Condo Media - November 2022 - 44

The Region MAINE
MAXIMIZING
Building Science: 101
Ventilation Best Practices
[By Jack Carr, P.E., LEED-AP]
With winter approaching, I am
often asked what cold-weather planning
the building committee should
be considering. Surprise is expressed
when I suggest the condominium's
attics and their venting. It is true an
attic only needs one-tenth the ventilation
in the winter than in the summer
to control moisture buildup and
temperature, but the wintertime
also has some unique issues.
l First, if the building committee
wants attics to be inspected by an
engineer or contractor, it is a lot
easier inspecting a cold attic than
an attic on an August afternoon.
l Secondly, if repairs are needed,
it is better to prepare bid documents
before the busy spring and summer
construction season.
42 CONDOMEDIA November 2022
l Finally, ice dams are caused by
excessive heat loss through an inadequate
attic insulation/vapor barrier.
If the attic ventilation system cannot
properly remove this hot air against
the underside of the roof sheathing,
a cycle of freezing and thawing of
expanding roof ice may infiltrate
through the roof surface, causing
interior water damage.
There are a lot of myths about what
makes good ventilation in an attic.
One venting myth to dispel is the best
natural ventilation is rising hot air venting
solely through ridge or gable vents.
This is sometimes called gravity ventilation.
Tests have shown this chimney effect
is negligible when compared to wind
movement, which has a much higher
efficiency and allows for a considerably
smaller net venting area to be successful.
WIND MOVEMENT
The difficulty with relying on wind
movement is areas of high and low
pressure will change with wind direction;
thus, existing buildings are
dependent on the structure's design
and orientation for determining the
type and location of vents. The best
designs have the outlet as high as
possible, such as a ridge vent, and the
inlet as low as possible, such as the
soffit area. To improve this airflow,
air chutes are often installed during
initial construction or later retrofitted.
These chutes are formed plastic
channels that are attached to the roof
joists and are butted up to the soffit
vents to act as a pathway conduit
for air coming through the soffit
vents. They also serve as a barrier
to prevent the attic insulation from
clogging the soffit vents.
Soffit vents are probably the most
important of all vents as they can act
as both an inlet and outlet for airflow.
That is why it is imperative they be
kept free of debris or other material
that could clog the vents. For this
reason, attic inspection should ensure
the attic's floor insulation is not
currently blocking the soffit vent's air
pathway. This blockage can occur as
early as the initial installation of the
attic floor insulation or after the insulation
is disturbed by recent work
on the sprinkler or electrical systems
found in the attic. Without soffit
venting, the ridge or high gable vents
would draw make-up air through the
ceiling instead of from outside. For
this reason, the soffit vent should
have at least 50 percent of the net
free area (NFA). This NFA rating is
stamped on vent products. A rule of
thumb is that the summer ventilation
requirement can be estimated by
determining the volume of attic
space and dividing by two, which
will produce the needed cubic feet
per minute (cfm) of ventilation.
Local building codes often require
1 square foot of venting for every
300 square feet of ceiling space.
Condo Media - November 2022

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