5.25 Alterations in Existing Buildings and Historic Structures
The goal of alteration projects is to meet the same standards described in this document for new projects. Equipment/systems at 20 years life or older must be demolished and new systems designed to meet the current usage of the facility. Renovation and rehabilitation designs must satisfy the immediate occupancy needs and anticipate additional future changes. Remodeling should make building systems become more flexible, not less. Parameters of reuse and disruption of service must be clearly specified in construction documents.
Alteration projects can occur at three basic scales: refurbishment of an area within a building, such as a floor or a suite; major renovation of an entire structure; and up-grade/restoration of historic structures.
In the first instance, the aim should be to satisfy the new requirements within the parameters and constraints of the existing systems. The smaller the area in comparison to the overall building, the fewer changes to existing systems should be attempted. In the second case, the engineer has the opportunity to design major upgrades into the mechanical, electrical and communications systems. The mechanical services can come close to systems that would be designed for a new building, within the obvious limits of available physical space and structural capacity.
Where a historic structure is to be altered, special documents will be provided by GSA to help guide the design of the alterations. The most important of these is the HBPP that identifies zones of architectural importance, specific character-defining elements that should be preserved, and standards to be employed. See Chapter 1: General Requirements, General Design Philosophy, Historic Buildings.
Modern standards for climate control developed for new construction may not be achievable or desirable for historic buildings. In each case, the lowest level of intervention needed to successfully accomplish the project should be selected. When a system is designed, it is important to anticipate how it will be installed, how damage to historic materials can be minimized, and how visible the new mechanical system will be within the restored or rehabilitated space.
The following guidelines should be followed for HVAC work in historic buildings:
- Reduce heating and cooling loads to minimize size and other impacts of modern equipment.
- Calculate the effect of historic building features such as wall thickness, skylights, and porticos, interior design features such as draperies, shutters and window shades, and existing site features such as landscaping.
- Add insulation where not visible and intrusive, such as attics or basements. Insulate walls only where it can be done without removal or covering of original visible elements.
- Add storm windows where they can be installed in a manner that will not detract from original visible elements.
- Use new replicated thermal windows only where it is not economically feasible to repair existing windows.
- Select system types, components, and placement to minimize alteration of significant spaces. In previously altered spaces, design systems to allow historic surfaces, ceiling heights, and configurations to be restored. Consider reuse of existing components when reuse will reduce architectural intrusion and achieve savings, without compromising overall performance and life cycle requirements. Reuse of HVAC system elements is only permitted with written documentation obtained from GSA Property Management by the A/E. Retain decorative elements of historic systems where possible. Ornamental grilles and radiators and other decorative elements shall be retained in place.
- Retain the original type of system where a new one cannot be totally concealed. For example, reuse existing radiators with new distribution piping or replace with modern heating-cooling units, rather than adding another type of system that would require the addition of new ceilings or other non-original elements.
- Use a number of smaller units in lieu of a few large ones. Insure that room is available to maintain and replace equipment without damaging significant features to the greatest extent possible, selecting components that can be installed without dismantling window or door openings.
- Place new distribution systems out of sight whenever possible by using closets, shafts, attics and basements.
- Use custom rather than commercial standard products where elements are exposed in formal areas.
- Select temperature and humidity conditions that will not accelerate deterioration of building materials.
- Where equipment is near significant features, insure that leakage from pipes and HVAC units will not cause deterioration. Use deeper condensate drain pans, lined chases and leak detectors.
- Design HVAC systems to avoid impacting other systems and historic finishes, elements and spaces.
- Place exterior equipment where it is not visible. Be particularly careful with new chimneys or vents and condensers, towers, solar panels and air intakes and discharges. Recess equipment from the edge of the roof to minimize visibility of the equipment from grade. Alternatively, explore creating a vault for easier access to large mechanical equipment. If equipment cannot be concealed, specify equipment housings in a color that will blend with the historic face. As a last resort, enclose equipment in screening designed to blend visually with the facade.
- Locate equipment with particular care for weight and vibration on older building materials. These materials may not accept the same stress as when the equipment is used in newer construction.
- If new ceilings must be installed, insure that they do not block any light from the top of existing windows or alter the appearance of the building from the outside. This is the area of highest natural illumination, and it can be used to reduce the need for artificial illumination, which will in turn reduce the size of HVAC systems. Original plaster ceilings in significant spaces such as lobbies and corridors should be retained, to the extent possible, and modified only as necessary to accommodate horizontal distribution. Use soffits and false beams where necessary to minimize alteration of overall ceiling heights.
- Locate pipes so that they do not damage or visually interfere with character-defining elements in historic structures such as windows, doors, columns, beams, arches, baseboards, wainscots, paneling, cornices, ornamental trim, decorative woodwork and other decorative treatments of doors, walls and ceilings.
- Vertical Distribution. If new risers are required, they should preferably be located adjacent to existing shafts.
- Horizontal Distribution. Many older buildings have high floor-to-floor heights, which permit an option to use an existing ceiling space.
- In buildings containing ornamental or inaccessible ceilings, piping and ductwork may have to be routed in furred wall space or exposed in the occupiable building area. Exposed ducts must be designed to complement the building architecture in forms and materials used. Use of exposed ducts is encouraged in locations where concealing ducts would obscure significant architectural surfaces or details, such as vaulted ceilings. Exposed ducts should also be considered in historic industrial buildings and open plan, tall ceiling, high window spaces suited to flexible grid/flexible density treatments.