Transcript 2003 AIA Seminar Steel Stud Walls

Moisture Management
in Steel Stud Walls
for
Severe Cold,
Cold and Mixed
Climates
Presented by Dow Building Materials
A Business unit of Dow Chemical Company and it’s subsidiaries
Learning Objectives
Participants will have the opportunity to consider the
following….
•Understand how moisture accumulates in steel
stud/frame walls cavities.
•Learn about the design challenges associated with
steel stud/frame walls.
•Evaluate and specify methods that reduce moisture
build-up in steel frame walls based specific
geographical locations.
Introduction
Steel Stud assemblies are gaining increased popularity in commercial
construction as part of an economical and versatile wall system.
Steel studs offer many advantages such as….
•low cost
•non-combustibility
•small footprint
•light weight
However………
When specifying steel stud wall systems, climate must be considered in
order to control moisture and energy flow …..
Introduction
Designers have accepted
that commercial buildings
must be responsive to
varying seismic risks, wind
loads and snow loads, yet
typically overlook variances
in temperature, rainfall,
exterior and interior
humidity and their
interaction.
The hygro-thermal regions
shown to the right are
determined by heating
degree days experienced
throughout a normal
calendar year.
SEVERE
COLD
COLD
HOT DRY
MIXED
HUMID
HOT HUMID
Introduction
¤ Severe Cold Climates
equal 8000 heating degree
days and higher.
¤ Cold climate equals 4500
- 8000 heating degree days.
SEVERE
COLD
¤ Mixed climate equals
4500 heating degree days
and less.
¤ All three climates
experience between 20 and
60 inches of precipitation
per year.
COLD
HOT DRY
MIXED
HUMID
HOT HUMID
Introduction
Annual Precipitation is
also an important
consideration and will
effect how humid a
climate is…
Introduction
Building envelopes and
mechanical systems
(HVAC) should be
designed to work
together based on each
specific hygro-thermal
climate.
There are five climates
that we have considered
for our system
recommendations…..
SEVERE
COLD
COLD
HOT DRY
MIXED
HUMID
HOT HUMID
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Course Agenda
¤
Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ What is Vapor Diffusion?
¤ Vapor Diffusion is the process by which water
vapor migrates through a wall system and it’s
components such as gypsum, concrete, insulation
and paint.
¤ Each component of the wall system has a perm
rating.
¤ The International Building Code says that a
material with a perm rating of 1.0 or less is a vapor
retarder.
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ Examples of the average perm ratings for different wall
components……
¤Gypsum
¤Plastic Sheet Barriers (2 - 10 mil)
¤Building Wrap (brands differ)
¤Peel & Stick Membranes
¤Paint (latex flat - 2 coats)
¤@ =< 35% Relative Humidity
¤@ > 35% Relative Humidity
¤Rigid Insulation
¤Brick Masonry 4” thick
¤Mortar (@ 1,2,4,mix)
12 - 50
.16 - .03
6 - 28
.02
perms
perms
perms
perms
1-5
5 - 8.6
perms
perms
5.8 - .03 perms
.8
perms
3.2
perms/inch
(.8 perms in
4”)
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ Diffusion from the
Warm Inside
interior side of the wall
is common.
¤ In winter conditions,
the vapor drive is
typically from inside to
outside.
¤ Moisture will always
diffuse into the wall
cavity, especially if the
perm rating(s) of the
material(s) on the
interior side of the steel
studs is greater than
1.0.
Vapor
Cold Outside
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ Real Life Example:
400 occupants in a
typical commercial
building can emit up
to 200 lbs..... of water
vapor into the air per
day.
¤ This water vapor
can diffuse into the
wall cavity.
Warm Inside
Vapor
Cold Outside
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ Diffusion from
the exterior side of
the wall is also
common.
¤ In spring,
summer and fall,
wind driven rain
can be absorbed by
brick veneer and
moisture is stored
by the masonry
material.
Wind Driven
Rain
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤After the storm as the
sun heats up the brick,
the moisture turns to
vapor in the brick
cavity and is driven
into the wall system.
¤ Moisture will always
diffuse into the wall
cavity, especially if the
perm rating(s) of the
material(s) on the
exterior side of the
steel studs is greater
than 1.0.
Vapor
Drive
Moisture in Steel Stud Assemblies
How Vapor Diffusion Works…..
¤ The air pressure difference
between the inside and the
outside of a building can effect
how much moisture gets driven
into the wall cavity.
