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Building Science of Walls
January 19, 2017
Kohta Ueno
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Background
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Environmental Separator
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What Separation Roles? Water control layer A.k.a. “drainage plane,” “water resistive barrier,”
“weather resistive barrier,” WRB
Housewraps, tar paper… more modern options
Air control layer A.k.a. “air barrier”
Drywall, sheathing, spray foam… and continuity
Vapor control layer A.k.a. “vapor barrier”—poly, Kraft paper, latex paint
Thermal control layer Insulation (fluffy in stud bays, continuous on outside)
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Climate Zone Map (BSC)
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Climate Zone Map (DOE)
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Water Control‐Hydrostatic Pressure
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Water Control and Drainage Gaps
Water control layer
Key is control of hydrostatic pressure
All about “the gap”
See “Mind the Gap” and “Hockey Pucks and Hydrostatic Pressure”
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Hydrostatic Pressure
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Wind Speed vs. Pressures
½” of “perched” water ≈ 35 mph wind force
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Why Rainscreen/Air Gap
“Sandwiched” water (surface tension) hangs up
Staying wet or wet/dry cycling Paint blow off
Damage over time
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Drainage from Lap Sidings
Added water between siding & housewrap
Lap sidings “self draining”
Window head flashings!
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Cladding Ventilation
Airflow behind cladding dries out both cladding & backup wall
Brick veneer example
Why vinyl siding and metal panel cladding work in cold climates
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Water Control Layers and Spaces
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Strapped Cavities/“Rainscreen Wall”
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Shingle Wall Rainscreen/Air Gap
Mesh style (Home Slicker, Keene Building Products)
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Water Control Layer Options
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Housewrap (Residential)
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Housewrap (Commercial)
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Billowing Housewrap
Is it really an air barrier (network airflow)?
Potential damage from cyclic loading
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Vapor-Impermeable Adhered Membrane
Cold climate + no exterior insulation = danger
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Vapor-Permeable Adhered Membrane
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Self-Adhered Membranes
Self-sealing
Air leakage improvement; no blow-off/billowing
No ‘hidden path’ water leakage/bypass
Reverse laps not as critical
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Taped Sheathings (WRB Surface)
Fast dry-in
Airtightness
Reliance on adhesive vs. laps? Surface prep
Rigid foam insulation too
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Taped Joints (Foam Sheathing)
Membrane-type flashing tape at joints
Horizontals more important than verticals
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Fluid-Applied WRBs
“Housewrap in a can” (GBA Column)
Continuous water control
Airtightness
Can be applied with air gun (paint sub)
Issues: surface prep, application temperature, substrate condition, etc.
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Reverse Lap Termination
“Termination mastic” at reverse lap condition
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Windows Flashings
Building Science 2008Windows and Curtainwalls No. 4480
EIFS & Windows ‐
Oops
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Sill Pan Flashings
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Backdams and Sloped Sills
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Building Science 2008Windows and Curtainwalls No. 4780
Formable Sub-sill Flashing
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Subsill
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Housewrap Installation Sequence
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Housewrap Installation Sequence
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Foam Sheathing Window Flashing
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Window Failure Examples
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Window Failure Examples
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Window Failure Repair
Stripped shingles and housewrap
Windows pulled, re-flashed (fluid-applied window ‘wrap’), and reinstalled
Fluid-applied WRB
Added rainscreen mat under shingles
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Wood Moves…
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Wood Moisture Movement
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Room for Expansion
Wood will move—let it expand For every 4" width of dry Certi-label Western Cedar
shingle material, the product will expand 1/8"
Wood floors indoors similar
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Stucco & Adhered Stone
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Stucco on Wood Frame Walls
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Stucco Failures (MN, PA)
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Stucco-to-Paper Bond
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Adhered Stone Veneer
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Air Flow
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Airflow Control: Why
Moisture control air leakage condensation
Comfort and Health Drafts Odors, particles, gases
Energy Heat transferred with air
Sound Required by some codes
If you can’t enclose air, you can’t condition it
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Driving Forces
1. Wind Pressures
2. Buoyancy (or stack effect)
3. HVAC
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Driving Forces
Wind Effect Stack Effect Combustion and Ventilation
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Wind Flow Patterns
Wind speedincreases
withheight
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2. Stack Effect: Cold Weather
Hot air rises
Tall Building in Winter = Heavy Balloon
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Stack Effect: Cold Weather “Perfect” Building equally leaky everywhere
Neutral Pressure Plane at mid-height
+
NPP
Air flows in at bottom
Air flows out at top
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Stack Effect: Warm Weather “Perfect” Building equally leaky everywhere
Neutral Pressure Plane at mid-height
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NPP
Air flows out at bottom
Air flows in at top
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Air Barriers
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Air Barrier Systems
Function: to stop airflow through enclosure
ABS can be placed anywhere in the enclosure
Must be strong enough to take wind gusts (code requirement)
Many materials are air impermeable, but most systems are not airtight
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Air Barrier Systems: Requirements
Continuous primary need, common failure
Strong designed for full wind load
Durable critical component - repair, replacement
Stiff control billowing, pumping
Air Impermeable (may be vapour permeable)
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“Trace the line”
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Poly can be (?) an air and vapour barrierButBEWARE when Air ConditioningDefinitely not in South
Use drywall, framing members• Seal with sealant, gaskets, etc.•Is stiff, strong•Often easier to ensure quality•Widely applicable to all forms of commercial, residential•Allows choice of vapor permeance
The Airtight Drywall Approach
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Air sealing around components: e.g., windows and wallsotherOpenings and penetrations
Typical Air Leakage Points
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Partition-Ceiling
Partition-Wall
Bathtub
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Big holes
Problem:Filter
Solution: Seal
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If you can see daylight it is not sealed
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Spray Foam as an Air Barrier
Spray foam doesn’t air seal where it isn’t there!
