Designing Cladding systems 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: Solution: The Forces of Gravity cause water to enter the cladding Solution: Slope joints/openings to the outside

Momentum Problem: Solution: The momentum from rain falling at an angle carries the water into the cladding Solution: Joint cover Labyrinth (maze)

Surface Tension Problem: Solution: Water adheres to joint & is drawn into the cladding Solution: Drip groove

Capillary Action Problem: Solution: Water pulled into the cladding Opening larger than a drop of water can bridge, or install a Capillary break

Air Pressure Problem: Solution: Differences in air pressure push, or pull water into cladding Solution: Pressure Equalization Chamber

Rainscreen Principle: Wall Rainscreen Principle: Design the Cladding to allow wind pressure differences between the outside and inside to neutralized themselves pressure Rainscreen Rainscreen Air Barrier Pressure Equalization Chamber

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 Joint too Deep Joint Correctly Fails with movement Joint too Deep Sealant Wasted Excess pressure on edges Joint Correctly Proportioned Bond Breaker

Joint Design & Installation Joint Cleaned Install Primer: (Optional) improves adhesion Install Backup Rod “fills” joint / support spongy material does not stick to sealant Install Sealant Tool Joint Joint Sealant Primer 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