SlideShare a Scribd company logo
[ ANOTHER BENEFIT FROM ABOVE-SHEATHING VENTILATION ! ]
                 Moisture Removal Benefits with Above-Sheathing Ventilation on Steep-Slope Metal Roofs
                                                       William (Bill) Miller, Ph.D., Achilles Karagiozis, André Desjarlais




    Providing an air space above the sheathing of a roof deck offers                       moisture-related problems in the roofing systems using ASV.
thermal benefits for stone-coated or standing seam metal roofs
that yield energy savings in the summer and winter while also                              The following modes of heat and moisture transport were included:
helping to remove unwanted moisture. The natural ventilation                               •  Vapor diffusion through all porous roof construction materials
above the sheathing improves the durability of the underlying                              •  Liquid transport through all porous roof construction materials
structure of the roof.                                                                     •  Air convection transport for both thermal and moisture
    Metal roofs are sometimes offset mounted from the roof                                    components
deck using a double-batten (counter-batten) construction. The                              •  Moisture storage in all roof construction materials
design provides an air space between the exterior face of the roof                         •  Radiative transport with nighttime sky conditions
sheathing and the underside of the roof cover so that a clear,                             •  Radiative transport within the air gap provided by the stone
albeit complex, air pathway exists beneath the roof cover. Solar                              coated metal roof
irradiance absorbed at the roof’s surface is conducted through                             •  Condensation and evaporation processes and freeze and
the metal roof and heats the air space. The warmer and therefore                              thawing processes with the associated latent heat exchanges
more buoyant air moves up the inclined air passage. The ventilation
scheme helps remove unwanted heat but it also removes unwanted
moisture from the roof deck, thereby improving the roof’s thermal                         [ ASV accelerated the removal of
performance as well as its durability. The thermally induced airflow                      unwanted moisture and reduced the
occurring in this air space is termed above-sheathing ventilation
(ASV).                                                                                    moisture content of the OSB sheathing
                                                                                          well below that of the OSB in a closed
                                                                                          cavity ]
                                                                                                In the simulation analysis, the exterior and interior
                                                                                           environmental loads were assumed for the climatic conditions
                                                                                           of Knoxville, Tennessee. The proposed ASHRAE SPC 160P, “Design
                                                                                           Criteria for Moisture Control,” was employed for both the exterior
                                                                                           and interior hygrothermal loading conditions. All simulations were
                                                                                           initiated using double the equilibrium moisture content (EMC) at
                                                                                           80% relative humidity. Both the ventilated and non ventilated cases
                                                                                           were simulated for a period of 2 years.
                                                                                                A snapshot of the moisture content in the sheathing board
                                                                                           {oriented strand board (OSB)} is given in Figure 2. The simulation
Figure 1: Batten and counter-batten system used to mount stone-coated metal roofs.         period started October 1, 2005, one of the more difficult periods of


    Field studies were conducted on several attic assemblies
having stone-coated metal shake roofs with and without cool
color (infrared reflective) pigments and with and without above-
sheathing ventilation. Stone-coated metal roofs are often offset
mounted from the roof deck using a batten and counter-batten
system. Here counter-battens made of nominal dimension wood
strips (1 by 4’s) are nailed to the roof deck from soffit to ridge, and
battens (2 × 2’s) are placed above the counter-battens and nailed
to the deck (Fig 1).

MOISTURE REMOVAL BENEFIT
    A moisture engineering analysis was performed on the roof
system depicted in Figure 1 using the MOISTURE-EXPERT model
(Karagiozis 2001) that has shown good agreement in ventilated
                                                                                          Figure 2. Comparison of moisture content of OSB layer as a function of ventilation strategy
wall systems. The intent was to show the potential for reducing                           (ventilated vs. non vented) for a 2-year period.

  Metal Construction News     |   Metal Architecture                                                                           C O O L M E TA L R O O F I N G F O R U M          |      19
The demand for electricity in the United
      States is increasing three times faster than the speed
      at which power plants and lines are being built. The
      U.S. energy demand is predicted to increase almost
      20 percent in the next decade.
      (North American Electric Reliability Council)


