In steel buildings, visible condensation occurs on exposed surfaces while
concealed condensation occurs within the insulation or roof and wall cavities.
Visible condensation
Visible condensation shows up when moisture appears on the inside surface. This
is the nuisance type of condensation you see on windowpanes and purlins that
results in dripping. Visible condensation can be controlled with ventilation and
the addition of reflective insulation. The insulation will reduce the cold
surface area where condensation is likely to occur. Running the insulation over
the inside of steel framing members will reduce this even further.
Ventilation to exchange indoor air with high moisture content for outdoor air of
lower moisture content will even further reduce visible condensation. Keeping
fans running to circulate the air will decrease the excess humidity in the air
as well. Installing a vapor barrier under the floor during construction will
also help prevent the problem.
Concealed condensation
Concealed condensation is a more difficult problem and can cause more damage
than visible condensation. Concealed condensation occurs when vapor has passed
through the insulation or the vapor barrier to condense either inside the
insulation or on the metal surface of the roof or exterior wall. It is best
controlled during construction by having proper ventilation in the roof and a
tight vapor barrier on the warm side of the wall.
Fiberglass batts will hold moisture inside as either water vapor or condensed
moisture both of which negate its effectiveness as an insulating material.
Removing moisture from fiberglass insulation once it gets inside is difficult
and replacement is usually called for.
Preventing concealed condensation is the most effective means of control. Proper
use of a vapor barrier is critical. As vapor barrier will significantly reduce
the movement of vapor transmission but not stop it completely, it is more
appropriately called a vapor retarder. The ability of a material to resist water
vapor is called permeance. A material must have a permeance of less than 1 grain
of vapor transmitted per hour per square foot per inch of mercury vapor pressure
difference in order to be considered suitable for use as a vapor retarder.
Aluminum foil reflective insulation has a perm rating of 0.05 making it an
excellent vapor barrier. When properly installed, waterproof reflective
insulation can reduce or eliminate condensation. As reflective insulation is
non-absorbent, it will not mildew or promote fungus growth. It can be stapled,
nailed, or glued in place. As the
Foil-Foam-Foil
and Foil-Bubble-Foil
types of reflective insulation products carry R-values of up to
14.5 (Details),
they can double as wall/roofing insulation and vapor barrier in one.
Other advantages of using reflective insulation over fiberglass and polytheylene
sheeting are that reflective insulation provides a superior aesthetic finish and
reflects light inside the building thereby reducing indoor lighting
requirements.
Click here for comprehensive information the different types of reflective insulation. |