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Chapter 19
Designing Cladding systems
Primary Functions of Cladding
Keeping water out (gravity, wind driven/air pressure)
Preventing air leakage
Controlling the passage of light (especially sunlight)
Controlling the radiation of heat (maintain an acceptable
interior surface temperature, comfortable for occupants)
Controlling the conduction of heat
Resist conduction of heat & cold
Avoid thermal bridges (EX metal studs exterior walls)
Controlling Sound
Secondary Functions of Cladding
Resisting Wind Forces
Controlling Water Vapor (retard passage)
Adjusting to Movement
Thermal expansion & contraction
 Within the cladding system - temperature differentials inside & out
 Between the cladding system & the structural frame
Moisture expansion & contraction
Structural movements (blg settlement,wind, quake, creep)
Resist Fire
Weather Gracefully (w/o streaking, oxidation, corrosion, freeze-thaw...)
Installation Requirements for Cladding
Should be easy to install
Attachment tolerances
Adequate dimensional
clearances
Back-up / secondary systems
Conditions required for Water Penetration
1) Water must be present at the wall surface
The 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
Conceptual Approaches for Watertight Wall
Keep water away from the wall
Broad overhang (but still have wind driven rain)
Eliminate wall openings (Barrier wall)
Seal every seam and opening
Difficulties:
 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
Forces Moving Water thru Cladding
Gravity
Momentum
Surface Tension
Capillary Action
Wind Currents
Gravity
Problem:
The Forces of
Gravity cause
water to enter the
cladding
Solution:
Slope
joints/openings to
the outside
Momentum
Problem:
The momentum
from rain falling
at an angle carries
the water into the
cladding
Solution:
Joint cover
Labyrinth (maze)
Surface Tension
Problem:
Water adheres to
joint & is drawn
into the cladding
Solution:
Drip groove
Capillary Action
Problem:
Water pulled into
the cladding
Solution:
Opening larger
than a drop of
water can bridge,
or install a
Capillary break
Air Pressure
Problem:
Differences in air
pressure push, or
pull water into
cladding
Solution:
Pressure
Equalization
Chamber
Rainscreen Principle
Wall
Rainscreen
Rainscreen Principle:
pressure
Design the Cladding to
allow wind pressure
differences between
the outside and inside
to neutralized
themselves
Designed to Counteract Wind Forces
(Air Barrier)
PEC
Designed to Counteract the “Five” Forces
Gravity:
•Sloped Sill
Momentum:
•Upturned Interior Sill
Surface Tension &
Capillary:
•Drip Grove
Wind:
•Interior Weatherstripping
Sealant Joints
Most cladding systems:
don’t exclusively use the rainscreen principle
Typically they incorporate sealants
Purpose - fill cladding joints to:
Prevent flow of air & water while:
Providing allowances for
 installation tolerances
 system movement
Sealant Materials
Types:
Gunnable & Solid
Gunnable Sealant
Materials
Viscous, sticky liquids
(mastics) injected into
joints
Applied with a caulking
gun
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
Joint Design
Joint too Narrow
•Fails with movement
Joint too Deep
•Sealant Wasted
•Excess pressure on edges
Joint Correctly
Proportioned
Bond Breaker
Joint Design & Installation
Joint Cleaned
Joint Sealant
Primer
Install Primer: (Optional)
•improves adhesion
Install Backup Rod
•“fills” joint / support
•spongy material
•does not stick to sealant
Install Sealant
Tool Joint
Backer Rod
Solid Sealant Materials
Gaskets
Preformed Tape
Less sensitive to installation problems - Widely used
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
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
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
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