Building Enclosure Detailing for Walls and Low-Sloped Roofs · 2018-12-17 · 1 of 63 Building...
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Building Enclosure Detailing for Walls and Low-Sloped Roofs
COLIN SHANE M.ENG., P.ENG. ASSOCIATE, SENIOR PROJECT MANAGER
RDH BUILDING SCIENCE INC.
Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board.
MARCH 2016
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This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written
permission of the speaker is prohibited.
© RDH Building Sciences Inc. 2015
Copyright Materials
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Wood-frame Building Enclosure Design Guides
! 2011 Building Enclosure Design
Guide – Wood-frame Multi-Unit
Residential Buildings
! Emphasis on best practices,
moisture and new energy codes
! 2013 Guide for Designing Energy-
Efficient Building Enclosures
! Focus on highly insulated wood-
frame assemblies to meet current
and upcoming energy codes
! CLT Handbook
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Building Science Basics
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! Separate indoors from outdoors, by controlling: ! Heat flow
! Air flow
! Vapor diffusion
! Water penetration
! Condensation
! Light and solar radiation
! Noise, fire, and smoke
! While at the same time: ! Transferring structural loads
! Being durable and maintainable
! Being economical & constructible
! Looking good!
Building Enclosure Design Fundamentals
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The Old Way
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The New Way – “Light & Tight”
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Old versus New
! Trend towards more energy efficiently building enclosures
! Air barriers now required in 2012 IECC and 2013 CEC
! Continuous insulation becoming more common
! Seeing more new building materials, enclosure assemblies and
construction techniques
! More insulation = less heat flow to dry out moisture ! “Marginal” assemblies that worked in the past may no longer
work ! Amount, type and placement of insulations matters, for vapor, air
and moisture control
! Need to fully understand the science and interaction of design parameters
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What do we know?
Building Enclosure
Control Air
Control Vapor
Control heat
Control Rain
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Building Enclosure Control Layers
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How do Walls get Wet and Dry?
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Water Penetration Control Strategies
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Rainscreen Cladding
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2013 CEC Air Barrier Requirement
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! Code requirement
! Moisture ! Air holds moisture that can be
transported and deposited
within assemblies.
! Energy ! Unintentional airflow through
the building enclosure can
account for as much as 50% of
the space heat loss/gain in
buildings.
Air Penetration Control – Why?
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Air, Vapor, or Water Barrier?
Tyvek
GoreTex
HDPE
! Air is made up of oxygen, nitrogen, and water vapor (water vapor is the smallest molecule)
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Air Leakage vs. Diffusion
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Why Does Air Leakage Cause Condensation?
70º
50% 50º
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Air Leakage Condensation Damage
Details, Details, Details!
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Types of Air Barrier Systems
Sealed Gypsum Sheathing – Sealant Filler at Joints
Loose Sheet Applied Membrane – Taped Joints & Strapping
Liquid Applied – Silicone sealants and silicone membrane at Joints
Sealed Plywood Sheathing –Sealant & Membrane at Joints
Sealed Sheathing – Membrane at Joints
Self-Adhered vapor permeable membrane
Plywood sheathing with taped joints (good tape)
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Airtightness Does Not Happen By Accident
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How to Tell the Membrane is Not the Air Barrier
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Definitely Not An Air Barrier… But What Is?
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Conductive Heat Loss Control
! Insulation between studs is most common heat control strategy
! Need to consider effective R-values
! Continuous insulation on exterior becoming more common
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2013 CEC Requirements
! U-0.059 = R-16.9
! U-0.042 = R-23.8 ! Zones 11, 14, 15, 16
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Framing Factor Impact on Effective R-values
Fram
ing @
16
” o.c
.
Fram
ing @
24
”o/c
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Cladding Attachment Through Exterior Insulation
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Thermally Improved Performance?