Warm
¤ If the air pressure is greater
on the warm side of the wall,
vapor will be driven into the wall
cavity.
Vapor
¤ It is best from a moisture
perspective to keep the warm
side of the wall at a slightly
lower pressure than the cold side
since cold air generally holds
less moisture than warm air.
Cold
Wall Assembly Recommendations
Air Barrier Tape….
¤ Tests show one system, 4” steel studs with 2” extruded polystyrene
insulated sheathing and joint tape, had excellent air retarding
properties; 0.0175 ft3/min./ft2 @ 0.02 in.hg.
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Moisture in Steel Stud Assemblies
The Effects of Air Infiltration/Exfiltration
¤ Exfiltration refers to the physical movement of indoor air (warm and
humid) from the interior towards the exterior as a result of an air
pressure difference. The pressure difference can be caused by wind
effects on the building envelope, stack effects, etc……
¤ While moisture diffusion occurs on a molecular level, moisture
movement by exfiltration occurs when the indoor air physically moves
through commonly occurring penetrations, unsealed openings and joints
in the interior vapor/air barrier, electrical outlets, etc….
¤ The potential for vapor movement by exfiltration is many times
higher than diffusion due to the slowness of the diffusion process.
Moisture in Steel Stud Assemblies
The Effects of Air Infiltration/Exfiltration
¤ Infiltration refers to the physical movement of outdoor air from the
exterior towards the interior as a result of an air pressure difference.
¤ While moisture diffusion occurs on a molecular level, moisture
movement by infiltration occurs when the outdoor air physically moves
through commonly occurring penetrations, unsealed sheathing joints,
joints in the exterior vapor/air barrier, window openings, flashings,
etc….
¤ Most Building Codes require a maximum of air infiltration rate of 0.06
ft3/minft2 for commercial buildings.
¤ The potential for vapor movement by infiltration is many times
higher than diffusion due to the slowness of the diffusion process.
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
R-19
Fiberglass Batt
¤ When the actual temperature
Sheathing
Interior
Gypsum
¤ The interior side of the exterior
wall sheathing is the most important
surface to consider.
¤ The R-19 fiberglass does a good
job of keeping the interior of the
building warm where there are no
studs, however the interior side of
the exterior sheathing is on the cold
side of the wall.
70
60
Temperature ºF
drops below the dewpoint
temperature inside the wall cavity,
condensation can occur if water
vapor is present.
Ext. Gypsum
Brick
50
40
30
20
10
0
24°F
Indoor Temperature = 70ºF
Outdoor Temperature = 14ºF
Cavity Insulation = R-19
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Dewpoint Chart: With a 70 degree indoor temperature and a 35%
relative humidity, our dewpoint comes in at 40 degrees.
% Relative Humidity
Temperature °F
100 95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
110
110 108 106 104 102 100 98
95
93
90
87
84
80
76
72
65
60
51
41
105
105 103 101 99
97
95
93
91
88
85
83
80
76
72
67
62
55
47
37
100
100 99
97
95
93
91
89
86
84
81
78
75
71
67
63
58
52
44
32
95
95
93
92
90
88
86
84
81
79
76
73
70
67
63
59
54
48
40
32
90
90
88
87
85
83
81
79
76
74
71
68
65
62
59
54
49
43
36
32
85
85
83
81
80
78
76
74
72
69
67
64
61
58
54
50
45
38
32
80
80
78
77
75
73
71
69
67
65
62
59
56
53
50
45
40
35
32
75
75
73
72
70
68
66
64
62
60
58
55
52
49
45
41
36
32
70
70
68
67
65
63
61
59
57
55
53
50
47
44
40
36
32
65
65
63
62
60
59
57
55
53
50
48
45
42
40
36
32
60
60
58
57
55
53
52
50
48
45
43
41
38
35
32
55
55
53
52
50
49
47
45
43
40
38
36
33
32
50
50
48
46
45
44
42
40
38
36
34
32
45
45
43
42
40
39
37
35
33
32
40
40
39
37
35
34
32
35
35
34
32
32
32
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ In our example, the temperature on
the inside of the exterior sheathing is
well below the dewpoint temperature.
chance moisture vapor exists in the
wall cavity through diffusion and air
infiltration/exfiltration.
¤ If water vapor exists in this wall
cavity condensation will occur on the
inside of the exterior sheathing and
the steel studs.
¤ There is a good chance of the wall
components becoming damaged ,
corroded or rotten if they get wet for a
prolonged period of time.