the year for the sheathing to dry out. ASV accelerated the removal                         of the air space yields about 80 air changes per hour. Therefore,
of unwanted moisture and reduced the moisture content of the                               the measured data is well within reason of the results from the
OSB sheathing well below that of the OSB in a closed cavity (Figure                        hygrothermal simulations.
2). Ventilating the roof deck dried the OSB within 200 days to safe
moisture limits in which fungal growth would not typically occur. In                       Summary
comparison, the closed roof deck required an additional 100 days to                            Moisture is a prevalent issue in all aspects of building design.
reach safe moisture content.                                                               Metal roofs employing above-sheathing ventilation show superior
     The number of air exchanges occurring within the ventilated                           hygrothermal performance when compared with a non vented
cavity (Figure 3) tells the story. The occurrence of air exchange                          roof system. Above-sheathing ventilation therefore adds yet
rates are displayed for the assumed air changes per hour (ACH),                            another feature to this ecologically sound building material.
which are dependent on both temperature and wind pressure                                  Providing the ventilation above the sheathing improves the
                                                                                           durability of the underlying structure of the roof. As a result, the
                                                                                           expected performance of metal roofing in high winds and hail
                                                                                           storms is further enhanced because of the improved hygrothermal
                                                                                           performance afforded by a metal roof system using above-
                                                                                           sheathing ventilation.
                                                                                               Future articles will address: (1) does above sheathing ventilation
                                                                                           reduce attic air temperatures and in turn reduce heat losses from
                                                                                           ducts installed in attics; (2) does increased spacing of the air gap
                                                                                           improve thermal performance; (3) what are the seasonal benefits
                                                                                           of above sheathing ventilation; and, (4) does above sheathing
                                                                                           ventilation help retard ice damming.
Figure 3. Period of time during 2-year simulation for cavity air changes per hour (wind-
and temperature-dependent).                                                                ACKNOWLEDGMENTS
                                                                                               Funding for this project was provided by the U.S. Department
flows acting along the roof ventilation cavity. Roughly 20-100                             of Energy under the supervision of Marc LaFrance of the Building
ACH are prevalent about 80% of the time during the 2-year                                  Technologies Program. The ORNL project team members are
simulation runs. The 60 ACH was the maximum incident air                                   André Desjarlais, William Miller, Tom Petrie, Jan Kosny and Achilles
exchange rate observed occurring about 25% of the time.                                    Karagiozis, all of ORNL’s Buildings Envelope Program. The Metal
Therefore, the potential moisture removal benefits afforded by                             Construction Association and its affiliate members provided the
ASV are evident from the vented compared to the non vented                                 stone-coated shake and S-mission roofs used in testing. Metro Roof
simulations.                                                                               Products constructed the attic assemblies and provided valuable
    As a check, Miller (2006) made field measurements of the                               assistance in installing the roofs on the steep-slope assemblies. The
airflow underneath stone-coated metal shake roofs (Figure                                  financial support of the Metal Construction Association, the Cool
1) by monitoring the decay rate of the tracer gas CO2 with                                 Metal Roofing Coalition and the guidance of Metro Roof Products
time and deducing the flow rate from a continuity balance for                              are greatly appreciated. cmrf
the concentration of CO2. The CO2 gas was injected into the
vent gap of the soffit to saturate the cavity. After a substantial                         Karagiozis, A. N. 2001. “Advanced Hygrothermal Modeling of Building Materials
buildup of concentration registered on a monitor, the gas                                  Using MOISTURE-EXPERT 1.0.” Pp. 39–¬47 in Proceedings of the International
injection was stopped, and the concentration was recorded                                  Particleboard/ Composite Materials Symposium.
at timed intervals. All measurements were made around solar                                Miller, W. A. 2006. The Effects of Infrared-Blocking Pigments and Deck Venting on
noon, when the roofs were at their highest temperatures.                                   Stone-Coated Metal Residential Roofs. ORNL/TM-2006/9. Oak Ridge, TN: Oak Ridge
Computed airflows were about 18 cfm, which for the volume                                  National Laboratory


20    |   C O O L M E TA L R O O F I N G F O R U M                                                                         Metal Construction News   |   Metal Architecture

More Related Content

Recently uploaded

一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
ghyke
 
Stealth attraction for mens gets her with your words
Stealth attraction for mens gets her with your wordsStealth attraction for mens gets her with your words
Stealth attraction for mens gets her with your words
ichettrisagar95
 
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptxAssignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
kirannaveed6
 
Understanding of Self - Applied Social Psychology - Psychology SuperNotes
Understanding of Self - Applied Social Psychology - Psychology SuperNotesUnderstanding of Self - Applied Social Psychology - Psychology SuperNotes
Understanding of Self - Applied Social Psychology - Psychology SuperNotes
PsychoTech Services
 
