Historic Preservation Principles
Historic preservation involves the careful maintenance and rehabilitation of buildings and structures that have historical, architectural, or cultural significance. The Secretary of the Interior Standards for Rehabilitation provide the framework for preservation work in the United States, establishing four treatment approaches: preservation, rehabilitation, restoration, and reconstruction. Preservation focuses on maintaining the existing form, integrity, and materials of a historic property. Rehabilitation acknowledges the need to alter or add to a historic property to accommodate continuing use while retaining the character-defining features. Restoration depicts the property at a particular period in history by removing evidence of other periods. Reconstruction re-creation of vanished or non-surviving portions of a property for interpretive purposes.
The evaluation of historic significance is the first step in determining appropriate preservation treatments. Properties listed on the National Register of Historic Places have been evaluated for their significance in American history, architecture, archeology, engineering, or culture. The significance may be associated with important events, important persons, distinctive architectural characteristics, or the potential to yield important information through archeological investigation. The period of significance defines the time span during which the property achieved its historic importance. Character-defining features including the overall form, materials, craftsmanship, detailing, interior spaces, and site features must be identified and preserved to the greatest extent possible during any intervention.
The approach to preservation work differs fundamentally from conventional construction because the goal is to retain historic fabric rather than replace it. Repair of deteriorated historic materials is preferred over replacement whenever feasible. When replacement is necessary due to extensive deterioration, the new materials should match the original in design, color, texture, and other visual qualities. The use of compatible materials and methods that do not damage historic fabric is essential. The National Park Service recommends that interventions be documented and reversible where possible, allowing future generations to undo today’s work if better preservation technology becomes available.
Rehabilitation of Historic Structures
The rehabilitation of historic structures must balance the preservation of historic character with the requirements of modern building codes and accessibility standards. Structural upgrades to meet current seismic requirements present particular challenges in historic buildings because the structural systems were typically not designed for the lateral forces required by modern codes. The addition of new structural elements must be carefully integrated to avoid damaging historic fabric. Steel moment frames concealed within existing walls can provide seismic resistance without altering the historic appearance. secretary of interior standards for historic rehabilitation. seismic retrofitting techniques for historic masonry buildings. repointing historic masonry with compatible mortar mixtures. Base isolation systems installed beneath historic masonry buildings provide seismic protection while preserving the existing structure above.
The upgrading of mechanical, electrical, and plumbing systems in historic buildings requires careful planning to minimize damage to historic finishes and features. New ductwork and piping should be routed through existing service chases, closets, and non-historic spaces rather than cutting through historic wall and ceiling finishes. The use of mini-split heat pump systems avoids the need for extensive ductwork in buildings where duct installation would damage historic fabric. Exposed mechanical systems in historically industrial buildings can be left visible as part of the historic character. The integration of modern fire suppression systems requires coordination with the historic preservation review to identify locations for sprinkler heads and piping that minimize visual impact.
Accessibility upgrades required by the Americans with Disabilities Act must be provided in historic buildings to the maximum extent feasible without threatening or destroying the historic significance. The placement of accessible entrances must respect the historic entry configuration while providing independent access for persons with disabilities. Interior accessibility improvements such as lifts or elevators must be located in areas that minimize impact on historic spaces and features. The consultation with the State Historic Preservation Office during the design process helps identify acceptable solutions that balance accessibility requirements with preservation goals.
Materials Conservation
The conservation of historic masonry involves cleaning, repointing, and repair of brick, stone, and mortar. Gentle cleaning methods using low-pressure water and soft bristle brushes remove dirt and biological growth without damaging the masonry surface. Abrasive cleaning methods such as sandblasting are never appropriate for historic masonry because they permanently erode the protective fired surface of brick and soften the detail of carved stone. Repointing, the process of removing deteriorated mortar and replacing it with new mortar, must use mortar that is compatible with the historic masonry. The new mortar should be softer and more permeable than the historic masonry units to allow moisture to escape through the mortar joints rather than through the masonry units.
Historic wood windows are character-defining features that should be repaired rather than replaced whenever possible. The repair of deteriorated wood sashes, frames, and trim using epoxy consolidants and wood patching compounds extends the service life of historic windows while retaining their original character. Weatherstripping added to repaired windows improves energy performance without the environmental cost of manufacturing new windows. Storm windows installed on the interior or exterior provide additional thermal performance while preserving the historic window appearance. Studies have shown that properly repaired historic wood windows with storm windows achieve thermal performance comparable to new double-glazed windows.
The preservation of historic metal features including cast iron storefronts, ornamental railings, and sheet metal cornices requires specialized knowledge of the metal type and appropriate conservation methods. Corrosion removal by gentle mechanical methods followed by application of appropriate protective coatings prevents further deterioration. Cast iron repair using metal stitching or epoxy patching restores structural integrity to cracked elements. The replication of missing decorative metal elements requires skilled craftspeople using traditional techniques such as sand casting or metal spinning.
Energy Performance Improvements
The improvement of energy performance in historic buildings must be accomplished without compromising the historic character or damaging historic materials. The addition of insulation in attic spaces and basement areas can significantly reduce energy consumption without affecting the historic appearance. Attic insulation added between and over the existing ceiling joists provides thermal improvement without altering interior spaces. Basement wall insulation applied to the interior side of foundation walls improves the thermal performance of the lowest level. The insulation of framed walls from the interior side is possible but requires careful detailing to maintain the vapor profile of the wall assembly and avoid moisture problems.
The replacement of historic heating and cooling systems with high-efficiency equipment reduces energy consumption while improving comfort. The equipment location must be selected to minimize visual impact on historic spaces. Ductless heat pump systems with discreet wall-mounted interior units and exterior compressors located in less visible areas provide efficient heating and cooling without extensive ductwork. Geothermal heat pump systems using ground loops buried beneath the site provide the highest efficiency and eliminate the need for outdoor condenser units on the building exterior. The installation of ground loops in historic landscapes must be done with minimal disruption to significant site features.
Lighting upgrades in historic buildings using LED technology reduce energy consumption while improving light quality. LED lamps that match the color temperature and appearance of historic incandescent lamps are available for use in historic light fixtures. The installation of occupancy sensors and daylight harvesting controls reduces lighting energy use in spaces that are not continuously occupied. The historic lighting fixtures themselves should be preserved and rewired as needed rather than replaced with modern fixtures. The combination of LED technology, controls, and behavioral changes can reduce lighting energy consumption in historic buildings by 50 to 75 percent compared to conventional systems.