R-19
Fiberglass
Batt
Interior
Gypsum
70
Temperature ºF
¤ We have noted that there is a good
Ext. Gypsum
Sheathing
Brick
60
50
40
30
20
10
0
24°F
Indoor Temperature = 70ºF
Outdoor Temperature = 14ºF
Cavity Insulation = R-19
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Insulated sheathing reduces the potential for condensation in a steel
stud frame walls. Condensation will not occur when the temperature of
the wall components are above dew point.
Indoor Temperature = 70ºF
Outdoor Temperature = 14ºF
Cavity Insulation = R-19
Temperature ºF
70
60
50
40
30
20
10
0
37°F
24°F
¤ Wall with R-5.0
¤ Wall with gypsum
insulated sheathing
sheathing
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ ASHREA formula
based Steel Stud
Condensation
Potential Tool.
¤ Program uses
weather data from the
designated ASHREA
city of your choice to
determine moisture
potential.
¤ Graphically
compare how two
different wall systems
will perform in the
same geography over
a 12 month period.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Steel Stud Condensation Potential Graphic Tool - Screens.
¤ First Screen: Specify two walls to evaluate.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Steel Stud Condensation Potential Graphic Tool - Screens.
¤ Second Screen: Graphically shows you when during the year
condensation is likely to occur in each wall system.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Lets Look @ a typical Steel Stud Wall...
¤ 6” 20Ga S.S. @ 24”o.c., R19 Batt + 1/2” Gyp.
Component
Int air film
Gypsum
Vapor retarder
R19 batt
Gypsum
Air space
Brick
Ext.air film
Wall Effective R-Value:
R-value
0.68
0.45
0.01
8.60
0.45
0.60
0.80
0.17
11.76
INTERIOR AIR FILM
GYPSUM BOARD
STEEL STUD
R-19 BATT INSULATION
EXTERIOR GYPSUM
AIR SPACE
MASONRY
EXTERIOR AIR FILM
The ASHRAE “PARALLEL PATH METHOD” was used to calculate the effective thermal performance for the
metal frame wall construction. The correction factors from ASHRAE Standard 90.1, 1989 User’s Manual were used
to assign reduced thermal resistance values for the insulated stud cavity.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ If we compare two walls that are identical except on the first we use
exterior gypsum sheathing and on the second we use 1” R-5.0 insulated
sheathing instead of the gypsum.
INTERIOR AIR FILM
¤ 6” 20Ga S.S. @ 24”o.c.,
GYPSUM BOARD
R19 Batt + 1/2” Gypsum
Effective R=11.76
STEEL STUD
R-19 BATT INSULATION
EXTERIOR GYPSUM
¤ 6” 20Ga S.S. @ 24”o.c.,
R19 Batt + 1” R5 EXPS
Effective R=16.31
AIR SPACE
MASONRY
EXTERIOR AIR FILM
¤ Location: Washington, DC
¤ Indoor Temperature: 70°F
Relative Humidity: 40%
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Lines represent the temperature of Cavity / Exterior sheathing
interface.
¤Even with R-5.0 insulated sheathing there is a chance for condensation
from late December to early February in this scenario.
R-5 Insulated Sheathing
Exterior Gypsum
DEWPOINT
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Gypsum Sheathing Model: The area below the dew point temperature
line represents condensation potential: Mid-Dec. to Early Mar.
R-5 Insulated Sheathing
DEWPOINT
Exterior Gypsum
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Lets look @ our same steel stud wall example but this time located in
Chicago...
INTERIOR AIR FILM
¤ 6” 20Ga S.S. @ 24”o.c.,
GYPSUM BOARD
R19 Batt + 1/2” Gypsum
Effective R=11.76
STEEL STUD
R-19 BATT INSULATION
EXTERIOR GYPSUM
¤ 6” 20Ga S.S. @ 24”o.c.,
R19 Batt + 1” R5 EXPS
Effective R=16.31
AIR SPACE
MASONRY
EXTERIOR AIR FILM
¤ Location: Chicago, IL
¤ Indoor Temperature: 70°F
Relative Humidity: 40%
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ line represent the temperature of Cavity / Exterior sheathing interface.
¤ Even with R-5.0 insulated sheathing there is a good chance for
condensation from late November to Mid-March in this scenario.
