Answer:

Heat insulation performance of thermal insulation is expressed by thermal resistance and thermal resistance is calculated by dividing "thickness of insulation material" by "thermal conductivity of insulation material". 

Thermal resistance (R-value) (㎡ · K / W) = thickness (m) / thermal conductivity (W / m · K)

The thermal insulation performance of the insulation is proportional to the thickness . 
In other words, if it is crushed to make it thin, the insulation performance will be lowered accordingly .

The insulation performance when crushed is calculated from the thermal conductivity according to the density (apparent density) when crushed and the thickness when crushed.

Example) 
Insulation material with a thickness of 100 mm crushed to 50 mm (apparent density: 20 K) 
Thermal insulation performance in normal condition 10 K 100 mm 0.10 (m) / 0.050 (W / m · K) = 2.0 (㎡ · K / W) 
Thermal insulation performance when crushed 20 K 50 mm 0.05 (m) / 0.042 (W / m · K) = 1.2 (m 2 · K / W)

As mentioned above, the insulation performance will be down by 40%. 
It is not recommended as a construction method because insulation performance will be reduced if it is crushed and made thin .

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