Continuous metal Z-girts
Fiberglass Clips & Hat-Tracks
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Building Science Basics: Sorption Isotherm
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Wall Design Evolution
40°F
70°F 65°F
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Wall Design Evolution
32°F
70°F
40°F
65°F
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Wall Design Evolution
32°F
70°F
40°F
65°F
55°F
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The ‘Perfect’ Assembly
! Rain penetration control: rainscreen cladding
over water barrier
! Air leakage control: robust air barrier system
! Heat control: continuous insulation layer
! Locate all barriers exterior of structure
! Keep structure warm and dry
! 50+ year old concept!
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Wood-Frame Assemblies – ‘Perfect’ Wall
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Wood-Frame Assemblies – ‘Perfect’ Roof
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Wall-to-Roof Detail
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Roof Design
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The Building Science of Roofs
! What is the function of a roof assembly?
! Control water, air, heat, vapor
! Support structural loads
! Provide an architectural finish
! The same as every other building enclosure assembly
! Nothing ‘magic’ or special about roofs, but…
! Lots of exposure to rain and sun
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The Building Science of Roofs
! Control layers
! Water control - roof membrane
! Air control - ?
! Vapor control - ?
! Thermal control – insulation
! Need to think about all control layers, not just the roof membrane (water)
! The order of the control layers affects design, performance, and construction
! And whether or not we vent
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Refresher: Low Slope Roof Types
! Protected Membrane (Inverted)
! Conventional
! Vented/Unvented (Compact)
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Inverted Roofs
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
! Control layers
! Water control - roof membrane
! Air control – roof membrane
! Vapor control – roof membrane
! Thermal control – moisture resistant insulation
! Membrane directly on sloped structure
! Insulation above membrane
! “Finish” – pavers, ballast
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
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Considerations for Inverted/PMR Roofs
How to keep insulation from becoming saturated below pavers, ballast or soil/green roofs
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Conventional Roofs
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Conventional Roof – Exposed Membrane
! Control layers
! Water control - roof membrane
! Air control
› Roof membrane?
› Or separate air barrier?
! Vapor control
› Roof membrane?
› Insulation?
› Or separate vapor barrier?
! Thermal control
› Rigid insulation
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Air Leakage vs. Diffusion
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Considerations for Conventional Roofs
! As always: details and durability of roof membrane
! Do I need a vapor barrier?
! Maybe, but often not in California
! Most of the time, this is the wrong question
! Do I need an air barrier?
! Yes!
! Location matters
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Vented Roofs
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Vented Roofs
! Control layers
! Water control - roof membrane
! Air control
› Interior drywall?
› Polyethylene sheet?
› Nothing?
! Vapor control
› Roof membrane
› Faced insulation?
! Thermal control
› Batt insulation
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Vented Roofs – Why do we vent?
! Because we put the insulation in the wrong spot
! To control air leakage / vapor diffusion condensation
! Roof membrane is a strong vapor barrier on ‘cold side’ of insulation
! The code tells us we have to
! Historically, this concept has worked (mostly) well in attic construction
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Attics – How they work
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Attics – How they work
! Heat loss from interior warms attic air, decreasing RH.
! Increased insulation levels have generally occurred along side increases in air tightness
! Less heat, but less moisture too
! The balance still works
! Buoyancy and wind ventilate the attic space with exterior air
! Effectiveness of ventilation is
highly variable
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What influences attic moisture issues & what can we control by design?
! Roof orientation / slope (solar radiation)
! Roofing material/color
! Adjacent buildings / trees – shading
! Outdoor climate
! Indoor climate
! Roof Leaks
! Insulation R-value
! Air leakage from house
! Duct leakage in attic
! Duct discharge location
! Vent area and distribution
! Sheathing durability
! Roof maintenance
! Other things
Influencers of Moisture Problems
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! Air leakage (ceiling details)
! Exhaust duct leaks & discharge location (roof, soffit, or wall)
! Inadequate venting provisions (amount, vent location, or materials)
! Outdoor moisture: night sky condensation on underside of sheathing
Where are we Seeing the Biggest Issues?
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Air Leakage Through Ducts
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Other Not So Great Ideas…
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Night Sky Radiation Condensation
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Solar and Night Sky Radiation
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Night Sky Radiation – Full Venting
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Night Sky Radiation – Full Venting
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Effectiveness of Venting
! Venting is ‘net drying’ in most climates, but isn’t a panacea for moisture problems
! There are many periods where venting will add moisture to roof assembly
! High RH winter evenings + night sky radiation
! Does your car have dew on it?
