low-slope wood-framed roofs enclosure design for durability€¦ · design criteria each assembly...
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Low-Slope Wood-Framed RoofsEnclosure Design for DurabilityCOLIN 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.
MAY 2016
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
“The Wood Products Council”is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516.
Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.
This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.__________________________________
Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
Course Description
à This seminar focuses on the effects of design decision for enclosure components such as insulation and control layers on low-slope wood-frame roofs. Venting requirements and detailing approaches to providing appropriate venting paths will be covered for roofs with attic insulation. Differences associated with roofs having continuous insulation on top of
the deck will also be reviewed, as will methods for ensuring durability of the both the structure and the insulation.
Learning Objectives
à Discuss the fundamental goals of building enclosures and the design criteria each assembly must achieve.
à Review how insulation and control layer placement in a wood-frame assembly affect moisture presence, movement, and evaporation.
à Explore the differences in enclosure design and ventilation
requirements for roofs with attic insulation vs. continuous insulation on top of the deck.
à Highlight detailing best practices for vented low-slope roofs to avoid moisture traps and ensure material longevity.
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Wood-frame Building Enclosure Design Guides
à 2011 Building Enclosure Design
Guide – Wood-frame Multi-Unit
Residential Buildings
à Emphasis on best practices,
moisture and new energy codes
à 2013 Guide for Designing Energy-
Efficient Building Enclosures
à Focus on highly insulated wood-
frame assemblies to meet current
and upcoming energy codes
à CLT Handbook
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Building Science Basics
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à Separate indoors from outdoors, by controlling:à Heat flow
à Air flow
à Vapor diffusion
à Water penetration
à Condensationà Light and solar radiation
à Noise, fire, and smoke
à While at the same time:à Transferring structural loads
à Being durable and maintainableà Being economical & constructible
à Looking good!
Building Enclosure Design Fundamentals
The Building Science of Roofs
à What is the function of a roof assembly?
à Control water, air, heat, vapor
à Support structural loads
à Provide an architectural finish
à The same as every other building enclosure assembly
à Nothing ‘magic’ or special about roofs, but…
à Lots of exposure to rain and sun
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The ‘Perfect’ Assembly
à Rain penetration control: rainscreen cladding
over water barrier
à Air leakage control: robust air barrier system
à Heat control: continuous insulation layer
à Locate all barriers exterior of structure
à Keep structure warm and dry
à 50+ year old concept!
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‘Perfect’ Roof
The Building Science of Roofs
à Control layers
à Water control - roof membrane
à Air control - ?
à Vapor control - ?
à Thermal control – insulation
à Need to think about all control layers, not just the roof membrane (water)
à The order of the control layers affects design, performance, and construction
à And whether or not we vent
Refresher: Low Slope Roof Types
à Protected Membrane (Inverted)
à Conventional
à Vented/Unvented (Compact)
Inverted Roofs
The ‘Perfect’ Roof – Inverted / PMR / IRMA
à Control layers
à Water control - roof membrane
à Air control – roof membrane
à Vapor control – roof membrane
à Thermal control – moisture resistant insulation
à Membrane directly on sloped structure
à Insulation above membrane
à “Finish” – pavers, ballast
The ‘Perfect’ Roof – Inverted / PMR / IRMA
The ‘Perfect’ Roof – Inverted / PMR / IRMA
Conventional Roofs
Conventional Roof – Exposed Membrane
à Control layers
à Water control - roof membrane
à Air control
› Roof membrane?
› Or separate air barrier?
› Insulation?
à Vapor control
› Roof membrane?
› Insulation?
› Or separate vapor barrier?
à Thermal control
› Rigid insulation
Considerations for Conventional Roofs
à As always: details and durability of roof membrane
à Do I need a vapor barrier?
à Maybe, but often not in California
à Most of the time, this is the wrong question
à Do I need an air barrier?
à Yes!
à Location matters
Vented Roofs
Vented Roofs
à Control layers
à Water control - roof membrane
à Air control
› Interior drywall?
› Polyethylene sheet?
› Nothing?
à Vapor control
› Roof membrane
› Faced insulation?
› Nothing?
à Thermal control
› Batt insulation
Surface Temperatures – Interior Insulation
35°F
70°F
32°FNight Day?
What happens if there is air leakage through the ceiling?
Surface Temperatures – Exterior Insulation
35°F
70°F
70°FNight
70°FDay
What happens if there is air leakage through the ceiling?
Vented Roofs – Why do we vent?
à Because we put the insulation in the wrong spot
à To control air leakage / vapor diffusion condensation
à Roof membrane is a strong vapor barrier on ‘cold side’ of insulation
à The code tells us we have to
à Historically, this concept has worked (mostly) well in attic construction
Attics – How they work
à Ventilation drying
à Heat loss from interior warms attic air, decreasing RH.