R-5 Insulated Sheathing
DEWPOINT
Exterior Gypsum
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
In some climates, while the insulated sheathing
reduces the potential for condensation it will not
prevent it.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Organizations such as state energy codes (MA) & Canada Mortgage
& Housing Corp (CMHC) already recognize the benefits of rigid
insulating sheathing on steel studs. CMHC even suggests removing the
batts all together when using closed cell moisture resistant insulated
sheathings.
¤ In some climates the only way to totally prevent condensation is to
remove the fiberglass insulation and increase the thickness of the
insulated sheathing.
Warm
Interior
Cold
Exterior
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
¤ Lets look @ our same steel stud wall example in Chicago...
¤ 6” 20Ga S.S. @ 24”o.c.,
R19 Batt + 1/2” Gypsum
Effective R=11.76
INTERIOR AIR FILM
GYPSUM BOARD
STEEL STUD
R-19 BATT INSULATION
¤ 4” 20Ga S.S. @ 16”o.c.,
Empty Cavity + 2” R-10.0 rigid
insulation Effective R=16.31
EXTERIOR GYPSUM
AIR SPACE
MASONRY
¤ Location: Chicago, IL
EXTERIOR AIR FILM
¤ Indoor Temperature: 70°F
Relative Humidity: 40%
This time we remove the batts ( & use 4” studs) and
increase the insulated sheathing to 2”R-10.0.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
R10 Insulated Sheathing
No Batts
Exterior Gypsum
R19 Batts
DEWPOINT
NO condensation ! And an added benefit when using 4”
studs instead of 6” is increased floor area inside the
building on each floor.
Moisture in Steel Stud Assemblies
Condensation/Dewpoint…..
Steel Stud Condensation Potential Tool
¤ Calculates the systems effective R-value
¤ Graphs when the potential for moisture accumulation is at it’s
¤
greatest.
¤ Analysis base on ASHRAE 90.1 formula.
¤ Heating degree days for each location are factored into
results.
We will be glad to run an analysis on your steel stud wall system.
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
¤ Case Study - Canadian
Moisture in Steel Stud Assemblies
Wetting/Drying…..
¤ All wall systems will experience moisture during their useful life.
¤ Diffusion, infiltration, exfiltration, leaks from window flashings
¤
¤
and roofs, defects in labor are all ways moisture gets into a wall
cavity.
Prolonged periods of severe weather conditions, hot or cold, can
produce large amounts of moisture build up.
One of the keys to designing an effective wall system is allowing
it to dry out when it gets wet.
Moisture in Steel Stud Assemblies
Wetting/Drying…..
¤ Drying can take a long time
¤
¤
to occur.
Our recommendation is to
seal the exterior side of the
wall as tight as possible
while leaving to inside
permeable enough to dry.
How can we accomplish
this?
Warm Inside
Cold
Outside
Moisture in Steel Stud Assemblies
Wetting/Drying…..
¤ The first thing we can do is
¤
¤
¤
take the fiberglass insulation
out of the wall cavity.
The fiberglass thermal
insulation decreases the flow
of energy through the wall
decreasing the rate of drying.
Without fiberglass insulation,
the wall cavity and all of it’s
components stay warmer and
are able to dry.
The wall cavity becomes
conditioned space which
leads to less moisture buildup and better drying
capabilities.
Warm Inside
Cold
Outside
Moisture in Steel Stud Assemblies
Wetting/Drying…..
¤ The second thing we can
¤
¤
do is apply an air/vapor
barrier on the exterior
side of the steel stud.
An air/vapor barrier will
prevent most of the
exterior moisture from
entering the wall cavity.
If some moisture does get
in,the wall cavity is
conditioned space which
leads to better drying
capabilities.
Air/Vapor Barrier
Warm Inside
Cold
Outside
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Mold…..
¤ Mold is not necessarily harmful.
¤ However, concerns have been raised regarding the health effects
¤
¤
of certain strains of mold on susceptible people.
Individuals with existing medical problems or suppressed immune
systems may risk infection. 1234
For more information on this subject please refer to the articles
below.
1. “Mold Resources” U.S. Environmental Protection Agency
www.epa.gov/iaq.pubs/moldresource.html, Site dated April 4, 2001
2. “Indoor Air Quality Info Sheet” California Department of Health Services
www.cal-iaq.org/mold9803.html.
3. “Mold Control in the house” Johns Hopkins University Asthma and Allergy
www.hopkins-allergy.org/rhinitis/therapeutics-house.html.
