principles of building envelope performance...2014/01/02 · source: graphic adapted from wood...
TRANSCRIPT
Learning Objectives
1. The purpose of the building envelope
2. The energy code’s definition of the building envelope
3. The exterior forces that act on the building envelope
4. The interior forces that act on the building envelope
5. The concept of continuous control layers to resist
exterior and interior forces
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The Building Envelope Defined
The building envelope is
the part of a building that
separates the controlled indoor
environment
from the uncontrolled outdoor
environment
3
The Building Envelope Defined
2009 International Energy Conservation
Code (IECC):
Building Thermal Envelope: The basement
walls, exterior walls, floor, roof, and any other
building element that enclose conditioned
space. This boundary also includes the
boundary between conditioned space and any
exempt or unconditioned space.
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The Building Envelope Defined
[2011 NYCECC]
Building Thermal Envelope: The basement walls,
exterior walls, floor, roof, and any other building
element that encloses conditioned space. This
also includes the boundary between conditioned
space and any exempt or unconditioned space
http://www.nyc.gov/html/dob/html/codes_and_reference_materials/nycecc_key_definitions.s
html
Building Envelope Design
Factors
Control heat flow
Control vapor diffusion
Be economical Be easy to build
Control radiation and light
Provide security
Control sound transmission
Control fire
Resist imposed loads
Control rain penetration
Control air flow
Accommodate movement
Be aesthetically pleasing Be durable
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Environmental Factors
Load: vapor pressure Load: air pressure
Load: temperature
Exterior Environment Temperature
Air Movement
Humidity
Rain
Snow
Light
Seismic
Soil Environment Temperature
Air Movement (radon gas)
Humidity
Seismic
Interior Environment Temperature
Air Movement
Humidity
Light
Occupant
activities
Mechanical
design
Air leakage
Snow cover
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Wind
Durability
• Ability to continue to perform functions over time
• Durability is a function of a material and its
environment
• E.g., heat, UV radiation, thermal cycles, wind, salt
spray, movement, water, biological growth, etc.
degrade building materials
• Moisture is the single biggest environmental factor
affecting durability
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Hydrothermal Design Factors Heat, air, and moisture control
Control
Heat
Flow
Control
Rain
Penetration
Control
Air Flow
Control
Vapor
Diffusion
Heat, air and
moisture flows are
interrelated
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Heat, air and moisture flows
are Interrelated
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Control
Air Flow
Control
Heat
Flow
Control
Vapor
Flow Control
Rain
Penetration
Water in the Environment
• “Water” exists in three phases:
• Liquid (rainwater, groundwater)
• Solid (snow and ice)
• Gas (water vapor)
• Building envelope
design must account
for water in all three
phases.
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Relevant terms related to
vapor
• Relative Humidity: defined as the ratio of partial
pressure of water vapor to the saturated vapor
pressure.
• Dew point Temperature: the temperature at which
air containing a constant amount of water vapor
reaches the saturation point. As the temperature
decreases, the air has a lower capacity to contain
moisture. Condensation can occur at or below the dew
point temperature.
• Condensation: Change of state of water from gas to
liquid, due to the water vapor cooling and contacting a
surface at or below the dew point.
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Water Vapor Diffusion
• Vapor diffusion is the process of water vapor
molecules moving through a material
(independent of air movements)
Source: Graphic adapted from Wood Frame Envelope in the Coastal Climate of British Columbia Best Practice Guide,
Canada Mortgage and Housing Corporation.
Inside Air
high water
content
Outside Air
low water
content
Direction of vapor flow
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Air Leakage
Air leakage is driven by the
air pressure difference across
the building envelope assembly.
