This Side Up: Installing Insulation Correctly
Picking the right face is essential with fiberglass insulation that has a vapor barrier (meaning with Kraft paper or foil attached on one side).
It should always face the warm-in-winter side. Thus, in cold climates, it should face the interior; in hot humid climates, it should face the exterior.
If a vapor barrier is installed on the wrong side, it can trap moisture inside wall cavities causing condensation to accumulate. This results in a musty smell, and eventually will cause wood to rot; it could even promote a serious mold problem.
Got insulation installed wrong side up? If so, remove it immediately, and inspect the subsurface for moisture or mold before reinstalling or replacing.