A slippery slope:
Solutions for the condensation challenges of metal roofing systems
Posted
10/1/2008
This white paper was developed by the Metal Construction
Association, Glenview, Ill. More information can be found at
www.metalconstruction.org.
Water is the most common cause of rot and corrosion in buildings,
with moisture being a major contributor to the formation of mold. A
2004 Centers for Disease Control and Prevention report,
commissioned in response to rising public concern about the
problem, found that damp or moldy buildings are linked to episodes
of sniffles, coughing and wheezing in otherwise healthy
people.
Any evidence of liquid water within the building commonly is
misunderstood by engineers and property owners as leakage or
failure of roof covering materials-but condensation is frequently
the cause.
The principal thermodynamic properties responsible for
condensation in buildings are identical for all building envelope
materials. Moisture condensation is not unique to metal roofing
systems but frequently is the cause of rotting timbers or corrosion
in metal decking, metal roof panels and metal fasteners.
The control of condensation in steep-slope metal roofing systems
is important toward maintaining the effectiveness of the insulation
and protecting the roof material from degradation.
Condensation in roof systems is based on the flow of water vapor
through the various layers of building materials to a point where
cooling forces moisture to condense. Each building component has
some resistance to heat and to vapor transfer. The choice in
composition and assembly techniques of building components is the
key to successful control of condensation. A building's
environmental conditions determine the moisture content of ambient
air. Internal temperature, building use and air movement all impact
the relative humidity of the interior air and dictate the design
requirements necessary to prevent condensation.
The current construction practice of making buildings "tighter" to
improve energy efficiency conversely traps moisture inside the
building envelope. In many cases, the building is completely sealed
off so moisture cannot escape. Moisture is generated inside a
building when inhabitants breathe, cook, wash or shower; another
less obvious source of moisture is liquid left behind within the
construction materials.
The thermodynamic principle behind condensation is based on a
phase change from a gaseous state to a liquid state when
moisture-laden air contacts a cooler surface. Cold air has less
capacity to hold water vapor than warm air. The temperature at
which this phase change occurs is called the dew point. In a
building, air that comes in contact with cooler surfaces at windows
or the underside of the roofing plane will condense when it reaches
the dew point. The result can be rotting wooden components,
corrosion of metal components and mold formation.
Low-slope commercial buildings, such as warehouses and
distribution facilities, typically are not vented and rely on
different types of construction to control moisture inside the
building. Steep slope buildings (> 3:12 slope), such as
residential dwellings, institutional facilities or retail
structures, usually have a higher human occupancy load, resulting
in the need for greater moisture control. Building codes require
passive ventilation for steep-slope roofing construction.
In a typical residential roofing assembly, several components are
recommended to minimize condensation in the attic space:
• A vapor retarder located on the attic side of a ceiling
board.
• Insulation located above the air barrier on the warm side of the
surface.
• Vented air space above the ceiling.
• A moisture barrier or underlayment located between the roof
covering material and the roof deck.
• A roof covering on top of the underlayment.
Vapor Retarders
Vapor retarders are an important roofing system component in areas
where the mean daily temperature is 40 F (4 C) or lower, and where
indoor relative humidity is greater than 45 percent. Vapor
retarders physically block the fl ow of moisture laden air, which
prevents it from contacting a cold surface. Vapor retarders are
generally thin flexible membranes with a perm rating of 0.50 or
less. Perm ratings are established by standard ASTM E96. The perm
rating is a measure of a material' resistance to the passage of
water vapor through it under specified duration, temperature and
humidity conditions. A rating higher than 0.50 perms generally is
considered too diffuse to serve as a vapor retarder. A 1.00 perm
rating indicates the passage of 1 grain of water vapor through 1
square foot (0.09 m2) of material in one hour for each inch of
mercury pressure differential that exists between the two sides of
material (1 pound of water vapor = 7,000 grains; 1 inch of mercury
= 0.491 pounds per square inch). ASTM C755 includes information
about the selection of vapor retarders for thermal
insulation.
Vapor retarders are effective when they are placed on the warm
side of the building insulation, so for most roofing applications,
the proper location would be above the building's drywall ceiling.
