There's no reason to guess about this any longer. Leaky ductwork often sucks pollutants into the house and can also increase the leakage in the house shell itself. Next, make sure the insulation is done correctly. This means no gaps, voids, compression, or places where outside air flows through the insulation. Typical insulation installation often leaves cold spots where condensation can occur, leading to mold growth.
Mold is one of the worst asthma triggers.
The most important thing to do to keep mold out of the house is to keep the house dry. Careful foundation drainage, damp proofing, gutters and downspouts, grading the land away from the house, and proper flashing all help to ensure water doesn't get into the house. A poly vapor barrier should be used on the ground of a crawlspace or under a concrete slab. Almost all the asthma triggers require relatively high moisture levels to grow and thrive. Keeping the moisture and relative humidity levels down are the most effective ways to reduce the amount of asthma triggers in the house.