Attic Ventilation

“To ventilate or not to ventilate?” is a
question sometimes discussed these days. Actually the discussion
more often goes along the lines of, “How much more ventilation
should be provided to do the job correctly?” With asphalt roofing
shingles, there are at least two primary reasons to provide
adequate below-deck ventilation: to reduce heat and moisture
build-up.
Typical shingle and structural problems experienced from
improper ventilation can be:
• Premature shingle failure,
• Shingle buckling due to deck movement,
• Deterioration of the wood components,
• Moisture accumulation in the deck and insulation, and
• Accumulation of airborne pollutants.
In addition, where energy conservation is important, it is
counterproductive to allow increased attic temperatures. Higher
temperature allows for heat intrusion into the conditioned interior
of the structure, which in turn causes increased energy usage.
Proper ventilation of the underside of the roof deck or attic cavity
helps to minimize heat build-up.
In areas of colder climates, lack of proper ventilation can lead
to moisture accumulation and ice damming. All structures (some
more than others) generate moisture from within. This warm,
moist air in the form of water vapor moves through the structure
and eventually contacts the cooler underside of the roof deck
where it can condense into liquid water. At the very least, this
will cause paint to peel, plaster to crack, and wood to rot.
20 • Interface August 2000
Photo 1: Poor ventilation on this roof resulted in a significant dimensional change in
the wood roof deck, which showed up as a ridge in the asphalt shingles.
Photo 2: These shingles were applied over an unventilated area and show evidence
of accelerated aging due to the resulting heat build-up.
BY RAYMOND L. CORBIN
A T T I C
V E N T I L A T I O N :
WWhhyy IItt??ss
Increases of moisture content can affect the wood
deck’s dimensional stability, possibly leading to
excessive deck movement with its consequent stress
upon the attached shingles. In addition, as the warm
air enters the non-ventilated attic, it causes snowcovered
roofs to melt at the deck surface. As the
snow melt flows down the deck, it reaches the cooler
overhang at the eaves and refreezes, causing an
ice dam. This can result in a roof leak unless the
structure has the proper self-adhering ice-and-water
membrane installed at the eaves.
Another heat and moisture problem is the cathedral
ceiling. In this type of construction it is common
to see high R-value thermal insulation applied
directly below the surface used to fasten the shingles.
This type of construction not only prevents
venting the underside of the shingle deck, it also
allows the insulation to keep the shingles hotter.
Prolonged heat exposure can have a negative effect
on the shingle’s coating asphalt, reducing its flexibility
and durability, thus reducing the life expectancy
of the shingle. A solution to this problem is to provide
continuous venting (from eaves to ridge)
through an air space located between the shingle
deck (nailing surface) and the insulation.
Additionally, an adequate vapor retarder must be
installed below the insulation to prevent moisture
build-up or, even worse, condensation at the shingle
and deck interface.
Another moisture-related issue is the potential for
shingle ridging on new construction. This problem can
arise whenever there is inadequate ventilation and no
vapor retarder. Often the interior moisture drive is sufficient
to cause buckling of the underlayment felt at
the joints in the plywood decking and, consequently,
the covering shingles. The
resulting pattern of ridging often
causes a concern to the owner.
Generally, but not always, the ridging
will settle or become less obvious after
the first year.
All of the afore-mentioned issues
relate to the structure or to the shingles
covering the structure. Consideration
must also be given to the interior environment
of the structure and how that
environment relates to the health of the
occupants. In the case of a non-vented,
sealed attic, the ability of the cavity to vent
moisture and prevent accumulation of
microbes and airborne pollutants is restricted.
The trapping of fungal-growing moisture and
microbial contaminants within the cavity and
the structure presents a health concern to the
occupants. Further study is a must before utilizing
such construction.
Photo 3: The shingles on this building (same roof, same elevation as Photo
2) were applied over a ventilated attic area and show no
evidence of accelerated aging.
August 2000 Interface • 21
22 • Interface August 2000
Summary
When evaluating the pros and cons of “To ventilate or not to
ventilate,” please consider the negative effect that prolonged
heat exposure can have upon the flexibility of the coating
asphalt, and thus, the shingles’ aging characteristics. Asphalt
roofing shingles are constructed with a blown (oxidized) asphalt
that normally performs well over a wide range of conditions.
How ever, logic should dictate that the hotter the shingles are
kept, the more likely that this oxidization or aging process will
continue, or even be accelerated. Allowing an increase in belowdeck
temperature only serves to elevate the temperature of the
shingle already being heated by the sun’s radiant energy, resulting
in a negative effect to its long-term durability. Lastly, let us
not forget that lack of proper ventilation within the structure
could adversely affect the health of its occupants.

Raymond L. Corbin is Director
of the Better Understanding of
Roofing Systems Institute (BURSI),
sponsored by Johns Manville. He
holds several United States roofing
shingle design and application
patents. Ray is a faculty member of
the Roofing Industry Educational
Institute (RIEI) and has served as
Chairman of the Code Committee
for the Asphalt Roofing
Manufacturers Association (ARMA).
ABOUT THE AUTHOR
RAYMOND L. CORBIN