à Increased insulation levels have generally occurred along side increases in air tightness
à Less heat, but less moisture too
à The balance still works
à Buoyancy and wind ventilate the attic space with exterior air
à Effectiveness of ventilation is
highly variable
à Excessive air leakage (ceiling details)
à Exhaust duct leaks & discharge location (roof, soffit, or wall)
à Inadequate venting provisions (amount, vent location, or materials)
à Outdoor moisture: night sky condensation on underside of sheathing
Where do we see attic problems?
Air Leakage Through Ducts
Other Not So Great Ideas…
Night Sky Radiation Condensation
Solar and Night Sky Radiation
Night Sky Radiation – Full Venting
Night Sky Radiation – Full Venting
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MC-FULL-N-312 MC-IN-SURF-N-312 T-IN-N-312-Plywood
T-N-312-Embedded Outdoor-Temperature Outdoor-Dewpoint
COND-N-312Sheathing SolarRadiation
Condensation
Effectiveness of Venting
à Venting is ‘net drying’ in most climates, but isn’t a panacea for moisture problems
à There are many periods where venting will add moisture to roof assembly
à High RH winter evenings + night sky radiation
à Does your car have dew on it?
à Minimizing moisture sources from interior space is critical
à Needs to be coupled with interior heat loss
Theoretical attic with no air leakage or heat loss into the attic and unrestricted ventilation
Effectiveness of Venting?
Considerations for… California?
Considerations for… California?
Low-Slope Vented Roofs vs. Attics
à What is different about low-sloped roofs?
à Typically on larger buildings with bigger
air pressures
à Greater occupancy moisture loads?
à More / bigger mechanical equipment?
à White roof membranes
à Minimal offset heights to promote
buoyance driven ventilation
à Discrete vents instead of continuous
à More complicated venting path
HIGHER MOISTURE LOADS
MINIMIZED DRYING
CEILING AIRTIGHTNESSIS
CRITICAL
Vented Roofs – California Code Requirements
à 2013 CBC / 2012 IBC:
à Can be reduced to 1/300 if:
à Top and bottom venting (3 feet offset)
à Class 1 or II Vapor barrier is installed
à What does this mean for conventional roofs?
NO MENTION OF CEILING
AIRTIGHTNESS
Vented Roofs – Residential Code
à 2013 CRC:
à Can be reduced to 1/300 if:
à Top and bottom venting (3 feet offset)
à Class 1 or II Vapor barrier is installed (Zone 14 and 16)
Vented Roofs – Residential Code Exceptions
à Residential Code contains additional exceptions:
à Venting not required when:
à Attic is enclosed by thermal envelope, and no interior vapor barrier
à Air-impermeable insulation (foam) below deck
à Air-impermeable insulation below deck + batt
à Batt below deck + insulation on the exterior
WARM SHEATHING
AIRTIGHTNESS
WARM SHEATHING
Air Leakage Condensation – Underside of Roof Deck
CODE COMPLIANT
VENTING
Air Leakage Condensation – Steel Deck
MORE INSULATION MORE PROBLEMS
Realities of Construction
à Details of venting path rarely shown in detail
à Faith based ventilation design
à How to achieve ceiling airtightness even more cryptic
à Who is responsible?
à Probably not the roofing contractor
Ceiling Airtightness - Options
à Airtight drywall approach
à Can work in theory, but difficult
à Often mistakenly seen as no different than any other drywall installation
à Sheet membrane
à It’s all about the details at penetrations and walls
à Construction sequencing
à Spray foam
à Installed from above ceiling, either at details or throughout
à Expensive
Vented Roofs – Differences in Perspective
Contractor/Owner Perspective
à “…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.”
à We’ll give you a warranty for 30 years
à As the ‘drying’ variables change, the airtightness requirements also have to change:
à White roof membranes
à R30+ depending on climate zone
à The roof contractor won’t warranty condensation
The Other Perspective
Split Insulation Approaches
à Good compromise of performance and cost
à If sheathing is sufficiently warm, venting not required
à Ratio of exterior insulation to interior insulation dependent on climate and interior humidity levels
à Pool in Tahoe requires more exterior insulation than apartment in San Diego
à Be prepared to convince your building official
Final Thoughts
à Making vented roofs perform well is harder than it used to be
à Air tightness of the ceiling is critical to good performance
à Details and performance requirements need to be explicit in the design
à Be very careful with ducts and other mechanical penetrations in attic and through ceiling
à Insulate on the exterior and sleep easy
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à rdh.com
Discussion + Questions
Colin Shane – [email protected]
This concludes The American Institute of Architects Continuing Education Systems Course