4. “Questions and Answers on Stachybotrys chartarum and other molds” Center
for Desease Control and Prevention (CDC)
www.cdc.gov/nceh/asthma/factsheets/molds/default.html dated July 6, 2001
Mold…..
¤ In general, molds demand a favorable combination of the
¤
¤
following conditions to germinate, sporulate, and grow:
1 Fungal spores settling on the surface
2 Oxygen availability
3 Optimal Temperatures
4 Nutrient availability
5 Moisture (liquid or relative humidity above 70%)
The first four conditions are met in almost every building.
The key remaining factor is moisture, which may be controlled
by adhering to the sound construction practices discussed in this
presentation.
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Moisture in Steel Stud Assemblies
Building Code Requirements
¤ The new IBC, ICBO, SBCCI and BOCA all refer to
ASHREA’s 90.1 code for energy compliance.
¤The International Building Code requires that we reference
ASHREA 90.1 - 1999 version
¤The 1999 version requires that walls in most Cold and
Mixed climates areas of the United States meet a
minimum R-Value requirement of 8.1.
¤The 1999 version requires that walls in most Severe
Cold climates areas of the United States meet a minimum
R-Value requirement of 15.6.
Moisture in Steel Stud Assemblies
Steel Stud Walls…...
¤ Steel studs transfer heat approximately 400 times faster than wood
studs.
¤ The use of steel studs creates a thermal bridge every 16” or 24” on
center in a commercial wall system.
Moisture in Steel Stud Assemblies
Steel Stud Walls…...
¤ Steel conducts heat much more efficiently than the other materials used
in a metal stud wall assembly - thus it has poor thermal resistance
properties.
R 5.0 Rigid Board
Thermal Performance
Steel Stud Walls…...
¤ The high heat conductivity of steel has a dramatic effect on the
Exterior wall temp. in C
¤
system R-Value of the wall system.
Temperature distribution on exterior wall in vicinity of metal wall
stud.
7
Direction along exterior wall
6
5
4
3
2
1
0
3"
2"
1"
0
1"
2"
3"
Cavity filled with
glass batt
1/2” gypsum bd.
3 1/2” steel stud
Thermal Performance
Steel Stud Walls…...
¤ The ASHRAE 90.1
correction factors consider
the heat loss through the
highly conductive steel
studs.
Thermal Performance
Steel Stud Walls…...
¤
Effective R-value = R-value x Correction Factor
¤ 2Recent analysis of tested assemblies indicates an R-value of 7.1 for
R-19 insulation in normal 6” framing at 16” on center, though the
correction factor published in Standard 90.1 currently offers higher
Thermal Performance
Steel Stud Walls…...
¤ Lets Look @ a Steel Stud Wall….
¤ 4” 20Ga S.S. @ 16”o.c., R11 Batt + 1/2” Gyp.
Component
Int air film
Gypsum
Vapor retarder
R11 batt
Gypsum
Air space
Brick
Ext.air film
Effective:
R-value
0.68
0.45
0.01
5.50
0.45
0.60
0.80
0.17
8.66
INTERIOR AIR FILM
GYPSUM BOARD
STEEL STUD
R-11 BATT INSULATION
EXTERIOR GYPSUM
AIR SPACE
MASONRY
EXTERIOR AIR FILM
The ASHRAE “PARALLEL PATH METHOD” was used to calculate the effective thermal performance
for the metal frame wall construction. The correction factors from ASHRAE Standard 90.1, 1989 User’s
Manual were used to assign reduced thermal resistance values for the insulated stud cavity.
Thermal Performance
Steel Stud Walls…...
¤ Lets Look @ a Steel Stud Wall….
¤ 6” 20Ga S.S. @ 24”o.c., R19 Batt + 1/2” Gyp.
Component
Int air film
Gypsum
Vapor retarder
R19 batt
Gypsum
Air space
Brick
Ext.air film
Effective:
R-value
0.68
0.45
0.01
8.60
0.45
0.60
0.80
0.17
11.76
INTERIOR AIR FILM
GYPSUM BOARD
STEEL STUD
R-19 BATT INSULATION
EXTERIOR GYPSUM
AIR SPACE
MASONRY
EXTERIOR AIR FILM
The ASHRAE “PARALLEL PATH METHOD” was used to calculate the effective thermal performance
for the metal frame wall construction. The correction factors from ASHRAE Standard 90.1, 1989 User’s
Manual were used to assign reduced thermal resistance values for the insulated stud cavity.