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Air infiltration occurs
where “smoke” is
sucked into building
“Smoke” puffer
Air Pressure Difference
Stack Effect Wind Mechanical
Pressurization
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Air Leakage
Importance of Air leakage control
• Air flow carries:
• heat (affects thermal performance),
• rain (increases risk of water leaks)
• water vapor (may result in condensation within
assemblies)
Durability
Characteristics of Climates in
Oregon– Mixed-Marine
• Mean Temperature of Coldest Month between 27°F
and 65°F
• Warmest month mean temperature less than 72°F
• At least 4 months mean temperature over 50°F
• Dry season in summer. Month with heaviest
precipitation in cold season has at least 3x as much
precipitation as the month with the least precipitation in
the rest of year. Cold season is October through
March.
• Heating Degree Days (base 65°F) between 3600 and
5400
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Characteristics of Climates in
Pacific Northwest – Cool-Dry
• Not marine and:
• Annual precipitation (inches) is less than 0.44 x
(annual mean temperature – 19.5)
• HDD (base 65°F) is between 5400 and 7200
• Less humidity
• Greater temperature extremes
• -40 for several days near Burns (1989)
• 117 degrees in Prineville, Pendleton (1998)
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Climate Zones – IECC, ASHRAE
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Climate Zones of OR State –
per 2009 IECC
C 5B
• 4C = Mixed-Marine
• 5B = Cool-Dry 23
CLIMATE ZONES PER 2010
OEESC
• Two Climate Zones are defined in the OEESC,
Section 301.1
• thermal performance requirements vary based on
Climate Zone
• Climate Zone 4C: West of the Cascades
Benton, Clackamas, Clatsop, Columbia, Coos, Curry, Douglas,
Jackson, Josephine, Lane, Lincoln, Linn, Marion, Multnomah, Polk,
Tillamook, Washington, Yamhill
• Climate Zone 5B: East of the Cascades
Baker, Crook, Deschutes, Gilliam, Grant, Harney, Hood River,
Jefferson, Klamath, Lake, Malheur, Morrow, Sherman, Umatilla, Union,
Wallowa, Wasco, Wheeler
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Rain Penetration Failures
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Rain Penetration Failures
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Rain Penetration Control
For control, resist these forces with • Deflection
• Drainage
• Drying
• Durable materials
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Momentum Capillarity can “pull” water “uphill”
Gravity Air pressure
Driving forces:
Rain Penetration Control
Design
Considerations
Deflection
Drainage
Drying
Durability
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Redundancy
is important to
success
Some common approaches
Mass wall
Face-sealed
Concealed barrier
Rainscreen
Rain Penetration Control
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Rain
Water Shedding
Absorbed Water
Rain Penetration Control
Mass Walls
Mass wall: relies on absorption & evaporation
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Rain
Water Shedding
Absorbed Water
Rain Penetration Control
Mass Walls
Mass wall: relies on absorption & evaporation
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Not all mass
walls are
created equal
Rain Penetration Control
Mass Walls
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Rain Penetration Control
Face Seals
Face seal (Barrier): relies on sealed exterior
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Rain Penetration Control
Face Seals
Face seal (Barrier): relies on sealed exterior
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Rain Penetration Control
Concealed Barriers
Concealed barrier: relies on multiple layers
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Rain Penetration Control
Concealed Barriers
Concealed barrier: relies on multiple layers
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Rain Penetration Control
Rainscreens
Rainscreen: relies on 2 layers with air space and drainage
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Rain Penetration Control
Rainscreens
Rainscreen: relies on 2 layers with air space and drainage
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Requirements of Rainscreen Walls
• Water shedding surface
(Rainscreen)
• Secondary moisture barrier
(water resistive barrier)
• Drainage path from water
barrier to outside
• Ventilated cavity increases
drying potential
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Water Resistive Barrier
Water Shedding Materials
• Asphalt impregnated paper (building paper)
• Spun-bonded polyolefin or similar “house wraps"
• Some fluid-applied membranes
• Water repellant coatings
Waterproof Materials
• some Fluid-Applied membranes
• some Self-Adhered membranes
• Torch Applied membranes
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Water Resistive Barrier
May also perform function of:
• Air barrier
• Vapor retarder • but must have insulation installed to exterior!
