Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 3 Conserving Energy In Historic Buildings

Procedure code:

0110004S

Source:

Preservation Briefs 3, National Park Service, Pad

Division:

General Requirements

Section:

Special Project Procedures

Last Modified:

09/05/2012

Details:

Preservation Briefs: 3 Conserving Energy In Historic Buildings



PRESERVATION BRIEFS: 3

CONSERVING ENERGY IN HISTORIC BUILDINGS

 

The link immediately below connects to the latest version of Preservation Brief 3:

 

http://www.nps.gov/history/hps/tps/briefs/brief03.pdf


Baird M. Smith, AIA

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01026-2
              Available ONLY in package sets
              Briefs 1-14 - $13.00

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


***INTRODUCTION***

With the dwindling supply of energy resources and new efficiency
demands placed on the existing building stock, many owners of
historic buildings and their architects are assessing the ability
of these buildings to conserve energy with an eye to improving
thermal performance.  This brief has been developed to assist those
persons attempting energy conservation measures and weatherization
improvements such as adding insulation and storm windows or
caulking of exterior building joints.  In historic buildings, many
measures can result in the inappropriate alteration of important
architectural features, or, perhaps even worse, cause serious
damage to the historic building materials through unwanted chemical
reactions or moisture-caused deterioration.  This brief recommends
measures that will achieve the greatest energy savings with the
least alteration to the historic buildings, while using materials
that do not cause damage and that represent sound economic
investments.


***INHERENT ENERGY SAVING CHARACTERISTICS OF HISTORIC BUILDINGS***

Many historic buildings have energy-saving physical features and
devices that contribute to good thermal performance.  Studies by
the Energy Research and Development Administration (see
bibliography) show that the buildings with the poorest energy
efficiency are actually those built between 1940 and 1975.  Older
buildings were found to use less energy for heating and cooling and
probably require fewer weatherization improvements.  They use less
energy because they were built with a well-developed sense of
physical comfort and because they maximized the natural sources of
heating, lighting and ventilation.  The historic building owner
should understand these inherent energy-saving qualities.

The most obvious (and almost universal) inherent energy saving
characteristic was the use of operable windows to provide natural
ventilation and light.  In addition, historic commercial and public
buildings often include interior light/ventilation courts, roof-top
ventilators, clerestories or skylights.  These features provide
energy efficient fresh air and light, assuring that energy
consuming mechanical devices may be needed only to supplement the
natural energy sources.  Any time the mechanical heating and air
conditioning equipment can be turned off and the windows opened,
energy will be saved.

Early builders and architects dealt with the poor thermal
properties of windows in two ways.  First, the number of windows in
a building was kept to only those necessary to provide adequate
light and ventilation.  This differs from the approach in many
modern buildings where the percentage of windows in a wall can be
nearly 100%.  Historic buildings, where the ratio of glass to wall
is often less than 20%, are better energy conservers than most new
buildings.  Secondly, to minimize the heat gain or loss from
windows, historic buildings often include interior or exterior
shutters, interior venetian blinds, curtains and drapes, or
exterior awnings.  Thus, an historic window could remain an energy
efficient component of a building.

There are other physical characteristics that enable historic
buildings to be energy efficient.  For instance, in the warmer
climates of the United States, buildings were often built to
minimize the heat gain from the summer sun.  This was accomplished
by introducing exterior balconies, porches, wide roof overhangs,
awnings and shade trees.  In addition, many of these buildings were
designed with the living spaces on the second floor to catch
breezes and to escape the radiant heat from the earth's surface.
Also, exterior walls were often painted light colors to reflect the
hot summer sun, resulting in cooler interior living spaces.

Winter heat loss from buildings in the northern climates was
reduced by using heavy masonry walls, minimizing the number and
size of windows, and often using dark paint colors for the exterior
walls.  The heavy masonry walls used so typically in the late 19th
century and early 20th century, exhibit characteristics that
improve their thermal performance beyond that formerly recognized.
It has been determined that walls of large mass and weight (thick
brick or stone) have the advantage of high thermal inertia, also
known as the "M factor."  This inertia modifies the thermal
resistance (R factor) of the wall by lengthening the time scale of
heat transmission.  For instance, a wall with high thermal inertia,
subjected to solar radiation for an hour, will absorb the heat at
its outside surface, but transfer it to the interior over a period
as long as 6 hours.  Conversely, a wall having the same R factor,
but low thermal inertia, will transfer the heat in perhaps 2 hours.
High thermal inertia is the reason many older public and commercial
buildings, without modern air conditioning, still feel cool on the
inside throughout the summer.  The heat from the midday sun does
not penetrate the buildings until late afternoon and evening, when
it is unoccupied.