Building owners are nevertheless advised to contact a design
professional to determine the proper design for and placement of a
vapor retarder. Calculations can be complex for more sophisticated
designs, such as when placing vapor retarders within layers of
insulation.
The vapor retarder can be placed anywhere in the building assembly
as long as its temperature remains above the dew point temperature.
This can be accomplished by installing a sufficient amount of
insulation on the outer side of the vapor retarder-and again,
consult a design professional for specific information on the need,
type and location of vapor retarder materials within a building
envelope.
The permeance rating of a vapor retarder material is an important
consideration when designing the building assembly. Condensation
can occur even with a properly chosen vapor retarder.
Moisture-laden air can leak through the joints of vapor retarder
membranes, through tears within the material or around
penetrations, such as lights, flues or ventilation piping. As a
result, joints in vapor retarder membranes must be sealed,
especially in colder northern climates where the moisture drive is
the strongest.
Other considerations when selecting vapor retarder materials
include:
• The chemical compatibility with the building's environment and
the other construction materials used.
• The effect of the vapor retarder materials on the fire
performance of the total system. FM Global, Johnston, R.I., lists
acceptable vapor retarder materials that do not affect the fire
ratings of roof systems when used properly.
• Tear resistance, tensile strength and resistance to rot and
mildew.
• The safety of the roofing installers. For example, a 6-mil
(0.15-mm) polyethylene sheet material is economical, easy to work
with and has an extremely low perm rating. However, installing this
slippery material on a steep-slope roof unwisely could subject
workers to hazardous conditions.
It is important to remember that vapor retarders can alleviate
condensation concerns, but they do not remove heat or prevent the
formation of ice dams. In some cases, moisture that is transported
through air leaks is more of a problem than diffusion through
nonvapor retardant membranes. In these cases, a properly installed
air barrier can provide more protection against trapped moisture
than vapor barriers.
Proper sealing of air barriers' side laps, end laps, window and
door penetrations, and foundation sills is critical to their
performance.
Insulation
In commercial roofing applications, the insulation material may be
loose-laid over the purlin or truss structure with the panel
attachment securing the insulation. A composite insulation board
wood panel (wood panel laminated to a rigid insulation) is also a
common substrate used under architectural metal panel roof systems.
Care must be taken not to compress and deform the insulation. A
metal or wood batten, counter-batten system also can be used to
create ventilation space below the metal panels where needed.
For residential construction, insulation in the attic cavity
minimizes the heat flow from the attic into the living space during
the warm seasons. It also minimizes the escape of warm heat from
the living space into the attic during the cold seasons. The heat
flow into the attic space below a roof comes from a number of
sources. Solar heating of the roof surface can raise the surface
temperature and conduct heat into the space below the deck. Heat
from chimneys, waste pipes and heating system duct work also raises
the temperature of the attic cavity. Finally, warm air from the
interior living space can escape through a ceiling into the attic
and raise the air temperature, as well.
In some modern homes where cathedral style or vaulted ceilings are
popular, insulation is placed between the roof rafters.
In construction where the insulation is placed above the
substrate, a vapor retarder may be needed between the roof
substrate and the insulation. In construction where the insulation
is placed below the substrate, a vapor retarder may be needed below
the insulation-on the warm side of the roof assembly.
Ventilation
Proper ventilation replaces the warm moist air with air that
contains less water vapor. Ventilation is relatively easy to
accomplish in conventional attic spaces; however, it is more
complicated when used in cathedral ceilings and with structural
insulated panels. Positive ventilation and air flow is necessary to
maintain a consistent temperature between the vented area and the
outside conditions.
Ventilation is generally expressed by the number of times per hour
the building air is replaced with outside air. This is referred to
as air changes per hour. The number of air changes required per
hour varies depending on the application. For commercial building
construction, general guidelines for whole building air exchanges,
such as in building types that do not have an attic space, are 3 to
5 air changes per hour for warehouses, 5 to 10 air changes per hour
for light manufacturing facilities and 10 to 20 air changes per
hour for heavy manufacturing. Air flow requirements can be
determined as shown in the graphic.
