It’s a Breeze: Understanding Proper Attic Ventilation of Sloped Roofs

It’s a Breeze: Understanding Proper Attic Ventilation of Sloped Roofs

nderstanding and
implementing proper
attic ventilation
design measures can provide
year-round benefits to owners
of sloped roof assemblies.
These benefits include:
getting the maximum service
life out of the building materials
used in the roof assembly,
added protection
against damage to wood
roof sheathing and structural
materials, minimizing ice
damming, a reduction in
energy consumption during
all four seasons of the year,
and added comfort for occupants.
A number of factors play
a role in the need for better
ventilation design measures.
One such issue is a result of
the 1970s energy crisis.
Since that time, there has
been an increased focus on
energy efficiency, with
buildings being designed
and constructed us ing more
insulation and less air infiltration.
Another reason is that
while the national code bodies
BOCA, ICBO, and
SBCCI have their place in
assuring that minimum standards
are met when buildings
are constructed, there
seems to be some debate as
to when it is necessary to
follow these codes (i.e.,
when determined to be necessary
by a building official’).
170°
Roof Sheath
Temperature ~
140″ Attic \
Temperature 11 s· Attic
Figure i: Unvented Radiant heat penetrating through roof sheath and attic
enters living areas of home. Vented With proper ventilation the heat is vented
out of the attic keeping living areas cooler.
)
Figure 2: Unvented Moisture rising up through the house condenses in the attic,
causing damage to studs, insulation, and other material. Vented A vented attic
allows moisture to escape.
1
As testing continues
and further research is performed
(such as studies by
The Small Homes Council
at the University of
Jllinois), some of these
requirements may be
changed. A higher minimum
standard and greater
uniformity would perhaps
be more suitable for
today’s energy-efficient
designs and materials.
Let’s start by establishing
what is considered
proper ventilation. Ventilation
comes from the Latin
word ‘findere,” “to fan ,” the
action word for causing air
to move. It is this concept
of air movement or
refreshing the air by constantly
replacing the
chambered air in the attic
with new fresh air on
which we want to focus.
For this article we shall
assume that the structure
has properly installed insulation
, that the attic space
is independent from the
conditioned air space, and
that all the bathroom ,
laundry room, and kitchen
exhausts are vented to the
outside of the structure
and not into the attic
space.
Our goal is to establish
a specific amount of air
movement to provide
& Adding to the problem are perhaps the outdated codes them-
9 selves, many of which refer to the guidelines set forth in the
Federal Housing Administration statutes of the Minimum Property
Standards, which appeared in the November I 958 edition.
year-round ventilation benefits.
In order to do this, system components must be carefully
sized and placed so they provide a constant, balanced flow of air
moving in a uniform direction.
To begin, let’s take a closer look at the physical properties
inherent in attic spaces that create the need to ventilate. These
November 1999 Interface • 11
Figure 3: Diagram of a balanced ven tilation system for
attic spaces. Soffit and ridge vents are shown.
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12 • Interface
About 50 percent of the
total installed ventilation
area
About 50 percent
of the total installed
ventilation area
physical properties during the
warmer temperatures are much
different than during the colder
temperatures.
Figure 1 depicts the effect
of heat transfer and build-up !I
that occurs during warmer
temperatures. Figure 2 represents
the effects of moisture
transfer and build-up that
occurs during colder temperatures.
Interestingly, while the
problem of attic heat and
moisture transfer and build up
involve distinctly different
causative properties, they
share a common solution.
The goal for designing this
common solution should be an
efficient, balanced, uniform
flow of air along the underside
of the roof sheathing. Hence,
“It’s a breeze.” The most efficient
way to achieve the
desired effect of a balanced system is to provide a continuous intake of air along
all available soffit areas (lowest points ) and a continuous exhaust vent along the
roof ridge areas (highest point).1 See Figure 3.
While this design is adequate for taking advantage of the thermal effect to
maintain circulation, the design would be greatly enhanced by the inclusion of
an electrical ly-powered, motorized fan. This should be connected to a thermo- S’
stat as well as a humidistat control. These fans are available in different sizes and ·
should be spaced to accommodate each different project. Specific manufacturers
should be consulted for proper use and placement of their products.
When determining ventilation needs, it is almost always best to design above
the minimum established guidelines in order to provide year-round benefits of
the ventilation system. In response to the need for greater energy conservation,
private research, university, and testing labs have developed an easy-to-follow,
five-step process for creating an effective and efficient ventilation system.
TO CREATE A BREEZE
For an efficient, fixed-vent system (passive)
I. Determine the square footage of attic ceiling area to be ventilated (length in
feet x width in feet = square feet of attic area).
2. Establish an airflow rate. [Note: tests conducted at the University of Illinois
established a suggested air flow rate of 1.5 cubic feet per minute
(CFM)/square foot of attic ceiling area].
3. Determine the net-free area required to establish the desired airflow (net-free
area is the total unobstructed area through which air can enter or exhaust a
non-powered ventilation component). This factor is usually measured in
square inches. (Square feet of attic ceiling x 1.5 square inches/square foot –
total net-free area required.)
4. Determine the amount of intake and exhaust net-free area required (divide
answer from step #3 in half).
5. Determine the number of intake and exhaust ventilation units to be used.
Check specifications for individual products to determine the actual net-free
vent area . (Note: the use of louvers and screens affects the actual net-free vent
area realized .)
November 1999
For an efficient power fan vent system (active)
1. Determine the fan capacity needed to provide
approximately 10 to 15 air exchanges
per hour (attic ceiling square feet x 0. 7 =
CFM capacity).’ Note: for roofs with an s/12
pitch or higher, add approximately 20% more
CFM to accommodate the larger volume of attic
space.
2. Determine the amount of intake venting
required (CFM rating of fan divided by
300 = square feet of intake ventilation
needed). To turn this number into square
inches (the common method of specifying
net-free area), multiply by 144 (square
inches in a square foot).
While certain elements of the actual construction
methods and materials used may influence
the results achieved (i.e. , thickness of insulation,
vapor barrier, climate and humidi ty), these ca lcu lations
can provide a benchmark va lue to assist in
developing proper attic ventilation systems.
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References
EQUIPMENT COMPANY
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“A Comprehensive Guide to Planning Attic Venti lation
Systems.” Principles of Attic Ventilation, sixth edition, 1997:
1-2, 5, and 14-16.
Feirer, John L. and Hutchins, Gilbert R. Carpentry and
Building Construction, third edition. California: Glencoe, 1986.
Hoke, John Ray, Ed. Architectural Graphics Standards, ninth edition.
New York: John Wiley and Sons, Inc., 1994.
Lile, Frank. “Let the Air Flow.” Professional Roofing, June 1998:
R6-R16.
NRCA Roofing and Waterproofing Manual, fourth edition. lllinois;
NRCA, 1996.
“Ventilation and Moisture Control for Residential Roofing.”
ARMA Technical Bulletin, March 1993: # 104-RR-86.
Footnotes
‘Lile, Frank. “Let the Air Flow.” Professional Roofing, June 1998:
R6-R16.
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November 1999
‘ Feirer, John L. & Hutchins, Gilbert R. Carpentry and Building
Construction, third edition. California: Glencoe, 1986.
‘”A Comprehensive Guide to Planning Attic Ventilation
Systems.” Principles of Attic Ventilation: Sixth Edition. 1997:
1-2, 5, and 14-1 6.
ABOUT THE AUTHOR
MARCH 24-28,
2000
Interface • 13