Moisture Control Guideline
Moisture vapor is present in all
buildings and is generated from a variety of sources that sometimes
cannot be eliminated. As metal buildings are made more airtight for
thermal efficiency, this moisture is less likely to be diluted by
dry outside air.
While having a positive effect on
energy consumption, an increase in the insulation levels in metal
buildings will not reduce moisture control problems. As insulation
becomes thicker, roof sheets are colder and more likely to be below
the dew point. With sufficient moisture in the building, some will
penetrate the vapor retarder and condense on
the roof sheets and purlins.
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Winter Median of Annual Extremes
|
Maximum Allowed Indoor Relative Humidity (Constant)
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Maximum Allowed Indoor Relative Humidity Daytime
(Intermittent)
|
|
Zone 1: -10°F
(-23°C)
or Lower
|
30%
|
40%
|
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Zone 2: -10°F
to +10°F
(-23°C
to –12°C)
or Lower
|
35%
|
45%
|
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Zone 3: +10°F(-12°C)
or Higher
|
45%
|
50%
|
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As
a means of reducing this problem, the insulation used in the walls
and roofs of metal buildings should have continuous vapor retarders
with sufficient perm ratings.
While
such vapor retarders can be effective in limiting water vapor
transmission, they are not totally impermeable membranes. And all
too often, they may be subjected to cuts, punctures, or improper
closure systems.
So
while good insulation vapor retarders are important means of
controlling moisture, they are only one factor in a good moisture
control design.
Before
a building is erected, construction conditions and end-use processes
– both of which are related to moisture generation – should be
carefully evaluated. A design professional, such as an HVAC
engineer, should be consulted to establish the equipment and
construction techniques necessary to reduce the moisture in the
building. The accompanying map and chart provide a prudent guideline
for interior humidity levels to minimize condensation problems.
Finally,
proper ventilation of both interior areas as well as air spaces
above insulation is essential. In all metal buildings, natural or
mechanically powered ventilation is the single most effective means
you can take to control moisture in interior spaces.
The information
contained herein is a technical guideline and should not be interpreted as guarantee to avoid moisture related problems.
It is
important the insulation system be designed or
approved by a qualified professional.
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