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotesProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
PsychoTech Services
 
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA CoursePersonal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
Tim Han Success Insider
 
Aggression - Applied Social Psychology - Psychology SuperNotes
Aggression - Applied Social Psychology - Psychology SuperNotesAggression - Applied Social Psychology - Psychology SuperNotes
Aggression - Applied Social Psychology - Psychology SuperNotes
PsychoTech Services
 
7 Habits of Highly Effective People.pptx
7 Habits of Highly Effective People.pptx7 Habits of Highly Effective People.pptx
7 Habits of Highly Effective People.pptx
gpangilinan2017
 
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
shahul62
 

Recently uploaded (9)

一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
一比一原版(mizzou毕业证书)美国密苏里大学毕业证如何办理
 
Stealth attraction for mens gets her with your words
Stealth attraction for mens gets her with your wordsStealth attraction for mens gets her with your words
Stealth attraction for mens gets her with your words
 
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptxAssignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
Assignment 1 (Introductions to Microsoft Power point 2019) kiran.pptx
 
Understanding of Self - Applied Social Psychology - Psychology SuperNotes
Understanding of Self - Applied Social Psychology - Psychology SuperNotesUnderstanding of Self - Applied Social Psychology - Psychology SuperNotes
Understanding of Self - Applied Social Psychology - Psychology SuperNotes
 
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotesProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
ProSocial Behaviour - Applied Social Psychology - Psychology SuperNotes
 
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA CoursePersonal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
Personal Growth Unleashed: Key Strategies from Tim Han’s LMA Course
 
Aggression - Applied Social Psychology - Psychology SuperNotes
Aggression - Applied Social Psychology - Psychology SuperNotesAggression - Applied Social Psychology - Psychology SuperNotes
Aggression - Applied Social Psychology - Psychology SuperNotes
 
7 Habits of Highly Effective People.pptx
7 Habits of Highly Effective People.pptx7 Habits of Highly Effective People.pptx
7 Habits of Highly Effective People.pptx
 
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
1-CIE-IGCSE-Additional-Mathematics-Topical-Past-Paper-Functions.pdf
 

Featured

2024 State of Marketing Report – by Hubspot
2024 State of Marketing Report – by Hubspot2024 State of Marketing Report – by Hubspot
2024 State of Marketing Report – by Hubspot
Marius Sescu
 
Everything You Need To Know About ChatGPT
Everything You Need To Know About ChatGPTEverything You Need To Know About ChatGPT
Everything You Need To Know About ChatGPT
Expeed Software
 
Product Design Trends in 2024 | Teenage Engineerings
Product Design Trends in 2024 | Teenage EngineeringsProduct Design Trends in 2024 | Teenage Engineerings
Product Design Trends in 2024 | Teenage Engineerings
Pixeldarts
 
How Race, Age and Gender Shape Attitudes Towards Mental Health
How Race, Age and Gender Shape Attitudes Towards Mental HealthHow Race, Age and Gender Shape Attitudes Towards Mental Health
How Race, Age and Gender Shape Attitudes Towards Mental Health
ThinkNow
 
AI Trends in Creative Operations 2024 by Artwork Flow.pdf
AI Trends in Creative Operations 2024 by Artwork Flow.pdfAI Trends in Creative Operations 2024 by Artwork Flow.pdf
AI Trends in Creative Operations 2024 by Artwork Flow.pdf
marketingartwork
 
Skeleton Culture Code
Skeleton Culture CodeSkeleton Culture Code
Skeleton Culture Code
Skeleton Technologies
 
PEPSICO Presentation to CAGNY Conference Feb 2024
PEPSICO Presentation to CAGNY Conference Feb 2024PEPSICO Presentation to CAGNY Conference Feb 2024
PEPSICO Presentation to CAGNY Conference Feb 2024
Neil Kimberley
 
Content Methodology: A Best Practices Report (Webinar)
Content Methodology: A Best Practices Report (Webinar)Content Methodology: A Best Practices Report (Webinar)
Content Methodology: A Best Practices Report (Webinar)
contently
 
How to Prepare For a Successful Job Search for 2024
How to Prepare For a Successful Job Search for 2024How to Prepare For a Successful Job Search for 2024
How to Prepare For a Successful Job Search for 2024
Albert Qian
 
