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Building Enclosure Design for Walls and Low-Sloped RoofsCOLIN SHANE M.ENG., P.ENG., P.E.
PRINCIPAL, 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.
APRIL 2018
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 session focuses on design considerations and best practices associated with detailing walls and low-slope roofs for durable wood structures. Starting with a review of building science fundamentals, it highlights durable, energy-efficient options for wood-frame wall assemblies. It then discusses low-slope roof design with an emphasis on ventilation, insulation placement, and the impact of each on long-term performance. Placement of control layers and dew point locations will be reviewed in the context of providing moisture protection while allowing drying, with the goal of achieving maximum durability. Details and case studies will be used to illustrate best practices and lessons learned.
Learning Objectives
1. Review building science fundamentals and building enclosure design considerations for light wood-frame buildings.
2. Explore the thermal benefits of utilizing wood-frame construction.
3. Review how insulation and control layer placement in a wood-frame assembly affect moisture presence, movement, and evaporation.
4. Discuss concepts for proper ventilation in low-slope roofs and review assembly and detailing lessons learned from case studies.
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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 Supportà Structural loads
à Controlà Heat flow
à Air flow
à Vapor diffusion
à Water penetration
à Condensation
à Light and solar radiation
à Noise, fire, and smoke
à Finishà Being durable and maintainable
à Being economical & constructible
à 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
à Trend towards more energy efficiently building enclosures
à Air barriers now required as of the 2012 IECC
à Continuous insulation becoming more common
à Seeing more new building materials, enclosure assemblies and
construction techniques
à More insulation = less heat flow to dry out moistureà “Marginal” assemblies that worked in the past may no longer
work
à Amount, type and placement of insulations matters, for vapor, air
and moisture control
à 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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Water Control Strategy
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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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Air Control Layer
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2015 IECC Air Barrier Requirement
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Air, Vapor, or Water Barrier?
Tyvek
GoreTex
HDPE
à Air is made up of oxygen, nitrogen, and water vapor (water vapor is the smallest molecule)
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à Code requirement
à Moistureà Air holds moisture that can be
transported and deposited
within assemblies.
à Energyà 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 Leakage vs. Diffusion
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Why Does Air Leakage Cause Condensation?
90º
70%75º
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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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Definitely Not An Air Barrier… But What Is?
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Thermal Control Layer
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Conductive Heat Loss Control
à Insulation between studs is most common heat control strategy
à Need to consider effective R-values
à Continuous insulation on exterior becoming more common
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0
2
4
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0 5 10 15 20 25 30 35 40
Overa
ll E
ffect
ive R
-Valu
e (
oF f
t2h
r/Btu
)
Framing Factor %
Steel 2x6 StudSteel 3 5/8 Stud
Framing Effect on R-values
Fram
ing @
16”
o.c
.
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2015 IECC Minimum R-Value Requirements
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Exterior Insulation Selection (Vapor Control)
à Rigid exterior foam insulations (XPS, EPS,
Polyiso, closed cell SPF) are vapor impermeable
(in thicknesses of 2”+)
à Rules of thumb: Vapor barrier on ‘warm’ side
à Fibrous insulations (mineral fiber / glass fiber)
are vapor permeable
à Allows drying to the exterior
à Often safer in cold and mixed climates
à Vapor permeance properties of WRB/air barrier
membrane is also very important
à Air barrier much more important
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Building Science: Wetting and Drying
à How can we keep the sheathing and other materials dry?
à Don’t let them get as wet
à Create air space to promote drying
à Design for vapor diffusion drying
à Keep sheathing warm
Drying
WettingSafe Storage Capacity
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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 dry
à 50+ year old concept!
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‘Perfect’ Wall
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‘Perfect’ Roof
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Details – Connect the Control Layers
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Roof Design
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
Refresher: Low Slope Roof Types
à Protected Membrane (Inverted)
à Conventional
à Vented/Unvented (Compact)
The ‘Perfect’ Roof – Inverted / PMR / IRMA
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
à The code tells us we have to
à Historically, this concept has worked (mostly) well in attic construction
Attics – How they work
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
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
Air Leakage Condensation – Underside of Roof Deck
CODE COMPLIANT
VENTING
Air Leakage Condensation – Steel Deck
MORE INSULATION MORE PROBLEMS
à Sprinklers – Projects using NFPA are required to have sprinklers in concealed spaces unless they meet spatial and material requirements
à Need to be carefully consider venting cavities
à How to deal with over-framed crickets?
à Conceptually, sprinkler code trying to limit air flow and venting
à Building code / moisture performance at odds with this
Other Considerations for Vented Roofs
à Coordination of venting (1:150 vent area) with the roof structure
Other Considerations for Vented Roofs
Other Considerations for Vented Roofs
WHAT DO WE DO AT SHEARWALLS WHERE FULL BLOCKING IS REQUIRED?
HOW DO WE CREATE SLOPE?
Unvented (Compact) Roofs
à CBC/IBC allows unvented roofs that meet the following conditions
à If only insulating below the
deck, air impermeable
insulation (spray foam) is
required
à Unvented roofs can work well, but how can you tell if there is a leak in the roof membrane?
Split Insulation Approaches
à Good compromise of performance and cost
à Ratio of exterior insulation to interior insulation dependent on climate and interior humidity levels
à Building code has some guidance here for ‘typical’ buildings
Summary: Low Slope Roof Types
à Protected Membrane (Inverted)
à Conventional
à Vented/Unvented (Compact)
Summary
à rdh.com | buildingsciencelabs.com
Discussion + Questions
FOR FURTHER INFORMATION PLEASE VISIT
This concludes The American Institute of Architects Continuing Education Systems Course
Colin Shane – [email protected]