Although these characteristics may not typify all historic
buildings, the point is that historic buildings often have thermal
properties that need little improvement. One must understand the
inherent energy-saving qualities of a building, and assure, by
re-opening the windows for instance, that the building functions as
it was intended.

To reduce heating and cooling expenditures there are two broad
courses of action that may be taken.  First, begin passive measures
to assure that a building and its existing components function as
efficiently as possible without the necessity of making alterations
or adding new materials.  The second course of action is
preservation retrofitting, which includes altering the building by
making appropriate weatherization measures to improve thermal
performance.  Undertaking the passive measures and the preservation
retrofitting recommended here could result in a 50% decrease in
energy expenditures in historic buildings.


***PASSIVE MEASURES***

The first passive measures to utilize are operational controls;
that is, controlling how and when a building is used.  These
controls incorporate programmatic planning and scheduling efforts
by the owner to minimize usage of energy consuming equipment.  A
building owner should survey and quantify all aspects of energy
usage, by evaluating the monies expended for electricity, gas, and
fuel oil for a year, and by surveying how and when each room is
used.  This will identify ways of conserving energy by initiating
operational controls such as:

    - lowering the thermostat in the winter, raising it in the
    summer
    - controlling the temperature in those rooms actually used
    - reducing the level of illumination and number of lights
    (maximize natural light)
    - using operable windows, shutters, awnings and vents as
    originally intended to control interior environment (maximize
    fresh air)
    - having mechanical equipment serviced regularly to ensure
    maximum efficiency
    - cleaning radiators and forced air registers to ensure proper
     operation

The passive measures outlined above can save as much as 30% of the
energy used in a building.  They should be the first undertakings
to save energy in any existing building and are particularly
appropriate for historic buildings because they do not necessitate
building alterations or the introduction of new materials that may
cause damage.  Passive measures make energy sense, common sense,
and preservation sense!


***PRESERVATION RETROFITTING***

In addition to passive measures, building owners may undertake
certain measures that will not jeopardize the historic character of
the building and can be accomplished at a reasonable cost.
Preservation retrofitting improves the thermal performance of the
building,resulting in another 20%- 30% reduction in energy.

When considering retrofitting measures, historic building owners
should keep in mind that there are no permanent solutions.  One can
only meet the standards being applied today with today's materials
and techniques.  In the future, it is likely that the standards and
the technologies will change and a whole new retrofitting plan may
be necessary.  Thus, owners of historic buildings should limit
retrofitting measures to those that achieve reasonable energy
savings, at reasonable costs, with the least intrusion or impact on
the character of the building.  Overzealous retrofitting, which
introduces the risk of damage to historic building materials,
should not be undertaken.

The preservation retrofitting measures presented here, were
developed to address the three most common problems in historic
structures caused by some retrofitting actions that necessitated
inappropriate building alterations, such as the wholesale removal
of historic windows, or the addition of insulating aluminum siding,
or installing dropped ceilings in significant interior spaces.  To
avoid such alterations, refer to the Secretary of the Interior's
"Standards for Historic Preservation Projects" which provide the
philosophical and practical basis for all preservation retrofitting
measures.  The second problem area is to assure that retrofitting
measures do not create moisture-related deterioration problems.
One must recognize that large quantities of moisture are present on
the interior of buildings.

In northern climates, the moisture may be a problem during the
winter when it condenses on cold surfaces such as windows.  As the
moisture passes through the walls and roof it may condense within
these materials, creating the potential for deterioration.  The
problem is avoided if a vapor barrier is added facing in.