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Why Control Air Flow?
• Air leakage will carry moisture laden air into the envelope, and interstitial condensation can lead to mold, rot, corrosion
• Higher energy cost
• Poor thermal comfort (winter and summer)
• Uncontrolled indoor environment (humidity, outdoor contaminants)
• Larger forces for rain penetration
• Possible fire/smoke movement
• Increased sound transmission
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Air Leakage Problems
Failure at Roof Parapet
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What Happens Without Envelope Continuity?
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What Happens Without Envelope Continuity?
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Spray foam insulation for
air, vapor and thermal
continuity
Failure at Soffits and Decks
Air Leakage Problems
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Air Leakage Problems
Window Interface
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Categorizing Air Leakage
Orifice Flow: air flow through direct opening localized leakage path condensation and water intrusion
Pao Pai
Diffuse Flow: air flow through material large areas condensation over time
Channel Flow: air flow through tortuous path localized leakage path highest condensation potential
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Air Leakage Path and
Condensation
75% 32% 29% 71%
52 Percentage shows relative amount of moisture that can be deposited in a wall cavity based on leakage path.
Controlling Air Flow
What is an Air Barrier System?
The assembly installed to provide a
continuous barrier to the movement of air
across the building envelope
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Air Flow Control
Design Considerations
• Install ‘plane of air tightness’ using low air
permeance materials (low air flow)
• Design system to resist air pressure difference
especially wind loads (structural integrity)
• Maintain low air flow and structural integrity
across joints and junctions (continuity)
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Vapor Diffusion Control
Vapor retarder: the material(s) installed to control the
diffusion of water vapor
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Vapor Diffusion Control
• Low vapor permeance material must be warm
enough to avoid moisture accumulation from
condensation (installed to the interior of thermal
insulation in Pacific Northwest)
• Assembly should be more permeable towards
the low vapor pressure side of the envelope
assembly
• Vapor retarder need not be sealed to control
vapor diffusion (if there is an air barrier and if
conditioned air cannot “ bypass” the vapor
retarder to circulates behind it)
Design Considerations
interior
exterior
building envelope
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Heat Flow
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Heat Flow Problems
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Thermal Barrier (Insulation)
• The element that represents the majority of
resistance to conductive heat flow in an assembly
• Must be continuous but does not need to be sealed
• Best placed to the exterior of the structure to
minimize thermal bridging
• Ideally majority placed outboard of air barrier
• Majority must be placed outboard of vapor barrier
(except in cooling climates)
• Placed inboard of weather barrier unless water
insusceptible
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Heat Flow Control
Design Considerations
• Design for continuity of coverage
• Minimize thermal bridges
• Avoid air flow through/around insulation
• Keep low permeance material warm enough to
avoid condensation
Heat Flow Control – Thermal
Mass
Image from www.concretethinker.com
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Below Grade Design
Considerations
• Control of groundwater
• Control of radon gas
• Control of potential soil contaminates
• Continuity of barriers at transition from
below grade to above grade assemblies
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Architectural expressions impacting
building envelope performance
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Architectural expressions impacting
building envelope performance
Source: Glass and Metal Curtain Wall Best Practice Guide, Canada Mortgage and Housing Corporation.
Vertical mullion
Air space Rigid insulation
Flexible air barrier membrane
Snap cap
Pressure plate
Sealed glazing unit
Anti-rotation spacer Galvanized metal
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Architectural expressions impacting
building envelope performance
Slab projections/decks,
etc.
• Good for rainwater
penetration control: protects
walls and windows from
wind driven rain
• Bad for thermal control:
large thermal bridges
through insulation
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Architectural expressions impacting
building envelope performance
Projecting decorative
trim
• Can collect water,
increasing risk of water
penetration and
deterioration of
claddings/trim.
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