6 Commissioning Humidified Buildings
Because building envelope commissioning for humidification is a fairly recent concept, it becomes critically important to develop a contractual relationship between the client and the CA. The contract should describe and explain the responsibility of the CA with respect to the responsibilities of the other parties. The responsibility of the CA, consistent with the aim of commissioning, must be: (1) to meet the operational needs of the building, (2) to meet them within the capabilities of the design, (3) to meet the design documentation and the owner's functional criteria, and (4) to include preparation of operator personnel.
1. The description of the operational needs should be explicit. It should be rela-tively simple to be explicit about the operational needs in response to humidifica-tion; a humidified building should be designed, constructed, and operated in a way that the humidification needs are met and moisture problems in the building envelope are avoided.
2. The CA must be involved in building design because it is the design intent that determines whether or not the criteria can be met.
3. The CA must prepare a report that documents the testing and modeling procedures that will be used to meet the functional responsibility of the task.
4. The CA must remain involved following completion of the design and construction contracts to ensure that the building is being operated safely and as designed.
Of particular importance is the question whether acceptance of the work hinges on approval by the CA. In principle, if the client is resolute regarding building performance as a condition for acceptance, then the role of the CA is clear and the building cannot be considered substantially complete until the CA certifies that the building will perform as intended. But because the weight of design and construction traditionally lie with the delivery of a product rather than performance, commissioning must be accepted to be a matter of adjusting the performance of the product rather than a rewriting of the conditions of acceptance.
The CA is advised to negotiate a humidity range acceptable by parties. This negotiation is preferable to having the interested parties express dissatisfaction for any deviance from their parochial (narrow) ideal.
The only strong reasons for humidification are programmatic. What activities within the building require humidification? What are the consequences to these activities if the building is occasionally, seasonally, or regularly at a lower humidity? Is humidity a physical requirement (the process cannot go forward without humidity), a contractual or regulatory requirement (e.g., lending agree-ments for traveling museum exhibits), or a convenience? Are cost thresholds involved? For example, could a museum collection withstand lower humidity if a certain part of the collection is housed in a microclimate?
It is important for the CA to receive frank and explicit answers to these questions. Once a lower limit is established as a target or threshold value, those concerned about damage from excessively low humidity should describe what the expected damage might be and the extent to which their programs are impacted by occasional excursions past the limit.
In climates with cold winters, interior humidity is the strongest determinant of the likelihood and severity of moisture damage to the building envelope. The choice of upper limits to the allowable humidity range will have an effect on design constraints and equipment sizing.
Initial Meetings
A clear statement from the CA and the client regarding the role of commissioning and its importance to the project is critical to the success of building commissioning. It is at the programmatic meetings that design responsibilities are assigned and the humidification range is established. The initial meetings also set the tone for the level of cooperation to follow.
It is important for the CA to be seen as an advisor to all the parties, who is capable of assisting the designers to deliver a product - a building capable of humidification - which might otherwise be a challenge to the design team. A capable CA must inspire confidence.
Commissioning Brief
Soon after the initial meetings, the CA should develop the commissioning brief, which is incorporated into the planning documents. The purpose of the brief is to guide the design and it should contain the following:
· building humidification design intent
· the acceptable or negotiated range of humidity
· the CA's understanding of the design direction
· critical elements of design and construction
· means of testing compliance with the critical elements, both for design and construction
· anticipated future course of action in commissioning.
An opportunity for discussion should follow the distribution of the commission-ing brief.
Envelope
It is in the design phase that the use of hygrothermal modeling is most appropriate. As described earlier, steady-state models can be used as a "first cut" indicator of condensing conditions. Transient models (e.g., MOIST) can be used to estimate the performance in the absence of convection.
The moisture performance of insulation materials vary widely. Fiberglass insulation is typically of a low or light density and moisture in the insulation zone can diffuse easily. As noted above, the vapor barrier was developed precisely to accommodate the vapor protection needs of low-density insulation materials. Conversely, foam insulation has a high resistance to moisture transfer and is, in effect, its own vapor barrier.
Avoid unworkable specifications. The performance requirements of materials and assemblies should be clearly specified. In addition, guidelines that provide the means for achieving the desired performance should also be defined and detailed in the construction documents. If seams in the vapor barrier membrane are to be made airtight, the means for achieving this must be spelled out. A designer who requires "airtight" construction must acknowledge that, if inadequate provision is made for balanced intake and exhaust, the building skin will rupture. A common, and regrettable, design error is to simply require "good workmanship." Designers who are not capable of managing moisture transport in building envelope assemblies might be inclined to absolve themselves of performance responsibility with such a vague specification. A contractor who would read such a specification without amplification in a bid document would be well warned to include a very large contingency. A "good workmanship" specification for a vapor barrier is no assurance of good performance. However, a concise description of the sequence of application, with fastening and sealing details, together with test criteria can make for excellent performance. Or, better yet, a judicious selection of insulation material may remove the criticality from managing membrane materials.
A general, moisture-resistant wall assembly design guideline for humidified buildings in northern climates might consist of the following points:
· Design for airtightness - assure airtightness with mechanical joints, not sealant.
· Set a specification for airtightness; design for that specification; ensure construction to that specification - do not allow "workmanship" to be used as a substitute for a vapor/air barrier specification.
· Make provisions for outdoor air, both intake and exhaust - the volume of air moving through intake and exhaust should be a much higher percentage than the anticipated amount of air flowing through the incidental cracks in construction.
· Detail corners and other potential cold spots carefully, with attention to continuity of the thermal envelope.
· After assuring airtightness, design for diffusion protection.
· Walk through the sequence of assembly carefully (TenWolde and Rose 1995c).
As the design proceeds, the CA should identify which spots are likely to be the spots where construction may be difficult, and where moisture problems are most likely to appear.