Thermal Performance
Steel Stud Walls…...
¤ Lets take the same 6” wall and replace the exterior gypsum sheathing
with 1” R-5.0 rigid insulation.
Component
Int air film
Gypsum
Vapor retarder
R-19 batt
1” R-5.0
Air space
Brick
Ext.air film
Effective:
R-value
0.68
0.45
0.01
8.60
5.00
0.60
0.80
0.17
16.31
A 38 %
Improvement!
INTERIOR AIR FILM
1/2” GYPSUM BOARD
STEEL STUD
R-19 BATT INSULATION
1” EXPS
AIR SPACE
MASONRY
EXTERIOR AIR FILM
The ASHRAE “PARALLEL PATH METHOD” was used to calculate the effective thermal performance
for the metal frame wall construction. The correction factors from ASHRAE Standard 90.1, 1989 User’s
Manual were used to assign reduced thermal resistance values for the insulated stud cavity.
Thermal Performance
Steel Stud Walls…...
¤ The effective R-Value of steel stud wall systems can be
¤
¤
economically increased by placing rigid insulation outside of the
framing cavity.
Insulation placed on the steel framing reduces heat flow through
the studs.
The insulation breaks the Thermal Short caused by the steel
framing.
Thermal Performance
Steel Stud Walls…...
¤ The International Building
¤
Code requires that a rigid
board insulation used in this
application have a maximum
flame spread of 25 as tested by
ASTM Method E-84.
Make sure the flame spread of
the insulation you specify is 25
or less.
Moisture in Steel Stud Assemblies
Steel Stud Walls…...
¤ The conductivity of steel studs can
¤
¤
¤
¤
lead to “ghosting” on interior
wall surfaces:
Thermal shorts cause interior
surfaces on the gypsum board to
be cooler at stud locations.
Air moves slower over the cooler
areas.
Over time slower moving air
deposits dust at these locations.
This leads to poor aesthetics &
increased cleaning / maintenance.
Technical Series 00-138: PERFORMANCE
OF A BRICK VENEER/STEEL STUD WALL SYSTEM.
Moisture in Steel Stud Assemblies
Steel Stud Walls…...
¤ The same “ghosting”can happen to the exterior of a building!
Course Agenda
¤ Moisture is Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Wall Assembly Recommendations
Structural Performance…...
¤ Racking Strength: The racking strength requirement for commercial
¤
¤
buildings varies based on location and the height of the building.
¤ This requirement is usually minimal because the racking strength
of the structure is satisfied by the structural steel not the light
gauge steel.
Deflection: Deflection is the more critical requirement of a light
gauge steel stud wall panel. This requirement is usually satisfied by
the steel stud manufacturer or structural engineer, who designs the
light gauge steel wall system to meet the building code through cross
bracing and other materials. However, the exterior sheathing must
hold up to the negative wind loads experienced over the life of the
building. Most rigid insulated sheathings will meet this requirement as
long as they are installed with screws and washers.
Since steel studs have a tendency to rotate or twist when a load is
applied, horizontal bracing (also called lateral bridging) is used to
prevent this. The bracing may be accomplished by either of three
methods:
Wall Assembly Recommendations
Structural Performance…...
¤
Rigid sheathing
(gypsum) applied to
both sides (interior and
exterior) of the stud,
Wall Assembly Recommendations
Structural Performance…...
¤
Flat metal straps
welded to both sides of
the stud.
Wall Assembly Recommendations
Structural Performance…...
¤
Or a C channel welded
to the cut outs in the
center of the stud.
Typically
36” or 48”
AMERICAN IRON AND STEEL INSTITUTE COLD-FORMED STEEL DESIGN MANUAL
Wall Assembly Recommendations
Fastening Recommendations…...
¤ Insulated sheathing is fastened to the steel studs using self tapping
corrosion resistant screws. A large washer (at least 1 1/4” in
diameter) can also be used so that the foam cannot be easily pulled off
of the fastener on windy days.
Installation of 48” x 96” product
Installation of 16” x 96” product
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Wall Assembly Recommendations
Fire Performance…...
¤ The International Building Code requires UL
¤
¤
rated wall assemblies only when building are 30
feet or closer to each other.
In an informal study conducted by our company
over the calendar year 2002, architects agreed
that there is rarely a need for a 1, 2 or 3 hour
rated assembly for the exterior walls of a
commercial building.
When there is a need for a rated wall assembly,
please refer to the UL design manual.