! Minimizing moisture sources from interior space is critical
! Needs to be coupled with interior heat loss
Theoretical attic with no air leakage or heat loss into the attic and unrestricted ventilation
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Effectiveness of Venting?
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Considerations for… California?
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Considerations for… California?
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Low-Slope Vented Roofs vs. Attics
! What is different about low-sloped roofs?
! Typically on larger buildings with bigger
air pressures
! Greater occupancy moisture loads?
! More / bigger mechanical equipment?
! White roof membranes
! Minimal offset heights to promote
buoyance driven ventilation
! Discrete vents instead of continuous
! More complicated venting path
HIGHER MOISTURE LOADS
MINIMIZED DRYING
CEILING AIRTIGHTNESSIS
CRITICAL
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Vented Roofs – California Code Requirements
! 2013 CBC / 2012 IBC:
! Can be reduced to 1/300 if:
! Top and bottom venting (3 feet offset)
! Class 1 or II Vapor barrier is installed
! What does this mean for conventional roofs?
NO MENTION OF CEILING
AIRTIGHTNESS
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Vented Roofs – Residential Code
! 2013 CRC:
! Can be reduced to 1/300 if:
! Top and bottom venting (3 feet offset)
! Class 1 or II Vapor barrier is installed (Zone 14 and 16)
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Vented Roofs – Residential Code Exceptions
! Residential Code contains additional exceptions:
! Venting not required when:
! Attic is enclosed by thermal envelope, and no interior vapor barrier
! Air-impermeable insulation (foam) below deck
! Air-impermeable insulation below deck + batt
! Batt below deck + insulation on the exterior
WARM SHEATHING
AIRTIGHTNESS
WARM SHEATHING
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Air Leakage Condensation – Underside of Roof Deck
CODE COMPLIANT
VENTING
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Air Leakage Condensation – Steel Deck
MORE INSULATION MORE PROBLEMS
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Realities of Construction
! Details of venting path rarely shown in detail
! Faith based ventilation design
! How to achieve ceiling airtightness even more cryptic
! Who is responsible?
! Probably not the roofing contractor
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Ceiling Airtightness - Options
! Airtight drywall approach
! Can work in theory, but difficult
! Often mistakenly seen as no different than any other drywall installation
! Sheet membrane
! It’s all about the details at penetrations and walls
! Construction sequencing
! Spray foam
! Installed from above ceiling, either at details or throughout
! Expensive
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Airtight Drywall – Done Right?
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Vented Roofs – Differences in Perspective
Contractor/Owner Perspective
! “…typically all of the standard drywall construction details for sound, fire, and thermal have been sufficient to satisfy the air barrier requirements on past projects.”
! We’ll give you a warranty for 30 years
! As the ‘drying’ variables change, the airtightness requirements also have to change:
! White roof membranes
! R30+ depending on climate zone
! The roof contractor won’t warranty condensation
The Other Perspective
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Re-Roofing Considerations
! Rule #1: Don’t make it worse
! Consider the entire assembly, not just the membrane
! Are you changing the wetting / drying balance?
! Don’t fix a leaking problem and create a durability
problem
! Be careful when switching to white roof membranes
! Be extra careful when switching to mechanically fastened white roofs
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Re-Roofing Considerations
VENTED + BATT INSULATION
ADD AIR BARRIER & EXTERIOR INSULATION
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Split Insulation Approaches
! Good compromise of performance and cost
! If sheathing is sufficiently warm, venting not required
! Ratio of exterior insulation to interior insulation dependent on climate and interior humidity levels
! Pool in Tahoe requires more exterior insulation than apartment in San Diego
! Be prepared to convince your building official
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Final Thoughts
! Making vented roofs perform well is harder than it used to be
! Air tightness of the ceiling is critical to good performance
! Details and performance requirements need to be explicit in the design
! Be very careful with ducts and other mechanical penetrations in attic and through ceiling
! Insulate on the exterior and sleep easy