Mechanical Equipment
Operation outside of the intended humidification ranges could be particularly damaging to the building, to the activities, and to the health of the building occupants. The principal role of the CA during mechanical system design should be to assure safe operation, under both normal and accidental circumstances.
More damage is done from high humidity than from low humidity, so the control system should fail to the off condition. Humidified air supplies can cause localized high humidities unless the air is well mixed after its discharge into the space. In particular, the equipment and ductwork immediately downstream from the site of injection of humidity should be easy to inspect and maintain.
The role of the CA during the construction process should be to ensure that the design intent is achieved in the execution. At preliminary meetings with the contractor, there should be an opportunity for presentation and questions to the CA. In particular, the details and specifications that are intended to achieve airtightness need to be reviewed by the contractor. Any difficulties in execution should be reviewed with the design professionals well in advance of the work.
In normal project delivery, substantial completion is the contractual event that signals acceptance of the work by the client. Normally, the architect will certify substantial completion. It would be uncommon, given the weight of traditional project delivery, for the certification of the CA to have a critical bearing on the acceptance of the building and the certification of substantial completion. The client and architect may wish to have information regarding building perfor-mance before their certification, and the CA should comply to the extent possible. The tests for critical performance, however, can typically be done only in critical seasons. It is very difficult to predict wintertime performance in any season but winter. An acknowledgment by the CA that the building "should" perform as anticipated should not be considered to have the weight that physical test results would have.
The principal activity of the commissioning of humidified buildings takes place after substantial completion. It is the CA's responsibility to make sure the building continues to work properly. If the commissioning process is approached rigorously, the chances of success are greatly enhanced.
Performance Log Maintenance
A crucial activity in the first year after construction is recordkeeping. The CA should maintain a log of the building performance, from the time that the mechanical systems are put into service, up to the completion of the commissioning report. The CA will not be onsite during the entire startup period, so arrangements should be made with occupants or appropriate staff that can collect and record observations on the performance of the building.5 Simple observations during noncritical periods should include: the "feel" of the indoor environment (e.g., smells and sensations), water spots (especially window condensation and basement leaks), conditions in unoccupied or rarely occupied areas of the building such as basement and attic spaces, the performance of finishes (including cracking), and the suitability of the indoor environment for the intended purpose.
Other critical data regarding the indoor environment must also be collected at this time. This work usually involves programming the operations control sys-tem to regularly download performance data. It may also involve the collection of regular indoor temperature and humidity measurements. During this period, outdoor temperature and humidity information must also be collected. A spreadsheet-type file should be maintained and should contain at least the following information:
· Indoor temperature and RH at critical locations in the building. It may be noted that the sites of placement for mechanical system controls do not necessarily represent the environment at locations critical to the activity of the building.
· Outdoor temperature and RH
· A macro that uses temperature and humidity to calculate a measure of water concentration in the air, such as humidity ratio, mixing ratio, or dew point temperature - These terms and the means to perform these calculations are found in ASHRAE Handbook of Fundamentals, Chapter 6, Psychrometrics (ASHRAE 1993).
· Comments on the indoor and outdoor conditions, including at least daily summaries of rainfall.
Airtightness Testing
Once the building is complete and occupied, the airtightness should also be tested. It is best if airtightness is a performance criterion defined from the outset. A blower door is the most commonly used testing instrument. A blower door "pumps up" the indoor air pressure to measure the indoor/outdoor pressure differential and the building's total air leakage area.
Cold Weather Testing
Cold weather testing is important to determine the building's performance during the winter months. One important piece of equipment for this phase of testing is the infrared pyrometer, an expensive but indispensable piece of equipment for a humidification CA. A pyrometer is handheld and measures surface temperature. The cost of the equipment depends on the optical arc - an instrument with a wide angle (measuring temperature in a wide patch from a short distance) may cost $400, while an instrument with a fine arc (measuring a patch of a few inches diameter from a distance of several feet) may cost up to $2,000.
The pyrometer is used on the interior of the building to identify cold spots, which are important determinants of moisture-related problems. A pyrometer allows the CA to reduce the number of critical visual inspection sites down to a few. These critical areas can be continuously evaluated for moisture problems as humidity loads are increased over the course of the season. The pyrometer can also be used on the outside of the building to determine the location of hot spots, which correspond most typically with areas of low insulation or air leakage.
The pyrometer (or an infrared thermography camera) can be used very effec-tively during a blower door test to determine not only the amount of leakage area but also the site of leakage. This measurement is particularly helpful if the identification and repair of a few sites makes a difference between acceptable and unacceptable performance of the building envelope.
The performance of windows must be an important part of the cold-weather investigation. Humidity is more common on high windows and on exterior panes of storm windows. A window investigation should include pane temperatures and indicated directions for airflow. The patterns may also be recorded in photographs or drawings, which can help establish the relative contribution of pane conductivity and frame conductivity to the overall window performance. Most important is to distinguish film condensation on windows from running condensation.
The main focus of the CA is issuance of the commissioning report once the work is complete. The essential elements of the commissioning report should include, at least:
· restatement of the aim and intention of the commissioning activity, with results of the negotiated acceptable range of humidity
· discussion of the design of the building envelope and the mechanical equipment
· results of modeling, both steady-state and transient
· discussion of the suitability of the construction with respect to the design intent and the anticipated humidity performance
· results of the recordkeeping period showing the actual performance of the building during the period
· results of the calculation showing the concentration of moisture inside and out
· observations during the critical cold-weather visit
· conclusions from the study
· recommendations for operation and maintenance
· training schedules.
5 The first year a building is constructed is the "drying out" period for most construction materials - especially wood and concrete. This strengthens the argument for careful observation of building conditions during this period.
