assemblies is the 

subject of much debate. While there is no conclusive 

way to predict all moisture problems, especially 

in historic buildings, experts seem to agree on a 

few basic tenants. Exterior materials in insulated 

buildings become colder in the winter and stay wet 

longer following a rain event. While the wetness may 

not pose a problem for robust materials, it may speed 

the deterioration of some building materials, and 

lead to more frequent maintenance such as repainting 

of wood or repointing of masonry. Summer moisture 

problems are most commonly associated with 

excessive indoor cooling and the use of interior wall 

fi nishes that act as vapor retarders (paint buildup or 

vinyl wall coverings). Good air-sealing at the ceiling 

plane usually controls moisture in insulated attics.

Most problems are caused by poor moisture 

management, poor detailing which does not allow 

the building to shed water, or inadequate drainage. 

Therefore, a thorough assessment of the building’s 

ability to keep out unwanted moisture must be done 

before adding new insulation materials. Refer to 

Preservation Brief #39: Holding the Line: Controlling 

Unwanted Moisture in Historic Buildings for more 

information. Because of all the uncertainties 

associated with insulating walls, brick walls in 

particular, it may be advisable to hire a professional 

consultant who specializes in the many factors that 

affect the behavior of moisture in a building and 

can apply this expertise to the unique characteristics 

of a particular structure. Sophisticated tools such 

as computer modeling are useful to predict the 

performance of building assemblies, but they require 

interpretation by a skilled practitioner and the results 

are only as good as the data entered. It is important 

to remember, there are no reliable prescriptive 

measures to prevent moisture problems.4

Vapor Retarders (Barriers): Vapor retardants are 

commonly used in modern construction to manage 

the diffusion of moisture into wall cavities and 

attics. For vapor retardants to work properly, 

however, they must be continuous, which makes 

their installation diffi cult in existing buildings, and 

therefore generally not recommended. Even in new 

construction, installation of vapor retardants is not 

always indicated. Formerly, the recommended 

treatment was to install a vapor retardant toward 

the heated side of the wall (toward the interior 

space in cold climates and toward the exterior in hot 

climates). DOE now recommends that if moisture 

moves both to the interior and exterior of a building 

for signifi cant parts of the year, it is better not to use 

a vapor retarder at all.5

Fig. 21. The interior face of a brick masonry wall shows damage that 

resulted from the installation of a vapor retardant (foil facing) and 

thermal insulation. Photo: Simpson Gumpertz & Heger.
14

imbedded in the wall. Masonry walls of buildings that 

are heated during the winter benefi t from the transfer of 

heat from the inside to the outside face of the walls. This 

thermal transfer protects the exterior face of the wall by 

reducing the possibility of water freezing in the outer 

layers of the wall, particularly in cold and wet climates. 

The addition of insulation on the interior of the wall not 

only prolongs the drying rate of the exterior masonry 

wall, but keeps it colder as well, thereby increasing the 

potential for damage due to freeze-thaw cycles.6

Extreme swings in temperature may also have negative 

effects on a historic masonry wall. The addition of 

insulation materials to a historic masonry wall decreases 

its ability to transfer heat; thus, walls tend to stay warm 

or cold for longer periods of time. In addition, walls 

exposed to prolonged solar radiation during winter 

months can also be subject to higher swings in surface 

temperature during the day. Deleterious effects due 

to stress caused by expansion and contraction of the 

building assembly components can result. 

Buildings with masonry materials of higher porosity, 

such as those built with low-fi red brick, or certain 

soft stones, are particularly susceptible to freeze-thaw 

cycles and must be carefully evaluated prior to adding 

insulation. Inspection of the masonry in areas that are 

not heated such as parapets, exposed wing walls, or 

other parts of the building is particularly important. A 

noticeable difference in the amount of spalling or sanding 

of the masonry in these areas could predict that the same 

type of deterioration will occur throughout the building 

after the walls are insulated. Brick that was fi red at lower 

temperatures was often used on the inside face of the 

wall or on secondary elevations. Even masonry walls 

faced with more robust materials such as granite may 

have brick, rubble, mortar or other less durable materials 

as backing.

Spray foams are being used for insulation in many 

masonry buildings. Their ability to be applied over 

irregular surfaces, provide good air tightness, and 

continuity at intersections between, walls, ceilings, fl oors 

and window perimeters makes them well suited for use 

in existing buildings. However, the long-term effects 

of adding either open- or closed-cell foams to insulate 

historic masonry walls as well as performance of these 

products have not been adequately documented. 

Use of foam insulation in buildings with poor quality 

masonry or uncontrolled rising damp problems should 

be avoided. 

Periodic monitoring of the condition of insulated 

masonry walls is strongly recommended regardless of 

the insulation material added.

