7 Historic Structures
Although the commissioning process is typically associated with new construction, it can also be applied to historic structures. Historic structures were designed based on the technology of the time and, therefore, were seldom humidified. When modern mechanical systems are introduced during adaptive reuse projects, a proliferation of moisture-related problems can result. In these cases, the commissioning process can be applied to ensure both interior comfort and protection of the historic building envelope.
An example of such a project is the historic Stockade Building (#216) at Fort Riley, KS. The Stockade is slated for conversion into a museum curation facility. As part of this adaptive reuse project, USACERL recently initiated the envelope commissioning process to ensure the proper balance between the needs of the facility's historic artifacts and the constraints of the historic building envelope.
A new chapter has been proposed for the ASHRAE Handbook of Applications (1999 edition). The proposed chapter refers to methodologies for conditioning the indoor environment of historic structures used as museums, libraries, or archive buildings. This chapter warns against controlling the indoor environment in ways that might be harmful to the historic fabric of the building.
In a historically unconditioned structure such as the Fort Riley Stockade, the indoor environment varies seasonally and with changes in the weather. Such a floating indoor environment is typically more beneficial to the building envelope than one designed for fixed setpoints of humidity and temperature. However, traditional mechanical engineering design often begins with specified indoor conditions of temperature and humidity and associated tolerances. Then, in traditional practice, a building envelope design is imposed, to accommodate the indoor environment specification. This typically imposes thermal insulation, vapor retarders, and infiltration control.
An alternate mechanical engineering design approach is being pursued for the Stockade. It may be described as identifying the temperature and moisture "signature" for an existing historic structure, then using that signature as a starting point for equipment design.
The temperature (T) and RH will be monitored outdoors and at several locations in the interior of the building. The T and RH values will then be converted to moisture concentration or other measure of absolute humidity. The conditions of absolute humidity are generally more uniform throughout the building than measured T and RH, and they can be used to disclose rates of moisture genera-tion and dilution. The moisture concentration may disclose the amount of buffer-ing of temperature (and with greater difficulty, moisture), and the normal fresh air dilution. In a stone building with large mass such as the Stockade, the enthalpy exchange - i.e., the temperature modulation by sorption and desorp-tion - may be of considerable interest.
The product of this envelope performance assessment will be a report showing the ranges of T and RH for the measured locations. The report will show, to the extent possible, how heat, air, and moisture move through the building. That information will form the baseline for determining what changes are necessary to affect the interior conditions.
Range of error in these measurements is considerable, as only a few measuring sites are used, and the monitoring campaign cannot extend through all possible conditions of exterior climate. Nonetheless, this information can be very valuable. The standard literature for the mechanical engineering field - the ASHRAE Handbooks - does not address conditions in existing buildings. Thus, mechanical engineers may not be familiar with using monitored building performance data. But, the use of such data may be considered similar to the use of drawings of an existing building by restoration architects.
The objective of the work to be performed at Fort Riley is to identify the envelope performance "signature" of the historic Stockade Building and provide recom-mendations for the pre-design phase of the building commissioning process. These recommendations will be applied toward the design and commissioning of the mechanical and building envelope systems during the adaptive reuse project. At that time, recommendations for the continuation of the process will be outlined.
To begin the development of the building performance "signature," the CA must conduct an initial site visit. A visual (nondestructive) inspection and assessment of the building's thermal and moisture conditions should be made. From these observations and discussions with building staff, locations for sensor placement can be defined. A minimum of five T and RH dataloggers and two multi-channel thermocouple dataloggers should be placed in various locations throughout the building. These locations should represent typical conditions and potential problem areas identified during the visual inspection.
A second site visit will be conducted by the CA to download data from the dataloggers and to review and verify the preliminary findings from the initial site visit. Again, a meeting with the building staff to discuss the preliminary findings and their experience with temperature and moisture problems in the building can provide valuable insight.
With the data in hand, the CA then prepares the Envelope Performance Report. This report should present the findings in a format that will facilitate discussion of acceptable and unacceptable indoor environmental conditions and should do, at a minimum, the following:
· summarize the temperature and humidity findings
· summarize the findings of temperature profile through the wall assembly
· estimate moisture concentrations and track humidity balance
· estimate thermal and moisture buffering
· describe building environment performance "signature."
Through discussion of the findings in the Envelope Performance Report, an acceptable range of indoor environmental conditions is determined. Based on this range, the CA prepares the Building Envelope Commissioning Guidelines. The guidelines should estimate the form and severity of potential building envelope failure under conditions of elevated indoor humidity and should accurately describe the design, equipment, and control measures required to avoid future moisture problems. Once the Building Envelope Commissioning Guidelines are complete, the CA will submit them for review and acceptance to the building owner, the project design team, and other contractors and con-sultants involved in the commissioning process.
