The original compression of the insulation was 25% at its nominal
joint width of 2.00 inches. When the joint opens to its maximum joint
width of 2.25 inches the change in compression of the insulation
decreases to ≈19%. Since the mass of the insulation is constant
and in the perimeter joint width increases which in turn causes an
increase the volume of the perimeter joint, therefore the density of
the insulation compressed in the perimeter joint decreases.
Effective Density
Unlike static perimeter joint protection, the effective density of a
dynamic perimeter joint protection decreases when the perimeter
joint opens from the nominal joint width. To understand changes in
effective density consider the following equations.