Install cool roofs and green roofs: Cool roofs and 

vegetated “green roofs” help to reduce the heat gain 

from the roof, thereby cooling the building and its 

environment. Cool roofs include refl ective metal roofs, 

light-colored or white roofs, and fi berglass shingles 

that have a coating of refl ective crystals. All of these 

roofi ng materials refl ect the sun’s radiation away from 

the building, which lessens heat gain, resulting in a 

reduction of the cooling load. Cool roofs are generally 

not practical in northern climates where buildings 

benefi t from the added heat gain of a dark-colored 

roof during colder months. Cool and green roofs are 

appropriate for use on historic buildings only when they 

are compatible with their architectural character, such 

as fl at roofs with no visibility. A white-colored roof that 

is readily visible is not appropriate for historic metal 

roofs that were traditionally painted a dark color, such 

as green or iron oxide red. A white refl ective roof is most 

suitable on fl at roofed historic buildings. If a historic 

building has a slate roof, for example, removing the 

slate to install a metal roof is not a compatible treatment. 

It is never appropriate to remove a historic roof if the 

material is in good or repairable condition to install 

a cool roof. However, if the roof has previously been 

changed to an asphalt shingle roof, fi berglass shingles 

with special refl ective granules may be an appropriate 

replacement. 

A green roof consists of a thin layer of vegetation 

planted over a waterproofi ng system or in trays installed 

on top of an existing fl at or slightly sloped roof. Green 

roofs are primarily benefi cial in urban contexts to reduce 

the heat island effect in cities and to control storm water 

run-off. A green roof also reduces the cooling load of 

the building and helps cool the surrounding urban 

environment, fi lters air, collects and fi lters storm water, 

and can provide urban amenities, including vegetable 

gardens, for building occupants. The impact of increased 

Fig. 22. Installation of both cool and green roofs in an urban 

environment.
15

structural loads, added moisture, and potential for 

leaks must be considered before installing a green roof. 

A green roof is compatible on a historic building only if 

the plantings are not visible above the roofl ine as seen 

from below. 

Alternative Energy Sources 

Although not the focus of this publication, alternative 

energy sources are dealt with in more detail in The 

Secretary of the Interior’s Standards for Rehabilitation & 

Illustrated Guidelines on Sustainability for Rehabilitating 

Historic Buildings and other NPS publications. 

Devices that utilize solar, geothermal, wind and other 

sources of energy to help reduce consumption of 

fossil fuel-generated energy can often be successfully 

incorporated in historic building retrofi ts. However, if 

the alterations or costs required to install these devices 

do not make their installation economically feasible, 

buying power generated off site from renewable 

sources may also be a good alternative. The use of most 

alternative energy strategies should be pursued only 

after all other upgrades have been implemented to make 

the building more energy effi cient because their initial 

installation cost is usually high. 

Solar Energy: Man has sought to harness the power 

of solar energy to heat, cool, and illuminate buildings 

throughout history. Construction techniques and design 

strategies that utilize building materials and components 

to collect, store, and release heat from the sun are 

described as “passive solar design.” As previously 

discussed, many historic buildings include passive solar 

features that should be retained and may be enhanced. 

Compatible additions to historic buildings also offer 

opportunities to incorporate passive solar features. 

Active solar devices, such as solar heat collectors and 

photovoltaic systems, can be added to historic buildings 

to decrease reliance on grid-source fossil-fuel powered 

electricity. Incorporating active solar devices in existing 

buildings is becoming more common as solar collector 

technology advances. Adding this technology to historic 

buildings, however, must be done in a manner that 

has a minimal impact on historic roofi ng materials and 

preserves their character by placing them in locations 

with limited or no visibility, i.e., on fl at roofs at a low 

angle or on a secondary roof slope. 

Solar collectors used to heat water can be relatively 

simple. More complex solar collectors heat a fl uid or 

air that is then pumped through the system to heat or 

cool interior spaces. Photovoltaic panels (PV) transform 

solar radiation into electricity. The greatest potential 

for the use of PV panels in historic buildings is on 

buildings with large fl at roofs, high parapets, or roof 

confi gurations that allow solar panels to be installed 

without being prominently visible. The feasibility 

of installing solar devices in small commercial and 

residential buildings will depend on installation costs, 

conventional energy rates, and available incentives, all of 

which will vary with time and location. The same factors 

apply to the use of solar collectors for heating water, but 

smaller installations may meet a building’s need and the 

technology has a considerable track record. 

Geothermal Energy: The use of the earth’s heat is 

another source of readily-accessible clean energy. The 

most common systems that utilize this form of energy 

are geothermal heat pumps, also known as geoexchange, earth-coupled, ground-source, or water-source 

heat pumps. Introduced in the late 1940s, geothermal 

heat pumps rely on heat from the constant temperature 

of the earth, unlike most other heat pumps which use 

the outside air temperature as the exchange medium. 

This makes geothermal heat pumps more effi cient than 

conventional heat pumps because they do not require an 

electric back-up heat source during prolonged periods of 

cold weather. 

