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Stylized Text: Priorities.

Priorities for commercial buildings have been identified by FSEC and noted below. Click on each individual step to find out further information on how to control and prevent moisture problems from occurring during commercial building construction.

 

Control Moisture Problems

  • Keep building materials dry during construction
  • If they get wet, take steps to dry immediately

Dry the Slab

  • Concrete normally reaches 70% of its strength after 30 days
  • After this, excess moisture in the concrete should be removed
  • After 30 days of curing, there are four choices regarding the floor
    1. Add plasticizers to reduce the amount of water
    2. Apply the flooring and wall board and hope that moisture and mold damage will not occur
    3. Allow natural drying -› costly delays
      • This can take 8 to 16 weeks
    4. Seal the concrete surface to trap moisture
    5. Dry the concrete
      • Dehumidifiers can reduce moisture emission rate from 8 to 3 lbs/day-1000 ft2 in 3 to 6 weeks

Construct a Tight Envelope

  • In the range of 2 to 6 ACH50
    • Typically larger buildings have smaller ACH50
  • Can also be expressed as CFM50/ft2
    • 2 to 6 ACH50 -› 0.79 to 2.37 CFM50/ft2
  • Existing standards
    • ASHRAE – 0.30 CFM50/ft2
    • British2 – 0.27 CFM50/ft2

Build Appropriate Walls

  • Based on Climate
    • In cold climates, install the air and vapor barriers to interior, and let wall assembly dry to outdoors
    • In hot/humid climates, install the air and vapor barriers to exterior, and let wall assembly dry to indoors
    • In mixed climate, install vapor barrier in middle of assembly
  • Place vapor barrier on warm side of wall assembly
    • In cold climate, that is toward indoors
    • In hot climate, that is toward outdoors
  • Vinyl wallpaper in hot and humid weather is a problem
  • Select appropriate wall materials
    • Use vapor permeable materials in wall assembly (at exterior in cold climates and at interior in h/h climates)
    • Avoid vinyl wall paper
    • Use unbacked batts, blown insulation, or rock wool
    • Avoid two vapor barriers in a wall

Locate Ceiling/Roof Air & Thermal Barriers at the Same Location

  • Do not vent the ceiling space/attic space in hot and humid climates
    • This venting only allows water vapor into the building
    • Cold surfaces in the ceiling/attic space may have condensation problem
    • The suspended T-bar ceiling allows air transport into the conditioned space
  • Locate the air boundary and thermal boundary at the roof deck
    • The ceiling space is then located inside the air and thermal boundaries of the building
    • This is a forgiving design
      • The impacts of various forms of uncontrolled air flow are minimized

Create Positive Pressure

  • This requires more incoming air (make-up air and outdoor air) than exhaust air
  • This requires a reasonably tight building envelope
  • +2 pascals is often considered to be sufficient
  • Practical considerations – tight envelope
    • Make the building envelope reasonably tight
      • ASHRAE Fundamentals – 0.08, 0.23, and 0.47 CFM50/ft2 are tight, average, and leaky walls
      • The ASHRAE tightness standard (average) equates to about 0.4 ACH50 in a large building
      • According to survey of 17 large commercial buildings (>4 stories), envelope leakage is 0.5 cfm/ft2 (2.5 l/s-m2) [Proskiw and Phillips, 2001
  • Practical considerations -- air flow balance:
    • For kitchen exhaust systems, design make-up air to be 80% of exhaust air
    • Design outdoor ventilation air to be remaining 20+%
    • Catch 22 – how to make outdoor air a reliable form of make-up air
      • “auto” fan control causes outdoor air to unreliable
      • “on” fan control often causes poor humidity control
      • therefore, need to select an AC system that can provide good humidity control with continuous fan operation
    • Recognize that VAV systems do not provide reliable outdoor air – it varies with return side pressure
      • Consider using a dedicated outdoor air fan in conjunction with VAV
    • Consider separating the V from HAC – use a dedicated outdoor air system that conditions and distributes the ventilation air
      • Consider super conditioning of the ventilation air so that the central AC system does not need to meet latent load
  • Maintaining positive pressure in zones
    • Balanced return air
      • Duct the return air and balance
      • Provide transfers that limit pressure drops from one zone to another to 2.5 pascals (0.1 inWG). (Florida Building Code)
      • To achieve 2.5 pascals, provide 70 in2 per 100 cfm of supply air.
    • Exhaust and make-up air must be balanced in each zone of the building
  • The ideal
    • Identify the necessary building ventilation rates – OA, EA, and MA – to achieve air quality objectives
    • Minimize EA flow rates – they are the "tail that wags the dog"
      • Size bathroom exhausts at only 50 cfm per fixture (new standard)
      • Use hoods and other means of exhaust capture and containment that minimizes EA requirements
      • Operate these EA systems only when required, or place on variable capacity operation control
    • Determine the net air flow ("in" versus "out") that results from the identified necessary ventilation rates – make sure it is positive
    • Identify a building airtightness which will achieve an effective level of positive pressure (e.g., +2 pascals)
    • Specify the desired building airtightness
    • Test to verify compliance with both air flows and building airtightness
    • Provide outdoor ventilation air through a dedicated system
    • Use enthalpy recovery to reduce the space conditioning load of the ventilation air
    • Consider super conditioning of the dedicated outdoor air to meet all of the building’s latent cooling load

