If you do not have enough insulation it is better to fill the cavity than to add to the outside. The primary benefit of the insulation is that it provides a smooth surface for the application of the siding.
House wrap is designed to break airflow and drafts.
Its performance is questionable, but the cost is so low it is generally worthwhile. The telltale sign of excessive interior moisture is condensation on windows if you are in a cold area. If you do not have enough insulation it is better to fill the cavity than to add to the outside. The primary benefit of the insulation is that it provides a smooth surface for the application of the siding. House wrap is designed to break airflow and drafts.