There are many reasons that geothermal heat pumps 

are well suited for use in historic buildings. They 

can reduce the amount of energy consumption and 

emissions considerably, compared to the air exchange 

systems or electric resistance heating of conventional 

HVAC systems. They require less equipment space, 

have fewer moving parts, provide better zone space 

conditioning, and maintain better internal humidity 

levels. Geothermal heat pumps are also quieter because 

they do not require external air compressors. Despite 

higher installation costs, geothermal systems offer 

long-term operational savings and adaptability that 

may make them a worthwhile investment in some 

historic buildings. 

Fig. 23. Solar collectors installed in a compatible manner on low 

sloping sawtooth monitors. Top Photo: Neil Mishalov, Berkeley, CA.
16

Wind Energy: For historic properties in rural areas, 

where wind power has been utilized historically, 

installation of a wind mill or turbine may be suitable to 

the historic setting and cost effective. Before choosing to 

install wind-powered equipment, the potential benefi t 

and the impact on the historic character of the building, 

the site and surrounding historic district must be 

analyzed. In order for the turbines to work effectively, 

average wind speeds of 10 mph or higher are necessary. 

This technology may not be practical in more denselypopulated areas sheltered from winds or regions where 

winds are not consistent. In cities with tall buildings, 

there is potential for installing relatively small rooftop 

turbines that are not visible from the ground. However, 

because of the initial cost and size of some turbines, it is 

generally more practical to purchase wind power from 

an off-site wind farm through the local utility company. 

Summary

With careful planning, the energy effi ciency of historic 

buildings can be optimized without negatively 

impacting their historic character and integrity. 

Measuring the energy performance of buildings after 

improvements are completed must not be overlooked, 

as it is the only way to verify that the treatments 

have had the intended effect. Ongoing monitoring of 

buildings and their components after alterations to 

historic building assemblies are completed can prevent 

irreparable damage to historic materials. This, along 

with regular maintenance, can ensure the long-term 

preservation of our historic built environment and the 

sustainable use of our resources.

Acknowledgements

Jo Ellen Hensley, Senior Architectural Historian, LEED Green Associate, and Antonio Aguilar, Senior Historical Architect, Technical 

Preservation Services Branch, National Park Service, revised Preservation Brief 3: Conserving Energy in Historic Buildings, written 

by Baird M. Smith, FAIA and published in 1978. The revised Brief contains expanded and updated information on the subject of 

energy effi ciency in historic buildings. A number of individuals and organizations have contributed their time and expertise in the 

development of this Brief, beginning with the participants of the “Improving Energy Effi ciency in Historic Buildings—A Round Table 

Symposium,” Washington, DC, 2002. Special thanks go to Mike Jackson, FAIA, Illinois Historic Preservation Agency; Edward Minch,

Energy Services Group; William B. Rose, Research Architect, University of Illinois; Bradford S. Carpenter, P.E., LEED AP; and Mark 

Thaler, AIA, for their technical advice. The Advisory Council on Historic Preservation’s Sustainability Task Force, the General Services 

Administration’s Center for Historic Buildings, and our colleagues at the National Center for Preservation Technology and Training 

commented on the manuscript. In addition, the Technical Preservation Services professional staff, in particular Anne Grimmer, 

Michael J. Auer and John Sandor, provided critical and constructive review of the publication.

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary 

of the Interior to develop and make available information concerning historic properties. Additional information about the programs of 

Technical Preservation Services is available on the website at www.nps.gov/tps. Comments about this publication should be addressed 

to: Charles E. Fisher, Technical Preservation Publications Program Manager, Technical Preservation Services, National Park Service, 

1201 Eye Street, NW, 6th Floor, Washington, DC 20005. This publication is not copyrighted and can be reproduced without penalty. 

Normal procedures for credit to the authors and the National Park Service are appreciated. The photographs used in this publication 

may not be used to illustrate other publications without permission of the owners. Cover photograph: Farmhouse with energy effi cient 

historic storm windows.

ISBN: 978-0-16-089762-7

U.S. Government Printing Offi ce Stock Number: 024-005-01294-0 

December 2011

End Notes

1

 John Krigger and Chris Dorsi, “Air Leakage,” in Residential 

Energy: Cost Savings and Comfort for Existing Buildings. Helena, 

Montana: Saturn Resource Management, 2004, p. 73.

2 Measured Winter Performance of Storm Windows. A 2002 study done 

by Lawrence Berkeley National Labs.

3 Midwest Weatherization Best Practices Field Guide. Prepared for the 

U.S. Department of Energy Weatherization Assistance Program, 

May 2007, p. 157.

4

 Adapted from comments provided by William B. Rose, Research 

Architect, University of Illinois, April 2011.

5

 U.S. Department of Energy, Insulation Fact Sheet, DOE/CE-0180, 

2008, p.14.

6

 Bradford S. Carpenter, P.E., LEED AP et al., The Designer’s 

Dilemma: Modern Performance Expectations and Historic Masonry 

Walls (paper presented at the RCI 2010 Symposium on Building 

Envelope Technology, San Antonio, Texas).