In southern climates, insulation and vapor barriers are handled
quite differently because moisture problems occur in the summer
when the moist outside air is migrating to the interior of the
building.  In these cases, the insulation is installed with the
vapor barrier facing out (opposite the treatment of northern
climates).  Expert advice should be sought to avoid
moisture-related problems to insulation and building materials in
southern climates.

The third problem area involves the avoidance of those materials
that are chemically or physically incompatible with existing
materials, or that are improperly installed.  A serious problem
exists with certain cellulose insulations that use ammonium or
aluminum sulfate as a fire retardant, rather than boric acid which
causes no problems.  The sulfates react with moisture in the air
forming sulfuric acid which can cause damage to most metals
(including plumbing and wiring), building stones, brick and wood.
In one instance, a metal building insulated with cellulose of this
type collapsed when the sulfuric acid weakened the structural
connections!  To avoid problems such as these, refer to the
recommendations provided here, and consult with local officials,
such as a building inspector, the better business bureau, or a
consumer protection agency.  

Before a building owner or architect can plan retrofitting
measures, some of the existing physical conditions of the building
should be investigated.  The basic building components (attic,
roof, walls and basement) should be checked to determine the
methods of construction used and the presence of insulation.  Check
the insulation for full coverage and whether there is a vapor
barrier.  This inspection will aid in determining the need for
additional insulation, what type of insulation to use (batt,
blown-in, or poured), and where to install it.  In addition,
sources of air infiltration should be checked at doors, windows, or
where floor and ceiling systems meet the walls.  Lastly, it is
important to check the condition of the exterior wall materials,
such as painted wooden siding or brick, and the condition of the
roof, to determine the weather tightness of the building.  A
building owner must assure that rain and snow are kept out of the
building before expending money for weatherization improvements.


***RETROFITTING MEASURES***

The following list includes the most common retrofitting measures.
Some measures are highly recommended for a preservation
retrofitting plan, but, as will be explained, others are less
beneficial or even harmful to the historic building:

    - Air Infiltration
    - Attic Insulation
    - Storm Windows
    - Basement and Crawl Space Insulation
    - Duct and Pipe Insulation
    - Awnings and Shading Devices
    - Doors and Storm Doors      
    - Vestibules
    - Replacement Windows
    - Wall Insulation-Wood Frame
    - Wall Insulation-Masonry Cavity Walls
    - Wall Insulation-Installed on the Inside
    - Wall Insulation-Installed on the Outside
    - Waterproof Coatings for Masonry

The recommended measures to preservation retrofitting begin with
those at the top of the list.  The first ones are the simplest,
least expensive, and offer the highest potential for saving energy.
The remaining measures are not recommended for general use, either
because of potential technical and preservation problems, or
because of the costs outweighing the anticipated energy savings.
Specific solutions must be determined based on the facts and
circumstances of the particular problem.  Therefore, advice from
professionals experienced in historic preservation, such as,
architects, engineers and mechanical contractors should be
solicited.

AIR INFILTRATION:  

Substantial heat loss occurs because cold outside air infiltrates
the building through loose windows, doors, and cracks in the
outside shell of the building.  Adding weatherstripping to doors
and windows, and caulking of open cracks and joints will
substantially reduce this infiltration.  Care should be taken not
to reduce infiltration to the point where the building is
completely sealed and moisture migration is prevented.  Without
some infiltration, condensation problems could occur throughout the
building.  Avoid caulking and weatherstripping materials that, when
applied, introduce inappropriate colors or otherwise visually
impair the architectural character of the building.  Reducing air
infiltration should be the first priority of a preservation
retrofitting plan.  The cost is low, little skill is required, and
the benefits are substantial.

ATTIC INSULATION:  

Heat rising through the attic and roof is a major source of heat
loss, and reducing this heat loss should be one of the highest
priorities in preservation retrofitting.  Adding insulation in
accessible attic spaces is very effective in saving energy and is
generally accomplished at a reasonable cost, requiring little skill
to install.  The most common attic insulations include blankets of
fiberglass and mineral wool, blown-in cellulose (treated with boric
acid only), blowing wool, vermiculite, and blown fiberglass.  If
the attic is unheated (not used for habitation), then the
insulation is placed between the floor joists with the vapor
barrier facing down.  If flooring is present, or if the attic is
heated, the insulation is generally placed between the roof rafters
with the vapor barrier facing in.  All should be installed
according to the manufacturer's recommendations.  A weatherization
manual entitled, In the Bank...or Up the Chimney (see the
bibliography), provides detailed descriptions about a variety of
installation methods used for attic insulation.  The manual also
recommends the amount of attic insulation used in various parts of
the country.  If the attic has some insulation, add more (but
without a vapor barrier) to reach the total depth recommended.

