One of the most effective means to attenuate the sound from an air handler is to affix sound absorbing lining to the interior walls of its associated duct systems. This sound absorbing lining is typically glass fiber. For many applications, a thin non-porous cover is placed over the glass fiber to protect and maintain the integrity of the material. However, this thin impermeable layer adds mass that is acoustically significant. Moreover, gluing the thin layer to the fiber further increases the mass and likely adds damping and stiffness to the layer. The objective of this research is to better understand the acoustic impact of this thin protective layer without and with glue. Accordingly, the insertion loss of a scaled down duct was measured with a thin protective layer affixed to the glass fiber. If the protective layer is glued to the fiber, it is demonstrated that the mass layer does little to improve the attenuation at low frequencies and adversely affects attenuation at higher frequencies. However, the attenuation is greatly improved at lower frequencies if the mass layer is pinned instead of glued to the fiber. In addition, finite element simulation was used to investigate the effect of varying the mass of a cover on insertion loss. The combined measurement and simulation results suggest that sound absorbing material manufacturers should work to produce linings that are spot tacked to the glass fiber instead of glued.