Social Media Marketing Trends 2024 // The Global Indie Insights
Social Media Marketing Trends 2024 // The Global Indie InsightsSocial Media Marketing Trends 2024 // The Global Indie Insights
Social Media Marketing Trends 2024 // The Global Indie Insights
Kurio // The Social Media Age(ncy)
 
Trends In Paid Search: Navigating The Digital Landscape In 2024
Trends In Paid Search: Navigating The Digital Landscape In 2024Trends In Paid Search: Navigating The Digital Landscape In 2024
Trends In Paid Search: Navigating The Digital Landscape In 2024
Search Engine Journal
 
5 Public speaking tips from TED - Visualized summary
5 Public speaking tips from TED - Visualized summary5 Public speaking tips from TED - Visualized summary
5 Public speaking tips from TED - Visualized summary
SpeakerHub
 
ChatGPT and the Future of Work - Clark Boyd
ChatGPT and the Future of Work - Clark Boyd ChatGPT and the Future of Work - Clark Boyd
ChatGPT and the Future of Work - Clark Boyd
Clark Boyd
 
Getting into the tech field. what next
Getting into the tech field. what next Getting into the tech field. what next
Getting into the tech field. what next
Tessa Mero
 
Google's Just Not That Into You: Understanding Core Updates & Search Intent
Google's Just Not That Into You: Understanding Core Updates & Search IntentGoogle's Just Not That Into You: Understanding Core Updates & Search Intent
Google's Just Not That Into You: Understanding Core Updates & Search Intent
Lily Ray
 
How to have difficult conversations
How to have difficult conversations How to have difficult conversations
How to have difficult conversations
Rajiv Jayarajah, MAppComm, ACC
 
Introduction to Data Science
Introduction to Data ScienceIntroduction to Data Science
Introduction to Data Science
Christy Abraham Joy
 
Time Management & Productivity - Best Practices
Time Management & Productivity -  Best PracticesTime Management & Productivity -  Best Practices
Time Management & Productivity - Best Practices
Vit Horky
 
The six step guide to practical project management
The six step guide to practical project managementThe six step guide to practical project management
The six step guide to practical project management
MindGenius
 
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
RachelPearson36
 

Featured (20)

2024 State of Marketing Report – by Hubspot
2024 State of Marketing Report – by Hubspot2024 State of Marketing Report – by Hubspot
2024 State of Marketing Report – by Hubspot
 
Everything You Need To Know About ChatGPT
Everything You Need To Know About ChatGPTEverything You Need To Know About ChatGPT
Everything You Need To Know About ChatGPT
 
Product Design Trends in 2024 | Teenage Engineerings
Product Design Trends in 2024 | Teenage EngineeringsProduct Design Trends in 2024 | Teenage Engineerings
Product Design Trends in 2024 | Teenage Engineerings
 
How Race, Age and Gender Shape Attitudes Towards Mental Health
How Race, Age and Gender Shape Attitudes Towards Mental HealthHow Race, Age and Gender Shape Attitudes Towards Mental Health
How Race, Age and Gender Shape Attitudes Towards Mental Health
 
AI Trends in Creative Operations 2024 by Artwork Flow.pdf
AI Trends in Creative Operations 2024 by Artwork Flow.pdfAI Trends in Creative Operations 2024 by Artwork Flow.pdf
AI Trends in Creative Operations 2024 by Artwork Flow.pdf
 
Skeleton Culture Code
Skeleton Culture CodeSkeleton Culture Code
Skeleton Culture Code
 
PEPSICO Presentation to CAGNY Conference Feb 2024
PEPSICO Presentation to CAGNY Conference Feb 2024PEPSICO Presentation to CAGNY Conference Feb 2024
PEPSICO Presentation to CAGNY Conference Feb 2024
 
Content Methodology: A Best Practices Report (Webinar)
Content Methodology: A Best Practices Report (Webinar)Content Methodology: A Best Practices Report (Webinar)
Content Methodology: A Best Practices Report (Webinar)
 
How to Prepare For a Successful Job Search for 2024
How to Prepare For a Successful Job Search for 2024How to Prepare For a Successful Job Search for 2024
How to Prepare For a Successful Job Search for 2024
 
Social Media Marketing Trends 2024 // The Global Indie Insights
Social Media Marketing Trends 2024 // The Global Indie InsightsSocial Media Marketing Trends 2024 // The Global Indie Insights
Social Media Marketing Trends 2024 // The Global Indie Insights
 