Wood-to-wood connections
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Spray Foam as an Air Barrier
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Spray Foam as an Air Barrier
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Cold Weather Condensation in Walls
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Vapor Diffusion vs. Air Leakage
Vapor Diffusion more to less vapor
no air flow
flow through tiny pores
Air Convection more to less air pressure
flow through visible cracks and holes
vapor is just along for the ride
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Wall w/o Insulated SheathingAir leakage
Vapor Diffusion
Cold = Condensation
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Frosting on Sheathing
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Wall with Insulated SheathingAir leakage
Vapor Diffusion
Warm = no condensation
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Vapor Barriers and the Code
Class I: 0.1 perm or less (polyethylene)
Class II: 0.1 < perm ≤ 1.0 perm (Kraft facing, vapor retarder paint)
Class III: 1.0 < perm ≤ 10 perm(Latex paint)
Polyethylene = no inward drying
More open vapor control allows greater drying—more “forgiveness” in wall
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Vapor Barriers and the Code
Can just use latex paint (no vapor barrier) if you add enough insulation outside of the stud bay insulation. Safer -> controls diffusion and air leakage moisture. Zone 5A = 30%/70% R-value ratio
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Thermal Bridging at Framing
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Thermal Bridging at Steel Framing
Steel is 400 times more conductive than wood
Steel studs are about 40 times thinner
R=4+
Cold Hot
R<0.3
A 2x6 steel stud wall 16” OC with R-19 Fiberglass Batt = effective R-9 wall assembly.
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Thermal Bridging at Steel Framing
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Thermal Bridging at Steel Framing
Summertime/AC example
Sun is hitting the wall (southeast orientation)
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Exterior Rigid Foam
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Exterior Rigid Foam (Taped Seams)
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4” Polyisocyanurate Foam Retrofit
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4” Polyisocyanurate Foam
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Foam Sheathing Cladding Attachment
250 lbs/113 kg load (7.8 psf): <0.003” deflection
Wood siding ~2 psfFiber cement 2-3 psfStucco 8-10 psf Image c/o Petersen Engineering
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Foam Sheathing Cladding Attachment
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BSC Cladding Attachment Research
System Mechanics
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Full System Laboratory Tests
Looked at initial response full system capacity as well as long term sustained loading
Used full scale samples to limit variations in fastener installation
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Recommendations
Based on the results of the testing it is currently recommended to use a maximum load per fastener of no more than 10lbs for up to 4” of insulation
Cladding weight (psf)
16” oc Furring 24” oc Furring
5 18 12
10 9 6
15 6 4
20 4 3
25 3 2
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The “Perfect Wall”
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1. Air / Rain Barrier
2. Structural Support
3. Rain Shedding
4. Insulation
Design Info from the 1960’s (Canada)
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The Perfect Wall
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Window
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The “Perfect” Wall: Higher Performance
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The Commercial Steel Frame Wall
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“Perfect Wall” Advantages
Very robust enclosure—“500 year building” Structural portion in “interior” conditions
Institutional/long term buildings
No risk of interstitial condensation
Continuity of control layers Continuous thermal insulation outside
Inspectable and simple air barrier “wrap”
Water control layer/WRB inspectable before insulation
Any interior condition
Any exterior condition
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Questions?Kohta Uenokohta (at sign) buildingscience dot com
This presentation will be available at http://buildingscience.com/past-events
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Document Resources Building Science Digest 014: Air Flow Control in Buildings
http://www.buildingscience.com/documents/digests/bsd-014-air-flow-control-in-buildings
Building Science Digest 163: Controlling Cold-Weather Condensation Using Insulationhttps://buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation
Building Science Insight 001: The Perfect Wall http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall/
Building Science Insight 005: A Bridge Too Farhttp://www.buildingscience.com/documents/insights/bsi-005-a-bridge-too-far/
Building Science Insight 029: Stucco Woes—The Perfect Stormhttp://buildingscience.com/documents/insights/bsi-029-stucco-woes-the-perfect-storm
Building Science Insight 038: Mind the Gap, Eh!http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh/
Building Science Insight 048: Exterior Spray Foamhttp://www.buildingscience.com/documents/insights/bsi-048-exterior-spray-foam/
Building Science Insight 057: Hockey Pucks and Hydrostatic Pressurehttp://buildingscience.com/documents/insights/bsi-057-hockey-pucks-and-hydrostatic-pressure
Building Science Insight 062: Thermal Bridges Reduxhttp://www.buildingscience.com/documents/insights/bsi062-thermal-bridges-redux
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