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.
 Shutters can be used to minimize the problem of summer heat gain by shading the windows. Photo: NPS files. |
Older buildings were found to use less energy for heating and
cooling and hence 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, rooftop 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,
a historic window could remain an energy efficient component of a building.
 This 19th c. building in Massachusetts employed several energy-conserving features in its historic design, including shade trees, roof overhangs, awnings and shutters. Photo: HABS collection, NPS. |
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) (1) 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.
"Conserving Energy" an Historic Preservation Brief