Trends In Paid Search: Navigating The Digital Landscape In 2024
Trends In Paid Search: Navigating The Digital Landscape In 2024Trends In Paid Search: Navigating The Digital Landscape In 2024
Trends In Paid Search: Navigating The Digital Landscape In 2024
 
5 Public speaking tips from TED - Visualized summary
5 Public speaking tips from TED - Visualized summary5 Public speaking tips from TED - Visualized summary
5 Public speaking tips from TED - Visualized summary
 
ChatGPT and the Future of Work - Clark Boyd
ChatGPT and the Future of Work - Clark Boyd ChatGPT and the Future of Work - Clark Boyd
ChatGPT and the Future of Work - Clark Boyd
 
Getting into the tech field. what next
Getting into the tech field. what next Getting into the tech field. what next
Getting into the tech field. what next
 
Google's Just Not That Into You: Understanding Core Updates & Search Intent
Google's Just Not That Into You: Understanding Core Updates & Search IntentGoogle's Just Not That Into You: Understanding Core Updates & Search Intent
Google's Just Not That Into You: Understanding Core Updates & Search Intent
 
How to have difficult conversations
How to have difficult conversations How to have difficult conversations
How to have difficult conversations
 
Introduction to Data Science
Introduction to Data ScienceIntroduction to Data Science
Introduction to Data Science
 
Time Management & Productivity - Best Practices
Time Management & Productivity -  Best PracticesTime Management & Productivity -  Best Practices
Time Management & Productivity - Best Practices
 
The six step guide to practical project management
The six step guide to practical project managementThe six step guide to practical project management
The six step guide to practical project management
 
