Adjust Insulation for Compression

Calculating the impact of compression on thermal resistance (R-value) for low-density open-cell insulation (polyester, stone wool or glass wool) is a complex process that involves considering the material's specific properties.

When low-density open-cell insulation is compressed, its structure changes, reducing the number of air pockets or voids within the material. These air pockets are essential for trapping air, which is a poor conductor of heat and vital to providing insulation.

To account for low-density open-cell insulation compression, the material properties need to be updated to create a Custom Material. For each 25.4 mm (1 inch) of compression, approximately 7% of the insulation thermal resistance can be considered lost. For simplification, it can be stated that per 5 mm of compression, approximately 1.3% of the insulation thermal resistance can be considered lost.

Let's look at an example and assume 75 mm of the low-density (11 kg/m3) glass wool insulation will be compressed into a 64 mm steel frame. The original supplier stated insulation thermal conductivity is 0.04 W/m·K for 75 mm. Based on 1.3% per 5 mm, the compression rate and lost thermal resistance are 2.86% for 11 mm of compression.

Supplier stated glass wool Insulation

0.075 m / 0.040 W/m·K

= R 1.875 m²·K/W

Compressed glass wool insulation

0.075 m / 0.040 W/m·K * (1-0.0286)

= R 1.821 m²·K/W

Speckel custom material Input

0.064 m / 0.03414 W/m·K

= R 1.821 m²·K/W

In all cases, we recommend speaking to your supplier for testing data related to compression. This is an approximation that accounts for the increased installed density only.

To adjust insulation for compression, hover over any generic material on the left of the screen and left-click the material icon in the top left corner. A detailed dropdown will then be revealed. Left-click the material property and add a revised thermal conductivity, as per above. Click Update.

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