chapter 19 designing cladding systems. primary functions of cladding keeping water out (gravity,...
TRANSCRIPT
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Chapter 19Designing Cladding systems
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Primary Functions of Cladding
Keeping water out (gravity, wind driven/air pressure)
Preventing air leakageControlling the passage of light (especially sunlight)
Controlling the radiation of heat (maintain an acceptable interior surface temperature, comfortable for occupants)
Controlling the conduction of heatResist conduction of heat & coldAvoid thermal bridges (EX metal studs exterior walls)
Controlling Sound
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Secondary Functions of Cladding
Resisting Wind ForcesControlling Water Vapor (retard passage)
Adjusting to MovementThermal expansion & contraction
Within the cladding system - temperature differentials inside & out Between the cladding system & the structural frame
Moisture expansion & contractionStructural movements (blg settlement,wind, quake, creep)
Resist FireWeather Gracefully (w/o streaking, oxidation, corrosion, freeze-thaw...)
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Installation Requirements for Cladding
Should be easy to install
Attachment tolerances
Adequate dimensional clearances
Back-up / secondary systems
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Conditions required for Water Penetration
1) Water must be present at the wall surfaceThe case in all except small buildings with overhangs
2) The cladding system must have an opening Design without openings - Barrier Wall (difficult to achieve because)
Sealed Joints must be perfect - Precise & consistent field installation Building movement tear or pull sealant loose Sealant exposed to the destructive forces of the elements
Therefore the Design often has Internal drainage and/or secondary lines of defense Example - Brick cavity wall
3) A force must exist to move the water through the opening
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Conceptual Approaches for Watertight Wall
Keep water away from the wallBroad overhang (but still have wind driven rain)
Eliminate wall openings (Barrier wall)Seal every seam and openingDifficulties:
Sealant likely not perfectly installed Sealant may fail over the life of the building
Remedies: Internal drainage or secondary defense
Eliminate/neutralize the forces that move water
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Forces Moving Water thru Cladding
Gravity
Momentum
Surface Tension
Capillary Action
Wind Currents
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Gravity
Problem: The Forces of
Gravity cause water to enter the cladding
Solution: Slope
joints/openings to the outside
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Momentum
Problem:The momentum
from rain falling at an angle carries the water into the cladding
Solution:Joint coverLabyrinth (maze)
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Surface Tension
Problem:Water adheres to
joint & is drawn into the cladding
Solution:Drip groove
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Capillary Action
Problem:Water pulled into
the cladding
Solution:Opening larger
than a drop of water can bridge, or install a
Capillary break
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Air Pressure
Problem:Differences in air
pressure push, or pull water into cladding
Solution:Pressure
Equalization Chamber
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Rainscreen Principle
pressure
Pre
ssur
e E
qual
izat
ion
Cha
mbe
rR
ain
scre
en
Air
Bar
rier
Wall
Rainscreen Principle:
Design the Cladding to
allow wind pressure differences between
the outside and insideto neutralized
themselves
Rai
nscr
een
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Designed to Counteract Wind Forces
PEC
(Air Barrier)
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Designed to Counteract the “Five” Forces
Gravity:•Sloped Sill
Momentum:•Upturned Interior Sill
Surface Tension & Capillary:
•Drip Grove
Wind:•Interior Weatherstripping
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Sealant Joints
Most cladding systems:don’t exclusively use the rainscreen principleTypically they incorporate sealants
Purpose - fill cladding joints to:Prevent flow of air & water while: Providing allowances for
installation tolerances system movement
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Sealant Materials
Types:Gunnable & Solid
Gunnable Sealant MaterialsViscous, sticky liquids
(mastics) injected into joints
Applied with a caulking gun
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Categories of Gunnable Sealants:
Low-range sealants - caulks limited elongation Uses: filling cracks & secondary joints (not cladding)
Medium-range sealants Often butyl rubber or acrylic elongation - 5 to 10% Uses: seal “non-working joints (mechanically fastened)
High-range sealants two part mixtures (polysulfides, polyurethanes, silcones) elongations - 50% +/- Uses: Working joints in cladding
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Joint DesignJoint too Narrow
•Fails with movement
Joint too Deep•Sealant Wasted•Excess pressure on edges
Joint CorrectlyProportioned
Bond Breaker
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Joint Design & Installation
Joint Sealant
Backer Rod
Primer
Joint Cleaned
Install Primer: (Optional)
•improves adhesion
Install Backup Rod•“fills” joint / support•spongy material•does not stick to sealant
Install Sealant
Tool Joint
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Solid Sealant Materials
Gaskets Preformed Tape
Less sensitive to installation problems - Widely used
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Curtainwall System
“an exterior cladding system supported at each story by steel or concrete frame, rather than bearing its own load to the foundations”
Therefore: It can be thin and light regardless
of its height
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Curtainwall Testing
Cladding System Mockup built & tested for:Air infiltration test
Static water penetration test
Dynamic water penetration test
Structural performance under loading
Necessary changes (test failures) incorporated into the finalized design
Fabrication commences subsequent to testing
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Cladding & Building Codes
Primarily Concerned with: Structural Strength
Strength / stiffness, integrity of the system Attachment to the building frame
Fire Resistance Combustibility of the system Design of parapets, spandrels Firestops
Energy Efficiency Thermal resistance, vapor retarder, air leakage
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Sustainability Issues
Because of its effect on energy consumption, glass should be used in moderation.
Operable windows may reduce energy costs
Properly insulate opaque areas (& spandrels)
Eliminate thermal bridges
Design for air tightness
Consider (use) building orientation to reduce energy consumption
Consider photovoltaic cells