Problems occur if the attic space is not properly ventilated.  This
lack of ventilation will cause the insulation to become saturated
and lose its thermal effectiveness.  The attic is adequately
ventilated when the net area of ventilation (free area of a louver
or vent) equals approximately 1/300 of the attic floor area.  With
adequate attic ventilation, the addition of attic insulation should
be one of the highest priorities of a preservation retrofitting
plan.

If the attic floor is inaccessible, or if it is impossible to add
insulation along the roof rafters, consider attaching insulation to
the ceilings of the rooms immediately below the attic.  Some
insulations are manufactured specifically for these cases and
include a durable surface which becomes the new ceiling.  This
option should not be considered if it causes irreparable damage to
historic or architectural spaces or features. However, in other
cases, it could be a recommended measure of a preservation
retrofitting plan.

STORM WINDOWS:

Windows are a primary source of heat loss because they are both a
poor thermal barrier (R factor of only 0.89) and often a source of
air infiltration.  Adding storm windows greatly improves these poor
characteristics.  If a building has existing storm windows (either
wood or metal framed), they should be retained.  Assure they are
tight fitting and in good working condition.  If they are not in
place, it is a recommended measure of a preservation retrofitting
plan to add new metal framed windows on the exterior.  This will
result in a window assembly (historic window plus storm window)
with an R factor of 1.79 which outperforms a double-paned window
assembly (with an air space up to 1/2") that only has an R factor
of 1.72.  When installing the storm windows, be careful not to
damage the historic window frame.  If the metal frames visually
impair the appearance of the building, it may be necessary to paint
them to match the color of the historic frame.

Triple-track metal storm windows are recommended because they are
readily available, in numerous sizes, and at a reasonable cost.  If
a pre-assembled storm window is not available for a particular
window size, and a custom- made storm window is required, the cost
can be very high.  In this case, compare the cost of manufacture
and installation with the expected cost savings resulting from the
increased thermal efficiency.  Generally, custom-made storm
windows, of either wood or metal frames, are not cost effective,
and would not be recommended in a preservation retrofitting plan.

Interior storm window installations can be as thermally effective
as exterior storm windows; however, there is high potential for
damage to the historic window and sill from condensation.  With
storm windows on the interior, the outer sash (in this case the
historic sash) will be cold in the winter, and hence moisture may
condense there.  This condensation often collects on the flat
surface of the sash or window sill causing paint to blister and the
wood to begin to deteriorate.  Rigid plastic sheets are used as
interior storm windows by attaching them directly to the historic
sash.  They are not quite as effective as the storm windows
described previously because of the possibility of air infiltration
around the historic sash.  If the rigid plastic sheets are used,
assure that they are installed with minimum damage to the historic
sash, removed periodically to allow the historic sash to dry, and
that the historic frame and sash are completely caulked and
weatherstripped.

In most cases, interior storm windows of either metal frames or of
plastic sheets are not recommended for preservation retrofitting
because of the potential for damage to the historic window.  If
interior storm windows are in place, the potential for moisture
deterioration can be lessened by opening (or removing, depending on
the type) the storm windows during the mild months allowing the
historic window to dry thoroughly.  

BASEMENT AND CRAWL SPACE INSULATION:

Substantial heat is lost through cold basements and crawl spaces.
Adding insulation in these locations is an effective preservation
retrofitting measure and should be a high priority action.  It is
complicated, however, because of the excessive moisture that is
often present.  One must be aware of this and assure that
insulation is properly installed for the specific location.  For
instance, in crawl spaces and certain unheated basements, the
insulation is generally placed between the first floor joists (the
ceiling of the basement) with the vapor barrier facing up.  Do not
staple the insulation in place, because the staples often rust
away.  Use special anchors developed for insulation in moist areas
such as these.