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
 

Summary Benefits of Above Sheathing Ventilation

  • 1. [ ANOTHER BENEFIT FROM ABOVE-SHEATHING VENTILATION ! ] Moisture Removal Benefits with Above-Sheathing Ventilation on Steep-Slope Metal Roofs William (Bill) Miller, Ph.D., Achilles Karagiozis, André Desjarlais Providing an air space above the sheathing of a roof deck offers moisture-related problems in the roofing systems using ASV. thermal benefits for stone-coated or standing seam metal roofs that yield energy savings in the summer and winter while also The following modes of heat and moisture transport were included: helping to remove unwanted moisture. The natural ventilation • Vapor diffusion through all porous roof construction materials above the sheathing improves the durability of the underlying • Liquid transport through all porous roof construction materials structure of the roof. • Air convection transport for both thermal and moisture Metal roofs are sometimes offset mounted from the roof components deck using a double-batten (counter-batten) construction. The • Moisture storage in all roof construction materials design provides an air space between the exterior face of the roof • Radiative transport with nighttime sky conditions sheathing and the underside of the roof cover so that a clear, • Radiative transport within the air gap provided by the stone albeit complex, air pathway exists beneath the roof cover. Solar coated metal roof irradiance absorbed at the roof’s surface is conducted through • Condensation and evaporation processes and freeze and the metal roof and heats the air space. The warmer and therefore thawing processes with the associated latent heat exchanges more buoyant air moves up the inclined air passage. The ventilation scheme helps remove unwanted heat but it also removes unwanted moisture from the roof deck, thereby improving the roof’s thermal [ ASV accelerated the removal of performance as well as its durability. The thermally induced airflow unwanted moisture and reduced the occurring in this air space is termed above-sheathing ventilation (ASV). moisture content of the OSB sheathing well below that of the OSB in a closed cavity ] In the simulation analysis, the exterior and interior environmental loads were assumed for the climatic conditions of Knoxville, Tennessee. The proposed ASHRAE SPC 160P, “Design Criteria for Moisture Control,” was employed for both the exterior and interior hygrothermal loading conditions. All simulations were initiated using double the equilibrium moisture content (EMC) at 80% relative humidity. Both the ventilated and non ventilated cases were simulated for a period of 2 years. A snapshot of the moisture content in the sheathing board {oriented strand board (OSB)} is given in Figure 2. The simulation Figure 1: Batten and counter-batten system used to mount stone-coated metal roofs. period started October 1, 2005, one of the more difficult periods of Field studies were conducted on several attic assemblies having stone-coated metal shake roofs with and without cool color (infrared reflective) pigments and with and without above- sheathing ventilation. Stone-coated metal roofs are often offset mounted from the roof deck using a batten and counter-batten system. Here counter-battens made of nominal dimension wood strips (1 by 4’s) are nailed to the roof deck from soffit to ridge, and battens (2 × 2’s) are placed above the counter-battens and nailed to the deck (Fig 1). MOISTURE REMOVAL BENEFIT A moisture engineering analysis was performed on the roof system depicted in Figure 1 using the MOISTURE-EXPERT model (Karagiozis 2001) that has shown good agreement in ventilated Figure 2. Comparison of moisture content of OSB layer as a function of ventilation strategy wall systems. The intent was to show the potential for reducing (ventilated vs. non vented) for a 2-year period. Metal Construction News | Metal Architecture C O O L M E TA L R O O F I N G F O R U M | 19
  • 2. The demand for electricity in the United States is increasing three times faster than the speed at which power plants and lines are being built. The U.S. energy demand is predicted to increase almost 20 percent in the next decade. (North American Electric Reliability Council) the year for the sheathing to dry out. ASV accelerated the removal of the air space yields about 80 air changes per hour. Therefore, of unwanted moisture and reduced the moisture content of the the measured data is well within reason of the results from the OSB sheathing well below that of the OSB in a closed cavity (Figure hygrothermal simulations. 2). Ventilating the roof deck dried the OSB within 200 days to safe moisture limits in which fungal growth would not typically occur. In Summary comparison, the closed roof deck required an additional 100 days to Moisture is a prevalent issue in all aspects of building design. reach safe moisture content. Metal roofs employing above-sheathing ventilation show superior The number of air exchanges occurring within the ventilated hygrothermal performance when compared with a non vented cavity (Figure 3) tells the story. The occurrence of air exchange roof system. Above-sheathing ventilation therefore adds yet rates are displayed for the assumed air changes per hour (ACH), another feature to this ecologically sound building material. which are dependent on both temperature and wind pressure Providing the ventilation above the sheathing improves the durability of the underlying structure of the roof. As a result, the expected performance of metal roofing in high winds and hail storms is further enhanced because of the improved hygrothermal performance afforded by a metal roof system using above- sheathing ventilation. Future articles will address: (1) does above sheathing ventilation reduce attic air temperatures and in turn reduce heat losses from ducts installed in attics; (2) does increased spacing of the air gap improve thermal performance; (3) what are the seasonal benefits of above sheathing ventilation; and, (4) does above sheathing ventilation help retard ice damming. Figure 3. Period of time during 2-year simulation for cavity air changes per hour (wind- and temperature-dependent). ACKNOWLEDGMENTS Funding for this project was provided by the U.S. Department flows acting along the roof ventilation cavity. Roughly 20-100 of Energy under the supervision of Marc LaFrance of the Building ACH are prevalent about 80% of the time during the 2-year Technologies Program. The ORNL project team members are simulation runs. The 60 ACH was the maximum incident air André Desjarlais, William Miller, Tom Petrie, Jan Kosny and Achilles exchange rate observed occurring about 25% of the time. Karagiozis, all of ORNL’s Buildings Envelope Program. The Metal Therefore, the potential moisture removal benefits afforded by Construction Association and its affiliate members provided the ASV are evident from the vented compared to the non vented stone-coated shake and S-mission roofs used in testing. Metro Roof simulations. Products constructed the attic assemblies and provided valuable As a check, Miller (2006) made field measurements of the assistance in installing the roofs on the steep-slope assemblies. The airflow underneath stone-coated metal shake roofs (Figure financial support of the Metal Construction Association, the Cool 1) by monitoring the decay rate of the tracer gas CO2 with Metal Roofing Coalition and the guidance of Metro Roof Products time and deducing the flow rate from a continuity balance for are greatly appreciated. cmrf the concentration of CO2. The CO2 gas was injected into the vent gap of the soffit to saturate the cavity. After a substantial Karagiozis, A. N. 2001. “Advanced Hygrothermal Modeling of Building Materials buildup of concentration registered on a monitor, the gas Using MOISTURE-EXPERT 1.0.” Pp. 39–¬47 in Proceedings of the International injection was stopped, and the concentration was recorded Particleboard/ Composite Materials Symposium. at timed intervals. All measurements were made around solar Miller, W. A. 2006. The Effects of Infrared-Blocking Pigments and Deck Venting on noon, when the roofs were at their highest temperatures. Stone-Coated Metal Residential Roofs. ORNL/TM-2006/9. Oak Ridge, TN: Oak Ridge Computed airflows were about 18 cfm, which for the volume National Laboratory 20 | C O O L M E TA L R O O F I N G F O R U M Metal Construction News | Metal Architecture