Select Proper Control System

  • Do not overcool the space
    • Can lowering the thermostat produce lower relative humidity?
  • Overcooling produces more cold surfaces on which condensation can occur
    • Ducts and air handlers in the attic get colder
    • The building wall assemblies get colder
      • This raises the surface relative humidity of materials within the wall assemblies
      • This creates cool surfaces on which condensation can occur
  • Humidistat control can be dangerous to the health of your building
    • Parallel control, either the thermostat or humidistat can turn on the AC
      • If the humidistat setting is too low, then the AC may not turn off
      • If the humidistat is not calibrated, then the AC may not turn off
      • If the humidistat dead band is too large, then the AC may not turn off

Choose Efficient HVAC System

  • Select a cooling system that can effectively dehumidify the room air
  • Some units do not dehumidify effectively
    • DX with constant flow continuous fan
    • CW with constant flow and modulating valve
  • Some units do dehumidify effectively
    • DX with constant flow cycled fan
    • CW with face and bypass
    • CW with VAV
  • There are two methods for modulating the cooling output of AC systems
    • Modulate the air flow rate (variable flow or on/off operation
    • Modulate the temperature of the coil
  • Simple rule: select a cooling system which maintains a cold coil whenever air is flowing across the coil

Control Air Flows

  • Condition outdoor ventilation air prior to injecting into the room or cooling system
    • If put into the room without conditioning, the ventilation air increases indoor RH
    • If the ventilation air is mixed with the return air, then room RH is lower
    • If the ventilation air is conditioned before entering the cooling system, then the latent removal effectiveness is increased
  • Lower air flow rates will enhance the dehumidification performance of cooling systems
    • Very true for DX systems
    • Somewhat true for CW systems
  • The following technologies can be used to enhance dehumidification of room air
    • Heat pipes
    • Runaround coils
    • Subcool /reheat systems
    • Enthalpy recovery AC
    • Dehumidifiers
  • The first four have several advantages
    • Lower equipment SHR
    • Less potential for overcooling the space when used to condition outdoor air
    • Lower RH in the supply ductwork, reducing mold growth potential in the ducts
  • Enthalpy exchange enhanced AC provides remarkable levels of system efficiency and dehumidification performance
  • Dehumidifiers are energy inefficient, and should be used only where other approaches are not available or practical
  • Specify building tightness, and then have it tested to verify compliance.
  • Avoid uncontrolled air flows in buildings
    • Duct leakage
    • Unbalanced return air
    • Unbalanced exhaust air
  • Specify duct construction and sealing materials, and require airtightness testing
  • Design the return air system to avoid greater than 2.5 pascals pressure drop across interior partitions
  • Minimize EA flow rates, and then get the building (and zones) to positive pressure by providing more OA than EA.

Plan for Unoccupied Periods

  • Select a good HVAC strategy for unoccupied periods
    • Remember to turn off ventilation when unoccupied
    • Remember that nights are poor latent performance periods – shut off the system at night
    • System schedule for weekends or vacant periods
    • Maybe a dedicated system to serve the unoccupied periods
  • Design flexibility into the HVAC system to account for partial and intermittent building use, especially during "unoccupied" periods
    • Schools: office area occupied 12 months
    • Schools: evening and weekend use of just small parts of the facility
  • Evaluate the moisture contribution of various facility cleaning options
  • Include cleaning guidelines with the building "operation manual"
    • Clean carpets with the least amount of moisture possible
    • Plan for the moisture and other contaminant impacts, and design with these in mind
  • Provide ventilation only when needed
    • Schedule: shut off ventilation when the people go home
    • Carbon dioxide: use CO2 controllers to bring in ventilation air only when needed.
      • This will reduce energy bills
      • This will greatly enhance humidity control