Course Agenda
¤ Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤ How Vapor Diffusion Works...
¤ The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
¤ Mold
¤ Thermal Performance - Building code requirements
¤ Wall Assembly Recommendations
¤ Structural Requirements
¤ Fire Requirements
¤ Sound Transmission
¤ Economics
Wall Assembly Recommendations
Sound Transmission and Economics
¤ In order to give a comparison of sound transmission and economics,
we need to set up a baseline wall system commonly used in
commercial buildings today...
R-19 Fiberglass Batt
6” Steel Stud 16”OC
Exterior Gyp Sheathing
Weather Barrier Film
Wall Assembly Recommendations
Sound Transmission and Economics
¤ Sound Transmission Coefficient (STC)of this wall system is 63.
¤ Below are the cost ranges for both materials and labor for each
¤
¤
¤
¤
¤
component installed...
6”, R-19 unfaced fiberglass
.38 - .45 per square foot
Exterior Gyp Sheathing
.55 - .72 per square foot
Weather Barrier Film
.60 - .88 per square foot
Baseline insulation/sheathing system 1.53 - 2.05 per square foot
For this exercise, we’ll use the cost numbers at the high end of the
range @ 2.05 per square foot...
R-19 Fiberglass Batt
6” Steel Stud
Exterior Gyp Sheathing
Weather Barrier Film
Wall Assembly Recommendations
Sound Transmission and Economics
¤
¤
¤
¤
Against our baseline wall we
will compare 3
recommendations in each of the
severe cold, cold and mixed
climates.
Design A: Takes fiberglass out
of the wall and insulates with
rigid sheathing on the exterior
side over gypsum sheathing.
Design B: Takes the fiberglass
and exterior gyp sheathing out
of the wall and insulates with
rigid sheathing on the exterior
side directly over steel studs.
Design C: Takes the gyp
sheathing out of the wall and
insulates with both fiberglass
and rigid insulation on the
exterior side.
SEVERE
COLD
COLD
HOT DRY
MIXED
HUMID
HOT HUMID
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design A”…
¤ Empty steel stud cavity
¤ Exterior grade Gypsum board
¤ Rigid Insulated Sheathing
¤ Air Barrier Membrane
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design A”…
¤
¤
¤
¤
Component
Baseline Severe Cold
Cold
Mixed
Exterior Grade Gypsum
Rigid Insulated Sheathing
.72
.00
.72
.725
.72
.725
.72
.475
(R 12.5)
(R12.5)
(R 7.5)
¤ R-Value
¤
¤
Air Barrier Membrane
Fiberglass Insulation
.88
.45
.88
.00
.88
.00
.88
.00
¤
Total Cost/ sq.. ft.
$2.05
$2.325
$2.325
$2.075
¤
¤
¤
Cost Difference
STC
Effective R-Value10.02
$ .00
63
15.82
$ .275
60
$ .275
60
10.82
$ .025
60
15.82
Wall Assembly Recommendations
Condensation Potential….
¤
“Design A”… Severe Cold Climate Minneapolis, MN
Wall Assembly Recommendations
Condensation Potential….
¤
“Design A”… Cold Climate Philadelphia, PA
Wall Assembly Recommendations
Condensation Potential….
¤
“Design A”… Mixed Climate St. Louis, MO
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design B”…
¤ Empty steel stud cavity
¤ No Exterior grade Gypsum
¤ Rigid Insulated Sheathing
¤ Joints of Insulated Sheathing
are sealed with peel & stick
modified asphalt and
polyethylene air and vapor
barrier tape
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design B”…
¤
¤
¤
¤
Component
Baseline Severe Cold
Cold
Mixed
Exterior Grade Gypsum
Rigid Insulated Sheathing
.72
.00
.00
.725
.00
.725
.00
.475
(R 12.5)
(R 12.5)
(R 7.5)
¤ R-Value
¤
¤
Air Barrier Membrane
Fiberglass Insulation
.88
.45
.30
.00
.30
.00
.30
.00
¤
Total Cost/ sq.. ft.
$2.05
$1.025
$1.025
$ .775
¤
¤
¤
Cost Difference
STC
Effective R-Value10.02
$ .00
63
16.15
($ 1.025)
52
($ 1.025) ($ 1.275)
51
50
11.15
16.15
Wall Assembly Recommendations
Condensation Potential….
¤
“Design B”… Severe Cold Climate Minneapolis, MN
Wall Assembly Recommendations
Condensation Potential….
