When warm air escapes through a gap in your insulation, it encounters a cold surface. This can be an exterior wall or somewhere inside the attic. When the warm air meets the colder surface, it creates condensation. The condensation will drip back down into your attic, wetting the insulation and wood, creating a breeding ground for mold and mildew. The same can happen behind your walls if the insulation or vapor barrier has gaps.
One sure sign your attic insulation is inadequate is melting snow during the cold winter months. As the hot air escapes from your home, the attic warms the roof to above the freezing mark. As a result, the snow melts from underneath and then refreezes when it meets the cold eves. At first, you may only notice small icicles, but the ridge of ice will back up onto your roof over time. This can jeopardize the roof’s structural integrity if the ice gets under the shingles and allows water to infiltrate the attic.