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Vapor
Laws:
1. The higher the temperature,
the more vapor the air can hold; the lower the temperature,
the less vapor.
2. The larger the space, the more vapor it can hold; the smaller
the space, the less vapor it can hold.
3. The more vapor in a given space, the greater will be its
density.
4. Vapor will flow from areas of greater vapor density to
those of lower vapor density.
5. Permeability of insulation is a prerequisite for vapor
transmission; the less permeable, the less vapor transfer.
The average water vapor saturation is about 65%. If a room
were vapor-proofed, and the temperature were gradually lowered,
the percentage of saturation would rise until it reached 100%,
although the amount of vapor would remain the same. If the
temperature were further lowered, the excess amount of the
vapor for that temperature in that amount of space would fall
out in the form of condensation. This principle is visibly
demonstrated when we breathe in cold places. The warm air
in our lungs and mouth can support the vapor, but the quantity
is too much for the colder air, and so the excess vapor for
that temperature condenses and the small particles of water
become visible.
In conduction, heat flows to cold. The under surface of a
roof, when cold in the winter, extracts heat out of the air
with which it is in immediate contact. As a result, that air
drops in temperature sufficiently to fall below the dew point
(the temperature at which vapor condenses on a surface). The
excess amount of vapor for that temperature that falls out
as condensation or frost attaches itself to the underside
of the roof.
Water vapor is able to penetrate plaster and wood readily.
When the vapor comes in contact with materials within walls
having a temperature below the dew point of the vapor, moisture
or frost is formed within the walls. This moisture tends to
accumulate over long periods of time without being noticed,
which in time can cause building damage.
To prevent condensation, a large space is needed between outer
walls and any insulation which permits vapor to flow through.
Reducing the space or the temperature converts vapor to moisture
which is then retained. The use of separate vapor barriers
or insulation that is also a vapor barrier are alternate methods
to deal with this problem. Aluminum is impervious to water
vapor and with the trapped air space is immune to vapor condensation.
Testing Thermal Values
U FACTOR is the rate of
heat flow in BTU's in one hour through one sq. ft. area of
ceilings, roofs, walls or floors, including insulation (if
any) resulting from a 1 degree F. temperature difference between
the air inside and the air outside.
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MEMORY JOGGER: U = BTU'S flowing ONE hour, through ONE
sq. ft. for ONE degree change.
R FACTOR or RESISTANCE to heat flow is the reciprocal
of U; in other words, 1/U. The smaller the U factor
fraction, the larger the R factor, the better the insulation's
ability to stop conductive heat flow. Note: Neither
of these factors include radiation or convection flow.
There are at present two kinds of techniques generally
used by accepted laboratories to measure thermal values:
the guarded hot plate and the hot box methods. The results
obtained seem to vary between the two methods. Neither
technique simulates heat flow through insulation in
actual everyday usage. Thermal conductivity measurements,
as made in the completely dry state in the laboratory,
will not match the performance of those same insulations
under actual field conditions. Most mass type insulating
materials become better conductors of heat when the
relative humidity increases because of the absorption
of moisture by the insulator. (Try keeping your feet
in a pair of wet socks.) Therefore, mass insulations,
which normally contain at least the average amount of
moisture which is in the air, are first completely dried
out before testing. In aluminum insulation, there is
no moisture problem. Aluminum foil is one of the few
insulating materials that is not affected by humidity,
and consequently, its insulating value remains unchanged
from the "bone dry" state to very high humidity
conditions. The R Value of a mass type insulation is
reduced by over 35% with only a 1-1/2% moisture content,
(i.e.: from R13 to R8.3). The moisture content of insulation
materials in homes typically exceeds 1 -1/2%!
In spite of the advances made by space technology in
insulation systems based on understanding and modifying
the effects of radiation, no universally accepted laboratory
method has yet been devised to measure and report the
resistance to heat flow of multi-layer foil. Until such
a method that will satisfy rigorous laboratory demands
is devised, we must be content to make our judgments
on the basis of common sense and experience.
There are many different types, grades, and qualities
of aluminum foil insulation designed for a variety of
applications. Matching the correct foil product to the
specific job is extremely important to maximize final
performance.
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