¤
“Design B”… Cold Climate Philadelphia, PA
Wall Assembly Recommendations
Condensation Potential….
¤
“Design B”… Mixed Climate St. Louis, MO
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design C”…
¤ R 19 fiberglass Insulation in
Steel Stud Cavity
¤ No Exterior Grade Gypsum
¤ Rigid Insulated Sheathing
¤ Joints of Insulated Sheathing
are sealed with peel & stick
modified asphalt and
polyethylene air and vapor
barrier tape
Wall Assembly Recommendations
Sound Transmission and Economics
¤ “Design C”…
¤
¤
¤
Component
Exterior Grade Gypsum
Rigid Insulated Sheathing
Baseline Severe Cold
.72
NA*
.00
NA*
¤ R-Value
Cold
.00
.60
Mixed
.00
.35
(R 10.0)
(R 5.0)
¤
¤
Air Barrier Membrane
Fiberglass Insulation
.88
.45
NA*
NA*
.30
.45
.30
.45
¤
Total Cost/ sq.. ft.
$2.05
NA*
$1.35
$1.10
¤
¤
¤
Cost Difference
STC
Effective R-Value10.02
$ .00
63
NA*
NA*
NA*
($ .70)
54
16.35
($ .95)
53
*
Using fiberglass insulation in severe cold climates does not allow the interior
side of the exterior sheathing to get above dewpoint temperature during the
colder months. Therefore the likelihood of moisture condensing is good and
we are not comfortable with recommending the“Design C” concept.
21.35
Wall Assembly Recommendations
Condensation Potential….
¤
“Design C”… Cold Climate Philadelphia, PA
Wall Assembly Recommendations
Condensation Potential….
¤
“Design C”… Mixed Climate St. Louis, MO
Moisture Management
with
Insulated Sheathing
in
Steel Stud Walls
Course Summary
Moisture Management in Steel Stud Walls
Summary….
¤
Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤
¤
How Vapor Diffusion Works...
The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
We’ve discussed how through diffusion,
infiltration/exfiltration, flashings and roof leaks water
vapor will always work it’s way into a wall cavity….
Moisture Management in Steel Stud Walls
Summary….
¤
Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤
How Vapor Diffusion Works...
¤
The effects of Air Infiltration/Exfiltration
¤ Condensation/Dewpoint
¤ Wetting/Drying
Once vapor is in the wall cavity it will condense into
water if the temperature drops below dewpoint…..
The component most likely to be cold is the interior side
of the exterior sheathing…..
Moisture Management in Steel Stud Walls
Summary….
¤
Moisture in Steel Stud Assemblies - The science behind wall assemblies...
¤
How Vapor Diffusion Works...
¤
¤
The effects of Air Infiltration/Exfiltration
Condensation/Dewpoint
¤ Wetting/Drying
It is inevitable that the cavity will experience moisture.
The key is to design the wall system so it will dry
effectively….
The best way to insure drying success is to eliminate the
fiberglass batts to create a conditioned cavity space and
specify a good weather barrier film on the exterior side
of the wall…..
Moisture Management in Steel Stud Walls
Summary….
¤
Mold
In general, molds demand a favorable combination of the following
conditions to germinate, sporulate, and grow:
Fungal spores settling on the surface
Oxygen availability
Optimal Temperatures
Nutrient availability
Moisture (liquid or relative humidity above 70%)
The first four conditions are met in almost every building.
The key remaining factor is moisture, which may be controlled by
adhering to the sound construction practices discussed in this
presentation.
Moisture Management in Steel Stud Walls
Summary….
¤
Thermal Performance -
Because Steel Studs are good conductors of heat, ASHREA
has written into the 90.1 code correction factors for using
fiberglass in steel stud wall assemblies…
The new IBC, ICBO, SBCCI and BOCA all refer to
ASHREA’s 90.1 code for energy compliance.
Refer to www.bcap-energy.org for continuous updates on
your state’s energy code.
Moisture Management in Steel Stud Walls
Summary….
¤
Wall Assembly Recommendations
¤ Structural Requirements
¤
Fire Requirements
¤ Sound Transmission
¤ Economics
Specifying steel stud wall systems based on climate will
increase your projects chance to perform long term.
Moisture Management in Steel Stud Walls
Summary….
Let us know what we can do to help you on your next
project…..
Moisture Management
with
Insulated Sheathing
in
Steel Stud Walls
QUESTIONS