In heated basements, or where the basement contains the heating
plant (furnace), or where there are exposed water and sewer pipes,
insulation should be installed against foundation walls.  Begin the
insulation within the first floor joists, and proceed down the wall
to a point at least 3 feet below the exterior ground level if
possible, with the vapor barrier facing in.  Use either batt or
rigid insulation.

Installing insulation in the basement or crawl space should be a
high priority of a preservation retrofitting plan, as long as
adequate provision is made to ventilate the unheated space, perhaps
even by installing an exhaust fan.

DUCT AND PIPE INSULATION:  

Wrapping insulation around heating and cooling ducts and hot water
pipes, is a recommended preservation retrofitting measure.  Use
insulation which is intended for this use and install it according
to manufacturer's recommendations.  Note that air conditioning
ducts will be cold in the summer, and hence moisture will condense
there.  Use insulation with the vapor barrier facing out, away from
the duct.  These measures are inexpensive and have little potential
for damage to the historic building.

AWNINGS AND SHADING DEVICES:

In the past, awnings and trees were used extensively to provide
shade to keep buildings cooler in the summer.  If awnings or trees
are in place, keep them in good condition, and take advantage of
their energy saving contribution.  Building owners may consider
adding awnings or trees if the summer cooling load is substantial.
If awnings are added, assure that they are installed without
damaging the building or visually impairing its architectural
character.  If trees are added, select deciduous trees that provide
shade in the summer but, after dropping their leaves, would allow
the sun to warm the building in the winter.  When planting trees,
assure that they are no closer than 10 feet to the building to
avoid damage to the foundations.  Adding either awnings or shade
trees may be expensive, but in hot climates, the benefits can
justify the costs.


DOORS AND STORM DOORS:

Most historic wooden doors, if they are solid wood or paneled, have
fairly good thermal properties and should not be replaced,
especially if they are important architectural features.  Assure
that the frames and doors have proper maintenance, regular
painting, and that caulking and weatherstripping is applied as
necessary.

A storm door would improve the thermal performance of the historic
door; however, recent studies indicate that installing a storm door
is not normally cost effective in residential settings.  The costs
are high compared to the anticipated savings.  Therefore, storm
doors should only be added to buildings in cold climates, and added
in such a way to minimize the visual impact on the building's
appearance.  The storm door design should be compatible with the
architectural character of the building and may be painted to match
the colors of the historic door.

VESTIBULES:

Vestibules create a secondary air space at a doorway to reduce air
infiltration occurring while the primary door is open.  If a
vestibule is in place, retain it.  If not, adding a vestibule,
either on the exterior or interior, should be carefully considered
to determine the possible visual impact on the character of the
building.  The energy savings would be comparatively small compared
to construction costs.  Adding a vestibule should be considered in
very cold climates, or where door use is very high, but in either
case, the additional question of visual intrusion must be resolved
before it is added.  For most cases with historic buildings, adding
a vestibule is not recommended.

REPLACEMENT WINDOWS:  

Unfortunately, a common weatherization measure, especially in
larger buildings, has been the replacement of historic windows with
modern double paned windows. The intention was to improve the
thermal performance of the existing windows and to reduce long-term
maintenance costs.  The evidence is clear that adding exterior
storm windows is a viable alternative to replacing the historic
windows and it is the recommended approach in preservation
retrofitting.  However, if the historic windows are severely
deteriorated and their repair would be impractical, or economically
infeasible, then replacement windows may be warranted.  The new
windows, of either wood or metal, should closely match the historic
windows in size, number of panes, muntin shape, frame, color and
reflective qualities of the glass.

WALL INSULATION-WOOD FRAME:

The addition of wall insulation in a wood frame building is
generally not recommended as a preservation retrofitting measure
because the costs are high, and the potential for damage to
historic building materials is even higher.  Also, wall insulation
is not particularly effective for small frame buildings (one story)
because the heat loss from the uninsulated walls is a relatively
small percentage of the total, and part of that can be attributed
to infiltration.  If, however, the historic building is two or more
stories, and is located in a cold climate, wall insulation may be
considered if extreme care (as explained later) is exercised with
its insulation.

