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Air Infiltration
Air Infiltration, in essence, bypasses insulation. It transfers
heat by the gross flow of air between the exterior and
the interior. The underlying force behind air infiltration
is the air pressure difference between the exterior and
the interior. Air pressure differences can be caused by
wind or stack effect.
Air infiltration may be eliminated by air sealing the
building envelope.
Conventional insulations traditionally use “house
wraps” in an attempt to air seal the building envelope.
Vapor retards may also serve to reduce air infiltration.
ASTM E 779 - Standard Test
Method for Determining Air Leakage Rate by Fan Pressurization
Air infiltration
can be measured using the “Blower
Door Test." A fan is installed in an exterior door of the
house to be tested (and all other doors, windows, etc.
are closed or sealed). The fan speed is adjusted to create
a calibrated pressure differential between the outside
air and the inside air. The air flow through the fan needed
to maintain this pressure difference is measured and used
to determine the air leakage rate.
Air Intrusion
Air intrusion is
also called “wind wash.” Unlike
air infiltration, wherein air moves from the exterior to
the interior, air intrusion occurs when air enters the
insulation from the exterior and exits back to the exterior.
There is no drafting of air to the interior of the building
but the thermal gradient of the insulation is disrupted.
In effect, air intrusion introduces forced convection into
the building envelope (wall, ceiling, etc.). Air intrusion
is caused by wind.
Spray polyurethane foam
For the same reasons
spray foam stops air infiltration, it will stop air intrusion.
Summary
Closed-cell spray polyurethane foam effectively blocks
the three primary and the three secondary mechanisms of
heat transfer, offering insulation efficiencies well beyond
that suggested by its high R-value alone. Spray foam’s
insulating efficiency remains high when other insulations’ efficiencies
drop due to wind, high temperature differences, and moisture.
Read our article about
R-value to learn more. |
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