If insulation sized for 6 ¼ inches is used in a 5 ½-inch wall cavity, what happens to its effectiveness?

When insulation that is sized for 6 ¼ inches is installed in a wall cavity that is only 5 ½ inches deep, the insulation will be compressed. This compression reduces the insulation's effectiveness because it does not fill the space properly, leading to gaps that allow air movement and thermal bridging. Insulation performs best when it is installed to fit the space it is intended for without being compressed, as this allows the material to trap air effectively, which is key to its insulating properties.

When insulation is installed in a space that is too small for its intended thickness, the R-value—the measure of insulation's ability to resist heat flow—decreases. A lower R-value means that the insulation will allow more heat transfer, thereby diminishing its effectiveness in maintaining temperature and energy efficiency within the building.

In contrast, insulation that perfectly fits the cavity enhances the thermal resistance and leads to better energy conservation. Thus, the overall thermal performance decreases when insulation is improperly sized for the wall cavity.