The installation of wall insulation in historic frame buildings can
result in serious technical and preservation problems.  As
discussed before, insulation must be kept dry to function properly,
and requires a vapor barrier and some provision for air movement.
Introducing insulation in wall cavities, without a vapor barrier
and some ventilation can be disastrous.  The insulation would
become saturated, losing its thermal properties, and in fact,
actually increasing the heat loss through the wall.  Additionally,
the moisture (in vapor form) may condense into water droplets and
begin serious deterioration of adjacent building materials such as
sills,  window frames, framing and bracing.  The situation is
greatly complicated, because correcting such problems could
necessitate the complete (and costly) dismantling of the exterior
or interior wall surfaces.  It should be clear that adding wall
insulation has the potential for causing serious damage to historic
building materials.

If adding wall insulation to frame buildings is determined to be
absolutely necessary, the first approach should be to consider the
careful removal of the exterior siding so that it may later be
reinstalled.  Then introduce batt insulation with the vapor barrier
facing in into the now accessible wall cavity.  The first step in
this approach is an investigation to determine if the siding can be
removed without causing serious damage.  If it is feasible,
introducing insulation in this fashion provides the best possible
solution to insulating a wall, and provides an excellent
opportunity to view most of the structural system for possible
hidden structural problems or insect infestations.  A building
owner should not consider this approach if it would result in
substantial damage to or loss of historic wooden siding.  Most
siding, however, would probably withstand this method if reasonable
care is exercised.

The second possible approach for wall insulation involves injecting
or blowing insulation into the wall cavity.  The common insulations
are the loose fill types that can be blown into the cavity, the
poured types, or the injected types such as foam.  Obviously a
vapor barrier cannot be simultaneously blown into the space.
However, an equivalent vapor barrier can be created by assuring
that the interior wall surfaces are covered with an impermeable
paint layer.  Two layers of oil base paint or one layer of
impermeable latex paint constitute an acceptable vapor barrier.
Naturally, for this to work, the paint layer must cover all
interior surfaces adjacent to the newly installed wall insulation.
Special attention should be given to rooms that are major sources
of interior moisture-the laundry room, the bathrooms and the
kitchen.

In addition to providing a vapor barrier, make provisions for some
air to circulate in the wall cavity to help ventilate the
insulation and the wall materials.  This can be accomplished in
several ways.  One method is to install small screened vents (about
2 inches in diameter) at the base of each stud cavity.  If this
option is taken, the vents should be inconspicuous as possible.  A
second venting method can be used where the exterior siding is
horizontally lapped.  Assure that each piece of siding is separated
from the other, allowing some air to pass between them.  Successive
exterior paint layers often seal the joint between each piece of
siding.  Break the paint seal (carefully insert a chisel and twist)
between the sections of exterior siding to provide the necessary
ventilation for the insulation and wall materials.

With provisions for a vapor barrier (interior paint layer) and wall
ventilation (exterior vents) satisfied, the appropriate type of
wall insulation may then be selected.  There are three recommended
types to consider: blown cellulose (with boric acid as a fire
retardant), vermiculite, or perlite.  Cellulose is the preferred
wall insulation because of its higher R factor and its capability
to flow well into the various spaces within a wall cavity.

There are two insulation types that are not recommended for wall
insulation: urea-formaldehyde foams, and cellulose which uses
aluminum or ammonium sulfate instead of boric acid as a fire
retardant.  The cellulose treated with the sulfates reacts with
moisture in the air and forms sulfuric acid which corrodes many
metals and causes building stones to slowly disintegrate.  This
insulation is not appropriate for use in historic buildings.

Although urea-formaldehyde foams appear to have potential as
retrofit materials (they flow into any wall cavity space and have
a high R factor) their use is not recommended for preservation
retrofitting until some serious problems are corrected.  The major
problem is that the injected material carries large quantities of
moisture into the wall system.  As the foam cures, this moisture
must be absorbed into the adjacent materials.  This process has
caused interior and exterior paint to blister, and caused water to
actually puddle at the base of the wall, creating the likelihood of
a formaldehyde smell.  In addition, the advertised maximum
 


Preservation Brief 3,conserving energy, insulating historic building,insulation,saving energy,storm window,vapor barrier, improving energy efficiency