In residential construction, ventilation requirements are
dependent on the size of the attic and the placement and rating of
the vents. The two types of vents are intake and exhaust vents. The
combination of these two types of venting provides free-flow
ventilation, which is the most efficient way to move hot and moist
air in enclosed areas. Most building codes require that a minimum
free-flow ventilation area equal to 1 square foot (0.09 m2) per 150
square feet (14 m2) of attic floor area must be designed and
properly installed to provide proper ventilation. Where a properly
designed, 50/50 balanced eave and ridge ventilation system is used,
or with the use of a vapor retarder (such as in new construction),
a free-flow ventilation area equal to at least 1 square foot (0.09
m2) per 300 square feet (28 m2) of attic floor area is often
sufficient. The best ventilation technique is to combine eave and
ridge venting. This allows air in the entire attic to be pulled
from the soffits in the low areas of the eave to the vents at the
ridge in the high areas.
Homes with cathedral-style or vaulted ceilings must be designed
with free-flow ventilation beneath the roof deck through the use of
vent baffles or chutes, which create a space between the roof deck
and insulation. Alternatively, a ventilated deck subassembly can be
installed over or in place of the existing deck. Insulation should
never block the eave intake vents or be in contact with the
underside of a roof deck. Where residential construction does not
include an attic space, the International Building Code calls for a
minimum 1-inch (25-mm) vented airspace beneath the roof deck. In
Canada, the ventilation space requirement for this type of
construction is 1 1/2 inches (38 mm).
Venting air and water vapor from a building can be effective
against condensation, but ventilation alone does not cure the
problem.
Underlayments or Moisture Barriers
On roof applications over solid decking, an underlayment (with a
slip sheet as needed) is recommended as a secondary barrier against
water penetration through a roof system. The underlayment should
"breathe" wherever possible, allowing air to pass through while
shedding moisture.
The National Roofing Contractors Association, Rosemont, Ill.,
recommends a minimum of one #30 asphalt-saturated felt for use as
underlayment with architectural metal roof systems. NRCA also
recommends that one layer of rosin-sized sheathing paper or other
slip sheet material be used over the asphalt-saturated felt prior
to panel installation where adhesion or bonding of the metal panel
to the felt underlayment may occur. In cold climates, an ice dam
protection membrane is recommended around the perimeter, at valleys
and around penetrations; this measure often is required by local
building codes in addition to the felt underlayment. A minimum
amount of coverage also may be required in areas on the roof
surface where ice potentially could build up.
Moisture Balance
If a balance between wetting and drying is maintained, moisture
will not accumulate and condensation related problems in roofing
are unlikely. The extent and duration of wetting, storage and
drying must be considered when assessing the risk of moisture
damage. The concept of using materials with greater drying
potential and storage capacity has begun to receive more attention.
A quick reference guide using this Moisture Balance Approach is
shown below.
Condensation Control Materials
A benefit of metal roofing is that it does not absorb condensed
water like other roofing materials. Certain fleece materials are
available to absorb liquid condensate into capillary pores and
release it back by evaporation or drainage at a later point when
conditions favor evaporation. The water vapor then gets transported
out of the system by convection and diffusion. These self-adhesive
fleece materials are typically laminated to the underside of metal
roof panels during manufacturing but before forming. The adhesive
layer acts as a barrier to prevent the moisture from contacting the
surface of the metal roof underside. In many cases, these materials
can replace anti-condensation blankets and vapor retarders.
Careful design of roof systems can prevent or at least minimize
condensation in building assemblies. The proper use of vapor
retarders, insulation, ventilation and moisture barriers can reduce
the presence of moisture in a building and control the formation of
condensation. Building codes and ASTM offer some guidance in the
selection of these building components. However, it is always
recommended that a moisture control or roof design professional be
consulted for specific information about how to design metal roof
systems to minimize condensation problems.
References
NRCA, "Metal Roofing and Waterproofing Manual."
MBMA, "Metal Roofing Systems Design Manual."
ARMA, "Technical Bulletin: Ventilation and Moisture Control for
Residential Roofing."
Straube, J.F., "Moisture, Materials & Buildings," HPAC
Engineering.
Straube, J.F., "Moisture in Buildings," ASHRAE Journal, January
2002.
Hens, H. and F. Vaes, "The Influence of Air Leakage on the
Condensation Behavior of Lightweight Roofs," Air Information
Review, Volume 6, No. 1, November 1984
www.metalconstruction.org