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R-Value
Insulation is rated
in terms of thermal resistance, called R-value,
which indicates the resistance to heat flow. The
higher the R-value, the greater the insulating effectiveness.
The R-value of thermal insulation depends on the
type of material, its thickness, and its density.
In calculating the R-value of a multi-layered installation,
the R-values of the individual layers are added.
The effectiveness
of an insulated ceiling,
wall
or floor
depends on how and where the insulation is installed.
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Insulation
which is compressed will not give you its full
rated R-value. This can happen if you add denser
insulation on top of lighter insulation in an
attic. It also happens if you place batts rated
for one thickness into a thinner cavity, such
as placing R-19 insulation rated for 6 1/4 inches
into a 5 1/2 inch wall cavity.
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Insulation placed between
joists, rafters, and studs does not retard heat
flow through those joists or studs. This heat
flow is called thermal bridging. So, the overall
R-value of a wall or ceiling will be somewhat
different from the R-value of the insulation
itself. That is why it is important that attic
insulation cover the tops of the joists and
that is also why we often recommend the use
of insulative sheathing on walls. The short-circuiting
through metal framing is much greater than that
through wood-framed walls; sometimes the insulated
metal wall's overall R-value can be as low as
half the insulation's R-value.
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