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VAPOR INTRUSION:
HOW A BUILDING WORKS AND BREATHS
The “Breathing Building”
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The top five floors of this seven-story
structure are covered in 734 gold, copper and
bronze-colored steel plates that can be raised
or lowered via 82 computerized motors.
The plates function as a second skin for the
building, increasing its ability to permit
natural ventilation and lowering energy
consumption by cutting down on the use of
air conditioning.
The plates are peppered with micro-holes to
allow visibility from within. Geox occupies the
ground and first two floors, while the
remaining four floors house non-retail
tenants.
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Goals for Today
1. Understand how buildings are working
2. What can effect movement of vapor through a building
3. Understand how subtle the air movement can be
through a building
4. What are the different forces that affect the air
movement.
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At what pressures do our buildings
function?
 Blower door tests and COE testing 75 Pa.
 Ductwork and ventilation system 1”-2” water column
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What are the pressure equivalents?
 1 atmosphere
 2.96077 e-5 atmosphere
 14.69 psi
 .000435 psi
 2116 psf
 .06 psf
 406.79 inches of water
column
 .012 inches of water
column
 101325 Pa
 3 Pa
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What happens in a building
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Convection
Stack Effect
Mechanical ventilation
Static Pressure
Natural Wind pressure
Adjacent spaces
Fresh air intakes (leakage and powered)
Recent building control innovations
Buoyancy
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What do we do for Ventilation?
 Building codes require:
 Living areas - 0.35 air changes /hr or > 15
cfm/person
 Garages-100 cfm/car
 Enclosed parking .05 cfm/sf AND capable of
1.5 cfm/sf
 Commercial Buildings – 4-10 air changes /hr
 ASHRAE-ventilation design is anticipated
to be thru infiltration. Energy issues have
changed that.
 For tight houses .5-.41 ACH
 For loose insulated homes 1.1-1.47 ACH
Westminster Palace venting
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Building Occupancy
Differs with how the building is used:
 Health care
 Higher air volumes
 Required to be full ducted return
 Office space
 Could be return plenum ceiling
 Apartments and hotels
 Usually parking below and furnace inside
 Does not have make up air system except in corridor
 Make up air is via opening a window
 Often referred to as Magic Pak or Ptac
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Convection
 Warm air rises, cold air falls
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Stack Effect
 Areas like stairwells, atriums
and elevator shafts show
the volume of air that is
moving vertically through
the building.
 E.g. IDS Building, 52nd floor.
Difficult to open the door to
the mechanical room on the
52nd floor
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Mechanical ventilation
 Air moves by the difference in
pressure
 Fan forced “push” or “pull
 Plenum spaces versus full ducting
 Think about the Metrodome. Air
pressure is only about 0.04-.4 psi
 Air balancing is a vital requirement.
 The in and the out need to be about
equal
 Try blowing or sucking air out of a
pop bottle
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Static pressure differential
 Air in ducts is measured in inches of water
column
 Different zones require
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Natural wind pressure
 Wind air pressure can be in the 20#+ psf (.13 psi)
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Building Air Leakage Case Study
Buildings:
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Two – 3 story barracks buildings
Steel frame construction
Composite concrete decks
Metal studs/exterior sheathing air barrier/rigid
insulation/brick
PTO roof membrane over rigid Insulation
Test Requirements:
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USACE Protocol for TESTING Air Leakage
in Occupied Buildings
ASTM E779 – Standard Test Method for
Measuring Air Leakage Rate by Fan Pressurization
Pressure boundary: total area of floor slab,
exterior walls & roof
Allowable CFM: 25% of pressure boundary area
Passing grade: 0.25cfm/sq ft @75 Pa (0.3 inches
H2O)
Ft Leonard Wood Barracks Building
Air Leakage Testing
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Building Air Leakage
Building Air Leakage
Air Pressure inside a building on a calm day
Depressurization Test Cycle -75Pa to -25Pa with Bias pressure readings pre & post test
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Building Air Leakage
Air pressure inside the same building on a windy day
Depressurization Test Cycle -75Pa to -25Pa with Bias pressure readings pre & post test
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Adjacent spaces
 Balancing Issues
 Air transfer in Roof Deck flutes between building
tenants
 Air transfer at material joints
 Air barrier installation (not “weather barrier” or
“vapor retarder”)
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Fresh air intake
 Normal fan powered units pass a 6-10% fresh air or # cfm per
occupant.
 Fin tube radiation has no fresh air unless a separate system is
provided.
 PTAC and Magic Pak units have a small amount of fresh air, but
unit must be running and balanced with the exhaust fan
 Pressurization
 Supply air-air loss=>return air + fresh air. Normally building is slightly
positive in pressure.
 Apartments and condos
 Corridor has make up air, but stopped from getting to the units due
to fire gasketing
 Opening locations
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Building Control system innovations
 LEED monitoring
 Low level conditioning
 Occupied versus unoccupied modes
 CO2 Sensors
 Mixed return air or air exchange systems and
heat wheels
 Breathable Buildings
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Buoyancy
 Warm air makes a difference
 Chemicals also make a difference.
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Vapor prevention
 Consider particle/molecular size- “Coffee Filter”
 Water proofing versus vapor retarding
 Think of Gore-Tex
 Vapor passes through
 Water beads up
 Issue is really molecule size
 Water liquid 90 molecules in size
 Water vapor 2 molecules in size
 If vapor is larger than water it will not go through normally
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Slab on grade and foundations
 Negative pressure zone under floor
 Membrane selection and sealing
 Materials
 Concrete block is porous and will allow air/smoke/vapor
through it
 Concrete block can also transport air vertically and up into the
second floor
 Vapor retarder Quality material, not lumber yard poly
 Virgin polyethylene. Stego, Raven etc.
 Exterior wall waterproofing
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EPA moisture control
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PNC Breathable Building in Pittsburgh.
Worlds Greenest Building
 Incorporates Convection, stack
effect, and natural conditions
 This allows the building to
operate with no mechanical
system 45% of the time.
https://youtu.be/z8ScZsOBB7k
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Underground garages
 Make up air and exhaust
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Radon and passive systems
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Conclusions
 There are many effects taking place in the building and they
vary by season.
 Not all buildings are the same. The ventilation systems,
design and operation all impact the vapor potential.
 Nature affects the way buildings operate.
 The mitigation system needs to work with the mechanical
system, if possible.
 The engineer needs to deal with all of these different issues
in the design of the building and systems.
 Remember there is a potential for the buildings to operate
outside of the norm. Mainly due to human interference.
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What can get in the way?
 Weather
 Other building systems or openings for fresh air
 Insulation systems that want to add a porous material to the
perimeter of the building.
 Making a connection between all the membranes through
structure.
 Future occupants
 Trying to save on the costs of energy, modifying the building by
disabling or shutting off systems.
 Tenant changes (pizza ovens; high volume exhaust etc)
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