Chapter 19Designing Cladding systems

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

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...)

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 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

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

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 coverLabyrinth (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

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

Designed to Counteract Wind Forces

PEC

(Air Barrier)

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 principleTypically 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 MaterialsViscous, 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 DesignJoint too Narrow

•Fails with movement

Joint too Deep•Sealant Wasted•Excess pressure on edges

Joint CorrectlyProportioned

Bond Breaker

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

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